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Discomfort caused by wide-source glare
Energy and Buildings, 15 - 16 (1990/91) 391 . 398
391
Discomfort Caused by Wide-source Glare TOSHIE IWATA and KEN-ICHI KIMURA
Department of Architecture, Waseda University, Tokyo (Japan)" MASANORI SHUKUYA
Department of Architecture, Musashi Institute of Technology, Tokyo (Japan) KYOSUKE TAKANO
Nikken Sekkei Ltd., Osaka (Japan)
ABSTRACT
We report here on experiments to measure subjective response to intense light, or glare caused by a wide source. We have investigated three glare indices: the Building Research Station glare index; the CIE glare index and the Cornell daylight glare index. We have also examined the glare vote and have proposed a new glare evaluation scale, as well as asking the subject to vote on the condition's acceptability. The Cornell formula most accurately predicts glare discomfort, but it is found to be inadequate for a range of wide-source glare conditions. Both the discomfort sensation vote and the glare vote which we proposed correlate well with the percentage of subjects dissatisfied when looking directly at the light source.
INTRODUCTION
The development of new window materials may make it possible to control the solar radiation transmitted through windows to the desired levels. From the viewpoint of controlling the glare caused by windows, it is important to be able to assess the magnitude of the glare caused by daylight. Even with an ideally controllable window material, we will never be able to use daylight fully if we do not understand the discomfort caused by glare. This paper describes the results of preliminary experiments to measure glare discomfort from windows. The general goal is to predict the extent of occupant discomfort by knowing general features about the light levels, orientation and perhaps other factors. The specific 0378-7788/91/$3.50
purpose of this work was to examine the practical capability and limitations of existing glare indices. In particular, we sought to evaluate the accuracy and usefulness of earlier research in predicting glare discomfort and satisfaction. The first problem with earlier research is that only the contrast effect was considered. It is not clear if this assumption can be applied to wide-source glare, such as that from a window. The second problem is that the glare criterion scale is not useful for practical design because it combines sensation and dissatisfaction. The results of these experiments will guide our future work in visual comfort.
GLARE INDICES
There are several existing methods to evaluate glare. The Building Research Station (BRS) glare index derived from a study by Petherbridge and Hopkinson [1] is perhaps the best known, and is defined as 16. 08 vL~w °~ BRS glare index = 10 log 0.478 ~., Lbp1.6
(1)
where L s (cd/m 2) is the luminance of each glare source in the field of view, w (sr) is the solid angle of the glare source subtended at the eye, and L b (cd/m 2) is the average luminance of the visual field in which the glare sources are seen. P is a function of the angle between the direction of the light source and the observer's direction of viewing [2]. The weakness of the Hopkinson formula lies in its mathematical inconsistency; a large glare source cannot be subdivided for the purpose of © Elsevier Sequoia/Printed in The Netherlands
392
summing up glare contributions. Therefore the so-called Einhorn formula, presented in ref. 3 as the 'CIE formula' was developed:
TABLE 1 Relationship between Hopkinson glare criterion and two glare indices
CIE glare index
Glare criterion
=8 log 2[ ! + (Ed/500)] V g~w ~ j ~ p2
(2) Imperceptible Just perceptible Perceptible Just acceptable Acceptable Just uncomfortable Uncomfortable Just intolerable Intolerable
where Ed(lx) is the direct vertical illuminance at the eye due to all sources, and Ei(lx) the indirect illuminance at the eye. Hopkinson modified the BRS glare formula in eqn. (1) to the following equation in order to be able to assess the glare from large sources [4]. This is called the daylight glare index or Cornell index:
BRS glare index (BRS GI)
Daylight glare index (DGI)
l0 13 16 19 22 25 28
16 18 20 22 24 26 28
daylight glare index L~.6~0.s = 10 log 0.478 ~ Lb + 0.07 W°'5 L s
We modified this criterion so that it could be used as a scale to evaluate glare sensation. One additional difficulty is that the Japanese language does not have a specific word for 'just'. Our modified scale avoids this problem. We call the modified scale the glare vote. It is shown in Fig. 1 as Questionnaire 1. However, both the original Hopkinson glare criterion and the modified glare vote combine the subject's feelings regarding both perception and discomfort. These scales make it difficult to predict levels of dissatisfaction. To separate these feelings, we developed two new scales. One questionnaire asks about sensation. We call this the discomfort sensation vote (DSV); it is shown as Questionnaire 2 in
(3)
where ~ is the solid angle subtended by the source modified for the effect of the position of its elements in diffel:ent parts of the field of view in the way put forward by Petherbridge and Longmore [5].
GLARE VOTE, DISCOMFORT SENSATION VOTE AND PERCENT SATISFIED
Hopkinson linked a subjective glare criterion for the BRS index to the daylight glare index. This relationship is shown in Table 1. Questionnaire
Questionnaire
1
2
Dkscomfort ,Sensation
Glare Vote
Vote (osv)
intolerable
4
3
very uncomfortable
uncomfortable _ _.
acceptable 1
1
uncomfortable
-
slightly uncomfortable
perceptible
-
0 __
not uncomfortable
-
imperceptible Questionnaire 3 Accep~b~
[]
acceptable
[]
not acceptable
Fig. 1. Questionnaires used for glare vote, discomfort sensation vote and acceptability.
393
Fig. 1. This scale is intended to ask the subject only about the degree of discomfort. The other questionnaire asks about satisfaction; this is called the percent dissatisfied and is shown as Questionnaire 3 in Fig. 1. Since the glare vote and the discomfort sensation vote scales have only one direction (monopolar scale), it is difficult to determine the borderline between acceptable and unacceptable levels of glare. Thus the percent dissatisfied is the most reasonable way to find the borderline.
METHOD
For this experiment, we used Waseda University's environmental test chamber, which is
equipped with a simulated window. Figures 2 and 3 show the plan of the chamber. The window was illuminated from the rear by a bank of metal halide lamps. The intensity of the light was adjusted by the number of sheets of paper over the glass. This also served to diffuse the light. The window size is 80 × 80 cm. Two subjects were tested at one time, although their positions were different. The subject in position A looks directly at the wall on which there is the simulated window (shown in Fig. 2). The subject in position B has a line of sight perpendicular to the light from the window (Fig. 3). Figure 4 shows the experimental procedure. In each test, two subjects sat in the chamber, looked down at sheets of paper on their desks
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Fig. 3. Plan and section of chamber with subjects in position B (dimensions in ram).
394
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(1) adaptation (lookingat desk) (2) head up, Jooklngstralghtahead, early vote (Glare Vote or DSV, and Acceptablllty) (3) keep lookingstralghtahead (30 seconds) (4) late vote Fig. 4. Procedure used to test subjects.
and performed a task. At certain intervals, they raised their heads and looked at the fixation point straight in front and were asked to evaluate the glare on a voting sheet. After looking at the point for 30 s, the subjects were again asked to evaluate the discomfort of the light. Each subject repeated the test at five different luminances: 40000, 13 000, 7000, 4000 and 2500 cd/m 2. The different light levels were presented randomly so as to control for the effect of presentation. The questionnaires were written in Japanese, but the exact words in the questionnaire corresponded closely to those English words shown in Fig. 1. Questionnaire 3 (on acceptability) was administered in every test, but Questionnaires 1 and 2 were administered in alternate tests. Altogether, 80 paid subjects (40 males, 40 females) participated in the experiments. The average age of the subjects was 22 years.
RESULTS We first compared the results of the experiment in terms of traditional glare indices. Tables 2 and 3 show the assumptions and calculations for the glare values. Figures 5 - 7 show the relationships between the glare vote by subjects and the three glare indices in position A, together with the dashed line showing the results of the previous study by Hopkinson. BRS and CIE formulae were calculated for the position index at the centroid of the window. Table 1 was used to convert the glare index values to the Hopkinson glare criterion. We found that the Cornell index most accurately predicted the subjects' votes with the Hopkinson glare criterion (see Discussion). Figures 8 and 9 show the relationships between the glare vote and our two new glare evaluations, the DSV and the percent dissatisfied. The percent dissatisfied means the per-
395 TABLE 2 Measurements and calculations for the glare values in position A
Screen
Seat of subject
Vertical illuminance (lx)
Background luminance (cd/m 2)
Maximum luminance (cd/m 2)
Minimum luminance (cd/m 2)
Average luminance (cd/m 2)
BRS glare
CGI glare
Cornell glare
1 1 5 5 9 9 13 13 17 17
left right left right left right left right left right
6000 6500 2100 2200 1200 1300 720 760 500 510
740 790 290 300 180 180 100 110 85 85
55000 61000 16000 17000 8300 9600 5200 5600 3000 3300
6200 6200 5600 5300 3200 3200 2000 2000 1300 1200
39000 43000 13000 13000 6600 7500 4200 4400 2400 2600
32 32 28 28 25 26 24 25 22 22
43 44 36 36 32 33 30 30 26 27
28 29 25 25 23 23 22 22 20 20
#
TABLE 3 Measurements and calculations for the glare values in position B # Screen
Seat of subject
Vertical illuminance (lx)
Background luminance (cd/m 2)
Maximum luminance (cd/m 2)
Minimum luminance (cd/m 2)
Average luminance (cd/m 2)
BRS glare
CGI glare
Cornell glare
1 1 5 5 9 9 13 13 17 17
le~ right leh right le~ right le~ right le~ right
3400 3300 1300 1300 840 800 590 570 460 450
790 680 320 290 210 200 160 150 130 120
47000 65000 14000 17000 8000 8900 4600 5200 2700 3000
8200 7900 5400 5300 3100 3100 1900 1900 1100 1200
35000 46000 11000 13000 6400 7000 3700 4100 2200 2400
20 22 16 14 14 15 11 12 8 9
29 32 23 24 20 21 17 18 14 15
17 19 14 15 12 12 10 10 7 8
centage of subjects who marked 'not accepta b l e ' o n Q u e s t i o n n a i r e 3. F i g u r e 10 s h o w s t h e r e l a t i o n s h i p b e t w e e n t h e D S V , w h i c h w a s intended to reflect the discomfort sensation, and
o--
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/"
the percent dissatisfied, which was intended reflect acceptability of the environment. We then compared our new evaluations the Cornell index, which best conformed
• - - . - - eorly vote O
late vote //
..."
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. •
,
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1'0 l's 2'0 2'5 3'0 3'5 B R S Glare Index (from Hopkinson)
Fig. 5. Glare vote by subjects vs. BRS glare index.
2'o 2'5 3'o 3'5 4o C I E Glare Index (from Einhorn)
Fig. 6. Glare vote by subjects vs. CIE glare index.
to to to
396
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10 5
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0 10
15
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25
30
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40
• ./y.e +J¢ /4(• •
Daylight Glare Index (by Cornell formula) Discomfort Sensation Vote
Fig. 7. Glare vote by subjects vs. daylight glare index.
Fig. 10. Percent dissatisfied vs. discomfort sensation vote.
poelUon A, early vote
4
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Discomfort Sensation Vote Fig. 8. Relationship between glare vote and our proposed discomfort sensation vote.
J
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10
15
20
25
30
35
Daylight Glare Index (by Cornell formula) Fig. 11. Discomfort sensation vote vs. daylight glare index.
95
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35
397 our data. This is shown in Figs. 11 and 12 for both the DSV and the percent dissatisfied. The votes by subjects in the two positions are plotted separately to show the difference due to the direction of viewing.
DISCUSSION
Figures 5 - 7 show that the Cornell index (Fig. 7) was the most accurate predictor of the glare vote from wide-source glare. (It is unreasonable to expect a perfect agreement with any scale, because the words used in the Japanese questionnaire differed slightly in their meanings from the English equivalents.) The CIE glare index (Fig. 6) was the least accurate in this lighting environment. It consistently predicted higher, that is, more severe, glare votes than actually occurred. The BRS criterion also greatly overpredicted glare sensation votes. These results are not surprising, because the BRS and CIE indices were originally determined for point-source light rather than wide-source glare. Figures 5- 7 also show the effect of eye adaptation. The differences between early and late votes show that adaptation occurs, so that the subjects judged the light to be less uncomfortable even after only 30 s. This suggests that the most serious glare problems occur during the transition, that is, the time immediately after exposure to the glare source. The Cornell formula was created to measure wide-source glare and accurately predicts discomfort in these conditions; however, it is inadequate for a range of wide-source glare conditions because it does not include parameters for adaptation and the luminance of the desk surface. We tested our proposed glare evaluations, the DSV and percent dissatisfied, in several ways. First, we compared the glare vote with our DSV and percent dissatisfied (Figs. 8 and 9). Next, we compared our two scales with each other (Fig. 10). There is a relatively good relationship between discomfort sensation and percentage dissatisfied for a single position. However, the relationship between discomfort and dissatisfaction changed with the subject's position. It appeared that there was a better relationship for position A, that is, facing the glare source. Unfortunately, the subjects in
position B were exposed to a narrower range of light levels, so it is difficult to make a strong correlation. Our proposed glare evaluations, discomfort sensation vote and percent dissatisfied, are plotted against the Cornell daylight glare index in Figs. 11 and 12. Both Figures show that the Cornell formula is not directly applicable to a window parallel to the subject's line of sight. This confirms the results reported by Chauvel et al. [6]. When the window is parallel to the subject's line of sight, the weighted solid angle, f2, in the Cornell formula is about 1/20 of that for facing the source directly. The new discomfort sensation vote did not have a significantly better relationship to percent dissatisfied than to the glare vote.
CONCLUSIONS We tested the existing glare indices and evaluation methods for wide-source glare in an environmental chamber. We defined new glare evaluations, the discomfort sensation vote and percentage dissatisfied, because the previous glare criterion (by Hopkinson) combined glare sensation and discomfort sensation. Neither the BRS glare index nor the CIE glare index could reliably predict discomfort sensation from a large light source. The Cornell formula was more accurate, but was found to be inadequate for a range of widesource glare conditions because it does not include parameters for adaptation and the luminance of the desk surface. Both our discomfort sensation vote and the glare vote correlated well with the percent dissatisfied when subjects looked directly at the wall on which there is the simulated window. However, the relationships were different and poorer when the subject's line of sight was parallel to the plane including the window.
LIST OF SYMBOLS Ls luminance of each glare source in the field of view (cd/m 2) w solid angle of the glare source subtended at the eye (sr)
398 Lb a v e r a g e l u m i n a n c e of the visual field in w h i c h the glare sources are seen (cd/m e) P a f u n c t i o n of the a n g l e b e t w e e n the direction of the light s o u r c e and the o b s e r v e r ' s d i r e c t i o n of v i e w i n g Ed direct vertical i l l u m i n a n c e at the eye due to all sources (lx) Ei indirect i l l u m i n a n c e at the eye (lx) solid angle of the s o u r c e modified for the effect of the position of its elements in different p a r t s of the field of view in the w a y put forv~ard by P e t h e r b r i d g e a n d L o n g m o r e [5].
REFERENCES 1 P. Petherbridge and R. G. Hopkinson, Discomfort glare and lighting of buildings, Trans. Illum. Eng. Soc., 15 (1950) 39- 79. 2 M. Luckiesh and S. K. Guth, Brightness in visual field at borderline between comfort and discomfort (BCD), Illum. Eng. (NY), 44 (1949) 650- 670. 3 CIE, Discomfort glare in the interior working environment, CIE Report No. 55, 1983. 4 R. G. Hopkinson, Architecture Physics: Lighting, HMSO, London, 1963. 5 P. Petherbridge and J. Longmore, Solid angles applied to visual comfort problems, Light Light., 47(1954) 173- 177. 6 P. Chauvel, J. B. Collins, R. Dognlaux and J. Longmore, Glare from windows: current view of the problem, Light. Res. Technol., 14 (1) (1982) 31-46.
391
Discomfort Caused by Wide-source Glare TOSHIE IWATA and KEN-ICHI KIMURA
Department of Architecture, Waseda University, Tokyo (Japan)" MASANORI SHUKUYA
Department of Architecture, Musashi Institute of Technology, Tokyo (Japan) KYOSUKE TAKANO
Nikken Sekkei Ltd., Osaka (Japan)
ABSTRACT
We report here on experiments to measure subjective response to intense light, or glare caused by a wide source. We have investigated three glare indices: the Building Research Station glare index; the CIE glare index and the Cornell daylight glare index. We have also examined the glare vote and have proposed a new glare evaluation scale, as well as asking the subject to vote on the condition's acceptability. The Cornell formula most accurately predicts glare discomfort, but it is found to be inadequate for a range of wide-source glare conditions. Both the discomfort sensation vote and the glare vote which we proposed correlate well with the percentage of subjects dissatisfied when looking directly at the light source.
INTRODUCTION
The development of new window materials may make it possible to control the solar radiation transmitted through windows to the desired levels. From the viewpoint of controlling the glare caused by windows, it is important to be able to assess the magnitude of the glare caused by daylight. Even with an ideally controllable window material, we will never be able to use daylight fully if we do not understand the discomfort caused by glare. This paper describes the results of preliminary experiments to measure glare discomfort from windows. The general goal is to predict the extent of occupant discomfort by knowing general features about the light levels, orientation and perhaps other factors. The specific 0378-7788/91/$3.50
purpose of this work was to examine the practical capability and limitations of existing glare indices. In particular, we sought to evaluate the accuracy and usefulness of earlier research in predicting glare discomfort and satisfaction. The first problem with earlier research is that only the contrast effect was considered. It is not clear if this assumption can be applied to wide-source glare, such as that from a window. The second problem is that the glare criterion scale is not useful for practical design because it combines sensation and dissatisfaction. The results of these experiments will guide our future work in visual comfort.
GLARE INDICES
There are several existing methods to evaluate glare. The Building Research Station (BRS) glare index derived from a study by Petherbridge and Hopkinson [1] is perhaps the best known, and is defined as 16. 08 vL~w °~ BRS glare index = 10 log 0.478 ~., Lbp1.6
(1)
where L s (cd/m 2) is the luminance of each glare source in the field of view, w (sr) is the solid angle of the glare source subtended at the eye, and L b (cd/m 2) is the average luminance of the visual field in which the glare sources are seen. P is a function of the angle between the direction of the light source and the observer's direction of viewing [2]. The weakness of the Hopkinson formula lies in its mathematical inconsistency; a large glare source cannot be subdivided for the purpose of © Elsevier Sequoia/Printed in The Netherlands
392
summing up glare contributions. Therefore the so-called Einhorn formula, presented in ref. 3 as the 'CIE formula' was developed:
TABLE 1 Relationship between Hopkinson glare criterion and two glare indices
CIE glare index
Glare criterion
=8 log 2[ ! + (Ed/500)] V g~w ~ j ~ p2
(2) Imperceptible Just perceptible Perceptible Just acceptable Acceptable Just uncomfortable Uncomfortable Just intolerable Intolerable
where Ed(lx) is the direct vertical illuminance at the eye due to all sources, and Ei(lx) the indirect illuminance at the eye. Hopkinson modified the BRS glare formula in eqn. (1) to the following equation in order to be able to assess the glare from large sources [4]. This is called the daylight glare index or Cornell index:
BRS glare index (BRS GI)
Daylight glare index (DGI)
l0 13 16 19 22 25 28
16 18 20 22 24 26 28
daylight glare index L~.6~0.s = 10 log 0.478 ~ Lb + 0.07 W°'5 L s
We modified this criterion so that it could be used as a scale to evaluate glare sensation. One additional difficulty is that the Japanese language does not have a specific word for 'just'. Our modified scale avoids this problem. We call the modified scale the glare vote. It is shown in Fig. 1 as Questionnaire 1. However, both the original Hopkinson glare criterion and the modified glare vote combine the subject's feelings regarding both perception and discomfort. These scales make it difficult to predict levels of dissatisfaction. To separate these feelings, we developed two new scales. One questionnaire asks about sensation. We call this the discomfort sensation vote (DSV); it is shown as Questionnaire 2 in
(3)
where ~ is the solid angle subtended by the source modified for the effect of the position of its elements in diffel:ent parts of the field of view in the way put forward by Petherbridge and Longmore [5].
GLARE VOTE, DISCOMFORT SENSATION VOTE AND PERCENT SATISFIED
Hopkinson linked a subjective glare criterion for the BRS index to the daylight glare index. This relationship is shown in Table 1. Questionnaire
Questionnaire
1
2
Dkscomfort ,Sensation
Glare Vote
Vote (osv)
intolerable
4
3
very uncomfortable
uncomfortable _ _.
acceptable 1
1
uncomfortable
-
slightly uncomfortable
perceptible
-
0 __
not uncomfortable
-
imperceptible Questionnaire 3 Accep~b~
[]
acceptable
[]
not acceptable
Fig. 1. Questionnaires used for glare vote, discomfort sensation vote and acceptability.
393
Fig. 1. This scale is intended to ask the subject only about the degree of discomfort. The other questionnaire asks about satisfaction; this is called the percent dissatisfied and is shown as Questionnaire 3 in Fig. 1. Since the glare vote and the discomfort sensation vote scales have only one direction (monopolar scale), it is difficult to determine the borderline between acceptable and unacceptable levels of glare. Thus the percent dissatisfied is the most reasonable way to find the borderline.
METHOD
For this experiment, we used Waseda University's environmental test chamber, which is
equipped with a simulated window. Figures 2 and 3 show the plan of the chamber. The window was illuminated from the rear by a bank of metal halide lamps. The intensity of the light was adjusted by the number of sheets of paper over the glass. This also served to diffuse the light. The window size is 80 × 80 cm. Two subjects were tested at one time, although their positions were different. The subject in position A looks directly at the wall on which there is the simulated window (shown in Fig. 2). The subject in position B has a line of sight perpendicular to the light from the window (Fig. 3). Figure 4 shows the experimental procedure. In each test, two subjects sat in the chamber, looked down at sheets of paper on their desks
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Fig. 2. Plan and section of chamber with subjects in position A (dimensions in ram).
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Fig. 3. Plan and section of chamber with subjects in position B (dimensions in ram).
394
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30
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(1) adaptation (lookingat desk) (2) head up, Jooklngstralghtahead, early vote (Glare Vote or DSV, and Acceptablllty) (3) keep lookingstralghtahead (30 seconds) (4) late vote Fig. 4. Procedure used to test subjects.
and performed a task. At certain intervals, they raised their heads and looked at the fixation point straight in front and were asked to evaluate the glare on a voting sheet. After looking at the point for 30 s, the subjects were again asked to evaluate the discomfort of the light. Each subject repeated the test at five different luminances: 40000, 13 000, 7000, 4000 and 2500 cd/m 2. The different light levels were presented randomly so as to control for the effect of presentation. The questionnaires were written in Japanese, but the exact words in the questionnaire corresponded closely to those English words shown in Fig. 1. Questionnaire 3 (on acceptability) was administered in every test, but Questionnaires 1 and 2 were administered in alternate tests. Altogether, 80 paid subjects (40 males, 40 females) participated in the experiments. The average age of the subjects was 22 years.
RESULTS We first compared the results of the experiment in terms of traditional glare indices. Tables 2 and 3 show the assumptions and calculations for the glare values. Figures 5 - 7 show the relationships between the glare vote by subjects and the three glare indices in position A, together with the dashed line showing the results of the previous study by Hopkinson. BRS and CIE formulae were calculated for the position index at the centroid of the window. Table 1 was used to convert the glare index values to the Hopkinson glare criterion. We found that the Cornell index most accurately predicted the subjects' votes with the Hopkinson glare criterion (see Discussion). Figures 8 and 9 show the relationships between the glare vote and our two new glare evaluations, the DSV and the percent dissatisfied. The percent dissatisfied means the per-
395 TABLE 2 Measurements and calculations for the glare values in position A
Screen
Seat of subject
Vertical illuminance (lx)
Background luminance (cd/m 2)
Maximum luminance (cd/m 2)
Minimum luminance (cd/m 2)
Average luminance (cd/m 2)
BRS glare
CGI glare
Cornell glare
1 1 5 5 9 9 13 13 17 17
left right left right left right left right left right
6000 6500 2100 2200 1200 1300 720 760 500 510
740 790 290 300 180 180 100 110 85 85
55000 61000 16000 17000 8300 9600 5200 5600 3000 3300
6200 6200 5600 5300 3200 3200 2000 2000 1300 1200
39000 43000 13000 13000 6600 7500 4200 4400 2400 2600
32 32 28 28 25 26 24 25 22 22
43 44 36 36 32 33 30 30 26 27
28 29 25 25 23 23 22 22 20 20
#
TABLE 3 Measurements and calculations for the glare values in position B # Screen
Seat of subject
Vertical illuminance (lx)
Background luminance (cd/m 2)
Maximum luminance (cd/m 2)
Minimum luminance (cd/m 2)
Average luminance (cd/m 2)
BRS glare
CGI glare
Cornell glare
1 1 5 5 9 9 13 13 17 17
le~ right leh right le~ right le~ right le~ right
3400 3300 1300 1300 840 800 590 570 460 450
790 680 320 290 210 200 160 150 130 120
47000 65000 14000 17000 8000 8900 4600 5200 2700 3000
8200 7900 5400 5300 3100 3100 1900 1900 1100 1200
35000 46000 11000 13000 6400 7000 3700 4100 2200 2400
20 22 16 14 14 15 11 12 8 9
29 32 23 24 20 21 17 18 14 15
17 19 14 15 12 12 10 10 7 8
centage of subjects who marked 'not accepta b l e ' o n Q u e s t i o n n a i r e 3. F i g u r e 10 s h o w s t h e r e l a t i o n s h i p b e t w e e n t h e D S V , w h i c h w a s intended to reflect the discomfort sensation, and
o--
- -- eady vote
/"
the percent dissatisfied, which was intended reflect acceptability of the environment. We then compared our new evaluations the Cornell index, which best conformed
• - - . - - eorly vote O
late vote //
..."
.~
,,'/
. •
,
I
1'0 l's 2'0 2'5 3'0 3'5 B R S Glare Index (from Hopkinson)
Fig. 5. Glare vote by subjects vs. BRS glare index.
2'o 2'5 3'o 3'5 4o C I E Glare Index (from Einhorn)
Fig. 6. Glare vote by subjects vs. CIE glare index.
to to to
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40
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Fig. 10. Percent dissatisfied vs. discomfort sensation vote.
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DISCUSSION
Figures 5 - 7 show that the Cornell index (Fig. 7) was the most accurate predictor of the glare vote from wide-source glare. (It is unreasonable to expect a perfect agreement with any scale, because the words used in the Japanese questionnaire differed slightly in their meanings from the English equivalents.) The CIE glare index (Fig. 6) was the least accurate in this lighting environment. It consistently predicted higher, that is, more severe, glare votes than actually occurred. The BRS criterion also greatly overpredicted glare sensation votes. These results are not surprising, because the BRS and CIE indices were originally determined for point-source light rather than wide-source glare. Figures 5- 7 also show the effect of eye adaptation. The differences between early and late votes show that adaptation occurs, so that the subjects judged the light to be less uncomfortable even after only 30 s. This suggests that the most serious glare problems occur during the transition, that is, the time immediately after exposure to the glare source. The Cornell formula was created to measure wide-source glare and accurately predicts discomfort in these conditions; however, it is inadequate for a range of wide-source glare conditions because it does not include parameters for adaptation and the luminance of the desk surface. We tested our proposed glare evaluations, the DSV and percent dissatisfied, in several ways. First, we compared the glare vote with our DSV and percent dissatisfied (Figs. 8 and 9). Next, we compared our two scales with each other (Fig. 10). There is a relatively good relationship between discomfort sensation and percentage dissatisfied for a single position. However, the relationship between discomfort and dissatisfaction changed with the subject's position. It appeared that there was a better relationship for position A, that is, facing the glare source. Unfortunately, the subjects in
position B were exposed to a narrower range of light levels, so it is difficult to make a strong correlation. Our proposed glare evaluations, discomfort sensation vote and percent dissatisfied, are plotted against the Cornell daylight glare index in Figs. 11 and 12. Both Figures show that the Cornell formula is not directly applicable to a window parallel to the subject's line of sight. This confirms the results reported by Chauvel et al. [6]. When the window is parallel to the subject's line of sight, the weighted solid angle, f2, in the Cornell formula is about 1/20 of that for facing the source directly. The new discomfort sensation vote did not have a significantly better relationship to percent dissatisfied than to the glare vote.
CONCLUSIONS We tested the existing glare indices and evaluation methods for wide-source glare in an environmental chamber. We defined new glare evaluations, the discomfort sensation vote and percentage dissatisfied, because the previous glare criterion (by Hopkinson) combined glare sensation and discomfort sensation. Neither the BRS glare index nor the CIE glare index could reliably predict discomfort sensation from a large light source. The Cornell formula was more accurate, but was found to be inadequate for a range of widesource glare conditions because it does not include parameters for adaptation and the luminance of the desk surface. Both our discomfort sensation vote and the glare vote correlated well with the percent dissatisfied when subjects looked directly at the wall on which there is the simulated window. However, the relationships were different and poorer when the subject's line of sight was parallel to the plane including the window.
LIST OF SYMBOLS Ls luminance of each glare source in the field of view (cd/m 2) w solid angle of the glare source subtended at the eye (sr)
398 Lb a v e r a g e l u m i n a n c e of the visual field in w h i c h the glare sources are seen (cd/m e) P a f u n c t i o n of the a n g l e b e t w e e n the direction of the light s o u r c e and the o b s e r v e r ' s d i r e c t i o n of v i e w i n g Ed direct vertical i l l u m i n a n c e at the eye due to all sources (lx) Ei indirect i l l u m i n a n c e at the eye (lx) solid angle of the s o u r c e modified for the effect of the position of its elements in different p a r t s of the field of view in the w a y put forv~ard by P e t h e r b r i d g e a n d L o n g m o r e [5].
REFERENCES 1 P. Petherbridge and R. G. Hopkinson, Discomfort glare and lighting of buildings, Trans. Illum. Eng. Soc., 15 (1950) 39- 79. 2 M. Luckiesh and S. K. Guth, Brightness in visual field at borderline between comfort and discomfort (BCD), Illum. Eng. (NY), 44 (1949) 650- 670. 3 CIE, Discomfort glare in the interior working environment, CIE Report No. 55, 1983. 4 R. G. Hopkinson, Architecture Physics: Lighting, HMSO, London, 1963. 5 P. Petherbridge and J. Longmore, Solid angles applied to visual comfort problems, Light Light., 47(1954) 173- 177. 6 P. Chauvel, J. B. Collins, R. Dognlaux and J. Longmore, Glare from windows: current view of the problem, Light. Res. Technol., 14 (1) (1982) 31-46.