CIE-95-1992.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 Copyright International Commission on Illumination Provided by IHS under license with CIELicensee=IHS Employees/1111111001, User=leee, leee Not for Resale, 11/24/2007 08:04:00 MSTNo reproduction or networking permitt
3、ed without license from IHS -,-,- 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 Internationale de IEclairage for study and application.
4、 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 document is advisory and not mandatory. The latest
5、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 Commission Internationale de IEclairage, pour tude e t appl
6、ication. Le document traite des connaissances courantes et de lexprience dans ie domaine spcifique indiqu de la lumire e t de 16clairage, e t il est tabli pour lusage des membres de la CIE et autres groupements intresss. II faut cependant noter que ce document est indicatif e t non obligatoire. Pour
7、 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 Internationale de IEclairage gebilligt worden. Das Dokument
8、 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. There exist different defini- tions of contrast de
9、pending 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 = background luminance. Here the difference of neighbo
10、uring 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 studied. In the Weber-Fechner re- gion C is c
11、onstant. 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 contrast values is avoided if the difference betw
12、een neighbouring lu- minances is related to their sum or mean (apart from a factor 2). The relative contrast sensitivity Copyright International Commission on Illumination Provided by IHS under license with CIELicensee=IHS Employees/1111111001, User=leee, leee Not for Resale, 11/24/2007 08:04:00 MST
13、No reproduction or networking permitted without license from IHS -,-,- -4- c VL= - c is designated as the visibility level. It would represent the ratio of actual contrast C and threshold contrast c.The threshold contrast is usually taken as the contrast when P = 0.5 where P is the probability of th
14、e 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 conditions. In Eq. (1.3) Lt and Lb have the same meaning as above. The expression (1.3) has a physi
15、ological 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 expression (1.3) defines also the modulation. Between Eq. (1 .l) and Eq. (1.3) there exist the follow
16、ing 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 displays, e.g. VDUs, are usually defined by the ratio of two K = - Lb for L, Lb for Lb Lt b K = -
17、L, (1.5) where Lt and Lb are defined as above. S o 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 modulation of rasters istheoretically and experi- Copyright International Commission on Illuminati
18、on Provided by IHS under license with CIELicensee=IHS Employees/1111111001, User=leee, leee Not for Resale, 11/24/2007 08:04:00 MSTNo reproduction or networking permitted without license from IHS -,-,- CIE 95 92 U 9006345 0004488 648 - 5 - mentally analysed by outer and mean contrast of characters b
19、y 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 = maximal luminance of a bright character L2, L1 = luminances of two adjacent raster points. The inner contrast of critical detail is one of the de
20、terminants 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 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.
21、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. Here the luminances Li, L2, Lm and Lb are localized. The mean contrast K is 15. On the basis of detection, identification and search experiment
22、s 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 Detection probability per glimpse The threshold contrast in Eq. (1.2) has to be measured under reference condit- ions according to CIE Publ.
23、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 being seen. P = 0.5 usually defines the threshold contrast e. For a presentation time of approx. 0.3 sa probability per qlimpse P , can be def
24、ined be- cause this presentation time simulates a single fixational pause. A plot of Pg as a function of VL or log VL is usually called a frequency-of-seeinq curve. The visibility VL is defined by Eq. (1.2). A great number of such frequency-of-seeing curves have Copyright International Commission on
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