NBSIR 77-1306

Levels of Illumination and

Legibility

Gary T. Yonemura

William M. Benson

Robert Tibbott

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

November 1977

Prepared for Architectural and Engineering Systems Branch

Division of Building and Community Systems

Office of the Assistant Secretary

Conservation and Solar Applications

Department of Energy

Washington, D.C. 20545

NBSIR 77-1306

LEVELS OF ILLUMINATION AND

LEGIBILITY

Gary T. Yonemura

William M. Benson

Robert Tibbott

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

November 1977

Prepared for Architectural and Engineering Systems Branch

Division of Buildings and Community Systems

Office of the Assistant Secretary

Conservation and Solar Applications

Department of Energy

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

FOREWORD

The National Bureau of Standards (NBS) research described in

this document was jointly supported by the Department of Commerce (DoC)

and the Department of Energy (DoE) . The Department of Energy has

continued its support to verify and extend these results. The followon effort is under the DoE/NBS Building Energy Conservation Criteria

Program, Task Order No. A008-BCS to DoE/NBS Interagency Agreement

No. EA-77-A-01-6010. The Illumination Engineering Research Institute,

supported by the University of Virginia, is currently associated with

the NBS in the follow-on research activity.
' • , ' ,£ . . , vi | £ 700 • - - < ^ ; ... , ,, •
CONTENTS

Page

1. Introduction 2

2. Method 6

3. Results 9

4. Discussion 10

5. Conclusions 14

6. References 15

Appendix A: Instructions 16

Levels of Illumination and Legibility

Gary T. Yonemura, William M. Benson and Robert L. Tibbott

Abstract: The visibility of tasks encountered in the working world

ranges from easy to difficult. The assumption that experiments

performed for threshold targets (difficult to see) can be

extrapolated to higher contrast tasks (easy to see) was tested

The experiments indicate that threshold level studies should not be

extrapolated to suprathreshold levels. The performance of the eye is

not the same at the two levels. The threshold function is monotonic,

that is, contrast required for detection decreases monotonically as

luminance is increased, whereas the suprathreshold experiments result

in a function with a minimum or optimum luminance level. Recommendations

are made to expand the empirical base from which lighting level

recommendations are derived to include the more commonly occurring

situation involving visual task performance for suprathreshold tasks.

Key Words: Lighting; lighting design; illumination levels; task

lighting; energy conservation.

1
INTRODUCTION

The visual tasks encountered in the working world range from highly

visible to tasks that are difficult to see even under the best lighting

conditions. In office work the visual tasks vary from high contrast

printed materials to barely legible fifth carbon reproductions. The

frequency of occurrence of these different levels of difficulty will

vary from office to office, but that they all exist at some frequency

can be recalled from our everyday experience. In some cases, the task

may be inherently difficult, e.g., the detection of defects in manufactured

goods (detection of cracks in a crankshaft) . This is a difficult visual

task where the eye is performing in its simplest mode—perceiving the

presence or absence of a nonuniformity in a homogeneous field. We call

this level detection. We can require that the observer not only detect

the presence of an object, but recognize it as a member of a specific

class of objects (e.g., recognizing that the object is a ring, square or

letter). This level of visual performance we call recognition. The next

higher level of visual capacity requires the ability to discriminate

between members of the same class, as in "That letter is an E and not a

B, a Landolt C because of the break in the ring, etc". This level we

call identification. All of the above can be and are, generally, studied

at threshold levels, and the response measured is usually the number of

correct responses. An example of a laboratory investigation of detection

is the minimum contrast required to detect the presence of a disc against

a uniform background. The minimum separation between two parallel bars

and the discrimination of form are examples of acuity threshold and

recognition studies. The Snellen Chart is a well known example of

identification, the task being to name a letter correctly at a distance

of 20 feet. All of the above are threshold tasks — just barely being

2
able to perform at some criterion level. In most everyday tasks involving

predominantly a visual component, we are not working at threshold

or would be unhappy if we had to. For example, a reproduction which is

so poor that all you can state is that something is on the paper (detection)

is impossible to accept. If the reproduction were slightly better

so that we could state that it was printed matter, it would still be

intolerable. One can have an improved copy such that we can with some

uncertainty identify the letters, i.e., discriminate between an E and B,

etc. But just barely being able to discriminate is still unsatisfactory.

What we really want is the situation where we can state that the visual

conditions are acceptable, that is, "I can read the printed page with

little effort". The criterion for visual performance should be goodness

of seeing, i.e., "I see better under condition A than condition B", and

not just barely being able to detect, recognize or identify an object.

This level will be defined as the suprathreshold level. The experimental

basis from which lighting recommendations are derived should resemble

the end-use conditions, or be valid although the experimental conditions

differ significantly from the real world. The criteria for acceptance

should meet the requirements or needs of the user.

The current lighting level recommendations are based on empirical

data obtained from experiments in which the observers were required to

detect the presence of a luminous disc, that is, the detection threshold

level.'*' The analogy in office work would be a reproduction so poor that

all the worker can say is : "There is something on the paper , but I have

no idea what it is". The visual performance data from acuity threshold

studies, more specifically the threshold for determining the gap in the

3
Landolt Ring, are also considered. The analogy in the real-world work

I situation would be the ability to just discriminate between two symbols,

e.g., an E and a B. Both the detection and acuity thresholds are visual

conditions that would be judged unacceptable by most workers. In most

cases it would not be possible to raise the visibility of a reproduction

that poor to an acceptable level by increasing luminance.

The preceding discussion has argued that the present lighting level

recommendations are based on empirical data obtained under threshold

conditions that differ significantly from most real-world conditions.

The obvious conclusion would be to base light level recommendations on

data obtained by having workers perform routine visual tasks in the real

working environment. The controversy over the validity of studies done

in the laboratory as opposed to studies done in the working world using

working world tasks had its heyday in the late forties. The two primary

2 antagonists were Luckiesh and Tinker. Luckiesh based his recommendations

3 on threshold acuity experiments, whereas Tinker based his recommendations

on task performance under actual conditions using real tasks, e.g.,

reading comprehension. Luckiesh felt that the levels recommended by

Tinker were too low, and Tinker felt that Luckiesh's recommendations

4

were too high, with Bitterman pointing out the difficulties or limitations

of their methods in arriving at a valid recommendation.

A similar controversy exists today. There are designers who feel

that the current recommended levels are too high. They argue that

workers performing under levels lower than those recommended by the IES

perform satisfactorily. The IES rebuts by stating that their recommendations
are based on years of systematic laboratory studies. Their studies

indicate that visual performance continuously increases as luminance is

2 increased, up to the luminance level investigated, 10,000 cd/m (2920 fL) .

A compromise between the two former antagonists Tinker and Luckiesh

is an approach that might also be acceptable to the present-day antagonists. Tinker felt that threshold studies should not determine

is lighting level requirements since the visual element of most tasks

not the type that require responding to something that is barely detectable.

Luckiesh strongly felt that production level indices are influenced by

factors other than sensory (physiological) parameters. He argued that

visual performance measured by production, reading comprehension and

other whole-task performance measures is confounded by variables not

under the control of the experimenter. Both points of view are legitimate

objections. What is needed is a study in which visual performance is

investigated by a psychophysical technique, under laboratory-controlled

conditions, but at suprathreshold levels. It is also essential that

subjective contributions, e.g., brightness preference, mood, motivation,

etc., as well as other nonvisual components (motor skills, intelligence,

etc.) be controlled, or at the least minimized. The laboratory experiment

should be of the form: For a definitely detectable task (non threshold)

is it more legible under illumination level A or B?

Yonemura and Kohayakawa^ conducted a study that fulfilled the above

requirement. They had their observers equate the contrast of gratings

of different mean luminance but equivalent in all other respects; this

was done for both sinusoidal and square-wave gratings at near threshold

and suprathreshold levels. The methodology differs significantly from

5
the threshold technique in that the grating patterns were visible at all

times. The observers were asked to adjust the contrast of one grating

pattern so that it appeared to be equal to an adjacent grating pattern,

that is, equal in contrast — hence the term equal apparent contrast

contour .

The results of their study agreed with laboratory studies at threshold,

but not at suprathreshold . For targets near threshold (just barely

visible) the data resembled the classical threshold data. As luminance

was increased the contrast required for equality of contrast decreased.

However, a departure from classical data was observed in the equality of

apparent contrast contours for suprathreshold contrast levels. Instead

of a monotonically decreasing function, the equal apparent contrast

contour initially showed a decrease in contrast required for a match as

luminance was increased, but further increases resulted in a reversal,

more contrast was required. That is, there was an optimum luminance

level, above or below which the apparent contrast of the grating being

measured decreases.

The following study was performed to validate the equality of

apparent contrast methodology for words, a frequently encountered realworld task. The procedure is similar to the earlier grating study, but

since the methodology is an important aspect of the compromise between

two opposing points of view, the methodology will be described in detail.

METHOD

Observers

The observers were three males and one female. Three of them were

experienced in visual psychophysical experiments, and their ages ranged

from the middle twenties to forties.

6
Stimuli

The targets were semigloss photographic reproductions of the Jaeger

Chart used in eye tests at reading distance.* The Jaeger Chart is shown

in Fig. 1. Two sets with a IX magnification were reproduced, one set

with a background reflectance of 0.82 and the second with a background

reflectance of 0.41. See Figures 2 and 3. Each set was composed of

about 35 charts with contrast ranging from approximately .04 to 0.94 in

roughly equal steps. Contrast is defined as: (L^ - L^/L^, where is

the background luminance and is the luminance of the target. Contrast

was measured by a micro-photometer that was capable of measuring a

portion of the target subtending 0.4 x 10 min of visual angle, or 20 x 500

micrometers. The three sizes used in this study, Jaeger 12, 8 and 6,

subtended 6.5, 4 and 3.2 min of visual angle, respectively. The Jaeger

Chart subtended 27 x 34 degrees and the reflectance of the area surrounding

the charts was 0.63, approximately the average of the light (0.82) and

dark (0.41) backgrounds.

Apparatus

The light source was a single U-shaped T-12 40W coolwhite instant

start fluorescent lamp mounted under the viewing platform. Reflectors

2 were added so that the illuminance was equal over a 100 cm central area.

Photometric measurements were made over the task area at the beginning of

the study and after several months of running. No appreciable change in

the absolute luminance levels was observed, and the distribution of light

over the task area was found to be homogeneous in both instances.

*We gratefully acknowledge the contribution of William Smallwood of the

Optical Physics Division, Institute for Basic Standards for reproducing

the charts.

7
Stimuli were viewed through a 5 x 15 cm window located 33 cm from the

task. The luminance was varied by Wratten neutral density gelatin films

placed over the viewing aperture.

Procedure

The observer was seated so that he viewed the target at a distance

of 38 cm and at an angle of 25 degrees from the normal to the task surface.

Two charts, one from each set, were placed side by side so that the

angular separation between the test word on the two charts was 3.3 degrees. Th

observer (0) was instructed to look at the target word on the standard

(high reflectance background chart) and compare its apparent contrast

with that of the corresponding word on the comparison (low reflectance

background chart) . If he thought the word on the comparison chart had

more contrast than on the standard chart, he was to ask the experimenter

for a new comparison chart having less contrast. If 0 felt the word on

the comparison chart had less contrast than on the standard, then he was

to ask for a comparison chart with more contrast. The process continued

until the observer thought the word was equal in apparent contrast on the

two charts. Five such matches were made, and the median physical contrast

of the comparison charts chosen in these matches was recorded . The

standard chart was then replaced by one of the same physical contrast as

the median comparison chart contrast recorded above. The illumination

level was halved by adding a 0.3 neutral density filter to the viewing

window. Thus, the background luminance and the contrast of the target

word on the new standard chart were equal to those on the previous

median of the matched comparison charts. The subject then made five

more matches, a new standard was determined, and the luminance was again

8
halved. This process was repeated until 0 required more contrast for

his matches than was available in the charts. The range of luminances

2 2

in the experiment was from 810 cd/m to about 0.2 cd/m .

Each run lasted approximately one hour, interspersed with rest

periods of two minutes after each series of five matches while the

charts and filters were changed. In half the runs the high reflectance

background chart was on the left of the low reflectance one; in the

rest of the runs these positions were reversed. To check for an order

effect, several runs for each subject began at lower luminance levels

instead of the highest one. In these instances, the low reflectance

background chart always served as the standard and the high reflectance

one as the comparison. Each subject was run under at least nine

conditions (three target sizes x three contrast levels) . The first

three sessions were practice sessions where the concept of apparent

contrast was demonstrated followed by practice in making equal contrast

judgments. See Appendix A.

RESULTS

The results of the experiment are presented separately for each

of the four subjects in Figs. 4-7. The curves represent equal apparent

contrast contours, i.e., all of the connected data points are perceived

as being subjectively equal in contrast at different levels of illumination.

The three symbols represent different stroke widths of the letters,

where the circle = 6.5, square = 4 and cross = 3.2 min of visual

angle. Unless specified by a number in parenthesis on the right of

each curve, each data point is the result of a single trial. Where

the data points represent the mean of more than one trial, the number

in parentheses on the right gives the number of trials.

9
The form of the lowest set of curves appears to be different from

the others. In general, for the lowest set of curves as we increase

luminance the contrast required for the letters to be subjectively equal

in contrast decreases monotonically . This observation holds for all of

the observers. See lowest set of curves in Figs. 4-7. But for the equal

apparent contrast contours at higher contrast levels, the contrastluminance relationship is different. The contrast decreases initially,

reaches a low point, then increases with further increases in luminance.

The shape of the curves is similar for all observers.

The results for all of the observers have been averaged for a given

size and are presented in Fig. 8. For the range of stroke widths used

in this study, 6.5 to 3.2 min of visual angle, the shape of the curves

are not significantly different. Therefore, the data were averaged over

the four observers and the three stroke widths and are presented in Fig.

9, which summarizes the principal findings of the experiment. At near

threshold contrast levels the function decreases monotonically as luminance

is increased, but at suprathreshold levels of contrast the curves exhibit

a minimum value.

DISCUSSION

The experiment indicates that the suprathreshold methodology gives

similar results when letters are used as the targets instead of gratings.

For near threshold contrast levels the function is monotonic, that is,

as luminance increases the contrast required for apparent equality of

contrast decreases in agreement with classical data. But for contrast

levels larger than near threshold levels, the equal apparent contrast

10
contours exhibit an optimum luminance level. As luminance is further

increased the contrast required for equality of apparent contrast decreases,

reaching a minimum, followed by increases in contrast as luminance is

further increased. In terms of practical visual task performance,

"goodness of seeing" increases as lighting level is increased, reaching

an optimum luminance level. However, increases in luminance beyond this

optimum level lead to a decrease in the clarity of the detail to be

perceived

.

What are the implications of the data from this new methodology?

First, they raise questions about the current basis for recommending levels

of illumination, but probably more importantly they provide an objective

basis for setting levels of illumination at plus or minus a given number

of footcandles rather than specifying a minimum as recommended by current

practices. The existence of an optimum rather than a continuously

increasing performance with increases in illumination levels should not

be surprising in the general context of sensory physiology. As Simonson

and Brozek point out, "... the other environmental components such as

temperature, humidity, barometric pressure, have a physiological optimum.

There is no reason why visual performance should behave differently."

The non-specification of a minimum will discourage over-design, encouraged

under current practices. That is, the consequence of a "more light,

better sight" mentality can be the addition of a significant "safety"

factor, the rationale being that the safety margin results in increased

performance.

11
We find that the controversy between Tinker and Luckiesh cited

j earlier could have resulted from the fact that the former used suprathreshold tasks, whereas the latter used threshold tasks as well as a

different measurement technique. The results of this study agree with Luckiesh,

in that, for near threshold tasks, visibility continuously increases as

illumination is increased, but they differ in that when the same methodology is used with suprathreshold tasks, the monotonic function changes

to one with an optimum illumination level. This study also agrees with

Tinker in that the visual performance on threshold and suprathreshold

tasks differs. Tinker found no change in reading performance (7-point

newspaper type) occurred above an illumination level of 7 fc. He did

not find a decrement in reading performance at higher illumination

levels, and furthermore his critical illumination level (the illumination

level beyond which no further change in reading performance occurs as

the illumination is increased) of 7 fc appears to be lower than that

found as the optimum in this study. No definitive explanation can be

given for this discrepancy, but it is possible that the differences may

be due to uncontrolled psychological variables. In comparing the

method of equal apparent contrast discussed in this paper with real-world

task performance measurements, two important distinctions are apparent.

First, in the equal contrast technique, subjects were asked to respond

directly to "goodness of seeing", that is, how well the details stood

out from the background. Second, since the methodology involved simultaneous

direct matching for equality, psychological variables affecting one

target should have equally affected the other. It is possible that

12
Tinker’s low critical illumination level resulted from a greater effort

by the subject to offset the lower visibility. The same explanation

can be postulated for the higher levels, where no decrement in performance

was observed. In the first case, the goodness of the task was directly

measured, whereas in the other, visibility was indirectly evaluated

through performance measures.

The performance of a worker is affected by physiological and psychological factors. In one case, we are describing the sensory capacity or

process and in the other, cognitive factors usually expressed as preferences.

The special case we are concerned with in this discussion is when the

two processes are the resultant of the same physical variable, e.g.,

luminance. As far as the sensory processes are concerned, an environment

in which the total visual field is of the same reflectance is optimum

for sensory performance, but we also know that this bland (limited light

and shadows) visual environment is not conducive to worker comfort, thus

indirectly affecting productivity. It is indirect in the sense that it

does not affect productivity by increasing or decreasing the sensory

capacity of the worker, but by -influencing his attitude toward the

visual environment. The difficulty with these preference variables is

that they are susceptible to individual differences. A preferred condition

for one individual may be an undesirable one for another. For example,

in color harmony preferences for clothing, some individuals may prefer a

subdued color combination, shades of gray, whereas others may prefer

extreme hue combinations like red and green.

13
What we are suggesting is that performance is a function of physiological and psychological factors. Subject response based on physiological

factors can generally be predicted with confidence, but the psychological

factors show large individual differences, in some cases the preferences

run to opposite extremes, which makes prediction difficult. We must face

the fact that they exist, with one important consideration. On the one

hand we have biological factors that have similar effects on most people,

and on the other hand we have highly individualistic preferences formed

by past experiences.

CONCLUSIONS

Threshold studies are not a valid basis for recommending levels of

illumination for tasks that are not at threshold levels. Real-world

performance studies also create difficulties in interpretation because

of the confounding by psychological variables not easily under the

control of the experimenter. Production and work performance measures

obtained from the working environment bring in nonsensory contributions

that make it difficult to separate that portion due to the visual process

from that contributed by psychological factors.

The empirical basis for recommending levels of illumination should

be obtained from controlled laboratory studies that approximate conditions

encountered in the real working environment. A single reference base,

however, may not be sufficient. At least two, and probably three,

visual performance criterion functions are needed to adequately handle

the different degrees of task legibility and the apparent differences in

the behavior of the eye for threshold and suprathreshold tasks.

14
References

1. Illuminating Engineering Society of North America, "IES Lighting

Handbook, 5th Ed.," edited by Kaufman, J., (Illuminating Engineering

Society of North America, 345 East 47th St., New York, N.Y. 10017,

1972)

2. Luckiesh, M., "Footcandle Levels Threshold, Ideal, Optimum and

Recommended", Illuminating Eng. 4_3, 395-415, 1948.

3. Tinker, M. A., "Trends in Illumination Standards", Illuminating

Eng. 43, 866-881, 1948.

4. Bitterman, M. E., "Lighting and Visual Efficiency: The Present

status of Research", Illuminating Eng. 4_3, 906-922, 1948.

5. Yonemura, G. T. and Kohayakawa, Y., "A New Look at the Research

Basis for Lighting Level Recommendations," NBS Building Science

Series 82, 1976. U.S. Government Printing Office, Washington, D.C.

20402, SC Cat. No. C13. 20/2:82.

6. Simonson, E. and Brozek, J., "Work, Vision and Illumination",

Illuminating Eng. 47_, 335-349, 1952.

15
APPENDIX A

INSTRUCTIONS

THIS IS AN EXPERIMENT ON CONTRAST, THAT IS HOW WELL WORDS STAND OUT

AGAINST THEIR BACKGROUNDS. I AM GOING TO CHANGE THE AMOUNT OF LIGHT

REACHING YOUR EYES BY COVERING THE RECTANGULAR SLOT IN FRONT OF YOU WITH

A VARIETY OF FILTERS. EVERY TIME A FILTER IS CHANGED, I SHALL ASK YOU

TO LOOK AT THE WORD "RANSOM" ON THE CHART ON YOUR LEFT AND COMPARE IT

WITH THE WORD "RANSOM" ON THE CHART ON YOUR RIGHT.

E PUTS IN #15 , LIGHT

#32, DARK

THE WORD ON THE WHITE CHART HAS MORE CONTRAST THAN THE WORD ON THE

GREY CHART. WHEN THIS HAPPENS, SAY "MORE" AND I SHALL PUT IN A DARK

CHART THAT HAS MORE CONTRAST.

ON THE OTHER HAND, THE DARK CHART MAY LOOK MORE LIKE THIS.

E TAKES OUT #32, DARK AND

PUTS IN #3, DARK

"RANSOM" NOW STANDS OUT MUCH MORE ON THE DARK CHART THAN IT DOES

ON THE LIGHT CHART. WHEN THIS HAPPENS, SAY "LESS", AND I SHALL PUT IN

ANOTHER DARK CHART THAT HAS LESS CONTRAST.

NOW THE WORD ON THE DARK CHART IS MORE EQUAL IN CONTRAST TO THE

WORD ON THE LIGHT CHART.

E PUTS IN #15, DARK

16
LET'S LOOK AT A FEW MORE CARDS

E TAKES OUT BOTH LIGHT AND DARK

CARDS, PUTS IN #28 LIGHT AND

#35 DARK

AGAIN YOU SEE THAT THE WORD ON THE LIGHT CARD STANDS OUT MORE THAN

THE WORD ON THE DARK CARD.

HERE IS THE DARK CARD THAT WAS PREVIOUSLY VERY SIMILAR TO THE

PREVIOUS LIGHT CARD.

E PUTS IN #15 DARK

NOW "RANSOM" ON THE DARK CARD STANDS OUT MORE THAN "RANSOM" ON THIS

LIGHT CARD.

REMEMBER THAT WE WANT YOU TO FIND A CARD THAT LOOKS EQUAL IN

CONTRAST TO THE OTHER CARD. THIS CARD OBVIOUSLY DOES NOT HAVE ENOUGH

CONTRAST.

#32 DARK AGAIN

AND THIS ONE OBVIOUSLY HAS TOO MUCH CONTRAST.

#3 DARK

SO YOU WOULD HAVE TO FIND A CARD SOMEWHERE BETWEEN THE TWO TO MATCH IT.

LET’S TRY TO FIND ONE THAT MATCHES.

17
'
Near V. E. t?c \ 5 Jaeger U

man oxen

Near V. E. % : 10 Jaeger 12 raw see van

Near V. E. % : 15 == Jaeger 10 noon even mew

Near V. E. %: 20 == Jaeger 8 war use worm eve

avenue ransom err

24 Point

25 37 84 90

18 Point

90 89 76 60 54

14 Point

38 72 80 93 54 60 76

12 Pn 'vt 40 53 8

809 42i

Acuity: Approx . O. / 2 {20/ 170)

.Sight- saving texts. oxxo oxxx

Acuity: Approx. O.l 5 (20/1 JO) Books , children 7-8 yrs. XOXXX 0X000

Acuity: Approx. 0.2 (20/100)

Books, children 8-9 yrs. XXOX XXXX OOOX

Acuity: Approx. 0.25 (20/80) Books, children 9-12 yrs. XOOX 000X0 ooxxx

remo romero suma

Near V. E. %: 30

: ~ Jaeger 7 scum crease nervous 98 67 45

cocoon cannon saucer 456 309 5

Acuity: Approx. 0.28 (20/70) Adult textbooks. XOOO OXXO OXXX XXXX

saca zarco cascarron

Near V. E. %: 40 =p Jaeger 6 arrow scour noose razor 35 98 20 82 zone reverence sorceress 740 203 9fi£

Acuity: Approx. 0.3 (20/65) Magazines. xxxo ooxx oxoox xoxox

retnanso semana asma

Near V. E. % : 50 Jaeger 5 amaze wares curve scarce 82 34 65 90 snooze caress sewer wax 428 397 564

Acuity: Approx. 0.4 (20/50) Newspaper text OXXO OOOO XOXX XXXX OOOX

remesa suave arrancar

Near V. E. 90 —~ Jaeger 3 comma worse reason measure vase 7s 23 68 «o 44 census arrears recover crane now 890 375 294 83

Acuity: Approx. 0.5 (20/40) Telephone directory. XOXX OOXO OXXO OXOO XXOX OOOO

reaaca carecer crecer sazonar

Near V. E. %: 95 === Jaeger 2 success numerous assurance consume 59 87 92 30 47 cocoa convex morocco uncommon err 209 384 872 40

Acuity: Approx. 0.6 (20/30) Want ads. XOXX OOOX OOXO OOXXX XOXO XXOX OXOO

sesoa sucesor vaso zamarra azar

Near V. E. %: 100

7==. Jaeger 1 orrumnet nemmor* auc cctaof romance worm 77 48 40 84

craie arron crew amoroua ecow aroma aamorar 784 484 304

Acuity: Approx. 0.8 (20/25) Small bibles. XXXOX XXOOX OOXX© XOXOX 001X0 XOXX comarcano roaca serrano amaneewr

Comparable:

Jaeger 1 +

3 Point Acuity: Approx. 1.0 ' Mailorder <• aaMaaaaawniastaaaMoaamaa 80,10 Xxooo 00000 00X00 Meant aerraao aranca

letter height

0.3493 mm. 2 Point

Fig. 1 Jaeger Chart
o<3

-

ISVdlNOO

Fig. 2 Contrast of high reflectance background targets. The target numbers

have been displaced by 4 for Jaeger 8 and 8 for Jaeger 6.
o

pen oo h- to in ro c\J —

O

— oododooo 0 o

1SVB1NOO

Fig. 3 Contrast of low reflectance background targets. The target

numbers have been displaced by 4 for Jaeger 8 and 8 for

Jaeger 6.

TARGET

NUMBER
CONTRAST

Fig. 4 Equal apparent contrast contours for three stroke widths of letters; circle

= 6.5, squares = 4 and triangles =3.2 min of visual angle. Subject WB.

The numbers in parentheses are the number of repetitions. The data

points are the arithmetic mean of the repetitions.
CONTRAST

Fig. 5 Equal apparent contrast contours for three stroke widths of letters:

circles = 6.5, squares = 4 and triangles = 3.2 min of visual angle. Subject

RT. The numbers in parentheses are the number of repetitions. The data

points are the arithmetic mean of the repetitions.
CONTRAST

o

QO 1.0 2.0 30

LUMINANCE (Log cd/m2

)

Fig. 6 Equal apparent contrast contours for three stroke widths of letters; circles

6.5, squares = 4 and triangles = 3.2 min of visual angle. Subject BG. The

numbers in parentheses are the number of repetitions. The data points are

the arithmetic mean of the repetitions.
CONTRAST

0.8

0.

00

! 1 10 20

LUMINANCE (Log cd/m2

)

3.0

Fig. 7 Equal apparent contrast contours for three stroke widths of letters: circles

= 6.5, squares = 4 and triangles = 3.2 min of visual angle. Subject BC . The

numbers in parentheses are the number of repetitions. The data points are

the arithmetic mean of the repetitions.
CONTRAST

Q8

0.0 1.0 20 3.0

LUMINANCE (Log cd/m2

)

Fig. 8 Equal apparent contrast contours for three letter sizes. The stroke widths of

the letters are: circles = 6.5, squares = 4 and trianges =3.2 min of visual

angle. The data points are averages of four subjects, except for curve (1)

which is the result of a single subject.
CON

TRAST

0.8

0.1

0.0 1.0 2.0 30

LUMINANCE (Log cd/m2

)

Fig. 9 Equal apparent contrast contours averaged over four subjects and three stroke

widths, except for curve (1) which is the result for a single subject.
NBS-114A IHEV. 7-73)

U.S. DEPT. QF COMM. 1. PUBLIC. ATION OR REPORT NO. 2. Gov’t Accession

BIBLIOGRAPHIC DATA hid c t n -it ,w No. SHEET NBSIR 77-1306

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Levels of Illumination and Legibility

5. Publication Date November 1977

6. Performing Organization Code

7. ^uT^oR^nemura , W.N. Benson and R. L. Tibbott

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Architectural & Engineering Systems Branch

Division of Buildings & Community Systems

Office of the Assistant Secretary, Conservation & Solar Applications, Department of Energy, Washington, D.C. 20545

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The visibility of tasks encountered in the working world ranges from easy to

difficult to see objects. The assumption that experiments performed for threshold

targets (difficult to see) can be extrapolated to higher contrast tasks (easy) was tested. The experiments indicate that threshold level studies should not be

extrapolated to suprathreshold levels. The performance of the eye is not the same

at the two levels. The threshold function is monotonic, that is, contrast

required for detection decreases monotonical ly as luminance is increased, whereas

the suprathreshold experiments result in a function with a minimum or optimum

luminance level. Recommendations are made to expand the empirical base from

which lighting level recommendations are derived to include the more commonly

occurring situation involving visual task performance for suprathreshold tasks.

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)

Energy conservation; illumination levels; lighting; lighting design; task lighting

18.

AVAILABILITY [J£ Unlimited

L For Official Distribution. Do Not Release to NTIS

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19. SECURITY CLASS

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21. NO. OF PAGES

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