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Eye movements and industrial inspection
Ergonomics of visual inspection
Applied Ergonomics 1979, 10.3, 145-154
Eye movements and industrial inspection E.D. Megaw and J. Richardson Department of Engineering Production, University of Birmingham
A brief survey is given of the eye movement parameters which contribute to visual search strategies. The literature on eye movements and industrial inspection is reviewed and some recent studies carried out at the University of Birmingham into the eye movements underlying four inspection tasks are described. These tasks included the inspection of tin cans, electrical edge connectors, men's briefs and woven fabric.
Introduction Only with comparatively recent technical developments has it been possible to record the eye movements of operators performing tasks outside the laboratory. In addition to industrial inspection these tasks include vehicle driving (Mourant and Rockwell, 1972; Rockwell and Zwahlen, 1977), various activities of aircraft pilots (Carbonnel, Ward and Senders, 1968; Stern and Bynum, 1970)and X-ray examination (Kundel and Wright, 1969; Kundel and La Follette, 1972). The number of published studies is, however, small when compared with the number of published laboratory tasks. The laboratory based tasks have included reading (Tinker, 1958; Rayner, 1975), facial recognition (Noton and Stark, 1971), comparative judgements (Buchsbaum, Pfefferbaum and Stillman, 1972; Gould and Peeples 1970), search tasks (Enoch, 1959; Nodine, Carmody and Kundel, 1978 ; Megaw and Richardson, 1979), observing pictorial scenes, (Yarbus, 1967) and vigilance (Mackworth. Kaplan and Metlay, 1964). Two important results arose from the last study. When subjects were required to monitor a dial for rarely occurring though easily discriminable signals, they often failed to report a signal although it had been visually fixated looking is not seeing. At the same time signals were sometimes reported which had not been presented (false alarms). That is to say, attentional and related factors cannot be ignored when considering search strategies.
cannot be assessed from performance data since inspectors may be adopting inappropriate search strategies. Alternative methods of assessment include subjective techniques, the use of a visibility meter (Eastman, 1968) and measures of effective useful field of view which are discussed later in the paper. The recording of eye movements provides valuable data to validate mathematical models which have been proposed to describe visual search behaviour. In the absence of such data researchers have relied heavily upon the observed cumulative distributions of search times to support the alternative models (Krendel and Wodinsky, 1960; Williams, 1966: Howarth and Bloomfield, 1968). While these models have proved useful in predicting inspection performance (Drury, 1975, 1978), they show several shortcomings which have been outlined by Megaw and Bellamy (1979). A less obvious but important reason for recording eye movements is the general interest that it stimulates in all those persons concerned with inspection procedures and quality control. This creates a favourable atmosphere in which to pursue ergonomic improvements. Additionally, the recording provides visual aid material which can be assin~ilated very readily into training programmes. It is not yet possible to evaluate the extent to which eye movement strategies can be taught to inspectors.
Eye movements and search strategies Why record the eye movement during inspection? The recording of eye movements can provide useful information on whether sufficient time is being allowed for the inspection of a product and. in particular, whether a product is being given adequate visual coverage. It is not, however, a simple exercise to establish standard times solely on the basis of eye movement recordings. Information is also required on fault difficulty or conspicuity since the easier faults are to detect the less thorough need be the coverage. The problem is further complicated by the fact that the expected fault severity for a particular product varies widely and different fault types are associated with different parts or areas of a product. Fault conspicuity
0003 6870/79/03 0145--10 $02.00 I~) IPC Business Press
From the point of industrial inspection the most relevant eye movements are the saccadic movements. These serve to bring different parts of the image of the product on to the fovea for detailed analysis. They are rapidly executed and usually range in amplitude between 5 ° and 40 °. Thus a 10° saccade typically has a duration of 35 ms and reaches a velocity of 400°/s. During the saccade the image becomes so blurred that the amount of detail that can be resolved is greatly reduced. The time elapsing between the presentation of a stimulus in the periphery and the onset of a saccade to fixate the stimulus is typically around 160 ms. Despite the favourable biomechanical properties of the oculomotor system, the frequency response of the saccadic control
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system is limited to around 2 Hz or 4 saccades/s (Hyde, 1959). Thus the average time elapsing between the end of one saccade and the beginning of the next, usually referred to as the fixation time, is rarely less than 200 ms. Nevertheless, shorter fixation times are observed. These precede small amplitude saccades which correct for under or overshoot o f the primary saccade (Megaw, 1975a, 1975b). If examiners are inspecting a product which is moving, the pursuit movement system becomes involved during the fixation phase. Pursuit movements function to stabilise the retinal image by matching the velocities of the eye and the moving stimulus. Up to display velocities of 20°/s the tracking error is very small, but as the velocity is further increased the eye velocity progressively lags behind and saccades are executed to compensate for the consequent error position. If subjects attempt to maintain their eyes stationary while observing a moving display, the eyes tend to follow the display although with a relatively low velocity (involuntary optokinetic nystagmus). Search strategy is a global term which reflects many parameters of the saccadic movements and these are described later. As a result of the publication of Visual Search Techniques (Morris and Home, 1960) there has been a tendency for those concerned with modelling search strategies to consider only two extreme strategies. These are the socalled random and systematic strategies (Howarth and Bloomfield, 1968). With a random search, successive eye fixations within a search trial are independent of each other and can therefore overlap each other. Over a series of trials this would result in an exponential cumulative distribution of search times. With a systematic strategy, on the other hand, a linear distribution would result since it is assumed that within a trial successive fixations do not overlap and the coverage is exhaustive. Because it is usual to find that inspection times are distributed exponentially (Drury), the random model has been incorporated into general models of inspection (Drury, 1975, 1978). However, as will be seen later, the underlying movements of experienced inspectors are obviously not random. A more interesting approach is taken by Williams (1956) who demonstrated how exponential distributions could arise even where subjects employ comparatively systematic strategies. e--
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Mens'
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Fixation time,
ms
Fig. 1
Distributions of fixation times from three experienced inspectors examining different products; data f r o m the Birmingham studies
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Applied Ergonomics September 1979
Central to the mathematical modelling approach is the concept of visual lobe area or effective visual field of view. This is defined as the area around the central fixation point from which specified information can be extracted. The field is shaped eUiptically, with the longer meridian extending in the horizontal plane. The effective visual field of view is the most objective method of assessing fault conspicuity. Because of the length of time it takes to measure lobe area it is more usual to measure the extent to which a particular task can be performed off-axis. Such measures are equivalent to measures of peripheral acuity. If the task is one of detecting a simple target on a plain background, as in classical perimetry, the target can be detected when presented 60 ° from the fovea. If recognition of the target is also required the maximum visual angle may drop to around 15 ° (Edwards and Goolkasian, 1974). The visual field is further reduced when the target is embedded in a complex background or is surrounded by irregularly positioned non-target items (Brown and Monk, 1975). In fact the field may be no greater than the area of the fovea (Mackworth, 1965). As with other measures of acuity, peripheral acuity demonstrates individual differences. There is some evidence to suggest that those subjects who obtain high peripheral acuity scores exhibit relatively short search times (Johnston, 1965). Eye movement parameters Before reviewing studies on eye movements m inspection it is necessary to describe briefly some of the parameters which contribute to search strategies. Fixation times Nearly all published studies include a descriptive statistic of fixation times. Normally these include a small variable component due to the saccades. Some typical distributions of fixations obtained from experienced inspectors are shown in Fig. 1. It is not possible to conclude whether the differences in mean and variance can be attributed to the nature of the inspection tasks or the experience and strategies of the individual inspectors. There is evidence to confirm the fixation times are dependent on task difficulty (Gould and Dill, 1969) and to support the claim of BOynton (1960) that the fixation times are shorter for experienced and skilled searchers (Krebs, 1975). However, one can be fairly confident that for most inspection tasks the average fixation time will be around 30Oms. It has already been pointed out why much shorter fixation times appear in distributions of times. Longer fixations are associated with the confirmation of the presence of a potential target (Bloomfield and Modrick. 1976) and with tasks where the search times are short (Gould and Schaffer. 1967). In addition, long times may be an artifact of the insensitivity ot' the recording device in detecting very small amplitude saccades. Ohtani (1971)has confirmed that the overall distributions like those shown in Fig. 1 are composed of two or three sub-distributions. Number of fixations The number of fixations is a much more critical parameter in determining search times than fixation times and is sensitive to both task and individual variables. For example, Kundel and La Follette (1972) found that experienced radiologists detected abnormalities in X-rays with fewer fixations than less experienced examiners.
Spatial distribution of fixations The coverage given to the stimulus material can be found by measuring the frequency of fixations falling in the elements of a grid superimposed over the display (Mackworth and Morandi, 1967) or by finding the frequency of fixations falling on specific features of the display (Williams, 1967). In both cases these frequencies correlate with the informativeness of the respective parts of the display as revealed by subjective estimates made by the searchers (Mackworth and Morandi, 1967; Kundel, 1974). In their analysis of driving behaviour, Mourant and Rockwell (1972) devised a concentration index ratio to define where a driver concentrates his gaze.
lnterfixation distances With static displays this measure is equivalent to the amplitude of the saccadic movements. Using this measure Mackworth and Brunet (1970) found that young children exhibited shorter distances than adults when examining pictures of familiar scenes. The authors argue that this result did not reflect differences in visual lobe area for the two groups of subjects but rather the lower rate of information processing of the children. On the other hand it is possible that when a comparatively systematic strategy is being employed, inter fixation distances may reflect the size of the useful field of view (Gould and Schaffer, 1965; Snyder and Taylor, 1976; Megaw and Richardson, 1979).
Direction of eye movements Mackworth and Brunet (1970) reported that horizontal saccades occurred more frequently than vertical ones and that this was not due to the organisation of the stimulus material. They also argue that this was not a result of the influence of a reading habit since 6-year old children showed tire same bias. Mackworth and Bruner suggest that the result reflects the elliptical shape of the effective useful field of view. A similar result was reported by Stern and Bynun (1970), although this may have been due to the nature of the search task which involved scanning the horizon as well as monitoring a helicopter instrument panel.
saccades. The eyes remained nearly stationary while the sheets passed in front of the inspector. The authors argue that this strategy gave a wider coverage to the sheets, but that the sheets were viewed under the less favourable conditions of dynamic visual acuity. With the former strategy inspectors were viewing under the optimal conditions of static acuity, but the coverage was restricted. These strategies were not specific to an inspector since inspectors 'alternated between the two strategies. No performance data were obtained to suggest which strategy was preferable. The strategies were unaffected by either the speed of movement of the sheets or their size. It is worth noting that at the fastest speed of 300 ft/min (1-5 m/s), inspectors would have been unable to devote more than two fixations to each of the smaller sheets which were 1 m 2 since the available viewing time was 620 ms. Ohtani (1969)investigated the same range of conveyor speeds as Moraal and found that fixation times were independent of the speed. A mean value of 300 ms was obtained. This contrasted with the results from another experiment where subjects inspected small items spaced out on a conveyor. Fixation times decreased as the conveyor speed increased since each item was fixated. The difference between these two tasks which may explain these results is that in the case of the discrete items inspectors are provided with distinct fixation points while with the homogeneous sheet material no obvious fixation points present themselves.
Inspection of empty bottles
The most popular of these is the scan-path which is a map of the locations of successive eye fixations within a search trial. Some people restrict the use of the term scanpath to those cases where the same sequence of fixations is repeated from one trial to the next. This is an important idea m the model of recognition proposed by Noton and Stark ( 1971 ). There are a few studies which have used some form of time series analysis to identify the sequential nature of search behaviour (Carbonnell, Ward and Senders, 1968; Buchsbaum, Pfefferbaum and Stillman, 1972).
Saito (1972)studied the strategies of female inspectors examining empty bottles which passed on a conveyor in front of them from left to right. A highly rated inspector employed a regular scan-patla which involved a high proportion of vertical saccades. Each scan included a number of fixations on the necks of the bottles where faults most frequently occurred, followed by some fixations on the bases of the bottles before repeating the cycle. As the pace of the inspection was increased from 150 to 300 bottles/ rain there was only a marginal increase in the duration of the scan-path so that the number of bottles inspected during a scan rose from 5 to 11. Saito argues that because the effective useful field of view was small for this task, horizontal saccades became more frequen| as the pace of inspection increased so that the effective coverage of the tops and bottoms of the bottles decreased. In addition, as the pace increased there was a very obvious increase in the number of bottles which passed by uninspected while a defective bottle was being rejected. Based on these two criteria, Saito recommends that speeds of 200 bottles/rain should not be exceeded. Saito quotes data on blink rate to support this conclusion.
Some previous inspection studies
Inspection of integrated circuit chips
Sequential indices
Inspection of steel sheet Moraal (1975) recorded the eye movements from experienced inspectors of steel sheet. Two distinct eye movement strategies were identified. In one the fixation phase was accompanied by a pursuit movement which matched the speed of the moving sheets. With this strategy saccades were executed either to redirect the eyes to the next sheet or to change the point of fixation within a sheet. With the other strategy there was a near total absence of
The eye movements from five experienced inspectors examining slides of circuit chips were recorded by Schoonard, Gould and Miller (1973). Each chip subtended 18° under the microscope. They observed that the edges of the drips were neglected although some fault types were likely to occur there. Generally fixations were most frequent in those areas where the stimulus features were most complicated. There was no evidence that inspectors employed their own scan-paths.
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On the contrary, whatever scan-paths the inspectors were intending to employ were soon modified by information acquired during each inspection trial. The more accurate inspectors required less fixations to complete the inspection cycle.
Inspection of tapered roller bearings This study (Anonymous, 1970) was carried out with the clearly defined objectives of increasing inspection accuracy and, if possible, reducing inspection time. A survey of 20 experienced inspectors revealed that the more accurate inspectors covered up to 85% of the bearing surface with foveal vision while the poorer ones covered only 30%. Unfortunately few details are given but it is claimed that by instructing the inspectors on the optimum scan-path and reversing the direction of the bearings' movements from a forwards to a backwards one, misses were reduced by 25% and inspection time by 50%. Some recent studies
Method of recording eye movements The reliability of the results from the previous studies depends largely upon the method of eye movement recording. The reader is referred to Young and Sheena (1975) and Laycock (1979) for extensive reviews of recording methods. In this section a brief account is given of the factors which influenced the choice made at the department in the University of Birmingham. Apart from cost, the following are some of the factors which influence the choice of equipment: Range of measurement Accuracy of measurement Frequency of response Head movements permitted? Glasses or contact lenses acceptable? Calibration and set-up time Subject discomfort and co-operation Form of output and speed of data analysis
Fig. 3
A general view of the inspection of tin cans
At Birmingham we were limited to a budget of £5000 and therefore could not invest large sums in development. We selected a commercially available corneal reflection method, the NAC eye camera which is illustrated in Fig. 2. With this method the eye spot which has been reflected from the cornea is optically mixed in the eye camera with the general field of view. A fibre optic bundle transfers the image to a TV camera fitted with a sensitive diode videcon tube. Finally the picture is recorded on video tape. The main advantages of the method are the ease of application, the speed of calibration, the freedom for head movements and the immediate production of an output recognisabte to all on the shop-floor. The eye camera is moderately comfortable for the subject for periods up to ~Ah. Within . l o. The the measurement range -¥ 15 o the accuracy Is main disadvantages of the method are the restriction to gross bodily movements imposed by the 1 m length of the fibre optic bundle, the loss of calibration that occurs during a recording due to the poor attachment of the eye camera to the subject's head. the probable reduction in the peripheral vision of the subject and the length of time taken to perform a frame-by-frame analysis of the recordings
Inspection of tin-plated cans Eye movements were recorded under normal production conditions from four female inspectors who worked a 4 h shift examining tin cans. Inspection was normally carried out on a sample basis. If a particular fault type became prevalent, 100% inspection was performed for that fault type. In this investigation, inspectors were looking for lap faults which were faults in the soldering of the seams along the length of the can. Fig. 3 provides a general view of the task. Inspectors removed cans from the ramp in front of them with their left hand and then rotated them with both hands to locate and inspect the seam before depositing them with their right hand. Although inspectors were paid on a flat rate they were expected to inspect at a rare of 20 cans/min.
Fig. 2
A view of the eye movement recording method used in the Birmingham studies
148
Applied Ergonomics September 1979
The results are summarised in Table 1. Only Inspectors A and C completed the inspection cycle within the unofficial standard time. Although the effects of wearing the eye camera were not evaluated in this study, it is
Table1: The results from fourinspectors examining tin cans Inspector A
B
C
D
Mean
372
337
274
379
SD
210
218
168
263
6-1 2'2
13'6 2"6
7"3 2"5
10-2 2'5
2.3
4'6
2"0
3'9
Fixation times (ms)
Number of fixations/can Mean SD Mean cycle time (s)
Table 2: The results from four inspectors examining electrical edge connectors Inspector E
F
G
H
Mean
317
343
418
458
SD
141
144
296
249
14
18
63
76
Side view
58
58
36
47
Top view
18
13
16
25
Bottom view
24
29
48
28
Fixation times (ms)
Mean number of fixations/ connector % fixations -
worth noting that both these inspectors had 10 years experience on can inspection while the other two inspectors had about 1 year's experience. The shorter cycle times of the experienced inspectors were mainly attributable to the smaller number of fixations they required to complete the cycle. The recordings indicated that each inspector used the same basic scan-path from one trial to the next although there were small differences between inspectors. For example, Inspector A directed her first three fixations at the open end of the can while rotating it to locate the seam. She then made one or two fixations on the other end of the seam and finally another two fixations while turning the can to inspect the seam inside the can. The less experienced inspectors used the same sequence of fixations but directed many more fixations along the length of the outside seam. In the absence of performance data one cannot decide whether this greater number of fixations resulted in more accurate performance or was a consequence of the lack of experience of the inspectors. A general conclusion which arose from this study was that a high proportion of the cycle time was spent locating the seam and that it would be useful to develop a method of presenting the cans to the inspectors in a fixed orientation.
Inspection of electrical edge connectors The inspection of two kinds of edge connectors was investigated during normal production conditions. Simple
connectors had two rows of 11 large spikes and the equivalent number of contacts. They were 6 cm long. The complex connectors were 14 cm long and had two rows of 54 smaller spikes. Both the spikes and the contacts of the complex connectors were gold-plated. Recordings were obtained from four inspectors, two inspecting the simple connectors (Inspectors E and F), the other two the complex items. Inspectors E and F had more than 4 years' experience inspecting connectors. Both normally worked under an incentive payment system. Inspector G was a patrol inspector with 7 years" experience while Inspector H was a technician in the quality control laboratory. Table 2 gives the data on fixation times and the number of'fixations to complete the inspection for the four inspectors. Not surprisingly, the number of fixations/ connector was much greater for the complex items. The fixation times were also longer for the complex connectors. There is no way of establishing whether this was due primarily to the complexity of the items or the experience of the inspectors. It should be pointed out that with the complex connectors the fixations were often placed very close together so that it was sometimes difficult to distinguish them. Each inspector had his own method of manually manipulating the connectors which largely determined the scan-paths. However, for any individual inspector there were small variations from one trial to another. For example, when viewing the spikes from the side there were variations in the end at which the inspector began to scan along the spikes. There were also variations from trial to trial in the number of recursive eye movements executed as inspectors scanned along the length of the connector. Table 2 includes data on the percentages of fixations associated with the three main viewing angles used by the inspectors. The two line inspectors showed close agreement while there were very marked differences in the distribution of fixations of the two inspectors examining the complex connectors.
Inspection of men's briefs This study was more detailed than the previous two. A batch of garments was prepared by someone from a large retail firm which was supplied by the company in which this study took place. The batch of 36 included seven faulty garments not acceptable to the retail firm. During the whole study the sequence of presentation of the garments was the same. All the examiners were female and had had at least five years experience with the company. On the first session, the batch was given to each of the examiners under normal conditions in order to obtain performance measures and subsequently to evaluate the effects of wearing the eye camera. The recording of eye movements was made in a second session which followed the original inspection by one week. The work cycle was complicated. A majority of the actual inspection was carried out while the garment was stretched over a wire frame which could be revolved to obtain different views of the garment. Examiners were not given specific instructions to adhere to the standard inspection rate of 2 rain 12 s/doz. During the first session it was found that all the examiners spent on average between 58 and 60% of the cycle time inspecting the garments on the frame. The cycle times were longer than the standard time for all the examiners. Table 3
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Table 3: The results from four examiners inspecting men's briefs
Remembering that the inspection times during this investigation were much longer than the standard time. one can appreciate that the selecti~m of fixation points under normal working conditions is critical in deterrmning outgoing quality. Research is continuing to see whether an optimum strategy can be taught to trainee examiners. It
Examiner D
M
J
S
Mean inspection time with garment on the frame (s) Without eye camera
12"5
8.7
9.1
13-4
With eye camera
16"7
14.5
9"0
23-8
Without eye camera
7
6
6
6
With eye camera
5
6
0
2
Without eye camera
7
5
5
1
With eye camera
2
2
0
0
Ri(J~ht
Front
Left
Left
Beck
Right
Number of critical faults detected, max = 7
Number of other garments rejected, max = 29
Fixation times (ms) Mean
321
332
284
389
SD
125
142
114
178
52
44
32
61
Mean number of fixations/ garment
shows that even at this rate, three of the four examiners missed one of the critical faults. As a possible consequence of the manner in which the study was conducted, the examiners rejected many garments which were acceptable to the person from the retail firm. However, these rejections could not be considered as false alarms since there was close agreement between the examiners as to which garments were rejected. This suggests that the examiners were not aware of the standards set by the retail firm.
S = stort C : COntlI'U¢~ E : end
Fig. 4
The basic scan-path adopted by Examiner D while inspecting men's briefs Right
Front
Left
When eye movements were recorded, Table 3 shows that inspection time with the garment on the frame was further increased for three of the examiners. In addition, the reject rate decreased for all the examiners to the extent that Examiners J and S failed to detect nearly all the critical faults. Only at this late stage of the study was it realised that these two examiners had poor vision. Although both had been provided with glasses, they were reluctant to wear them for long periods during normal inspection. As with all the studies that have been reported, differences in inspection times were mainly a result of differences in the number of fixations made to complete the inspection. All the examiners had their own methods of inspecting the garments on the frame and this was reflected in the analysis of their scan-paths. Figs. 4 and 5 illustrate the basic scan-paths for Examiners D and M. Individual fixation locations are not shown. The scan-paths closely matched the inspection performance. For example, Examiner M completed the inspection without viewing the back panel of the garment and consequently failed to reject the garment with a large ladder in the fabric of the rear panel. Similarly, the other critical misses could be attributed to the stereotyped scan-paths of the examiners.
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Applied Ergonomics September 1979
Left
Back
\\\
Right
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Fig. 5
c:
E=
~ooti~o~
The basic scan-path adopted by Examiner M while inspecting men's briefs
To show the effective coverage, samples of data from two examiners have been re-plotted in Fig. 8. In both cases the coverage was essentially a zig-zag one. The major difference between the examiners was in the number of saccades executed in passing from one side of the fabric to the other. This meant that Examiner B, who executed fewer fixations in passing across the fabric, gave a better general coverage. Examiner A gave detailed coverage to certain areas but left others completely unexplored. With this strategy it was unlikely that non-recurring faults such as dirt marks would have been detected should they have fallen in these areas. Apart from trying to encourage examiners to adopt a more random search strategy, the most obvious way to achieve better coverage is by reducing the speed of the fabric movement. As a start to improving the inspection task a suitably designed control to vary the speed during inspection should be provided. This would ensure that examiners would not have to inspect the fabric while it was outside the specialised lighting area.
Fig. 6
A general view of the inspection of woven fabric
To see the extent to which the scanning strategies were reflected in performance data, specially prepared rolls of
has been previously pointed out that to do this one needs knowledge of the relative importance and conspicuity of the various fault types.
Width of fabric
I 3174 5 216 18 6 14 IO 9 8 7 15 13 12 II 2019
Inspection of weft knitted fabric At this factory woven and dyed fabric was inspected for around 50 different fault types. The width of the fabric was 155 cm and it came in lengths of between 35 and 50 m. The general viewing conditions are shown in Fig. 6. The inspection method was to have the fabric advancing towards the examiner at a speed of 46 cm/s. There was no control to alter the speed during inspection although by ~playing' the foot controls which stopped and started the drive mechanism slower speeds could be achieved. Depending on the exact posture adopted by the examiners the angular velocity of the fabric varied between 20 ° and 30 30°/s. A viewing area was prescribed by a 45 cm wide transilluminated strip across the viewing table.
I
5 0 cm
However, with the occurrence of the pursuit movements there were several instances where the fixation point ended up outside this area. This was particularly apparent when the fixation time was long because the examiner was attempting to confirm the presence of a potential fault. Fig. 7 does not indicate the actual coverage given to the fabric since it takes no account of the fabric movement.
An example of the distribution within the viewing area of 20 successive fixations
Fig. 7
Examiner A
In the first part of this study five experienced examiners inspected two rolls of unpatterned production material. The mean fixation times of the examiners varied between 210 and 298 ms. These times are comparatively short and they confirm the conclusion that was drawn from the results of Ohtani (1969) that in the absence of distinct fixation points fixation times are shorter. Because of the movement of the fabric, 75% of the fixation phases were accompanied by pursuit movements. It was not possible to establish how closely they matched the velocity of the fabric. All the examiners demonstrated the same overall search strategy which is illustrated in Fig. 7. They executed a series of saccades so that the fixation points passed from left to right and back again across the width of the fabric. Initially the fixations fell within the area served by the transillumination.
Tronsiluminoted viewing area
Examiner B 19
19
18
18
17
16
17
15
16
14 15 14
~3
13
12 II 10 7 6 4
12 II 10
9
8 7
8
6
5
3
4 3
2
2
I I
50 cm
Fig. 8
Examples of the typical coverage given to a 2 m length of fabric by two examiners
material were used in the second part of the study. Four faults involving discolouring small areas (1 cm 2 ) of the fabric were added to each of five beige coloured rolls. Two different fault types were added. In one case tile faults were black and therefore very conspicuous while in the other they were yellow and much less conspicuous. Two
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151
rolls had all black faults, one roll all yellow faults and the other two had a mixture of faults. Four experienced examiners agreed to inspect all five rolls on three separate occasions, each separated by at least a week. The order of presentation of the rolls was randomised. During the first two sessions eye movements were recorded, but not during the third session. The last session was included to establish the extent to which the eye camera disrupted performance. Before inspecting each roll the examiner was informed of the fault characteristics. All the examiners exhibited the same search strategies that were reported in the first part of this study so that one can assume that large areas of the fabric were not covered by foveal fixations. Surprisingly, the eye camera did not interfere with performance since fault detection did not improve over the final session. None of the black faults was missed by any of the examiners, indicating that they were easily detected with peripheral vision. Performance was also very good on the roll containing all yellow faults and a hit probability of 0.94 was achieved. Only one of the faults was missed and this was positioned in the middle of the roll. Since two of the other yellow faults were positioned near the edges of the fabric, one must assume that even low contrast faults could be detected with peripheral vision. This finding, therefore, contradicts the prediction made following the first part of this study that small non-recurring faults would be very difficult to detect. On the other hand one should not forget that there is a lot of evidence that the peripheral visual system is specialised for detecting moving targets (Sharpe, 1974). This means that it is relatively difficult to predict the effects of search strategies on performance when studying the inspection of moving products. There remains one rather puzzling result. In the case of the rolls with both fault types, the detection probability of the yellow faults was only 0'44. There was one yellow fault positioned near the edge of the fabric which was detected on only five out of 24 separate inspections. It was unlikely that the low detection of the yellow faults on the mixed rolls compared with the performance on the all yellow roll was due solely to uncontrolled effects of fault position and fault contrast. An alternative explanation is in terms of the difficulty that examiners experience when inspecting simultaneously for fault types which vary widely in their conspicuity (Megaw and Richardson, 1979).
Conclusions All the studies that have been described should be regarded as exploratory. Only the last two attempted to introduce some necessary controls. By preparing special batches of a product which can be given to a sample of inspectors and by taking performance measures it was possible in those two studies to identify the extent to which the method of eye movement recording affected performance accuracy and to investigate individual differences in performance and eye movement strategies. One should be cautious in making comparisons between the studies if different recording methods were used. All the Birmingham studies used the same method.
in the number of fixations taken to complete the inspection cycle or to detect a fault rather than in differences in fixation times. With unpaced tasks, the experienced inspector employed relatively fewer fixations/cycle. Because standard times for inspection are established on an arbitrary basis, there is not an obvious correlation between the complexity of an inspection task and inspection time. In one firm, experienced inspectors took between 14 and 18 fixations to complete the inspection of simple electrical edge connectors while in another firm inspectors took between 9 and 15 fixations to examine complex electronic circuit chips. 2 Distributions of fixation times from individual inspectors were positively skewed and, therefore, if the results from several individuals over a range of tasks are compared, a positive correlation is found between means and standard deviations. There was some evidence to suggest that fixation times were comparatively short in the tasks where there were no distinct fixation points (steel sheet, woven fabric). While there was also evidence that experienced inspectors employed shorter fixation times, the effects of task complexity on fixation times were not identified. 3. Individual scan-paths were very pronounced in those cases where the product was manipulated as part of the work cycle, (bearings, tin cans, electrical connectors. men's briefs). It was implied that faults may have been missed because the scan-paths were too inflexible (men's briefs). The study on the inspection of electronic circuit chips did not reveal the presence of individual scan-paths. It has been pointed out that the task was complex but that the inspection times were surprisingly short. Under such conditions, it was possible that the sequence of eye movements was determined primarily by information acquired during the inspection of each chip. Systematic strategies were recorded from inspectors examining moving products (steel sheet, bottles, woven fabric). 4. Peripheral vision is particularly important for detecting potential faults when the product is moving and subtends a large visual angle (steel sheet, woven fabric).
Acknowledgements This work was sponsored by an SRC research contract. We would like to acknowledge the support given by the six companies who provided facilities for the research and we are grateful to Jane Alexander who analysed many of the eye movement recordings.
References Anonymous 1970 Factory Management and Maintenance. 1t 1, 88--89. Eye camera shows how to improve inspection techniques.
Bloomfield, J.R., and Modriek, J.A. 1976 Cognitive processes in visual search. Proceedings of the 6th International Ergonomics Association Conference. Maryland. 204-209.
Despite these shortcomings, the following conclusions emerged:
Boynton, R.M.
1. Large variations in inspection times reflected differences
1960 Summary and discussion. In: Visual search techniques
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(Edited by A. Morris and E.P. Home) (Washington: National Academy of Sciences - National Research Council). pp 23l 250.
Brown, B., and Monk, T.H. 1975 Human Factors, 17, 81--88. The effect of local target surround and whole background constraint on visual search times.
Buchsbaum, M., Pfefferbaum, A., and Stillman, R. 1972 Perceptual and Motor Skills, 35, 895 901. Individual differences in eye-movement patterns.
Carbonnell, J., Ward, J.L., and Senders, J.W. 1968 A queueing model of visual sampling experimental validation. IEEE Transactions on Man-Machine Systems, MMS-9, 82 87.
Orury, C.G. 1973 Applied Ergonomics, 4, 2-7. The effect of speed of working on industrial inspection accuracy.
Drury, C.G. 1975 Human Factors, 17, 257 265. The inspection of sheet materials - model and data. Drury, C.G. 1978 Human Factors, 20, 561 -572. Integrating human factors models into statistical quality control.
Edwards, D.C., and Goolkasian, P.A. 1974 Journal of Experimental Psychology, 102,244 249. Peripheral vision location and kinds of complex processing.
Eastman, A.A. 1968 Illuminating Engineering, 63, 37-40. A new contrast threshold visibility meter.
Enoch, J.M. 1959 Journal of the Optical Society of America, 49, 280-286. Effect of the size of a complex display upon visual search. Gould, J.D. 1973 Journal oflZxperimental Psychology, 98, 184-- 195. Eye movements during visual search and memory search.
Gould, J.D., and Dill, A.B. 1969 Perception and Psychophysies, 6, 311-320. Eye movement parameters and pattern discrimination.
Gould, J.D., and Peeples, D.R. 1970 Journal of ExperimentaIPsychology, 85, 51 55. Eye movements during visual search and discrimination of meaningless symbol and object patterns.
Gould, J.D., and Schaffer, A. 1965 Perceptual and Motor Skills, 20, 521 535. Eye movement patterns in scanning numeric displays. Gould, J.D., and Schaffer, A. 1967 Journal of Experimental Psyehology, 74, 225 229. Eye movement parameters in pattern recognition,
Howarth, C.I., and Bloomfield, J.R. 1968 Towards a theory of visual search. AGARD Conference Proceedings, No 41, London.
Hyde, J.E. 1959 American Journal of Opthalmology, 48, 85 94. Some characteristics of voluntary human ocular movements in the horizontal plane.
Johnston, D.M. 1965 Human Factors, 7, 528 535. Search performance as a function of peripheral acuity.
Kaplan, I.T., and Schoenfield, W.N. 1966 Journal of Experimental Psychology, 72, 447--451. Oculomotor patterns during solution of visually displayed anagrams. Krebs, M.J. 1975 Scanning patterns in real-time FLIR displays. Proceedings of the Human Factors Society 19th Annual Meeting, 418 422.
Krendel, H.L., and Wodinsky, J. 1960 Visual search in unstructured fields. In:Visual search techniques (Edited by A. Morris and E.P. Home) (Washington: National Academy of Sciences National Research Council). pp 151 169.
Kundel, H.L. 1974 Investigative Radiology, 9, 87 03. Visual sampling and estimates of the location of information on chest fihns.
Kundel, H.L., and La Follette, P.S. 1972 Radiology, 103, 523 528. Visual search patterns and experience with radiological images.
Kundel, H.L., and Wright, J.D. 1969 Radiology, 93, 315 320. The influence of prior knowledge on search strategies during the viewing of chest radiograph'..
Laycock, J. 1979 The measurement and analysis of eye movements. Proceedings of the Symposium entitled 'Search and the human observer'. Loughborough University of Technology, in press.
Mackworth, N.H. 1965 Psychonomic Science, 3, 67 68. Visual noise causes tunnel vision.
Mackworth, N.H., and Bruner, J.S. 1970 Human Development, 13, 149 177. How adults and children search and recognise pictures.
Mackworth, N.H., Kaplan, I.T., and Metlay, W. 1964 Perceptual and Motor Skills, 18, 397 402. Eye movements during vigilance.
Mackworth, N.H., and Morandi, A.J. 1967 Perception andPsychophysics, 2, 547 -551. The gaze selects informative details within pictures.
Megaw, E.D. 1975a Saccadic eye-movements and the refractory period. In: Attention and performance V (Edited by P.M.A. Rabbitt and S. Dornic) (London: Academic Press). pp 383 394.
Megaw, E.D. 1975b Factors underlying distributions of eye fixation times. In: Age et Constraintes de Travail (Edited by A. Laville, C. Teiger and A. Wisher) (Jouy-en-Josas: NEB Editions Scientifiques). pp 291 -305.
Megaw, E.D., and Bellamy, L. 1979 Eye movements and visual search. Proceedings of the Symposium entitled 'Search and the Human Observer'. Loughborough University of Technology, in pxe.ss.
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Megaw, E.D., and Richardson, J. 1979 Human Factors, 21,302-315. Target uncertainty and visual scanning strategies.
Saito, M. t972 Journal of the Science of Labour, 48, 395 -400. A study on bottle inspection.
Moraal, J. 1975 The analysis of an inspection task in the steel industry. In: Human reliability and quality control (Edited by C.G. Drury aod J.G. Fox) (London: Taylor and Francis). pp 217-230.
Schoonard, J.W., Gould, J.D,, and Miller, L.A. 1973 Ergonomics, 16,365 379. Studies of visual inspection
Morris, A., and Home, E.P. 1960 Visual search techniques (Washington: National Academy of Sciences - National Research Council). Mourant, R.R., and Rockwell, T.H. 1972 Human Factors, 14, 325-335. Strategies of visual search by novice and experienced drivers. Nodine, C.F,, Carmody, D.P., and Kundel, H.L. 1978 Searching for Nina. In: Eye movements and the higher psychological functions (Edited by J.W. Senders, D.F. Fisher and R.A. Monty) (Hillsdale: Lawrence Erlbaum Associates). pp 241-258. Noton, D., and Stark, L. 1971 Science, 171,308- 311. Scanpaths in eye movements during pattern perception. Ohtani, A. 1969 Eye movements during visual inspection task. Proceedings of the 16th International Congress on Occupational Health, Tokyo, 88-90. Ohtani, A. 1971 Ergonomics, 14, 167-174. Analysis of eye movements during a visual task. Rayner, K. 1975 Cognitive Psychology, 7, 65-81. The peripheral span and peripheral cues in reading. Rockwell, T., and Zwahlen, H. 1977 Eye movements research: Vehicle operations. Human Factors 21 st Annual Meeting, San Francisco.
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Sharpe, C.R. 1974 Vision Research, 14, 905 906. The contrast sensitivity, of the peripheral visual field to drifting sinusoidal gratings.
Snyder, H.L., and Taylor, D.F. 1976 Computerised analysis of eye movements during static display visual search. Aerospace Medical Research Laboratory Report AMRL-TR-75-91. Ohio. Stem, J.A., and Bynum, J.A. 1970 Aerospace Medicine, 41, 300-305, Analysis of visual search activity in skilledand novice helicopter pilots. Tinker, M.A. 1958 PsychologicalBulletin, 65, 215 231~ Recent studies of eye movements in reading. Williams, L.G. 1966 Human Factors, 8, 80-92. Target conspicuity and visual search. Williams, L.G. 1967 Acta Psychologica, 27, 355 -360. The effects of target specification on objects fixated during visual search.
Yarbus, A.L. 1967 Eye movements and vision (New York: Plenum Press). Young, L.R., and Sheena, D. 1975 Behaviour Research Methods and Instrumentation, 7, 397,429. Survey of eye movements recording methods.
Applied Ergonomics 1979, 10.3, 145-154
Eye movements and industrial inspection E.D. Megaw and J. Richardson Department of Engineering Production, University of Birmingham
A brief survey is given of the eye movement parameters which contribute to visual search strategies. The literature on eye movements and industrial inspection is reviewed and some recent studies carried out at the University of Birmingham into the eye movements underlying four inspection tasks are described. These tasks included the inspection of tin cans, electrical edge connectors, men's briefs and woven fabric.
Introduction Only with comparatively recent technical developments has it been possible to record the eye movements of operators performing tasks outside the laboratory. In addition to industrial inspection these tasks include vehicle driving (Mourant and Rockwell, 1972; Rockwell and Zwahlen, 1977), various activities of aircraft pilots (Carbonnel, Ward and Senders, 1968; Stern and Bynum, 1970)and X-ray examination (Kundel and Wright, 1969; Kundel and La Follette, 1972). The number of published studies is, however, small when compared with the number of published laboratory tasks. The laboratory based tasks have included reading (Tinker, 1958; Rayner, 1975), facial recognition (Noton and Stark, 1971), comparative judgements (Buchsbaum, Pfefferbaum and Stillman, 1972; Gould and Peeples 1970), search tasks (Enoch, 1959; Nodine, Carmody and Kundel, 1978 ; Megaw and Richardson, 1979), observing pictorial scenes, (Yarbus, 1967) and vigilance (Mackworth. Kaplan and Metlay, 1964). Two important results arose from the last study. When subjects were required to monitor a dial for rarely occurring though easily discriminable signals, they often failed to report a signal although it had been visually fixated looking is not seeing. At the same time signals were sometimes reported which had not been presented (false alarms). That is to say, attentional and related factors cannot be ignored when considering search strategies.
cannot be assessed from performance data since inspectors may be adopting inappropriate search strategies. Alternative methods of assessment include subjective techniques, the use of a visibility meter (Eastman, 1968) and measures of effective useful field of view which are discussed later in the paper. The recording of eye movements provides valuable data to validate mathematical models which have been proposed to describe visual search behaviour. In the absence of such data researchers have relied heavily upon the observed cumulative distributions of search times to support the alternative models (Krendel and Wodinsky, 1960; Williams, 1966: Howarth and Bloomfield, 1968). While these models have proved useful in predicting inspection performance (Drury, 1975, 1978), they show several shortcomings which have been outlined by Megaw and Bellamy (1979). A less obvious but important reason for recording eye movements is the general interest that it stimulates in all those persons concerned with inspection procedures and quality control. This creates a favourable atmosphere in which to pursue ergonomic improvements. Additionally, the recording provides visual aid material which can be assin~ilated very readily into training programmes. It is not yet possible to evaluate the extent to which eye movement strategies can be taught to inspectors.
Eye movements and search strategies Why record the eye movement during inspection? The recording of eye movements can provide useful information on whether sufficient time is being allowed for the inspection of a product and. in particular, whether a product is being given adequate visual coverage. It is not, however, a simple exercise to establish standard times solely on the basis of eye movement recordings. Information is also required on fault difficulty or conspicuity since the easier faults are to detect the less thorough need be the coverage. The problem is further complicated by the fact that the expected fault severity for a particular product varies widely and different fault types are associated with different parts or areas of a product. Fault conspicuity
0003 6870/79/03 0145--10 $02.00 I~) IPC Business Press
From the point of industrial inspection the most relevant eye movements are the saccadic movements. These serve to bring different parts of the image of the product on to the fovea for detailed analysis. They are rapidly executed and usually range in amplitude between 5 ° and 40 °. Thus a 10° saccade typically has a duration of 35 ms and reaches a velocity of 400°/s. During the saccade the image becomes so blurred that the amount of detail that can be resolved is greatly reduced. The time elapsing between the presentation of a stimulus in the periphery and the onset of a saccade to fixate the stimulus is typically around 160 ms. Despite the favourable biomechanical properties of the oculomotor system, the frequency response of the saccadic control
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system is limited to around 2 Hz or 4 saccades/s (Hyde, 1959). Thus the average time elapsing between the end of one saccade and the beginning of the next, usually referred to as the fixation time, is rarely less than 200 ms. Nevertheless, shorter fixation times are observed. These precede small amplitude saccades which correct for under or overshoot o f the primary saccade (Megaw, 1975a, 1975b). If examiners are inspecting a product which is moving, the pursuit movement system becomes involved during the fixation phase. Pursuit movements function to stabilise the retinal image by matching the velocities of the eye and the moving stimulus. Up to display velocities of 20°/s the tracking error is very small, but as the velocity is further increased the eye velocity progressively lags behind and saccades are executed to compensate for the consequent error position. If subjects attempt to maintain their eyes stationary while observing a moving display, the eyes tend to follow the display although with a relatively low velocity (involuntary optokinetic nystagmus). Search strategy is a global term which reflects many parameters of the saccadic movements and these are described later. As a result of the publication of Visual Search Techniques (Morris and Home, 1960) there has been a tendency for those concerned with modelling search strategies to consider only two extreme strategies. These are the socalled random and systematic strategies (Howarth and Bloomfield, 1968). With a random search, successive eye fixations within a search trial are independent of each other and can therefore overlap each other. Over a series of trials this would result in an exponential cumulative distribution of search times. With a systematic strategy, on the other hand, a linear distribution would result since it is assumed that within a trial successive fixations do not overlap and the coverage is exhaustive. Because it is usual to find that inspection times are distributed exponentially (Drury), the random model has been incorporated into general models of inspection (Drury, 1975, 1978). However, as will be seen later, the underlying movements of experienced inspectors are obviously not random. A more interesting approach is taken by Williams (1956) who demonstrated how exponential distributions could arise even where subjects employ comparatively systematic strategies. e--
--
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Woven
fobric
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,~ 4c
._8 o
~- ....
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Mens'
plotes
briefs
/\
"6 5c c
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Fixation time,
ms
Fig. 1
Distributions of fixation times from three experienced inspectors examining different products; data f r o m the Birmingham studies
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Applied Ergonomics September 1979
Central to the mathematical modelling approach is the concept of visual lobe area or effective visual field of view. This is defined as the area around the central fixation point from which specified information can be extracted. The field is shaped eUiptically, with the longer meridian extending in the horizontal plane. The effective visual field of view is the most objective method of assessing fault conspicuity. Because of the length of time it takes to measure lobe area it is more usual to measure the extent to which a particular task can be performed off-axis. Such measures are equivalent to measures of peripheral acuity. If the task is one of detecting a simple target on a plain background, as in classical perimetry, the target can be detected when presented 60 ° from the fovea. If recognition of the target is also required the maximum visual angle may drop to around 15 ° (Edwards and Goolkasian, 1974). The visual field is further reduced when the target is embedded in a complex background or is surrounded by irregularly positioned non-target items (Brown and Monk, 1975). In fact the field may be no greater than the area of the fovea (Mackworth, 1965). As with other measures of acuity, peripheral acuity demonstrates individual differences. There is some evidence to suggest that those subjects who obtain high peripheral acuity scores exhibit relatively short search times (Johnston, 1965). Eye movement parameters Before reviewing studies on eye movements m inspection it is necessary to describe briefly some of the parameters which contribute to search strategies. Fixation times Nearly all published studies include a descriptive statistic of fixation times. Normally these include a small variable component due to the saccades. Some typical distributions of fixations obtained from experienced inspectors are shown in Fig. 1. It is not possible to conclude whether the differences in mean and variance can be attributed to the nature of the inspection tasks or the experience and strategies of the individual inspectors. There is evidence to confirm the fixation times are dependent on task difficulty (Gould and Dill, 1969) and to support the claim of BOynton (1960) that the fixation times are shorter for experienced and skilled searchers (Krebs, 1975). However, one can be fairly confident that for most inspection tasks the average fixation time will be around 30Oms. It has already been pointed out why much shorter fixation times appear in distributions of times. Longer fixations are associated with the confirmation of the presence of a potential target (Bloomfield and Modrick. 1976) and with tasks where the search times are short (Gould and Schaffer. 1967). In addition, long times may be an artifact of the insensitivity ot' the recording device in detecting very small amplitude saccades. Ohtani (1971)has confirmed that the overall distributions like those shown in Fig. 1 are composed of two or three sub-distributions. Number of fixations The number of fixations is a much more critical parameter in determining search times than fixation times and is sensitive to both task and individual variables. For example, Kundel and La Follette (1972) found that experienced radiologists detected abnormalities in X-rays with fewer fixations than less experienced examiners.
Spatial distribution of fixations The coverage given to the stimulus material can be found by measuring the frequency of fixations falling in the elements of a grid superimposed over the display (Mackworth and Morandi, 1967) or by finding the frequency of fixations falling on specific features of the display (Williams, 1967). In both cases these frequencies correlate with the informativeness of the respective parts of the display as revealed by subjective estimates made by the searchers (Mackworth and Morandi, 1967; Kundel, 1974). In their analysis of driving behaviour, Mourant and Rockwell (1972) devised a concentration index ratio to define where a driver concentrates his gaze.
lnterfixation distances With static displays this measure is equivalent to the amplitude of the saccadic movements. Using this measure Mackworth and Brunet (1970) found that young children exhibited shorter distances than adults when examining pictures of familiar scenes. The authors argue that this result did not reflect differences in visual lobe area for the two groups of subjects but rather the lower rate of information processing of the children. On the other hand it is possible that when a comparatively systematic strategy is being employed, inter fixation distances may reflect the size of the useful field of view (Gould and Schaffer, 1965; Snyder and Taylor, 1976; Megaw and Richardson, 1979).
Direction of eye movements Mackworth and Brunet (1970) reported that horizontal saccades occurred more frequently than vertical ones and that this was not due to the organisation of the stimulus material. They also argue that this was not a result of the influence of a reading habit since 6-year old children showed tire same bias. Mackworth and Bruner suggest that the result reflects the elliptical shape of the effective useful field of view. A similar result was reported by Stern and Bynun (1970), although this may have been due to the nature of the search task which involved scanning the horizon as well as monitoring a helicopter instrument panel.
saccades. The eyes remained nearly stationary while the sheets passed in front of the inspector. The authors argue that this strategy gave a wider coverage to the sheets, but that the sheets were viewed under the less favourable conditions of dynamic visual acuity. With the former strategy inspectors were viewing under the optimal conditions of static acuity, but the coverage was restricted. These strategies were not specific to an inspector since inspectors 'alternated between the two strategies. No performance data were obtained to suggest which strategy was preferable. The strategies were unaffected by either the speed of movement of the sheets or their size. It is worth noting that at the fastest speed of 300 ft/min (1-5 m/s), inspectors would have been unable to devote more than two fixations to each of the smaller sheets which were 1 m 2 since the available viewing time was 620 ms. Ohtani (1969)investigated the same range of conveyor speeds as Moraal and found that fixation times were independent of the speed. A mean value of 300 ms was obtained. This contrasted with the results from another experiment where subjects inspected small items spaced out on a conveyor. Fixation times decreased as the conveyor speed increased since each item was fixated. The difference between these two tasks which may explain these results is that in the case of the discrete items inspectors are provided with distinct fixation points while with the homogeneous sheet material no obvious fixation points present themselves.
Inspection of empty bottles
The most popular of these is the scan-path which is a map of the locations of successive eye fixations within a search trial. Some people restrict the use of the term scanpath to those cases where the same sequence of fixations is repeated from one trial to the next. This is an important idea m the model of recognition proposed by Noton and Stark ( 1971 ). There are a few studies which have used some form of time series analysis to identify the sequential nature of search behaviour (Carbonnell, Ward and Senders, 1968; Buchsbaum, Pfefferbaum and Stillman, 1972).
Saito (1972)studied the strategies of female inspectors examining empty bottles which passed on a conveyor in front of them from left to right. A highly rated inspector employed a regular scan-patla which involved a high proportion of vertical saccades. Each scan included a number of fixations on the necks of the bottles where faults most frequently occurred, followed by some fixations on the bases of the bottles before repeating the cycle. As the pace of the inspection was increased from 150 to 300 bottles/ rain there was only a marginal increase in the duration of the scan-path so that the number of bottles inspected during a scan rose from 5 to 11. Saito argues that because the effective useful field of view was small for this task, horizontal saccades became more frequen| as the pace of inspection increased so that the effective coverage of the tops and bottoms of the bottles decreased. In addition, as the pace increased there was a very obvious increase in the number of bottles which passed by uninspected while a defective bottle was being rejected. Based on these two criteria, Saito recommends that speeds of 200 bottles/rain should not be exceeded. Saito quotes data on blink rate to support this conclusion.
Some previous inspection studies
Inspection of integrated circuit chips
Sequential indices
Inspection of steel sheet Moraal (1975) recorded the eye movements from experienced inspectors of steel sheet. Two distinct eye movement strategies were identified. In one the fixation phase was accompanied by a pursuit movement which matched the speed of the moving sheets. With this strategy saccades were executed either to redirect the eyes to the next sheet or to change the point of fixation within a sheet. With the other strategy there was a near total absence of
The eye movements from five experienced inspectors examining slides of circuit chips were recorded by Schoonard, Gould and Miller (1973). Each chip subtended 18° under the microscope. They observed that the edges of the drips were neglected although some fault types were likely to occur there. Generally fixations were most frequent in those areas where the stimulus features were most complicated. There was no evidence that inspectors employed their own scan-paths.
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On the contrary, whatever scan-paths the inspectors were intending to employ were soon modified by information acquired during each inspection trial. The more accurate inspectors required less fixations to complete the inspection cycle.
Inspection of tapered roller bearings This study (Anonymous, 1970) was carried out with the clearly defined objectives of increasing inspection accuracy and, if possible, reducing inspection time. A survey of 20 experienced inspectors revealed that the more accurate inspectors covered up to 85% of the bearing surface with foveal vision while the poorer ones covered only 30%. Unfortunately few details are given but it is claimed that by instructing the inspectors on the optimum scan-path and reversing the direction of the bearings' movements from a forwards to a backwards one, misses were reduced by 25% and inspection time by 50%. Some recent studies
Method of recording eye movements The reliability of the results from the previous studies depends largely upon the method of eye movement recording. The reader is referred to Young and Sheena (1975) and Laycock (1979) for extensive reviews of recording methods. In this section a brief account is given of the factors which influenced the choice made at the department in the University of Birmingham. Apart from cost, the following are some of the factors which influence the choice of equipment: Range of measurement Accuracy of measurement Frequency of response Head movements permitted? Glasses or contact lenses acceptable? Calibration and set-up time Subject discomfort and co-operation Form of output and speed of data analysis
Fig. 3
A general view of the inspection of tin cans
At Birmingham we were limited to a budget of £5000 and therefore could not invest large sums in development. We selected a commercially available corneal reflection method, the NAC eye camera which is illustrated in Fig. 2. With this method the eye spot which has been reflected from the cornea is optically mixed in the eye camera with the general field of view. A fibre optic bundle transfers the image to a TV camera fitted with a sensitive diode videcon tube. Finally the picture is recorded on video tape. The main advantages of the method are the ease of application, the speed of calibration, the freedom for head movements and the immediate production of an output recognisabte to all on the shop-floor. The eye camera is moderately comfortable for the subject for periods up to ~Ah. Within . l o. The the measurement range -¥ 15 o the accuracy Is main disadvantages of the method are the restriction to gross bodily movements imposed by the 1 m length of the fibre optic bundle, the loss of calibration that occurs during a recording due to the poor attachment of the eye camera to the subject's head. the probable reduction in the peripheral vision of the subject and the length of time taken to perform a frame-by-frame analysis of the recordings
Inspection of tin-plated cans Eye movements were recorded under normal production conditions from four female inspectors who worked a 4 h shift examining tin cans. Inspection was normally carried out on a sample basis. If a particular fault type became prevalent, 100% inspection was performed for that fault type. In this investigation, inspectors were looking for lap faults which were faults in the soldering of the seams along the length of the can. Fig. 3 provides a general view of the task. Inspectors removed cans from the ramp in front of them with their left hand and then rotated them with both hands to locate and inspect the seam before depositing them with their right hand. Although inspectors were paid on a flat rate they were expected to inspect at a rare of 20 cans/min.
Fig. 2
A view of the eye movement recording method used in the Birmingham studies
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Applied Ergonomics September 1979
The results are summarised in Table 1. Only Inspectors A and C completed the inspection cycle within the unofficial standard time. Although the effects of wearing the eye camera were not evaluated in this study, it is
Table1: The results from fourinspectors examining tin cans Inspector A
B
C
D
Mean
372
337
274
379
SD
210
218
168
263
6-1 2'2
13'6 2"6
7"3 2"5
10-2 2'5
2.3
4'6
2"0
3'9
Fixation times (ms)
Number of fixations/can Mean SD Mean cycle time (s)
Table 2: The results from four inspectors examining electrical edge connectors Inspector E
F
G
H
Mean
317
343
418
458
SD
141
144
296
249
14
18
63
76
Side view
58
58
36
47
Top view
18
13
16
25
Bottom view
24
29
48
28
Fixation times (ms)
Mean number of fixations/ connector % fixations -
worth noting that both these inspectors had 10 years experience on can inspection while the other two inspectors had about 1 year's experience. The shorter cycle times of the experienced inspectors were mainly attributable to the smaller number of fixations they required to complete the cycle. The recordings indicated that each inspector used the same basic scan-path from one trial to the next although there were small differences between inspectors. For example, Inspector A directed her first three fixations at the open end of the can while rotating it to locate the seam. She then made one or two fixations on the other end of the seam and finally another two fixations while turning the can to inspect the seam inside the can. The less experienced inspectors used the same sequence of fixations but directed many more fixations along the length of the outside seam. In the absence of performance data one cannot decide whether this greater number of fixations resulted in more accurate performance or was a consequence of the lack of experience of the inspectors. A general conclusion which arose from this study was that a high proportion of the cycle time was spent locating the seam and that it would be useful to develop a method of presenting the cans to the inspectors in a fixed orientation.
Inspection of electrical edge connectors The inspection of two kinds of edge connectors was investigated during normal production conditions. Simple
connectors had two rows of 11 large spikes and the equivalent number of contacts. They were 6 cm long. The complex connectors were 14 cm long and had two rows of 54 smaller spikes. Both the spikes and the contacts of the complex connectors were gold-plated. Recordings were obtained from four inspectors, two inspecting the simple connectors (Inspectors E and F), the other two the complex items. Inspectors E and F had more than 4 years' experience inspecting connectors. Both normally worked under an incentive payment system. Inspector G was a patrol inspector with 7 years" experience while Inspector H was a technician in the quality control laboratory. Table 2 gives the data on fixation times and the number of'fixations to complete the inspection for the four inspectors. Not surprisingly, the number of fixations/ connector was much greater for the complex items. The fixation times were also longer for the complex connectors. There is no way of establishing whether this was due primarily to the complexity of the items or the experience of the inspectors. It should be pointed out that with the complex connectors the fixations were often placed very close together so that it was sometimes difficult to distinguish them. Each inspector had his own method of manually manipulating the connectors which largely determined the scan-paths. However, for any individual inspector there were small variations from one trial to another. For example, when viewing the spikes from the side there were variations in the end at which the inspector began to scan along the spikes. There were also variations from trial to trial in the number of recursive eye movements executed as inspectors scanned along the length of the connector. Table 2 includes data on the percentages of fixations associated with the three main viewing angles used by the inspectors. The two line inspectors showed close agreement while there were very marked differences in the distribution of fixations of the two inspectors examining the complex connectors.
Inspection of men's briefs This study was more detailed than the previous two. A batch of garments was prepared by someone from a large retail firm which was supplied by the company in which this study took place. The batch of 36 included seven faulty garments not acceptable to the retail firm. During the whole study the sequence of presentation of the garments was the same. All the examiners were female and had had at least five years experience with the company. On the first session, the batch was given to each of the examiners under normal conditions in order to obtain performance measures and subsequently to evaluate the effects of wearing the eye camera. The recording of eye movements was made in a second session which followed the original inspection by one week. The work cycle was complicated. A majority of the actual inspection was carried out while the garment was stretched over a wire frame which could be revolved to obtain different views of the garment. Examiners were not given specific instructions to adhere to the standard inspection rate of 2 rain 12 s/doz. During the first session it was found that all the examiners spent on average between 58 and 60% of the cycle time inspecting the garments on the frame. The cycle times were longer than the standard time for all the examiners. Table 3
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Table 3: The results from four examiners inspecting men's briefs
Remembering that the inspection times during this investigation were much longer than the standard time. one can appreciate that the selecti~m of fixation points under normal working conditions is critical in deterrmning outgoing quality. Research is continuing to see whether an optimum strategy can be taught to trainee examiners. It
Examiner D
M
J
S
Mean inspection time with garment on the frame (s) Without eye camera
12"5
8.7
9.1
13-4
With eye camera
16"7
14.5
9"0
23-8
Without eye camera
7
6
6
6
With eye camera
5
6
0
2
Without eye camera
7
5
5
1
With eye camera
2
2
0
0
Ri(J~ht
Front
Left
Left
Beck
Right
Number of critical faults detected, max = 7
Number of other garments rejected, max = 29
Fixation times (ms) Mean
321
332
284
389
SD
125
142
114
178
52
44
32
61
Mean number of fixations/ garment
shows that even at this rate, three of the four examiners missed one of the critical faults. As a possible consequence of the manner in which the study was conducted, the examiners rejected many garments which were acceptable to the person from the retail firm. However, these rejections could not be considered as false alarms since there was close agreement between the examiners as to which garments were rejected. This suggests that the examiners were not aware of the standards set by the retail firm.
S = stort C : COntlI'U¢~ E : end
Fig. 4
The basic scan-path adopted by Examiner D while inspecting men's briefs Right
Front
Left
When eye movements were recorded, Table 3 shows that inspection time with the garment on the frame was further increased for three of the examiners. In addition, the reject rate decreased for all the examiners to the extent that Examiners J and S failed to detect nearly all the critical faults. Only at this late stage of the study was it realised that these two examiners had poor vision. Although both had been provided with glasses, they were reluctant to wear them for long periods during normal inspection. As with all the studies that have been reported, differences in inspection times were mainly a result of differences in the number of fixations made to complete the inspection. All the examiners had their own methods of inspecting the garments on the frame and this was reflected in the analysis of their scan-paths. Figs. 4 and 5 illustrate the basic scan-paths for Examiners D and M. Individual fixation locations are not shown. The scan-paths closely matched the inspection performance. For example, Examiner M completed the inspection without viewing the back panel of the garment and consequently failed to reject the garment with a large ladder in the fabric of the rear panel. Similarly, the other critical misses could be attributed to the stereotyped scan-paths of the examiners.
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Applied Ergonomics September 1979
Left
Back
\\\
Right
///
\%_____jy
Fig. 5
c:
E=
~ooti~o~
The basic scan-path adopted by Examiner M while inspecting men's briefs
To show the effective coverage, samples of data from two examiners have been re-plotted in Fig. 8. In both cases the coverage was essentially a zig-zag one. The major difference between the examiners was in the number of saccades executed in passing from one side of the fabric to the other. This meant that Examiner B, who executed fewer fixations in passing across the fabric, gave a better general coverage. Examiner A gave detailed coverage to certain areas but left others completely unexplored. With this strategy it was unlikely that non-recurring faults such as dirt marks would have been detected should they have fallen in these areas. Apart from trying to encourage examiners to adopt a more random search strategy, the most obvious way to achieve better coverage is by reducing the speed of the fabric movement. As a start to improving the inspection task a suitably designed control to vary the speed during inspection should be provided. This would ensure that examiners would not have to inspect the fabric while it was outside the specialised lighting area.
Fig. 6
A general view of the inspection of woven fabric
To see the extent to which the scanning strategies were reflected in performance data, specially prepared rolls of
has been previously pointed out that to do this one needs knowledge of the relative importance and conspicuity of the various fault types.
Width of fabric
I 3174 5 216 18 6 14 IO 9 8 7 15 13 12 II 2019
Inspection of weft knitted fabric At this factory woven and dyed fabric was inspected for around 50 different fault types. The width of the fabric was 155 cm and it came in lengths of between 35 and 50 m. The general viewing conditions are shown in Fig. 6. The inspection method was to have the fabric advancing towards the examiner at a speed of 46 cm/s. There was no control to alter the speed during inspection although by ~playing' the foot controls which stopped and started the drive mechanism slower speeds could be achieved. Depending on the exact posture adopted by the examiners the angular velocity of the fabric varied between 20 ° and 30 30°/s. A viewing area was prescribed by a 45 cm wide transilluminated strip across the viewing table.
I
5 0 cm
However, with the occurrence of the pursuit movements there were several instances where the fixation point ended up outside this area. This was particularly apparent when the fixation time was long because the examiner was attempting to confirm the presence of a potential fault. Fig. 7 does not indicate the actual coverage given to the fabric since it takes no account of the fabric movement.
An example of the distribution within the viewing area of 20 successive fixations
Fig. 7
Examiner A
In the first part of this study five experienced examiners inspected two rolls of unpatterned production material. The mean fixation times of the examiners varied between 210 and 298 ms. These times are comparatively short and they confirm the conclusion that was drawn from the results of Ohtani (1969) that in the absence of distinct fixation points fixation times are shorter. Because of the movement of the fabric, 75% of the fixation phases were accompanied by pursuit movements. It was not possible to establish how closely they matched the velocity of the fabric. All the examiners demonstrated the same overall search strategy which is illustrated in Fig. 7. They executed a series of saccades so that the fixation points passed from left to right and back again across the width of the fabric. Initially the fixations fell within the area served by the transillumination.
Tronsiluminoted viewing area
Examiner B 19
19
18
18
17
16
17
15
16
14 15 14
~3
13
12 II 10 7 6 4
12 II 10
9
8 7
8
6
5
3
4 3
2
2
I I
50 cm
Fig. 8
Examples of the typical coverage given to a 2 m length of fabric by two examiners
material were used in the second part of the study. Four faults involving discolouring small areas (1 cm 2 ) of the fabric were added to each of five beige coloured rolls. Two different fault types were added. In one case tile faults were black and therefore very conspicuous while in the other they were yellow and much less conspicuous. Two
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rolls had all black faults, one roll all yellow faults and the other two had a mixture of faults. Four experienced examiners agreed to inspect all five rolls on three separate occasions, each separated by at least a week. The order of presentation of the rolls was randomised. During the first two sessions eye movements were recorded, but not during the third session. The last session was included to establish the extent to which the eye camera disrupted performance. Before inspecting each roll the examiner was informed of the fault characteristics. All the examiners exhibited the same search strategies that were reported in the first part of this study so that one can assume that large areas of the fabric were not covered by foveal fixations. Surprisingly, the eye camera did not interfere with performance since fault detection did not improve over the final session. None of the black faults was missed by any of the examiners, indicating that they were easily detected with peripheral vision. Performance was also very good on the roll containing all yellow faults and a hit probability of 0.94 was achieved. Only one of the faults was missed and this was positioned in the middle of the roll. Since two of the other yellow faults were positioned near the edges of the fabric, one must assume that even low contrast faults could be detected with peripheral vision. This finding, therefore, contradicts the prediction made following the first part of this study that small non-recurring faults would be very difficult to detect. On the other hand one should not forget that there is a lot of evidence that the peripheral visual system is specialised for detecting moving targets (Sharpe, 1974). This means that it is relatively difficult to predict the effects of search strategies on performance when studying the inspection of moving products. There remains one rather puzzling result. In the case of the rolls with both fault types, the detection probability of the yellow faults was only 0'44. There was one yellow fault positioned near the edge of the fabric which was detected on only five out of 24 separate inspections. It was unlikely that the low detection of the yellow faults on the mixed rolls compared with the performance on the all yellow roll was due solely to uncontrolled effects of fault position and fault contrast. An alternative explanation is in terms of the difficulty that examiners experience when inspecting simultaneously for fault types which vary widely in their conspicuity (Megaw and Richardson, 1979).
Conclusions All the studies that have been described should be regarded as exploratory. Only the last two attempted to introduce some necessary controls. By preparing special batches of a product which can be given to a sample of inspectors and by taking performance measures it was possible in those two studies to identify the extent to which the method of eye movement recording affected performance accuracy and to investigate individual differences in performance and eye movement strategies. One should be cautious in making comparisons between the studies if different recording methods were used. All the Birmingham studies used the same method.
in the number of fixations taken to complete the inspection cycle or to detect a fault rather than in differences in fixation times. With unpaced tasks, the experienced inspector employed relatively fewer fixations/cycle. Because standard times for inspection are established on an arbitrary basis, there is not an obvious correlation between the complexity of an inspection task and inspection time. In one firm, experienced inspectors took between 14 and 18 fixations to complete the inspection of simple electrical edge connectors while in another firm inspectors took between 9 and 15 fixations to examine complex electronic circuit chips. 2 Distributions of fixation times from individual inspectors were positively skewed and, therefore, if the results from several individuals over a range of tasks are compared, a positive correlation is found between means and standard deviations. There was some evidence to suggest that fixation times were comparatively short in the tasks where there were no distinct fixation points (steel sheet, woven fabric). While there was also evidence that experienced inspectors employed shorter fixation times, the effects of task complexity on fixation times were not identified. 3. Individual scan-paths were very pronounced in those cases where the product was manipulated as part of the work cycle, (bearings, tin cans, electrical connectors. men's briefs). It was implied that faults may have been missed because the scan-paths were too inflexible (men's briefs). The study on the inspection of electronic circuit chips did not reveal the presence of individual scan-paths. It has been pointed out that the task was complex but that the inspection times were surprisingly short. Under such conditions, it was possible that the sequence of eye movements was determined primarily by information acquired during the inspection of each chip. Systematic strategies were recorded from inspectors examining moving products (steel sheet, bottles, woven fabric). 4. Peripheral vision is particularly important for detecting potential faults when the product is moving and subtends a large visual angle (steel sheet, woven fabric).
Acknowledgements This work was sponsored by an SRC research contract. We would like to acknowledge the support given by the six companies who provided facilities for the research and we are grateful to Jane Alexander who analysed many of the eye movement recordings.
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