CIE 95-1992 Contrast and Visibility (1st Edition) (E)《对比度和能见度》.pdf
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1、 CIE 95 72 I 9006345 0004480 357 = ISBN 3 900 734 32 1 COMMISSION INTERNATIONALE DE LCLAIRAGE I NT ER NAT I O NA L C O M M I SS 1 O N O N 1 LLU M I NAT I ON INTERNATIONALE BELEUCHTUNGSKOMM ISSION CONTRAST AND VISIBILITY Pub. No. CIE 95 Ist Edition 1992 UDC: 159.931 Descriptor: Vision, psychology 61
2、2.843.63 Capacity of recognition 61 2.843.7 Visual perception, contrast, visibility CIE 95 92 I 9006145 0004481 273 ii This Technical Report has been prepared by CIE Technical Committee 1-1 7 of Division 1 Vision and Colour and has been approved by the Board of Administration of the Commission Inter
3、nationale de IEclairage for study and application. The document reports on current knowledge and experience within the specific field of light and lighting described, and is intended to be used by the CIE membership and other interested parties. It should be noted, however, that the status of this d
4、ocument is advisory and not mandatory. The latest CIE proceedings or CIE NEWS should be consulted regarding possible subsequent amendments. Ce rapport technique a 626 prpar par le Comit Technique CIE 1-17 de la Division 1 Vision et couleur et a t approuv par le Bureau dadministration de la Commissio
5、n Internationale de IEclairage, pour tude et application. Le document traite des connaissances courantes et de lexprience dans ie domaine spcifique indiqu de la lumire et de 16clairage, et il est tabli pour lusage des membres de la CIE et autres groupements intresss. II faut cependant noter que ce d
6、ocument est indicatif et non obligatoire. Pour connaitre dventuels amendements, consulter les plus rcents comptes rendus de la CIE ou le CIE NEWS. Dieser Technische Bericht ist vom CIE-Technischen Komitee 1-17 der Division 1 Sehen und Farbe ausgearbeitet und vom Vorstand der Commission International
7、e de IEclairage gebilligt worden. Das Dokument berichtet we will not consider chromatic discrimination and temporal variation of stimuli. 1.2 Definitions of contrast The contrast of a target is a measure of the difference in luminance between the target and its immediate surround or background. Ther
8、e exist different defini- tions of contrast depending on the shape of the targets. For simple, isolated tar- getson a uniform background, e.g. a disk or a Landolt ring, the (psychometric) contrast is given by the expression I c=t- Lt - L, Lb (1.11 where C = contrast Lt = target luminance Lb = backgr
9、ound luminance. Here the difference of neighbouring luminances is related to the background lu- minance to which the observer should be adapted. Eq. (1 .l) has a physiological meaning for the detection of threshold contrasts in visual psychophysics where the just perceptable luminance increments are
10、 studied. In the Weber-Fechner re- gion C is constant. The contrast definition in Eq. (1.1) leads to contrast values that range from O to infinity for targets brighter than their background and from O to 1 for targets that are darker than their background. The problem of an asymmet- ric range of con
11、trast values is avoided if the difference between neighbouring lu- minances is related to their sum or mean (apart from a factor 2). The relative contrast sensitivity -4- c VL= - c is designated as the visibility level. It would represent the ratio of actual contrast C and threshold contrast c.The t
12、hreshold contrast is usually taken as the contrast when P = 0.5 where P is the probability of the target being seen. For spatialty periodic stimuli like sine gratings, another contrast definition is com- monly used: IL, - L,I m= t Lb (1.3) which leads to contrasts that range from O to 1 for all cond
13、itions. In Eq. (1.3) Lt and Lb have the same meaning as above. The expression (1.3) has a physiological meaning if the adaptation luminance is governed by the mean luminance of target and background as could be possible for spatial periodic stimuli, e.g. a grating target. For such targets the expres
14、sion (1.3) defines also the modulation. Between Eq. (1 .l) and Eq. (1.3) there exist the following relations m = C/(2 + C) for targets brighter than their background, and C = 2m/(l - m) m =C/(2 - C) and C = 2m/(l + m) for targets darker than their background. The contrast of characters on active dis
15、plays, e.g. VDUs, are usually defined by the ratio of two K= - Lb for L, Lb for Lb Lt b K= - L, (1.5) where Lt and Lb are defined as above. So far, there is no accepted specification of the location and spatial extent of the area where the target luminance Lt should be measured. The luminance modula
16、tion of rasters istheoretically and experi- CIE 95 92 U 9006345 0004488 648 -5- mentally analysed by outer and mean contrast of characters by the following expressions: resulting in the definition of local inner, local nr outercontrast Ka = - L Lb =2 innercontrast Ki = - (1.6) (1.7) where Lm = maxim
17、al luminance of a bright character L2, L1 = luminances of two adjacent raster points. The inner contrast of critical detail is one of the determinants for the effect of contrast on visual identification tasks. The size of the areas, where the luminances Lm, L2 and Li should be measured, is only one
18、picture element (pixel). The mean contrast K is defined by Eq. (1.4) and (1 -5) where Lt is the mean luminance of the whole character. Fig. 1 - 1 shows an ex- ample of a raster character. The luminance profile through the middle part of the character M is depicted in the lower half of Fig. 1 .l. Her
19、e the luminances Li, L2, Lm and Lb are localized. The mean contrast K is 15. On the basis of detection, identification and search experiments on VDUs it is shown by that the inner local contrast of characters is the deter- minant for the effect of contrast on visual performance and acceptance. 1.3 D
20、etection probability per glimpse The threshold contrast in Eq. (1.2) has to be measured under reference condit- ions according to CIE Publ. 19.21.1. But one gets a fluctuation of the threshold con- trast e from observation to observation.Therefore one has to assume a probabili- ty P of the target be
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