• Aucun résultat trouvé

Visual performance, visibility and apparent contrast

N/A
N/A
Protected

Academic year: 2021

Partager "Visual performance, visibility and apparent contrast"

Copied!
8
0
0

Texte intégral

(1)

Publisher’s version / Version de l'éditeur:

Lighting design + application : LD + A., 7, 11, pp. 23-26, 1977-11

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Visual performance, visibility and apparent contrast

Levy, A. W.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=7cfccbd8-55e6-423d-b431-ca8b4fa35080

https://publications-cnrc.canada.ca/fra/voir/objet/?id=7cfccbd8-55e6-423d-b431-ca8b4fa35080

(2)

Ser

TH1

R l d

no.

755

bo. 2

National Research Council of Canada

m D G

Conseil national de recherches du Canada

VISUAL PERFORMANCE, VISIBILITY, AND

APPARENT CONTRAST

by A. W. Levy

Reprinted from

Lighting Design and Application

Vol.

7,

No. 1 1, November 1977

p. 23-26

DBR Paper No. 755

Division of Building Research

(3)

SOMMAIRE

L'article porte sur l'utilisation d'Ctudes de visi-

bilitC et de performance visuelle pour la conception

de l'kclairage, 1'Ctablissement des niveaux d'C-

clairement et 1'Cconomie de I'Cnergie. On Ctablit une

distinction particulibre entre les Ctudes sur le

contraste apparent ou subjectif et les Ctudes sur la

performance servant

i

mesurer la vitesse et la prC-

cision. On propose aussi un usage prCcis du terme

"visibilitC," ce qui entraine une nouvelle classifi-

cation des niveaux de visibilite; soit les niveaux

subliminal et supraliminaire et la saturation. On

explique en dCtail la terminologie relative

i

la

visibilitC et i la performance visuelle en vue de

mettre fin

2

la confusion qui a jusqu'i prCsent en-

tour6 ce sujet. On Ctablit la distinction entre les

paradigmes de visibilitC statique et dynamique tout

en soulignant la definition restreinte du terme

"niveau de visibilitC." Des Ctudes publikes sur la

constance des contrastes permettent d'expliquer les

rCsultats des Ctudes rCcentes du NBS sur la visibi-

lit&, que l'on a d'ailleurs proposC d'utiliser comme

autre base pour l'ktablissement des niveaux d'C-

clairement.

(4)

VISUAL PERFORMANCE

Visual performance,

visibility, and apparent

~ N A C Y Z E O

If

the NBS results are interpreted using present

knowledge of contrast constancy, there is no

contradiction between NBS results and those of

CIE Publication No.

19.

Each applies to a

different set of visual conditions-saturated

and

suprathreshold respectively

A. W.

Levy

Visual performance and visibility criteria offer a scientific and logical approach to lighting design and lighting energy con- servation. No longer is it desirable to design solely in terms of raw footcandles (lux) without first considering, in some detail, visibility and its effect on visual perfor- mance.

Since the appearance in 1972 of C I E Publication No. 19 (TC3.1), and for some years previous to that in North America, both enthusiasm and controversy have existed in the application of what have been

The author: National Research Council, Ottawa, Canada. This article is a contribution of the Div. of Building Research and is published with the approval of the director of the division.

[I] Sample accuracy curve: response probability data fitted by a normal curve.

Relative target contrast

LIGHTING DESIGN & APPLICATION / NOVEMBER 1977

mostly laboratory studies of visual per- formance to lighting design in buildings. Perhaps the most recent and fundamental criticism has arisen from the work a t the National Bureau of Standards (NBS), Washington, D.C.' For the sake of brevity the term "V.P. System" will be used, hereafter, to describe CIE Publication No.

19 and subsequent draft documents of CIE Technical Committee 3.1.

Few would criticize the human factor approach to artificial lighting design of- fered by visual performance, but legitimate concern has been expressed as to whether visual performance is the correct parame- ter to explore in detail when what is really being sought is a measure of total perfor- mance. Also, for practical application, a human factor approach must necessarily ignore or assume constant a number of important factors-e.g., intelligence, mo- tivation and fatigue. Some view this sit- uation with dismay, believing that these very factors are the most important to comprehend to obtain a satisfactory arti- ficially lighted interior.

It is suggested that a pragmatic ap- proach should be adopted. The V.P. Sys- tem is part of a rapidly growing science studying human behavior under various environmental conditions. Until a better alternative is produced, the V.P. System is the only scientifically based, engineering approach that takes into account human behavior under artificial lighting so as to maximize the effectiveness of the luminous environment for comfortable and accurate seeing. It is unreasonable to insist that this

(5)

performed under static viewing conditions. (This is how visibility levels are measured in the V.P. System and will be discussed fully later.) Instances of static viewing be completely understood before new lumination for optimum visibility now conditions are hard to find under normal methods can be introduced for practical means that the illuminating engineer must viewing circumstances.

use. possess more than a passing knowledge of Tasks requiring visual search and scan- One of the purposes of this article is to how eyesight is used for various visual ning or peripheral vision, or both, are per- indicate where the visual performance tasks. Unfortunately, there is a dearth of formed under dynamic viewing conditions. approach is most useful and appropriate to general information on this topic, and this Almost all of the most common and fre- use. Confusion over the terminology used is the main reason for the confusion con- quent visual tasks such as reading, writing, in the V.P. System has not only led to cerning the applicability of the visual per- typing, and tasks involving visual search controversy, but has seriously hindered formance criteria to prescriptive lighting would come under the category of dynamic useful discussions and implementation. levels. However, an engineering description viewing.

Because of this some of the most important of two separate types of viewing strategies Tasksinvolvingperipheralvisiononly-

parameters, visibility, threshold visibility, has emerged from the analysis of visual where no visual search and scanning take suprathreshold visibility, and visual'per- performance studies. Static and dynamic place-are included in the dynamic view- formance have been redefined and ex- viewing paradigms are distinguished2 that ing definition for good reasons. If sufficient plained. permit a clearer understanding of the time were available, or if sufficient time conditions under which the visual perfor- were allowed, the peripheral visual process mance methodology is most appropriate. could be substituted by one of search, scan,

Static and dynamic viewing Those visual tasks for which the observer and subsequent fixation. Peripheral vision Undoubtedly a major part of illumi- knows when and where to fixate, such as is only employed because of a self-imposed nating engineering should be renamed vi- the visibility reference task (a four-minute or externally imposed time limitation. If sion engineering. Providing the correct il- disc displayed for second), are said to be extra time were allowed to view a task under static viewing conditions, this would

1.0- w c m

E

a, a m '2 a, 2 + -

d

certainly lead to improved visual perfor-

- -

/I mance, but no search and scanning would

take place because by definition the subject knows when and where to fixate.

Visibility and apparent contrast 0 . 5 - - - -

i

Visibility is a word generally used to describe how well a scene or a particular

,

,

--,.

athreshold Saturi

---.--, object can be seen. For meteorological

I

purposes it can be measured as the greatest distance that selected objects or light

0 -

-

t

1

sources can be seen or identified under

Subthreshold particular conditions or observation. For

indoor situations, however, no such simple

V ~ s ~ b ~ l ~ t y level definition exists, and the reason is quite

[2] Three vie* conditions for visual performance: subthreshold, of what and for what'?

suprathreshold and saturation. In the meteorological application the object

is arbitrarily chosen as well as the condition

[3] Visual performance curve indicating static and dynamic thresh- of viewlng-the choice of these two ~ a r i -

olds. ables being fairly restricted by practical

1.0- 0.8 G) C 0.6

E

-

E - 0.4 '2 w -

-

m

-

B

0.2

A.e

" - . -

*~*/,-.+ T reality. In the case of indoor applications

there are a multitude of visual objects and

-

viewing conditions.

-

Fortunately the V.P. System brings some order to the chaos. Employing the

-

concepts of threshold visibility and equlv-

-

alent contrast provides a metric of v~sib~lity for indoor applications. It can be said that

Ad-

- - -

Thrcshoki

A is more vis~ble than B and by how much in terms of a contrast multiplier. Visibility

-

in this sense, however, is strictly in terms of

-

the visual performance that the particular task and viewing conditions will allow. In

-

other words, when the term visibility 1s

-

used in the V.P. System, visibility for per-

O V A

forming visual tasks is meant. Visibility in

this sense, therefore, has a restricted

'

L

-

L

meaning.

1 10 For those circumstances where no visual

Vlstb~l~ty level, VL performance is involved-there is no time LIGHTING DESfGN & APPLICATION 1 NOVEMBER 1977

(6)

limitation or levels of speed and accuracy involved-the term apparent contrast can be used as a metric of visibility. Apparent contrast is evaluated by asking a subject to adjust the contrast of a test object to be subjectively equal to that of a reference object. The inverse of luminance contrast is then taken as the apparent contrast. This is the type of experiment that was carried out a t NBS. In these experiments the subject is given unlimited time to perform the visual task and there is no score or measure of errors. How well a subject is performing cannot be assessed or mea- sured. In this sense visual performance is not involved; this is a visual scenario for which the V.P. System does not apply and was never intended to apply. Although it is not known exactly why (the dynamic range of the visual system may be involved), subjective or apparent contrast does not seem to be a good metric of visibility in relation to visual performance. That is, although task A may appear more con- trasty than task B under conditions of steady and unlimited fixation time, it does not follow that a higher level of perfor- mance can be achieved with task A . All the evidence so far is that visibility level is a better metric for performance and it has yet to be shown that performance ceases to improve up to a saturation value with in- creasing illuminance for a target of fixed contrast, all other things being equal.

Threshold and suprathreshold visibility

Threshold contrast is the value of con- trast when the luminance difference be- tween background and detail is so small that detail is barely perceptible: it is the minimum contrast that can be detected. There are a number of informational cri- teria suitable for threshold contrast de- termination but detection of presence is most suitable for the visibility reference task. It has become common practice to assign the term threshold to the value of the stimulus for which the resulting re- sponse is 50 percent correct. The justifi- cation for using the 50 percent criterion is that the frequency of detection (corrected for the rate of guessing) near threshold, as a function of increasing stimulus intensity, can be fitted by a normal probability inte- gral (Fig. [I]); it is convenient, therefore, (but arbitrary) to define the threshold condition as a probability of 0.5.

Problems in terminology arise when the frequency of detection moves above 50 percent but remains less than 100 percent.

The use of the term threshold is perhaps unfortunate in that it implies a step-func- tion transformation from threshold to su- prathreshold conditions; the shape of the is not so. The term suprathreshold is usu- ally applied to those values of stimulus or response above threshold. Furthermore, in vision and illumination research, it has generally been implied (and explicitly stated1) that suprathreshold conditions are those under which one normally operates in everyday viewing when relative response is always 100 percent. The recent work of Blackwel12 seriously questions these as-, sumptions. For most common visual tasks, under normal working conditions, people do not generally achieve 100 percent per- formance, but are operating in the range between 50 to 100 percent. N o confusion need exist if it were agreed that supra- threshold conditions describe only perfor- mance in this range (50 to 100 percent). At 100-percent performance and above, the term saturation could be applied to those values of stimulus for which there is no measurable improvement in response. The separate regimes are illustrated in Fig.

PI.

[4] Equal clarity contours as a functlon of mean spatial luminance for a 3.9 cpd square wave grating (from Yonemura and Kohayakawa, 1976).

Log ~ , ( c d / r n ~ )

(7)

In the V.P. System, stimulus is normally measured in units of visibility level (VL), which is the ratio of equivalent contrast to the value of the visibility reference func- tion, VLl, for a given luminance. Values of VL are measured under static viewing conditions. VLI, for instance, is the level of threshold visibility for static viewing of the visibility reference task. Values of VL greater than unity for this task alone mean that either suprathreshold or saturation conditions exist, but this will not neces- sarily be the case for other visual tasks.

For tasks where ocular search and scanning take place the parameter alpha

( a ) characterizes the dynamic threshold

for the task. Alpha is the logarithm of VL for which 50 performance is ob- tained. Thus the value of VL associated with a can certainly be greater than unity. Figure [3] summarizes the situation. The VL value that obtains a t the threshold condition for one task may correspond with suprathreshold or saturation conditions for another; it depends on the value of a .

It would appear that the definition of visibility level in CIE Publication No. 19

ought to be amended. In the glossary of terms and abbreviations, VL is defined as "a measure of suprathreshold visibility with reference to the standard visibility reference function, representing the nu- merical value of a contrast multiplier ap- plied to values of C,." With our proposed definition, VL is not a unique measure of suprathreshold visibility and will only de- scribe this state for certain tasks under certain viewing conditions.

NBS results

The NBS work involved subjects matching a test visual pattern to a refer- ence visual pattern of fixed contrast and spatial frequency. Equal clarity contours were generated such as those illustrated in Fig. [4]. The important feature of the contours is the minima a t relatively low light levels. The authors conclude the ex- istance of an optimum luminance level, a t which a minimum amount of contrast is needed to achieve a given degree of clarity. It is then inferred that the same clarity a t either a lower or higher luminance would require more contrast. Furthermore it was interpreted that an optimum luminance for

LIGHTING DE

suprathreshold visibility existed (their

meaning being normal everyday viewing), above which there is no improvement in visibility and therefore performance. If true, this would mean that an optimum il- luminance level existed for maximum vi- sual performance; the consequences for lighting energy conservation would be far-reaching. However, following the pre- vious discussion it is considered that ap- parent contrast measurements do not pre- dict correct values of visual performance. If the NBS results are interpreted as being applicable to nonperformance sit- uations, there is no contradiction between those results and the V.P. System. Each applies to a different set of visual condi- tions, saturated and suprathreshold re- spectively. For most practical lighting problems and especially energy conserva- tion, the V.P. System should be employed because most visual tasks in offices, schools, etc., are performed under supra- threshold conditions. The saturation state studies may be used for non-performance situations where the subjective appearance of artificially lighted interiors is most im- portant.

More precise use of the word visibility is required: the visibility of what and for what must always be clearly stated. The dearth of knowledge concerning the visual process for most common visual tasks se- riously hinders the correct and speedy ap- plication of the V.P. System. Much more effort must be expended in determining those tasks for which the V.P. System is most suitable to use to provide the correct and optimum illumination. Perhaps of all the stimulus variables-intensity, compo- sition, and spatial distribution-time du- ration may be the most important for vision engineering and the V.P. System. If this is so, then the categorization of visual tasks applicable to the V.P. System will be con- siderably easier.

REFERENCES

I . Yonemura, G . T. and Kohayakawa, Y., NBS Building Science Series 82. A new look at the research basis for lighting level recommenda- tions (1976).

2. Blackwell, H. R., Proceedings, the basis for effective management of lighting energy sym- posium, Federal Energy Agency, Washington,

D.C., p. 99 (1975).

(8)

This publication is being distributed by the Diviaion of Bullding R e a e a r c h of the National R e s e a r c h Council of

Canada. It should not b e reproduced i n whole o r in part without permimaion of the original publisher. T h e Di- vision would be glad to be of a s a i s t a n c a i n obtaining such permission.

Publications of the Division m a y b e obtained by m a i l - ing the a p p r o p r i a t e r e m i t t a n c e ( a Bank. E x p r e s s , o r P o s t Office Money O r d e r , o r a cheque, m a d e payable to the Receiver General of Canada, c r e d i t NRC) to the National R e s e a r c h Council of Canada, Ottawa. K1A OR6.

Stamps a r e not acceptable.

A l i s t of a l l publications of the Division i s available and m a y be obtained f r o m the Publications See tion, Division of Building Research, National R e s e a r c h Council of Canada. Ottawa. KIA OR 6.

Références

Documents relatifs

Figure 1: Examples of parts from a resulting input depth image (bottom row), with the corresponding parts from the reference color image (top row), showing the three issues

Whole area (large red polygon) and visible area (small green polygon) of the 3D object projection in the image plane. If too small the scheme may stay in IBVS mode. Similarly, a

It is estimated by a three-step processing of the pair of left and right stereo images: segmentation of the roadway on each image using color information, registration between

when the intensity λ of the underlying Poisson point process is a function λ(R) of the radius which goes to infinity when R → 0.. Let V λ,R be the total visibility associated with

In this paper we describe a unified data-structure, the 3D Vis- ibility Complex which encodes the visibility information of a 3D scene of polygons and smooth convex objects.. This

In previous global visibility algorithms, in particular those relating to aspect graph computations (e.g., [21, 10, 9]), and to antipenumbra [26] or discontinuity meshing [5,

right) vertex of the first (resp. last) face of the walk, but we just have to sweep them before (resp. after) the walk). We also see that vertices incident to f are swept correctly

(This plaza has no terrace café)---217 Table VI.9: Measurements of five isovist drawn from inside the limits of Zwaka plaza.----224 Table VI.10 Shows the number people of each type