- Email: [email protected]
Searching for Blemishes on Apples
Theoretical and Applied Aspects of Eye Movement Research A.G. Gale and F. Johnson (Editors) 0 Elsevier Science Publishers B.V. (North-Holland), 1984
36 1
SEARCHING FOR BLEMISHES ON APPLES J.R.C.
Hillen
Department o f Human Sciences Loughbor ou g h U n i vers it y o f Techno 1ogy England.
The views expressed i n t h i s paper are n o t n e c e s s a r i l y those o f the M i n i s t r y o f A g r i c u l t u r e F i s h e r i e s and Food o r o f the Department o f Human Sciences, Loughborough U n i v e r s i ty
.
The v i s u a l i n s p e c t i o n o f apples has o n l y r e c e n t l y come under human f a c t o r s s c r u t i n y i n t h i s country. This paper describes i n i t i a l work t o i n v e s t i g a t e t h e a p p l i e d problem u s i n g v i s u a l lobe measures f o r s p e c i f i c d e f e c t s and a simple ' b e s t case' model o f i n s p e c t i o n . Limited eye movement recordings are used t o show t h a t t h e ' b e s t case' i s n o t apparent i n r e a l l i f e and h i g h l i g h t c e r t a i n individual differences. INTRODJCTION Apples can boast one o f the e a r l i e s t references t o f r u i t , when Eve gave Adam one t o eat, b u t i t i s o n l y i n t h e p a s t few years t h a t the human f a c t o r s of apple h a r v e s t i n g and marketing have been i n v e s t i g a t e d (e.g. Burkhardt and O'Brien 1979). The work described here i s p a r t o f a p r o j e c t c a r r i e d o u t a t Loughborough, funded by t h e M i n i s t r y o f A g r i c u l t u r e Food and F i s h e r i e s , The research has been s p l i t w i t h the aim o f improving the B r i t i s h apple. i n t o t h r e e main areas:1
2 3
-
Harvesting
- V i s u a l I n s p e c t i o n i n the Packhouse - Grading Standards Used i n the Packhouse.
I t i s d i f f i c u l t t o separate the l a s t two items b u t t h i s paper w i l l attempt t o concentrate on p u r e l y the v i s u a l i n s p e c t i o n component.
TYPICAL GRADING PRACTICE Apples a r e graded when the market r e q u i r e s the f r u i t , i n sane cases t h i s i s d i r e c t l y a f t e r harvesting b u t i n the main the fruit i s taken from a c o l d store. Large wooden bins, c o n t a i n i n g sane 720 l b s . o f produce, a r e used I n the l a r g e r packt o t r a n s f e r the apples onto a grading conveyor b e l t . houses t h i s i s done by f l o a t i n g t h e f r u i t o u t o f t h e b i n s b u t f o r t h e The person i n charge o f the m a j o r i t y o f cases d r y t i p p i n g i s used.
J.R.C.Hillen
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t i p p i n g o p e r a t i o n i s o f t e n responsible f o r t a k i n g o u t the r o t s (bad apples which have gone r o t t e n i n store). The apples (mainly Cox's Orange P i p p i n ) may then be p o l i s h e d which removes much o f the d r i e d f u n g i c i d e d i p used t o increase l o n g e v i t y i n the c o l d store, and imparts a supermarket shine. The grading t a b l e , where v i s u a l i n s p e c t i o n takes place, i s u s u a l l y n e x t i n This i s a r o l l i n g t o p conveyor designed t o r o t a t e t h e apples the line. (See Fig. 1). t o e x h i b i t more o f the apple surface t o the examiners. The number standing a t a s i n g l e The examiners are t r a d i t i o n a l l y female. grading t a b l e may v a r y between one and fourteen, the most cmmon nunber Apples r o l l onto the conveyor i n an being one o r two placed i n s e r i e s . i r r e g u l a r manner and a t i r r e g u l a r flow rates. The c o l o u r o f the r o l l e r s v a r i e s from black t o grey although t h e M i n i s t r y recmmends mid-grey. A f t e r the grading t a b l e a mechanical s i z i n g machine channels the f r u i t i n t o appropriate packing areas where the apples a r e boxed and weighed ready f o r market. F i g u r e 1:
R o l l i n g Top Conveyor
THE EXAMINERS TASK The task a t t h e grading t a b l e c o n s i s t s o f d e t e c t i n g flawed apples c l a s s i f y i n g t h e f l a w and p l a c i n g f r u i t i n the a p p r o p r i a t e c l a s s lane. Decisions are r e q u i r e d on f o u r a t t r i b u t e s o f each apple, c o l o u r , shape, r u s s e t and blenish. The standards f o r each a t t r i b u t e vary f o r each apple type and t o Standards are sometimes checked by an i n some e x t e n t from farm t o farm. house i n s p e c t o r c a r r y i n g o u t a 1 percent sample o f a l l boxes d e s t i n e d f o r sale. If t h e farm i s a member o f a s p e c i f i c marketing scheme t h e scheme's own i n s p e c t o r s may v i s i t t h e packhouse once a f o r t n i g h t f o r spot checks. M i n i s t r y t r a i n e d i n s p e c t o r s are u l t i m a t e l y responsible f o r the upkeep o f the EEC standards. Despite these checks i t i s estimated t h a t approximately 22 percent o f fruit i s m i s c l a s s i f i e d . FIELD WORK During the '82-83' grading season v i s i t s were made t o several commercial packhouses throughout England t o i n v e s t i g a t e t h e problem. W h i l s t a t the packhouse a measure o f t h e examiners b i n o c u l a r v i s u a l lobe was attempted. (For a d e s c r i p t i o n o f the v i s u a l lobe see previous paper by L. Bellamy.) To s i m p l i f y t h e mathematics i n v o l v e d a ' h a r d s h e l l ' concept was used. The lobe i n t h i s case being a two dimensional area around a c e n t r a l f i x a t i o n p o i n t w i t h i n which o b j e c t s can be seen on a t l e a s t a given percent o f occasions on which they appear.
Searching for Blemishes on Apples To map comprehensively t h e lobe boundaries i s an exhaustive process. t h i s study o n l y the major and minor axes were measured o f f l i n e i n an approximate s i m u l a t i o n o f t h e on l i n e task o f grading apples.
363 In
A s e r i e s o f 32 s l i d e s o f a 9 x 9 g r i d o f cox apples were presented t a c h i s t o E i g h t y o f the apples were s c o p i c a l l y a t a viewing d i s t a n c e o f 900 mn. The p o s i t i o n o f t h e d e f e c t v a r i e d a t randcm. p e r f e c t , one was d e f e c t i v e . Subjects were asked t o i n d i c a t e where they f e l t t h e f a u l t was and t o g i v e a After a r a t i n g o f confidence on a seven p o i n t scale i n t h e i r judgement. f i v e minute r e s t a f u r t h e r 32 s l i d e s were shown. Two types o f d e f e c t were used, blemishes which consisted o f c r a c k i n g over an area o f 135 sa mn and a markedly d i s c o l o u r e d b r u i s e o f 100 sq mn. These d e f e c t s were chosen t o r e p r k e n t a range from e a s i l y d e t e c t a b l e fau t s t o t h e more s u b t l e appearance f a u l t , such as a bruise. he I n c l a s s i c a l t a r g e t a c q u i s i t i o n terms a b r u i s e i s d i f f i c u l t t o define. v i s i b i l i t y o f t h e t a r g e t i s a f f e c t e d by i t s p o s i t i o n on t h e apple and the The c o l o u r o f the b r u i s e i s dependent on i t s c o l o u r o f t h e background. aae. Recent b r u i s e s are o n l v v i s i b l e as i n d e n t a t i o n s on the skin. Bru ses four t o f i v e hours a r e ma&edly discoloured, whereas o l d e r b r u i s i n g produces a l i g h t e r corky d i s c o l o u r a t i o n j u s t below t h e surface. The c o l o u r across t h e b r u i s e v a r i e s and i s a l s o dependent on t h e o r i g i n a l s k i n colour. Therefore as a standard b r u i s e c o u l d n o t be used f o r t h e study t h e s l i d e s were taken of' a range o f concave bruises.
OF
The o b j e c t i v e data has been accumulated f o r 47 subjects and an average lobe c a l c u l a t e d f o r t h e l a r g e r d e f e c t i s shown i n Fig. 2. For a 100 sq mm b r u i s e the lobe reduces t o the s i z e o f a s i n g l e apple. F i g u r e 2:
'Average' Lobe f o r Cracking Defect
SIMPLE MODEL OF SEARCH Using the hard s h e l l lobe approximation a simple model based on c e r t a i n assunptions may be employed t o i n d i c a t e the n m b e r o f bruises t h a t can be seen by an examiner under s p e c i f i c conditions. R o t a t i o n o f t h e apples has an important bearing on t h e p r o b a b i l i t y o f s p o t t i n g a blemish. Assum i n g t h a t an examiner can examine 120 degrees o f an apple surface a t each glimpse i t w i l l r e q u i r e 5 f i x a t i o n s t o examine t h e e n t i r e surface o f t h e apple, i n c l u d i n g some o v e r l a p o f glimpses. Assuming t h e average diameter o f an apple i s 68 mn, t h e minimum r o t a t i o n a l d i s t a n c e r e q u i r e d t o e x h i b i t 5 sides of the apple i s 285 m. Unfortunately the d i s p l a y o f the f i v e sides r a r e l y occurs i n t h e d e s i r e d sequence. If it
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J. R. C. Hillen
i s assumed t h a t any of the three adjacent s i d e s i s equally l i k e l y t o appear next, then by Monte Carlo methods i t will require on average 9 s i d e s t o be displayed f o r the e n t i r e surface t o be exposed f o r examination. This requires the apple t o r o t a t e over a distance of 569 mm. In p r a c t i c e due t o the shape of f r u i t the assumption of each adjacent s i d e i s a s l i k e l y t o be shown i s unlikely t o hold t r u e . Again by Monte Carlo methods 60 percent of t h e apples will have been f u l l y exposed i n 569 mm, the remaining 40 percent requiring one o r two s i d e s s t i l l t o be displayed. On average 12 percent o f t h e apple surface will not be displayed i n t h e 569 mn which implies the examiner misses the same percentage of blemishes. The assumpt i o n being t h a t the examiner i s continuously engaged i n t h e search task. Assuming t h e search area f o r a s i n g l e examiner, a t a s p e c i f i c t a b l e , i s 450 mm (width of the b e l t ) by 569 mm and i s as f u l l as possible with f r u i t Allowing f o r a viewing angle of 45 (an a r r a y of 6 by 8 average apples). I t will require degrees the lobe f o r the major cracking i s 350 x 328 mm. 6 glimpses t o optimally cover t h e search area completely, allowing f o r considerable overlap. Assuming the ' b e s t c a s e ' of systematic search with f i x a t i o n times of.3 s (Megaw and Richardson 1979) the time required f o r For the conveyor i n question the time t o complete coverage i s 2 s. t r a v e r s e the search area i s 5.1 s , equivalent t o two and a half complete A t the centre of the b e l t where considerable overlap scans of the b e l t . occurs t h e r e will be s u f f i c i e n t glimpses t o cover the e n t i r e apple surface. A t t h e s i d e s only 7.5 glimpses occur and only about 83 percent of the s i d e a p p l e s ' surface will be seen. O n average, as each row of six apples passes through, only 94 percent of the a v a i l a b l e surface will be seen. Combining the lobe calculation with the one on r o t a t i o n i t can be deduced t h a t about 20 percent of the t o t a l surface will not be exposed f o r examinat i o n by a s i n g l e examiner. This calculation requires some very s i m p l i s t i c assumptions which a r e not r e a l i s t i c although the model indicates t h e r e i s a problem. Conditions vary markedly The above f i g u r e i s only applicable t o one farm. Flow r a t e s of apples per examiner may range from 60 i n o t h e r packhouses. t o 480 per minute and the physical measurements of the grading t a b l e may differ. Other f a c t o r s not taken account of i n t h e model will a l s o a f f e c t inspection perfotmance. Management s t y l e and r a t e s of pay a r e not uniform throughout the country. Pay schemes can be based on a f l a t r a t e , f l a t r a t e plus bonus Enviromental f a c t o r s play a l a r g e p a r t i n examiner o r piece work. perfotmance, the range of conditions encountered during t h e f i e l d work i s summarised i n the following table.
Searching for Blembhes on Apples
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Ranges o f E n v i r o m e n t a l Conditions Between Packhouses
Dry Bulb Temp. Noise Level Light Vertical L i g h t Horizontal
OC
Dba Lux Lux
Min
Max
7
22 80 2500 1100
65 300 150
EYE MOVEMENT RECORDINGS I n a d d i t i o n t o the t h e o r e t i c a l ' b e s t case' model i t was decided t o t a k e eye movement recordings o f examiners c a r r y i n g o u t the i n s p e c t i o n task. Due t o t h e l a c k o f time a v a i l a b l e t h i s was n o t an exhaustive study and few objecHowever t h e recordings can be employed t i v e conclusions can be reached. t o high1 i g h t i n d i v i d u a l d i f f e r e n c e s between examiners. The recordings were taken a t a M i n i s t r y farm, using one male and two female subjects, A NAC eyemark camera from towards the end o f a f o u r hour morning s h i f t . The examiner being the U n i v e r s i t y o f Birmingham was used f o r t h e study. recorded stood t h i r d i n s e r i e s a t t h e grading t a b l e due t o t h e l a c k o f space f o r equipnent i n the grading room. There were occasional complaints t h a t t h e h a l f s i l v e r e d m i r r o r s o f t h e facemask made i t v e r y d i f f i c u l t t o d e t e c t No performance data was c o l l e c t e d . bruising. From t h e tape i t appears t h a t none o f the i n s p e c t o r s used any systematic scan paths t o cover t h e b e l t . No obvious p a t t e r n c o u l d be discerned, due t o the i r r e g u l a r f l o w o f items t o be inspected. This observation, although n o t based on a l a r g e sample helps show t h a t t h e assumption i n c o r p o r a t e d i n the simple model i s f a l s e . I f t h e o t h e r assumptions h o l d t r u e t h e estimate o f blemishes missed i s e v i d e n t l y conservative. Two o f the subjects had over f i v e years experience o f grading whereas the remaining one had j u s t The naive grader made more f i x a t i o n s per u n i t time s t a r t e d t h i s season. than t h e o t h e r two, however about 15 percent o f f i x a t i o n s were d u p l i c a t i o n s I n b o t h o f t h e experienced operators such marked d u p l i c a o f e a r l i e r ones. There i s a p o s s i b i l i t y t h a t t h e subjects lobe i n t i o n was n o t present. creases w i t h experience although t h i s was n o t e v i d e n t from the f i e l d work. Rather t h e d e c i s i o n element may n o t be as a u t a n a t i c i n n a i v e graders as f o r those w i t h experience. This s u p p o s i t i o n s t r a y s i n t o t h e c o g n i t i v e danain o f the p r o j e c t . The naive examiner picked up a l a r g e r p r o p o r t i o n o f apples t o a i d i n s p e c t i o n A general and f i x a t e d more on the c l a s s 2 lane when t r a n s f e r r i n g f r u i t . philosophy behind grading apples i s n o t t o handle the produce more than necessary i n order t o reduce t h e chance o f b r u i s i n g .
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The n a i v e g r a d e r a l s o l o o k e d s t r a i g h t ahead a t t h e conveyor whereas t h e o t h e r s tended t o t a k e a more o b l i q u e view of t h e b e l t e n s u r i n g t h a t any rogue a p p l e s t a y s i n s i g h t f o r a l o n g e r p e r i o d . T h i s may be due t o e x p e r i e n c e o r personal preference. Summary o f t h e Three Examiners R e s u l t s Mean F i x a t i o n Time ( 5 . ) Sub A Sub B Sub C
0.45 0.25 0.24
S.D. F i x a t i o n Time ( s . )
0.23 0.20 0.18
Flow Rate Appl es/Mi n. 177 190 180
P
~
~
~
e
Fixated
70 80 55
SUMMARY Rudimentary l o b e measurements, s i m p l i s t i c m o d e l l i n g , and eye movement r e c o r d i n g s can be a u s e f u l c o m b i n a t i o n when t a c k l i n g an a p p l i e d i n s p e c t i o n Eye movements h e l p e d t o i n d i c a t e obvious d i f f e r e n c e s between problem. search s t r a t e g i e s i n a ' b e s t case' model and r e a l l i f e . The r e c o r d i n g s a l s o c o n t a i n e d i n f o r m a t i o n on i n d i v i d u a l d i f f e r e n c e s i n examiner behaviour. F u r t h e r work i s b e i n g c a r r i e d o u t a t Loughborough t o i n v e s t i g a t e d i f f e r e n t methods o f d i s p l a y i n g apples t o t h e examiners. ACKNOWLEDGEMENTS
I would l i k e t o thank D r E. Megaw f o r t h e l o a n o f h i s t i m e and t h e eye movement r e c o r d e r . REFERENCES (1)
Bellamy, L. 1981, Developnent of a Search Task f o r t h e Measurement o f P e r i p h e r a l A c u i t y , Ergonomics 24 ( 7 ) 497-509.
(2)
B u r k h a r d t , T.H. and O ' B r i e n , M. 1979, Hunan C o n s i d e r a t i o n s i n Mechanizing F r u i t and Vegetable Grading,Transactions o f A.S.A.E. 507 -509.
(3)
Megaw, E. and Richardson, J. 1979. Eye Movements and I n d u s t r i a l I n s p e c t i o n , A p p l i e d Ergonomics 1 0 ( 3 ) 145-154.
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36 1
SEARCHING FOR BLEMISHES ON APPLES J.R.C.
Hillen
Department o f Human Sciences Loughbor ou g h U n i vers it y o f Techno 1ogy England.
The views expressed i n t h i s paper are n o t n e c e s s a r i l y those o f the M i n i s t r y o f A g r i c u l t u r e F i s h e r i e s and Food o r o f the Department o f Human Sciences, Loughborough U n i v e r s i ty
.
The v i s u a l i n s p e c t i o n o f apples has o n l y r e c e n t l y come under human f a c t o r s s c r u t i n y i n t h i s country. This paper describes i n i t i a l work t o i n v e s t i g a t e t h e a p p l i e d problem u s i n g v i s u a l lobe measures f o r s p e c i f i c d e f e c t s and a simple ' b e s t case' model o f i n s p e c t i o n . Limited eye movement recordings are used t o show t h a t t h e ' b e s t case' i s n o t apparent i n r e a l l i f e and h i g h l i g h t c e r t a i n individual differences. INTRODJCTION Apples can boast one o f the e a r l i e s t references t o f r u i t , when Eve gave Adam one t o eat, b u t i t i s o n l y i n t h e p a s t few years t h a t the human f a c t o r s of apple h a r v e s t i n g and marketing have been i n v e s t i g a t e d (e.g. Burkhardt and O'Brien 1979). The work described here i s p a r t o f a p r o j e c t c a r r i e d o u t a t Loughborough, funded by t h e M i n i s t r y o f A g r i c u l t u r e Food and F i s h e r i e s , The research has been s p l i t w i t h the aim o f improving the B r i t i s h apple. i n t o t h r e e main areas:1
2 3
-
Harvesting
- V i s u a l I n s p e c t i o n i n the Packhouse - Grading Standards Used i n the Packhouse.
I t i s d i f f i c u l t t o separate the l a s t two items b u t t h i s paper w i l l attempt t o concentrate on p u r e l y the v i s u a l i n s p e c t i o n component.
TYPICAL GRADING PRACTICE Apples a r e graded when the market r e q u i r e s the f r u i t , i n sane cases t h i s i s d i r e c t l y a f t e r harvesting b u t i n the main the fruit i s taken from a c o l d store. Large wooden bins, c o n t a i n i n g sane 720 l b s . o f produce, a r e used I n the l a r g e r packt o t r a n s f e r the apples onto a grading conveyor b e l t . houses t h i s i s done by f l o a t i n g t h e f r u i t o u t o f t h e b i n s b u t f o r t h e The person i n charge o f the m a j o r i t y o f cases d r y t i p p i n g i s used.
J.R.C.Hillen
3 62
t i p p i n g o p e r a t i o n i s o f t e n responsible f o r t a k i n g o u t the r o t s (bad apples which have gone r o t t e n i n store). The apples (mainly Cox's Orange P i p p i n ) may then be p o l i s h e d which removes much o f the d r i e d f u n g i c i d e d i p used t o increase l o n g e v i t y i n the c o l d store, and imparts a supermarket shine. The grading t a b l e , where v i s u a l i n s p e c t i o n takes place, i s u s u a l l y n e x t i n This i s a r o l l i n g t o p conveyor designed t o r o t a t e t h e apples the line. (See Fig. 1). t o e x h i b i t more o f the apple surface t o the examiners. The number standing a t a s i n g l e The examiners are t r a d i t i o n a l l y female. grading t a b l e may v a r y between one and fourteen, the most cmmon nunber Apples r o l l onto the conveyor i n an being one o r two placed i n s e r i e s . i r r e g u l a r manner and a t i r r e g u l a r flow rates. The c o l o u r o f the r o l l e r s v a r i e s from black t o grey although t h e M i n i s t r y recmmends mid-grey. A f t e r the grading t a b l e a mechanical s i z i n g machine channels the f r u i t i n t o appropriate packing areas where the apples a r e boxed and weighed ready f o r market. F i g u r e 1:
R o l l i n g Top Conveyor
THE EXAMINERS TASK The task a t t h e grading t a b l e c o n s i s t s o f d e t e c t i n g flawed apples c l a s s i f y i n g t h e f l a w and p l a c i n g f r u i t i n the a p p r o p r i a t e c l a s s lane. Decisions are r e q u i r e d on f o u r a t t r i b u t e s o f each apple, c o l o u r , shape, r u s s e t and blenish. The standards f o r each a t t r i b u t e vary f o r each apple type and t o Standards are sometimes checked by an i n some e x t e n t from farm t o farm. house i n s p e c t o r c a r r y i n g o u t a 1 percent sample o f a l l boxes d e s t i n e d f o r sale. If t h e farm i s a member o f a s p e c i f i c marketing scheme t h e scheme's own i n s p e c t o r s may v i s i t t h e packhouse once a f o r t n i g h t f o r spot checks. M i n i s t r y t r a i n e d i n s p e c t o r s are u l t i m a t e l y responsible f o r the upkeep o f the EEC standards. Despite these checks i t i s estimated t h a t approximately 22 percent o f fruit i s m i s c l a s s i f i e d . FIELD WORK During the '82-83' grading season v i s i t s were made t o several commercial packhouses throughout England t o i n v e s t i g a t e t h e problem. W h i l s t a t the packhouse a measure o f t h e examiners b i n o c u l a r v i s u a l lobe was attempted. (For a d e s c r i p t i o n o f the v i s u a l lobe see previous paper by L. Bellamy.) To s i m p l i f y t h e mathematics i n v o l v e d a ' h a r d s h e l l ' concept was used. The lobe i n t h i s case being a two dimensional area around a c e n t r a l f i x a t i o n p o i n t w i t h i n which o b j e c t s can be seen on a t l e a s t a given percent o f occasions on which they appear.
Searching for Blemishes on Apples To map comprehensively t h e lobe boundaries i s an exhaustive process. t h i s study o n l y the major and minor axes were measured o f f l i n e i n an approximate s i m u l a t i o n o f t h e on l i n e task o f grading apples.
363 In
A s e r i e s o f 32 s l i d e s o f a 9 x 9 g r i d o f cox apples were presented t a c h i s t o E i g h t y o f the apples were s c o p i c a l l y a t a viewing d i s t a n c e o f 900 mn. The p o s i t i o n o f t h e d e f e c t v a r i e d a t randcm. p e r f e c t , one was d e f e c t i v e . Subjects were asked t o i n d i c a t e where they f e l t t h e f a u l t was and t o g i v e a After a r a t i n g o f confidence on a seven p o i n t scale i n t h e i r judgement. f i v e minute r e s t a f u r t h e r 32 s l i d e s were shown. Two types o f d e f e c t were used, blemishes which consisted o f c r a c k i n g over an area o f 135 sa mn and a markedly d i s c o l o u r e d b r u i s e o f 100 sq mn. These d e f e c t s were chosen t o r e p r k e n t a range from e a s i l y d e t e c t a b l e fau t s t o t h e more s u b t l e appearance f a u l t , such as a bruise. he I n c l a s s i c a l t a r g e t a c q u i s i t i o n terms a b r u i s e i s d i f f i c u l t t o define. v i s i b i l i t y o f t h e t a r g e t i s a f f e c t e d by i t s p o s i t i o n on t h e apple and the The c o l o u r o f the b r u i s e i s dependent on i t s c o l o u r o f t h e background. aae. Recent b r u i s e s are o n l v v i s i b l e as i n d e n t a t i o n s on the skin. Bru ses four t o f i v e hours a r e ma&edly discoloured, whereas o l d e r b r u i s i n g produces a l i g h t e r corky d i s c o l o u r a t i o n j u s t below t h e surface. The c o l o u r across t h e b r u i s e v a r i e s and i s a l s o dependent on t h e o r i g i n a l s k i n colour. Therefore as a standard b r u i s e c o u l d n o t be used f o r t h e study t h e s l i d e s were taken of' a range o f concave bruises.
OF
The o b j e c t i v e data has been accumulated f o r 47 subjects and an average lobe c a l c u l a t e d f o r t h e l a r g e r d e f e c t i s shown i n Fig. 2. For a 100 sq mm b r u i s e the lobe reduces t o the s i z e o f a s i n g l e apple. F i g u r e 2:
'Average' Lobe f o r Cracking Defect
SIMPLE MODEL OF SEARCH Using the hard s h e l l lobe approximation a simple model based on c e r t a i n assunptions may be employed t o i n d i c a t e the n m b e r o f bruises t h a t can be seen by an examiner under s p e c i f i c conditions. R o t a t i o n o f t h e apples has an important bearing on t h e p r o b a b i l i t y o f s p o t t i n g a blemish. Assum i n g t h a t an examiner can examine 120 degrees o f an apple surface a t each glimpse i t w i l l r e q u i r e 5 f i x a t i o n s t o examine t h e e n t i r e surface o f t h e apple, i n c l u d i n g some o v e r l a p o f glimpses. Assuming t h e average diameter o f an apple i s 68 mn, t h e minimum r o t a t i o n a l d i s t a n c e r e q u i r e d t o e x h i b i t 5 sides of the apple i s 285 m. Unfortunately the d i s p l a y o f the f i v e sides r a r e l y occurs i n t h e d e s i r e d sequence. If it
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i s assumed t h a t any of the three adjacent s i d e s i s equally l i k e l y t o appear next, then by Monte Carlo methods i t will require on average 9 s i d e s t o be displayed f o r the e n t i r e surface t o be exposed f o r examination. This requires the apple t o r o t a t e over a distance of 569 mm. In p r a c t i c e due t o the shape of f r u i t the assumption of each adjacent s i d e i s a s l i k e l y t o be shown i s unlikely t o hold t r u e . Again by Monte Carlo methods 60 percent of t h e apples will have been f u l l y exposed i n 569 mm, the remaining 40 percent requiring one o r two s i d e s s t i l l t o be displayed. On average 12 percent o f t h e apple surface will not be displayed i n t h e 569 mn which implies the examiner misses the same percentage of blemishes. The assumpt i o n being t h a t the examiner i s continuously engaged i n t h e search task. Assuming t h e search area f o r a s i n g l e examiner, a t a s p e c i f i c t a b l e , i s 450 mm (width of the b e l t ) by 569 mm and i s as f u l l as possible with f r u i t Allowing f o r a viewing angle of 45 (an a r r a y of 6 by 8 average apples). I t will require degrees the lobe f o r the major cracking i s 350 x 328 mm. 6 glimpses t o optimally cover t h e search area completely, allowing f o r considerable overlap. Assuming the ' b e s t c a s e ' of systematic search with f i x a t i o n times of.3 s (Megaw and Richardson 1979) the time required f o r For the conveyor i n question the time t o complete coverage i s 2 s. t r a v e r s e the search area i s 5.1 s , equivalent t o two and a half complete A t the centre of the b e l t where considerable overlap scans of the b e l t . occurs t h e r e will be s u f f i c i e n t glimpses t o cover the e n t i r e apple surface. A t t h e s i d e s only 7.5 glimpses occur and only about 83 percent of the s i d e a p p l e s ' surface will be seen. O n average, as each row of six apples passes through, only 94 percent of the a v a i l a b l e surface will be seen. Combining the lobe calculation with the one on r o t a t i o n i t can be deduced t h a t about 20 percent of the t o t a l surface will not be exposed f o r examinat i o n by a s i n g l e examiner. This calculation requires some very s i m p l i s t i c assumptions which a r e not r e a l i s t i c although the model indicates t h e r e i s a problem. Conditions vary markedly The above f i g u r e i s only applicable t o one farm. Flow r a t e s of apples per examiner may range from 60 i n o t h e r packhouses. t o 480 per minute and the physical measurements of the grading t a b l e may differ. Other f a c t o r s not taken account of i n t h e model will a l s o a f f e c t inspection perfotmance. Management s t y l e and r a t e s of pay a r e not uniform throughout the country. Pay schemes can be based on a f l a t r a t e , f l a t r a t e plus bonus Enviromental f a c t o r s play a l a r g e p a r t i n examiner o r piece work. perfotmance, the range of conditions encountered during t h e f i e l d work i s summarised i n the following table.
Searching for Blembhes on Apples
365
Ranges o f E n v i r o m e n t a l Conditions Between Packhouses
Dry Bulb Temp. Noise Level Light Vertical L i g h t Horizontal
OC
Dba Lux Lux
Min
Max
7
22 80 2500 1100
65 300 150
EYE MOVEMENT RECORDINGS I n a d d i t i o n t o the t h e o r e t i c a l ' b e s t case' model i t was decided t o t a k e eye movement recordings o f examiners c a r r y i n g o u t the i n s p e c t i o n task. Due t o t h e l a c k o f time a v a i l a b l e t h i s was n o t an exhaustive study and few objecHowever t h e recordings can be employed t i v e conclusions can be reached. t o high1 i g h t i n d i v i d u a l d i f f e r e n c e s between examiners. The recordings were taken a t a M i n i s t r y farm, using one male and two female subjects, A NAC eyemark camera from towards the end o f a f o u r hour morning s h i f t . The examiner being the U n i v e r s i t y o f Birmingham was used f o r t h e study. recorded stood t h i r d i n s e r i e s a t t h e grading t a b l e due t o t h e l a c k o f space f o r equipnent i n the grading room. There were occasional complaints t h a t t h e h a l f s i l v e r e d m i r r o r s o f t h e facemask made i t v e r y d i f f i c u l t t o d e t e c t No performance data was c o l l e c t e d . bruising. From t h e tape i t appears t h a t none o f the i n s p e c t o r s used any systematic scan paths t o cover t h e b e l t . No obvious p a t t e r n c o u l d be discerned, due t o the i r r e g u l a r f l o w o f items t o be inspected. This observation, although n o t based on a l a r g e sample helps show t h a t t h e assumption i n c o r p o r a t e d i n the simple model i s f a l s e . I f t h e o t h e r assumptions h o l d t r u e t h e estimate o f blemishes missed i s e v i d e n t l y conservative. Two o f the subjects had over f i v e years experience o f grading whereas the remaining one had j u s t The naive grader made more f i x a t i o n s per u n i t time s t a r t e d t h i s season. than t h e o t h e r two, however about 15 percent o f f i x a t i o n s were d u p l i c a t i o n s I n b o t h o f t h e experienced operators such marked d u p l i c a o f e a r l i e r ones. There i s a p o s s i b i l i t y t h a t t h e subjects lobe i n t i o n was n o t present. creases w i t h experience although t h i s was n o t e v i d e n t from the f i e l d work. Rather t h e d e c i s i o n element may n o t be as a u t a n a t i c i n n a i v e graders as f o r those w i t h experience. This s u p p o s i t i o n s t r a y s i n t o t h e c o g n i t i v e danain o f the p r o j e c t . The naive examiner picked up a l a r g e r p r o p o r t i o n o f apples t o a i d i n s p e c t i o n A general and f i x a t e d more on the c l a s s 2 lane when t r a n s f e r r i n g f r u i t . philosophy behind grading apples i s n o t t o handle the produce more than necessary i n order t o reduce t h e chance o f b r u i s i n g .
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The n a i v e g r a d e r a l s o l o o k e d s t r a i g h t ahead a t t h e conveyor whereas t h e o t h e r s tended t o t a k e a more o b l i q u e view of t h e b e l t e n s u r i n g t h a t any rogue a p p l e s t a y s i n s i g h t f o r a l o n g e r p e r i o d . T h i s may be due t o e x p e r i e n c e o r personal preference. Summary o f t h e Three Examiners R e s u l t s Mean F i x a t i o n Time ( 5 . ) Sub A Sub B Sub C
0.45 0.25 0.24
S.D. F i x a t i o n Time ( s . )
0.23 0.20 0.18
Flow Rate Appl es/Mi n. 177 190 180
P
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e
Fixated
70 80 55
SUMMARY Rudimentary l o b e measurements, s i m p l i s t i c m o d e l l i n g , and eye movement r e c o r d i n g s can be a u s e f u l c o m b i n a t i o n when t a c k l i n g an a p p l i e d i n s p e c t i o n Eye movements h e l p e d t o i n d i c a t e obvious d i f f e r e n c e s between problem. search s t r a t e g i e s i n a ' b e s t case' model and r e a l l i f e . The r e c o r d i n g s a l s o c o n t a i n e d i n f o r m a t i o n on i n d i v i d u a l d i f f e r e n c e s i n examiner behaviour. F u r t h e r work i s b e i n g c a r r i e d o u t a t Loughborough t o i n v e s t i g a t e d i f f e r e n t methods o f d i s p l a y i n g apples t o t h e examiners. ACKNOWLEDGEMENTS
I would l i k e t o thank D r E. Megaw f o r t h e l o a n o f h i s t i m e and t h e eye movement r e c o r d e r . REFERENCES (1)
Bellamy, L. 1981, Developnent of a Search Task f o r t h e Measurement o f P e r i p h e r a l A c u i t y , Ergonomics 24 ( 7 ) 497-509.
(2)
B u r k h a r d t , T.H. and O ' B r i e n , M. 1979, Hunan C o n s i d e r a t i o n s i n Mechanizing F r u i t and Vegetable Grading,Transactions o f A.S.A.E. 507 -509.
(3)
Megaw, E. and Richardson, J. 1979. Eye Movements and I n d u s t r i a l I n s p e c t i o n , A p p l i e d Ergonomics 1 0 ( 3 ) 145-154.
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