NBS BUILDING SCIENCE SERIES

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

ifWi II Ui ilnlij I A New Look at

the Research Basis for

Lighting Level Recommendations

.U58

NO. 82

1976

L C.2
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[See mailing list announcement on last page J
A New Look at the Research Basis

for Lighting Level Recommendations

\ \ \.

Gary T. Yonemura and

Yoshimi Kohayakawa

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secrefary

Dr. Betsy Ancker-Johnson, Assisiani Secreiary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acfing Direcior

Issued March 1976
Library of Congress Cataloging in Publication Data

Yonemura, Gary, 1924- A new look at the research basis for lighting level recommendations. (NBS building science series; 82) Bibliography: p. Supt. of Docs. No.: C 13.29/2:82

1. Electric lighting. 1. Kohayakawa, Yoshimi, joint author. 11. Title. III. Series: United States. National Bureau of Standards. Building science series; 82. TA435.U58 No. 82 [TK4175] 690'.021s [621.32'2] 76-136

National Bureau of Standards Building Science Series 82

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Contents

Page

1. Introduction 1

2. Experiment I. Sinusoidal gratings 3

2.1. Procedure 3

2.2. Results 4

3. Experiment II. Square-wave gratings 5

3.1. Procedure 5

3.2. Results 7

'

4. Discussion 7

5. References 8

iii

A New Look at the Research Basis for Lighting

Level Recommendations

G. T. Yonetnura and Y. Kohayakawa"'

• The validity of using threshold studies as the hasis for lighting level recommendations is questioned. The performance of the eye at suprathreshold levels was investigated with sine- and sijuare- wave gratings. The results of the study indicate that the behavior of the eye is significantly different at suprathreshold levels as opposed to threshold levels. For threshold studies, when contrast is plotted against luminance, the function is a monotonically decreasing function. At suprathreshold levels the function indicates the existence of a definite minimum, luminances greater or less requiring more

contrast to appear subjectively equal. It is recommended that lighting levels be based on laboratory studies that appraise visual requirements and performance simulating conditions encountered in real world environments.

Key words: Gratings; illuminating engineering; lighting; modulation transfer function: suprathreshold

visibility; visibility; vision.

1. Introduction

The determination of the quantity of illumination

required for visual task performance should be

based on empirical data obtained from the user. To this end, the Illuminating Engineering Society

(lES) has adopted a Visibility Reference Function

to serve as a standard or reference base for the

determination of recommended levels of illumination [1].^ See the solid curve in figure 1. The Visi- bihty Reference Function was obtained under

controUed laboratory conditions and is defined as the contrast required to detect a 4 min diameter

luminous disk presented for 1/5 s. as a function of

background luminance (L). The threshold contrast

is the contrast required to detect the luminous disk

50 percent of the time using a forced choice tech- nique. Contrast (C) is defined as: C={U-L,)IU, (1)

where the subscripts t and b stand for target (lumi- nous disk) and background, respectively. Usually

the larger of the two areas constitutes the background luminance and is often referred to as the

adaptation luminance. Equation (1) is often ex- pressed as: C=\M\IL,„ (la)

the numerator being the luminance difference be- tween target and background regardless of whether

the target or background luminance is greater.

The detection threshold is the borderline between

visibility and invisibility. Most routine visual tasks

are not performed at this borderhne of visibility

and invisibility. The next higher order of visual

*Gu€st worker at the \ational Bureau of Stamlards. Permanent address: Canon.

Inc., Tokyo 144 Japan.

' Figures in brackets indicate the literature references al the end of this paper.

BaCKGROUNO LUMIN.'NCE (Log cd/m^)

Figure 1. The visual performance reference base used by the Illuminating Engineering Society for recommending levels of

illumination.

The solid curve is the Visiliihty Reference Function and the dashed curve is the Visual Performance Criterion Function.

performance requires the observer not only to detect

the presence of an object, but also to recognize

it as a member of a group. This level will generally

require shape or form discrimination and is called the acuity threshold. Classical examples of this task are the Landolt C and the Snellen Chart. In an acuity test the observer is not only required to detect the presence of an object, but in addition

is required to detect the orientation of the gap in the landolt C or recognize the perceived image as

a specific alphabet member in the Snellen .Acuity Test. But in the majority of realistic task conditions,

the task is significantly above the acuity threshold.

For example, in reading we see (detect) some things on the page and recognize (acuity) them as specific

1
members of the alphabet. That is, under most

realistic conditions, the visual tasks are significantly above threshold. We generally see the

task — the only remaining question is, "how well can we see it?". Is it better under condition A

rather than B? In the lES system, extrapolation from

detection to realistic levels is accomplished by the

concept of visibility level. The Visibility Reference Function is called VLl

(visibility level 1). Higher levels of visibility (suprathreshold) are defined as VLi (visibility level i)- In terms of its physical correlates, contrast and

luminance, FLi is defined as:

CiiL) = i[C,iL)]; (2)

where CiiL) is the suprathreshold contrast at FL,

and Ci is the contrast threshold as given by the

Visibility Reference Function for a given background luminance level {L). The contrast threshold,

Ci, is the just noticeable difference ijnd). Therefore

VLi is i number of jnd steps above threshold, that is:

CdL)ICAL) = VLi{L). (3) The lES in its handbook (5th ed.) [1] defines

VLS (i = 8) as the Visual Performance Criterion

Function, shown by the dashed curve in figure 1. That is, VLS is the luminance contrast required at different levels of task background luminances

under conditions of "realistic visual work," or

alternately, the luminance required for a task of a given contrast to meet the criterion for visual task

performance.

VLS was obtained by considering three aspects

of realistic conditions that differed from those exist- ing in the detection experiments: (1) threshold

detection for a moving rather than a stationary

target, (2) redefinition of the threshold as detecting

the presence of the target 99 percent of the time

instead of 50 percent, and (3) the threshold for detecting a target whose apparent location was unknown as opposed to knowing exactly where the

target was going to appear. Experiments con- ducted with moving targets indicated that the contrast required to detect a moving target was 2.8 times greater than that required to detect a sta- tionary target. Similarly the contrast multiplier

required to change from 50 percent probability of

detection to 99 percent probability of detection was found to be 1.9. The contrast required to detect

a target whose appearance location was unknown

was 1.5 times greater than that required to detect

a target whose appearance location was known. The

product of these three contrast multipliers, (2.8) (1.9)(1.5) = 8.0, is the factor by which the Visibility Reference Function (Cj) has to be multiplied

in order to approach realistic conditions. The dashed

curve in figure 1 is the Visual Performance Criterion

Function (FL8), discussed earlier, and is obtained

by multiplying the Visibility Reference Function

(solid curve in fig. 1) by 8. The assumption is that since VLl represents contrasts that are subjectively

equal (equal detectability) VLS also represents

physical contrasts that are subjectively equal:

Cs{Li) = Cs{Lj), (4)

or the physical contrast for VLS at luminance i is matched by physical contrast for VLS at lumi- nance j. That is. for any given VL aU points falling on the curve of contrast plotted against luminance

have equal visibility. For a more complete description of the Reference Visibility Function and Visual

Performance Criterion Function see Blackwell [2]. There are two points in the extrapolation to higher

visibility levels that should be validated. First, all the data, including the contrast multipliers for cor- respondence to realistic conditions, were obtained

at threshold levels, therefore direct experiments

at suprathreshold levels (100% seeing) should be

conducted. Second, is the linear extrapolation (contrast multipliers) to higher visibility levels, eq (2), justified? Bodmann anticipated this difficulty, when

he suggests that eq (3) be defined as contrast factor, as "the term contrast factor would not anticipate

its interpretation and might therefore be preferred

by the reader" [3]. Cases can be cited from the

literature where predictions of suprathreshold

performance from threshold data did (can) lead to erroneous predictions. Poppel and Harvey [4] found

that subjective brightness as a function of retinal eccentricity predicted from threshold data differed

significantly from the empirical data obtained by

experiments performed at suprathreshold level. An example of this difference in photometry is the

spectral luminous efficiency function. We can dis- cuss spectral luminous efficiency in terms of retinal location, rods and cones, etc., but for purposes

of this discussion, the primary interest is the bright- ness sensitivity as a function of radiant energy

evaluated wavelength by wavelength. The function

describing the sensitivity of the eye to radiant

energy from different parts of the visible spectrum

is different when obtained at the detection (threshold) levels [5] as opposed to equal brightness contours obtained for suprathreshold stimuli [6\ In both examples, experiments conducted at supra- threshold levels indicate the inappropriateness

of predicting suprathreshold functions from threshold data. The grating (alternating light and dark bars) has

been in use for a long time as a resolution power

test in lens evaluation and as a target for psychophysical studies in vision. The resolution threshold

as a function of pertinent parameters has been

extensively investigated for several forms of visual

targets: Landolt C, single lines, disks, etc., as well

as gratings [7]. Bryndgdahl [8] investigated subjective contrast of gratings at suprathreshold levels,

but his range of luminances was limited, 5 to 20

cd/m-. Watanabe et al. [9] had subjects make equal2
ity of contrast judgments for gratings at mean

luminances of 17 to 171 cd/m''^ for many spatial

frequencies, but only at a single low contrast level, 0.032. The purpose of the present experiment was

to determine whether the function obtained under

visibility conditions significantly higher than de- tection or acuity thresholds would be similar in form to that obtained at threshold levels. The

present study will investigate visibility at supra- threshold levels over a large range of luminances,

0.06 to 630 cd/m', and contrasts, 0.08 to 0.7.

2. Experiment I. Sinusoidal Gratings

Two identical sinusoidal (luminance distribution of dark and light bars follows a sinusoidal pattern)

grating transparencies were projected through

separate projectors.^ The luminous intensities of

the projectors were controlled by neutral density

filters and neutral density wedges and balances.

Each grating subtended a visual angle of 7.5° (0.13 rad), and the gratings were projected adjacent

to each other on a vertical (frontal) plane with the

bars oriented horizontally. The gratings were seen

against a dark surround and the viewing distance was 145 cm. The contrasts of the gratings were

varied by defocusing the images. The contrast change was accompanied by a shght change in magnification and luminance of the grating, but

these changes were below threshold for the contrast ranges used in a given experimental run; consequently, they were ignored when the data were

analyzed. The modulation of the projected pattern,

defined as^ (^max ^min )/ (^max "^^ ^min) > (5) was measured for every new condition. M will be

used to denote contrast defined by eq (5) and C for contrast as defined by eq (1). The spatially averaged

luminance, defined as: Lm— (^max "I" -^min )/25 (6)

is the counterpart of in eq (1). Since a sinusoidal

distribution retains its sinusodial form after defocusing, nothing in the image changes except the

contrast. The modulation was evaluated by measuring the luminances at the centers of the dark and

light bars with a modified spot photometer.

2.1. Procedure

The reference grating of fixed spatial frequency,

contrast and spatially averaged luminance was

presented in the left visual field. The test target

presented in the right field was in all cases of the same spatial frequency as the reference field. In the

first trial, the luminances of the two fields were the

^ The gratings transparencies were furnished by the Optics and Micrometrology

Section, Institute for Basic Standards, NBS.

same. The Observer (O) was asked to vary the contrast of the test field until both fields appeared to be

of equal "clarity" or "distinctness." These adjustments were made by changing the focus of the test

field. When making these adjustments, O was asked

to look at the center of the grating patterns and not near the edge separating the test and reference

gratings. Since both fields were of the same lumi- nance and frequency, the average contrast obtained

for the test field should be the same as that for the

reference field. If 0 did not meet a 1 percent

criterion — that is, if the average of 10 match settings of the test field was not within 1 percent of the

reference stimulus contrast — O was given a practice

session. In all cases but one, practice sessions were

not required as the test matches were within 0.5 percent of the reference field contrast under

identical frequency and luminance conditions.

During the next phase of the study, a neutral

filter of density (D) 0.5 was added to the test field. O was asked to vary the contrast of the now dimmer

test field until the clarities of both gratings were

subjectively equal. After O made a setting, the ex- perimenter changed the contrast of the test field. In half of the cases the experimenter increased the

contrast over the level set by O, and in the other

half the contrast was decreased. O was asked to make another match. This procedure was repeated

for a total of 10 settings. The mean of these 10 test

field contrast settings was computed, the test field was set to this mean value and the contrast for this mean setting was measured with a modified spot

photometer. The luminance of the test field was

then decreased by an additional 0.5 D (a total of

1.0 D), making the luminance of the reference field 10 times greater than that of the test field. O was

again asked to make 10 "equaUty of clarity"

matches, and the average setting was measured with

the photometer. For the next trial, the spatially

averaged luminance and contrast of the reference

field were changed to the luminance and mean

contrast value of the test field based on the previous run and the same procedure was followed again and

again as described above. This cascade technique,

whereby the reference stimulus replaced the

previous test field, was continued in 1.0 D steps

until the apparatus limitations were reached. When

the mean luminance level of either field was less than 1.0 cd/m'-, the O was dark adapted for 5 minutes prior to the run. Either two or three starting reference contrasts were used for each frequency. The contrast values

ranged from 0.7 to 0.08. Three spatial frequencies:

1.8, 3.9 and 7.7 cycles per degree (cpd) were

investigated for spatially averaged luminances

between 0.06 and 630 cd/m-.

2.2 Results

The results of the experiment with sinusoidal

gratings are presented in figures 2—4. Each datum

3
point represents the mean value of 10 settings.

Each curve in these graphs represents a contour

of constant clarity, with the datum point at 630

cd/m^ being the reference starting contrast. The

outstanding feature of the most of these curves is the existence of an optimum luminance level, at

which a minimum amount of contrast is needed to achieve a given degree of clarity. Obtaining the same

clarity at either a higher or lower mean luminance

requires more contrast than is needed at this optimum luminance. This finding applies to gratings

presented at suprathreshold levels only. These

0.6

< 04

o

<-> 0.3

Log Ln^{cd/m2)

Figure 2. Equal-clarity contours as a function of mean spatial luminance for a sine- wave grating offrequency 1 .8 cpd.

The solid and dashed curves represent the results for two different observers.

0.1

1.0 Log Ln^ (cd/m2 j

Figure 3. Equal-clarity contours as a function of mean spatial luminance for a 3.9 cpd sine-wave grating.

The solid and dashed curves represent the results for two different observers. The curves with the circled data

points are for surround luminaces maintained at the mean spatial luminance of the grating with the higher luminance.

4
0.1

-1.0 0.0 1.0 2.0 3D

Log (cd/m2 )

Figure 4. Equal-clarity contours as a function of mean spatial luminancefor a 7.7 cpd

sine-wave grating.

The solid and dashed curves represent the results for two different observers.

optimum luminance levels appear to depend on the

frequency of the grating, shifting toward higher

luminance values as spatial frequency (fineness of

detail) increases. When the reference (starting) contrast approaches a level near the resolution

threshold, the equal-clarity contours assume a monotonicaDy decreasing form similar to that obtained at resolution thresholds. This is clearly seen in the low^est equal-clarity contour for 3.9 cpd, figure 3. The dark surround used in the experiment may

have had an important effect on the results. To

investigate this possibihty, a square surround field, 25.1 deg (0.438 rad) on each side, w^as presented

during the matches. The luminance of the surround was always the spatially averaged luminance of the

brighter grating. The results of this experiment are presented in figure 3 as circles. These circles indicate the occurrence of an optimum luminance

level. (It can be presumed that the needed contrast must rise at sufficiently low luminance levels, so

the optimum for the open-circle data must be below

the lowest luminance used.)

3. Experiment II. Square-Wave

Gratings

Most practical visual tasks involve targets with

sharp rather blurred edges. The spatial luminance

distribution of the sinusoidal grating has an edge

gradient that can be called blurred. Furthermore,

when the amplitude of a sinusoidal grating is changed, although the spatial luminance distribution is stiU sinusoidal, the slope of the edge gradient

is different. A square-wave grating by definition has

a sharp cutoff separating the dark and light bars. In order to be sure that the results of the experiments with sinusoidal gratings were not artifacts due to specific shape of the spatial luminance

distributions, analogous experiments were con- ducted with a square-wave grating.

3.1. Procedure

A single projector displayed a square-wave

grating transparency of 3.9 cpd with the Ught and

dark bars running horizontally. The lower half of

this grating was projected through a 0.7 D neutral

density filter and was used as the test field, with the

top half being the reference field. The observer was

thus presented with two grating patterns physically

similar in all respects except for the mean lumi- nances. The contrast of the grating was varied by

superimposed veiling lights, since defocusing

distorts square-waves. Two additional projectors were used to add homogeneous veiUng illumination

independently to the top and bottom gratings. The

luminous intensities from both veihng projectors were controlled by neutral density filters, wedges

and balances. By increasing or decreasing the

luminous intensity of the veiling lights, the grating contrast could be varied. The spatially averaged

luminance also changed as the contrast was

changed.

The reference field was kept constant at a given

spatially averaged luminance and contrast. The

contrast and mean luminance of the test field were

varied by the veiUng projector until a match with the

reference field in clarity was obtained. The setting of the veiling-light variable-density wedge was

recorded and randomly changed to a new setting

5
for the next trial. Ten observations were taken for each condition. The reference field was then

changed so that the contrast and mean luminance

corresponded to the mean value obtained for the test

field on the previous run. Because of apparatus

limitations, an exact match for luminance and contrast could not be obtained, but as can be seen in figures 5 and 6 a close approximation was obtained.

The mismatches appear as open spaces between

ends of straight line segments making up the curve.

0.0 1,0

Log (cd/m^ )

Figure 5. Equal-clarity contours as a function of mean spatial luminancefor a 3.9 cpd

square-wave grating.

Observer YK.

0.7

-1.0 0.0 1.0 2.0 3.0 Log Lm (cd/m2)

Figure 6. Equal-clarity contours as a function of mean spatial luminancefor a 3.9 cpd

square-wave grating.

Observer GY.

6
3.2. Results

The results for the square-wave gratings are shown in figures 5 and 6. The results are similar

to those obtained with sine-wave gratings. For

contrasts significantly above threshold, there is an optimum mean luminance level at which a minimum

amount of contrast is needed to achieve a given

degree of clarity. For grating clarities approaching

resolution threshold, the classical monotonically

decreasing function is obtained. For square-wave

gratings, as for sine-wave gratings, the functions

obtained at threshold levels are significantly different from those obtained at suprathreshold

levels. The left ends of the dashed lines in figures 5 and 6 represent contrasts which were at the maximum levels obtainable with the apparatus for the

given condition. The observer felt that this contrast value "almost" gave equal distinctness. That is, the subject felt that a shght increase in contrast would have been sufficient to meet the equality

criterion.

4. Discussion

The Visibihty Reference Function currently used

as the standard for recommending levels of illumination states that visibihty increases monotonically as luminance is increased. That is, as luminance

increases, the contrast required to detect the

presence of an object decreases. This study utilizing a different psychophysical technique confirms the

above, but only for the special condition where

visibility is at or near threshold levels. Near thres- hold the contrast required to match the same target

for clarity at a higher luminance decreases

monotonically as luminance is increased. But, when

the targets are significantly above detection and

acuity thresholds, the form of the equal-clarity contours changes from a monotonically decreasing

function to one indicating the presence of a minimum contrast level. There appears to be an optimum

luminance level where the target is seen with a given

degree of clarity at a minimum contrast level, or with maximum clarity for a fixed value of contrast. The observation that the equal-clarity contours near

threshold levels, hke the detection function, de- crease monotonically as luminance is increased,

indicates that the existence of an optimum luminance

at suprathreshold levels is not an artifact due to criterion differences or experimental methodology.

There appears to be a continuous transition from

the "monotonic" to the "optimum" form, and the

simple detection or acuity function can be identified

with seeing at a minimum level of clarity.

The spatial frequencies used in this study were

7.7, 3.9, and 1.8 cpd; the width of the fight or dark

bars being 3.90, 7.69, and 16.7 min, respectively.

These values can be compared to the 4 min luminous

disk used by BlackweU [2], the 2.5 and 1.6 min fine

thicknesses for numbers used by Bodmann [10] and

Weston's [11] 1.5 to 4.5 min gap in the Landolt C.

All of these earlier data were considered directly or indirectly to derive recommended illumination

levels. The results of the present study indicate

that for suprathreshold stimuli, the size effect is manifested by changes in the location of the optimum luminance level on the luminance axis. Figures

2-4 indicate that as spatial frequency is increased

(or bar-width is decreased) the optimum luminance

level is shifted toward higher luminance values. More data will have to be obtained at suprathreshold

levels to determine the slope of the function and

location of optimum luminance levels as influenced

by other variables that affect visual performance.

The present study was not an attempt to define

the absolute contrast and luminance required for visual task performance. Is VLS the correct visibility level to be used? This study did indicate that what- ever visibility level is decided on, extrapolation from

threshold data to performance criteria levels by a simple multiplicative constant may not be valid. The

most valid technique for determining standard

performance criteria is to empirically determine

them under work situations, but such determinations

may be impractical. This performance test should

minimize nonvisual components hke motor re- sponses and cognitive factors. An acceptable

laboratory technique may be to have the observers

choose the contrast level they feel is just acceptable,

similar to the criterion used in the concept of

borderline between comfort and discomfort (BCD)

in glare [12]. The dependent variable would be

contrast and not luminance. At a given luminance

level, for example, 150 cd/m'-, contrast is varied

until a just acceptable contrast level is obtained.

The equi-contrast contour falling on this point wiU

serve both as the Visibility Reference Function and

Visual Performance Criterion Function.

Tasks of differing sizes, shapes and edge gradients

will be equated to the standard task for equality

in subjective contrast and/or clarity. The Visual

Task Evaluator (VTE) for this purpose wiU require

a modification of Blackwell's [13] VTE, since equating two stimuli for contrast wiU be required rather

than bringing a single stimulus to threshold. There

may be advantages to asking observers to match

two stimuh for equality rather than bringing a single

task to threshold. In matching two stimuh for equality the reference stimulus is always present,

the observer's task being to vary the test stimulus

until it appears to be equal to the reference in the

dimension being compared. The advantage is that suprathreshold stimuli can lead to less within

observer variability (response variabihty for a single

observer) than threshold stimuli. When observers

are asked to color match one field with another, the

individual variabihty in color matches increases as

target conspicuity decreases [14, 15]. That is, the

standard deviation for color matches serves as an

index of color discriminabihty.
The lES Lighting Handbook, 5th Ed. states: "The

illumination levels shown in the table are intended

to be minimum on the task ..." and "in order to assure these values at all times, higher initial levels should be provided . . . ." If the aim of interior lighting is to insure adequate illumination levels

for the most difficult task encountered, then the

present Visibility Reference Function may be the

appropriate standard. In a typical visual environment the most difficult task (although encountered

infrequently) may be near threshold levels. But,

the majority of reahstic tasks are not near threshold

levels. The results of the present study indicate that,

in order to accomodate for this infrequent occur- rence a price must be paid. The higher luminance

levels recommended to satisfy the requirements for seldom occurring low contrast tasks may bring the

luminance level beyond the optimum luminance

level for tasks with good contrast. The above is an

instance of "less light, better sight" and seriously

questions the indiscriminate application, as a working rule, the popular notion "more hght, better

sight." The monotonicaUy decreasing contrast re- quirement as luminance is increased dictated by

the Visibihty Reference Function and the Visual

Performance Criterion Function derived from it reinforces the concept more hght, better sight. The

Visual Performance Criteria Function as the basis

for recommending hght levels may lead to an un- necessary expenditure of energy as well as a

loss in ease of seeing for many reahstic tasks. Why

should energy be wasted in setting lighting levels high enough to accommodate infrequently occurring

difficult tasks, especially if the consequences may

be an actual loss in ease of seeing for the many good

contrast tasks encountered in everyday visual

performance?

5. References

[1] Illuminating Engineering Society, lES Lighting Handbook,

5th ed.. edited by Kaufman, J., (Illuminating Engineering

Society, 345 East 47th St., New York, N.Y. 10017, 1972).

[2] Blackwell, H. R., Development and use of a quantitative

method for specification of interior illumination levels on

the basis of performance data. III. Eng. Soc. 54, 317-353

(1959).

[3] Bodmann, H. W., Visibility assessment in lighting engineering, J. 111. Eng. Soc. 2 , 437-444 (1973).

[4] Piippel, E., and Harvey, L. O., Jr., Differential threshold and

subjective brightness in the periphery, Psychol. Forsch, 36, 145-161 (1973).

[5] CLE. Proc. 1951, Comite D'Etudes sur la Lumiere et la Vision, rapporteur, LeGrand, Y. 1, Sec. 4, 1-31, Bureau

Central CLE., 57 rue Cuvier, Paris (1951).

[6] Gibson, K. S., and Tyndall, E. P. T., Visibihty of Radiant

Energy, Scientific Papers of the Bureau of Standards, No. 475, 1923.

[7] Riggs. L. A., Chap. 11, Visual Acuity, in Vision and Visual

Perception, edited by Graham, C. H., 321-349 (John Wiley and Sons, Inc., New York, 1969).

[8] Bryngdahl, O., Characteristics of the visual system: Psychophysical measurements of the response to spatial sine- wave stimuh in the photopic region, J. Opt. Soc. Am. 56,

811-821 (1966).

[9] Watanabe, A., Mori, T., Nagata, S., and Hiwatashi, K., Spatial sine-wave responses of the human visual system,

vision Res. 8, 1245-1263 (1968).

[10] Bodmann, H. W., Illumination levels and visual performance. Inter. Lighting Rev. 13, 41-47, (1962).

[11] Weston, H. C, Light, Sight and Work (H. K. Lewis, London,

1962).

[12] Guth. S. K.. A method for the evaluation of discomfort

glare. 111. Eng. Soc. 58, 351-364 (1968).

[13] BlackweU, H. R., Development of procedures in instrumentation for visual task evaluation, lU. Eng. Soc. 65 , 267-291

(1970).

[14] MacAdam, D. L., Visual sensitivities to color differences in daylight, J. Opt. Soc. Am. 32,247-274(1942).

[15] Yonemura, G. T., and Kasuya, M., Color, discrimination under reduced angular subtense and luminance, J. Opt.

Soc. Am. 59, 131-135 (1969).

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1 PIIRI If ATIOM OR RKPORT NO

NBS-BSS-82

2 • CJ oV t Accession

No.

3. Recipient s Acccs s ion N

.

A New Look at the Research Bas i for T.i p-h ^ i ncr T (^-v/^r^

l

Recommenda tions

.

5. Publication Date March 1976

6» Performing Organi/cation Code

7. AUTHOR(S)

Gary T. Yonemura and Yoshimi Kohayakawa

8. Performing Organ. Report No.

y. FhKrOKMlNG UKGANlZAllUN NAMh AND ADDRhaS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4634160

11. Contract/Grant No.

12a Sponsoring Organization Nflmc £tn(i Complete Address (StrcGt, ^i^y, S^aic, ^IP)

Same as 9.

13. Type of Report & Period CoveredFinal

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Number: 76-136

16. ABSTRACT (A 200-word or less (actual suwmary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The validity of using threshold studies as the basis for lighting level

recommendations is questioned. The performance of the eye at supra thresho Id

levels was investigated with sine- and , square-wave gratings. The results of the

study indicate that the behavior of the eye is significantly different at

suprathreshold levels as opposed to threshold levels. For threshold studies, when

contrast is plotted against luminance, the function is a monotonica 1 ly decreasing

function. At suprathreshold levels the function indicates the existence of a definite

minimum, luminances greater or less requiring more contrast to appear subjectively

equal. It is recommended that lighting levels be based on laboratory studies that

appraise visual requirements and performance simulating conditions encountered in real

world environments.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Gratings; illuminating engineering; lighting; modulation transfer function;

suprathreshold visibility; visibility; vision.

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