34

Specia/ Issue'LUX PACIFICA '97 3rd PACIFIC BASIN LIGHTING CONGRESS 'P, p ''

ImpressionS of Overall BrightneSS Bn a Non‑Uniformly
Illuminated Spaee
Shigeo KOBAYASHl
Departn;ent ot Butt E wironmei t
Tokyo Insutute ot Technotogy
4259 Nagatsuta Midori‑ku, Yokohama, 226

JAPAN

Yoshiki NAKAMURA
Departn)ent of Built Environment
Tokyo Institute of Technology

4259 Nagatstrta Midori‑ku, Yokohama, 226

iAPAN

Masao INUl
Department ofArchitecture
Musashi Institute of Technoiogy
1‑28‑ I Tan atutumi Setagayaku. Tokyo, 158 JAPANReceived Dec. 17. 1997

ABSTRACT

When an interior space is illuminated non‑uniformly, we perceive different levels of brightness. The
strongly illuminated areas give a bright impression while the weakly iiluminated areas give dark
impression. In other words, the whole space is divided into several portions which have different
levels of brightness. The purpose of this study is to estimate overall impression of brightness which is
divided into such portions which have different brightness.
Two experiments were carried out in this research to assess brightness of whole spaces those were
illuminated non‑uniformly. In the first experiment, Iuminance distribvtion images of an interior space
were vsed as stimuli. In the second experiment, actual interior spaces of non‑uniform lighting wereused as stimuli. From these results, it could be concluded that the whole space that is illuminated
non‑uniformly is perceived darker than the space illuminated uniformly. The overall impression ofbrightness of the space that is illuminated non‑uniformly could not explained on the basis of thequantitative average of the luminances. It could be corresponded with the psychological average ofall portions' brightness those were divided by the illumination level.

1. Introduction
The luminances in a non‑uniformly illuminated space is
not uniform. If the reflectance of the interior surfaces isvariations resulting from non‑uniform illumination. As ameans of investigating the model described above, first we

uniform, a strongly illuminated areas show highluminances, and weakly illuminated areas show low

used an image of varying luminance to experimentally

luminances. If we assume that individuals observing thethe result of the first experiment, similar investigations

inside of the interior space are doing so three‑were conducted involving experimental evaluations ofdimensionally, surfaces of high luminance will be seen as

brightness in actual interior spaces subject to non‑uniform

being strongly illuminated space, and surfaces of low

illumination.

evaluate the impression of brightness. Then, to confirm

luminance will be seen as being weakly illuminated space.

In previous researches (4) (5), the experimental results

In other words, the interior space will be seen as consisting

about the brightness in the non‑uniform lighting were notof portions of varying levels of brightness (see Fig. I ).

corresponded. The experiments in these researches

Under these conditions it is thought that the impressionof overall brightness in a non‑uniformly illuminated spaceis determined by the balance between the various portions

visual working. So, it would be considered that theof the space.

This research initially investigated spatial luminance

The I!Iuminating Engineerin.g Institute of Japansupposed the object room as the specific situation for
impressions of brightness were strongly influenced by the
specific area's luminances those were related to the visual
work.

35

J. Light & Vis. Env. Vol.22, No.1, 1998

r! ht Pe :ien$D r

S { trspa{{ l ivmir

ncespati i ju

nce va iatons

Added

hcy spaSia:}v ? inance va a iens

vency
iatonsportiens.

i,*t* ?

Hg.1 In a non‑uniformly il!uminated space, thespace is devided into several portions whichhave ditferent leve]s of brightness

2 Experiment with Luminance Distribution Images
2‑1 Experimental Outline
Luminance distribution images (measuring 29.4 cm x
2 1 .5 cm) based on interior models were used as stimuli toassess brightness. Luminance distribution images wereobtained from a model room of uniform surface reflectivity

and illuminated non‑uniformly. Because the room was
wrapped by uniform reflectivity, the lightness in this image

is dependent upon differences in the amount of light
exposed at interior surfaces. The lighting methods of theroom were two types, one is centralized lighting by thenarrow spotlight and the other is simulated daylighting byi!v

ini ncea etiens

Fig.2 Lumir}ance distribution image in which the two
types of spatial lumir}ance variations wereexpressed as variables(Lighting type : Centralized lighting)I tensity of iuminance vayiations
1 ,1Originai image

Added low freqvency
uminance variations

the parallel light through the window.

The characteristics of luminance variations in these¥';‑ , ‑*1***̲
. Addediimages are classified into two types.

(1) Spatial luminance variations in small areas of low
luminance such as the depressions and shadows on
f urniture.

igh frequency

lumin8nce variations

1 .05

Svbtracted low frequluminance variationsn{ y(2) Spatial luminance variations resulting from non‑uniform illumination of the space.
The low luminance districts caused by ( I ) do not spreadwide area. So, spatial luminance variations in (1) tend tobe high frequency. While the low luminance districts
caused by (2) spread wide area and spatial luminancevariations in (2) tend to be low frequency.

As shown in Fig.2, these two types of luminancevariations were extracted separately from the images.
Then, we added each extracted luminance variation to, or
subtracted from the original image. So, the luminancedistribution images were obtained in which the two typesof luminance variation were expressed as variables. Theseluminance distributions could be divided into a number ofcomponent sine waves of various frequencies using Fouriertransforms. With these frequencies, the spatial interval of
luminance variations were expressed (6). with the amplitudeof these sine waves, the intensity of luminance variations
can also be expressed. The characteristics of the intensityof luminance variations are shown in Fig.3.

O. 1 25cy
1eldeg 0,5cy Ie/deg ubtracted high frequer]cy
lumina e var'a ons

F equency ot iuminance va iations icycieldeg j<centraiized lighting>Rg 3 Luminance distributions divided into sine
waves of various frequencies
Because the brightness of a room interior can be assessed

only after the eye has adjusted to the luminance of thatinterior, the luminance distribution images used 'as stimuli
did not encompass the entire visual field. For this reason,a semicircular acrylic shield was placed around theluminance distribution images in order to alter the surface

luminance and thereby assure agreement between the eye's

adjusted luminance and the average luminance of eachstimulus. Each of the luminance distribution image wasassessed by a group of 20 observers. All observers self‑

The I!!uininating Engineering Institute of Japan

36

Specia/ Issue

reported normal or correct vision. The ME method wasused to assess brightness. Observer saw the standard
stimuli at first and it's impression of brightness was
specified as I OO. Another stimuli was evaluated on'LUX PA CIFICA '97 3･d PACIFIC BASIN LIGHTING CONGRESS 'luminance variations result from non‑uniform illuminationof the interior space, and the illumination is evaluated as

being dark. This suggests that even if the averageluminance of images were equal, uneven brightness caused

comparison with the standard. The order of presentation of

by uneven illumination was a factor that reduced

the different lighting conditions to each observer wasvaried to minimize any order effects. Care was taken to

impression of brightness.

ensure that there was a short adaptation time of a minutec.*tr*liz*d lighting

by the observer to each condition before assessment

High frequency
lvminance v riationscommenced.

Su tTactadd439‑0.3482‑2 Experimental Results and Discussion
The individual evaluations for a stimulus were not

Low frequency!urnin nce variations

p <0.01

i. ti tr b = :p <0.05

Da ylightin

Low frequency

Hig freql ency
!uminance vatiations

matched to the same value. But after examining the data indetail, relative evaluations between stimuli were almost

subtract

stable. So, to understand general tendencies, we usediuminance variations

add‑1.s02

average evaluation values for following explanations.

Table I Result of t‑test"""= "' '=100‑ 5Lightif̲*= >*

type : Centr iized iigh ir g

' '

o*
=>
"

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CLighting: ce tralized
lighting
LJ[>: Dayiighting

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o

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LU 1.02 1.04 i.06 1.08 1.1.12Intensity ot high frequency luminance variations
Freqvency : o.50cycleldeg

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5 60

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tJJ
1̲04 1̲06 1.08 1.1 1.12 1̲14
intef)sity of iow frequency i Jmin nce variations

Frequency : o.i25cycie/deg

Flg.4 Relationship between frequency luminancevariations and impression of brightness

The results of the experiment are shown in Fig.4.
Average luminance of these stimuli were the same. The
impression of brightness did not change very much withhigh frequency spatial luminance variations. On the other
hand, the impression of brightness was greatly affected by

Fig.S Sepa ate the spaces into portions

The result that uneven illumination effect the total
impression of brightness might be connected with theseparate impressions of brightness in the uneven
illuminated room. If we assume that individuals observing

the interior space are doing so three‑dimensionally,
surfaces of high luminance will be seen as being strongly
illuminated space, and surfaces of low luminance will beseen as being weakly illuminated space. So we suggested ahypothesis that the overall impression of brightness in theinterior space was determined f̲rom the sum of theimpressions of brightness for all portions of the space. In
order to test this hypothesis, the interior space used as thelow frequency spatial luminance variations. Stimuli instimulus was divided into equally illuminated portions ofwhich low frequency variations were increased were
evaluated as being very dark. Table I show the test

the space. The method used to separate the space into

results by each luminance variation. Low frequency spatial

The llluminating Engineerin8 Institute of Japan

portions is shown in Fig.5. The interior space was divided
into two portions in which the luminance was greater than,

37

J. Light & Vis. Env. Vol.22. No.1, 1998

and those portions in which the luminance was less than,of portions. The process of calculating this value isthe average luminance of the stimulus image. In thisexperiment, the average luminance of the stimulus wasequal to the adaptation luminance of the eye. Then, toshown in Fig.6. Table 2 shows the each value.

Fig.7 shows the distribution of the 'predicted overall

predict the each portion's impression of brightness, theimpression of brightness', and the experimentally evaluatedimpression of brightness. It is apparent that the 'predicted

Stevense equation

impression of brightness' approximates the evaluated7

was employed. Each portion's

impression of brightness was calculated by the averageluminance of the portions and the adaptation luminance ofthe eye. This impression of brightness is referred to as

impression of brightness.

That is, the reduction in the impression of brightness
resulting from the non‑uniform illumination of the space iscompensated for by the average perceived brightness of thethe 'predicted impression of brightness' and is distinct from

impression of brightness' for each portion of the space isspace. While the average luminance is a quantitative
average, the perceived average is a value close to thethen is weighted with each solid angle, and the overa]lgeometric average. As the latter is always less than the

average value calculated. The overall average iscalculated average. So, the non‑uniformly illuminatedcalculated on logarithm to take the psychological averagespace would be perceived darker.

the evaluated impression of brightness. The 'predicted

Configuration factor of poriion(1 )Steve s eqvatio

Quantitative average tumi ]ancer

7TT]portion(1 ) of portion (i )l> Predicted impression ofb ightness of portion (1 )Average luminance of visual fieid=Adaptation luminance of the eye

Av e age on lo

Predicted overall Impressionof brightness

Configuration factor of portion(2)･ j*"h I

Quantitative average lvminancestevens equationof portion (2)t> P edicted impression ofbrightness of portion (2)Average luminance of visual field=Adaptation iuminanee o

:LP

io L

the eyeJn(3)Process of calculating brightness

Fig.6

Evali a e

Unifofmjty
of heilivmination

Non‑uf]ifo mCentraa 'zed'zedlighiingng

Uniform

Non‑uniform

overa I
mpress on of brightness

lvminance o
the who!eCortfiguratioR

space

Average Impression offactorlumina tee brigh ness

Predicted

Table 2

Predicted :m ression oCon{igvratien factor;

Average luminaAeebrig tness

P ediet d overall
imp ession ofrightness

60 27

139 15o 31

369 80

1 7.37

o 6e

35.53

55.51

70.01

86.51

1 37.82o 31

287 37

07.06

o 69

70.63

7

.37

80 93

93.49

1 38.80o 31

1 s2 68 1 oo.71

o 69

1 32.56

95.16

96.8s

63.43

139 ooo 28

378.97 1 1 9.75

O 72

45 68

59 48

72.3s

83 19

39 04

o 28

285 1 3 I07.06

0.72

82 23

74 54

82 50

92 24

39 Io

o.28

1 58 ,)4 1 oo.7 1

o.72

131 74

95 16

96 69

Day lighting
ghting

Uniform

Portion of IOw llum nancePortion of higb iiiuminance

Ave age

P edicted impression of brightness of each portion

The I!luminating Engineering In.stitute of Japa,738Specia/ Isstie "LUX PACIFICA '97 3･d PACIFIC BASIN LIGHTING CONGRESS'

u)
(1)e)

1 ooo
:
a)

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3 Experiment with Actual Environments:i

3‑1 Experimental Outline

;

90o

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To confirm above results under actual visual
80o

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environments, next experiment evaluated the impression ofbrightness in the real interior spaces. The room consistedof five office working rooms. To ensure that a variety of

i
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distances and views of the spaces from the seats were

El!

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Lu

[]: Centralized iighting: Da li ht'n

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{

5

50

70 80 90 10060

Predicted overall impression of brlghtness

Fig.7 Relationship between predicted impression oi

brightness and evaluated impressior} ofavailable, 1 5 positions were selected. Plan views of eachspace, and locations of each position, are shown in Fig.8.
The experiment evaluated the extent of the impression ofbrightness from each position. Impressions of brightness
were evaluated in five stages from 'bright' to 'dark', with 2
or 3 subjects located at each position. In this experiment, a
total of 16 subjects participated . Subjects were different
group from the prior experiment.

Luminance distribution was measured by photometerusing an orthographic projection lens.

brightness

The numberof the positio
ositio

/

w

=

1

HJ, i
3

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4 I Wi ndow

C }v

A‑Office

WindowWs

L )[̲ ̲ ̲ ̲ L

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C:

Window)7

B‑OfficeJ

L

D‑Office//'
E‑Office
Fig.8

Plan location and orientation oi each seat position

The 11luminating Engineering Institute of Japan

o

5

1 OM

39

J. Light & Vis. Env. Vol.22, No.1, 1998

3‑2 Experimental Results and Discussion
C1 : C‑oftice, Seat‑1

O

Evalvated overaii impression of bfightness

2 OO
41 .

Avera e iuminanceo ,6 cd/rrf

Pred'cted
Averageum nance brightness

Conf iguration

iactor1

Portion 1)

Portion R

o.10

54.2

Por on 3

o 62

21 2

62 5

Portion 4

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47

brighiness

62.6

Predicted overall impressioR o

C3

)

83 .3

EvalUate overall impression of rightnessoAverage luminancc

70.56 cdlrrf

Conf iguration

Averageactor

iuminance

Predicted
brighiness1oPortion 1

o.04

936.6

198 3

Portion ( )0.09

i oo.481 7

Portion 3

o.17

54 2

6s.8

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o 71

19.8

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brlghtness

52.5

Predicted overaii impressio0

O CO OeO 10e C
ed/r

Hg.9 Example luminance distributions and divisions of the field‑of‑view
It was expected that some individual differences of therating scale exited. But in this experiment, fixed subjects
did not participated in each situation. So, the rating scores
were not standardized with subject's mean and variance. In

this case as well, overall impression of brightness wascalculated from the brightness for each portion of thespace. The field of view was split up based on theluminance level for each of the 15 positions for whichluminance distribution was measured. It has previously
been the case that the boundary of the field of view be
drawn by the subject on the basis of 'a change in level ofillumination'. However in this experiment, for the sake ofoverall impression of brightness was corresponded with thepsychological average. It was apparent that the relationshipof between the evaluated value and the 'predicted overall
impression of brightness' was improved.
(1)

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the luminance distribution from the viewing position, thedivision of the field of view, and the 'predicted impressionof brightness' for each portion. The overall impression was
calculated on logarithm to take the psychological average

.
IBi

(D
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ji }

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Dar

,'! I "

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i;
o

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of portions .

Fig.10 shows the relationship between the averageoverall impression of brightness' found by dividing up theif f } ij B3 . A

ij{

1:S

Fig.11 shows the relationship between the 'predicted

I,
i

(Q

with the same reflectivity. Fig.9 shows and example ofluminance in the field of view at each position, and theaverage evaluated value for the impression of brightness.

; iii

j i:

; i'2
i,i
;ii {

i ji icoe)convenience, the boundary was drawn on the basis of adifference in brightness of a factor of approximately fivei*iivisual field (cdl i)
Fig. I O

Relationship between the ave age luminanceand evalvated value for the impression ofb ightness

field of view, and the average evaluated value for the
impression of brightness. In real environment as well, theThe I!lun7inating E17gineering Institute of Japan

40

Specia/ Issue'LUX pACIFICA '97 3,d pACIFIC BASIN uGHTING CONGRESS 'Fig.12 shows two different
processes in a non‑uniform
R:L̲0.551" Iuminance distribution
space for overall impression ofhc: brightness.
That is, non‑uniform
illuminated space and2Q){L) A2 non‑uniform reflectance space. It appears that the overall
:

･i

[

15
impression ofj brightnessof the non‑uniformly illuminated
c4
i !i
B3
o"c interior spaceisbased
on separate impressions ofIAAIe) brightness for eachspace, bright portion from strongly
illuminated areas
and dark portion from weakly illuminatedEc'A Bi
A areas. The overall
impression of brightness determined! cl'I.
cl fromD thed3 'balanceofeach Impresslon of bnghtness. That
0'1 i E1 c2
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: c2jimpressiono:'is,d3theoloverall
could be explained on the
3 Dar D3 ; psychological
average of each portion. In comparison
D3 i i i= with, the space
i is illuminated uniformly, even if its> reflectivityi is
not uniform, separate impression of
CQ

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30 40 brightness
not 120
be exist. So, the overall impression of
50 60 70 would
80 90 Ioo
P edicted overaR impression of b ightness brightness would be explained on the quantitative averageof luminance.

Flg 1 1 Relatlonship between the caiculated
impression of brightness and evaluatedvalue for the impression of brightness
impressien ef (righ n es

Yerage ivminaneeee eN port;enthe port:ien, ad ptation lum{n nee oNe eye ,Non‑VnitOtm
i,e' s+, ef

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Rg.1 2 Ove all imptession of brightness in non‑uniform luminance spaces

The llluminatin.g Engineering Institute of JapanS @e$sheho aee,d8 tatien lurnj

ee O{ the eye )41

J. Light & Vis. Env. Vol,22, No.1, 1998

4 Conclusion

Ref erences

In this study, two experiments were conducted to assess(1) Shigeo Kobayashi, Yoshiki Nakamura and Masao

impression of brightness of whole spaces those were
illuminated non‑uniformly. In the first experiment,

Lighting Environment, CIE Proceedings of 23ed

luminance distribution images of an interior spac wereused as stimuli. In the second experiment, real interior
spaces of non‑uniform lighting were used as stimuli andresults obtained approximately supported the analysis of
the first experiment.

The experiment showed that the greater the degree of
non‑uniform illumination, the darker the interior space as awhole was evaluated, despite equal average luminance.
The overall impression of brightness of the interior spacecannot be explained on the basis of the average luminanceof the interior space, however it was found that the
impression of brightness for each level of illumination in

Inui : The Assessment of Brightness in the Interior
Session, pp.264‑265, 1995

(2) Gilchrist, A. L, and Jacobsen, A. : Perception oflightness and illumination in a world of one
reflectance, Perception, Vol, 1 3, pp.5‑19, 1984
(3) Flock, H. R, and Nusinowitz, S. : Visual structures for

achromatic color perceptions, Perception andPsychophysics, Vol.8, pp, 129‑1 36, 1984

(4) A.J. Shepherd, W.G. Julian and A.T. Purcell : Gloom
as a psychophysical phenomenon, Lighting Research
and Technology, Vol.21, pp.89‑97, 1989
(5) Tiller D K, Veitch J A : Perceived room brightness :Pilot study on the effect of luminance distribution,

the interior space resembled the psychological average.
While the average luminance is a calculated average, theLighting Research & Technology, Vol.27, N0.2,

perceived average is a value close to the geometric(6) Yoshiki Nakamura and Masao Inui : How to Express
Luminance Distribution, Proceedings of 8th LUX

average. As the latter is always less than̲ the calculated
average, the impression of brightness for a non‑uniformly
illuminated space cannot be explained in terms of averagevalue of luminances.

pp.93‑101, 1995

EUROPE. Vol.2, pp.9 1 7‑920, 1 993(7) Stevens, J.C.. Stevens, S.S. : Brightness Function
Effect of Adaption, Journal of the Optical Society ofAmerica, Vol.53, pp.375‑385, 1963

The I!haninating Engineering Instittite oj'Japan