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Lightness Contrast in CRT and Paper-And-Illuminant Displays

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Lightness Contrast in CRT and Paper-And-Illuminant Displays Perception & Psychophysics 1996,58 (2), 250-258 Lightness contrast in CRT and paper-and-illuminant displays TIZIANO AGOSTINI and NICOLA BRUNO University ofTrieste, Trieste, Italy The increased use of CRTmonitors for displaying and controlling stimuli in studies of surface color poses problems of comparability with data obtained with traditional paper-and-illuminant methods. A review of comparable studies using the two methodologies revealed that CRTstudies tend to re­ port larger contrast effects. To investigate factors that may be responsible for this difference, si­ multaneous lightness contrast was measured using both CRT and paper-and-illuminant presenta­ tions. The spatial distribution of luminance in the whole field of view and the visual angles subtended by the displays were controlled. The CRTpresentation yielded contrast effects twice as big as those measured for a paper surface in a homogeneously illuminated room. However, a paper display under Gelb lighting yielded almost exactly the same effect size as that measured in the CRTpresentation. These results demonstrate that contrast effects in both modes of presentation are affected by the spa­ tial distribution of luminance beyond the basic experimental stimuli. Increasingly, investigators ofachromatic surface color In simultaneous lightness contrast (SLC), two achro­ (lightness) have been reporting experiments based on matic surfaces are presented side by side on black-and­ judgments of surfaces simulated on CRT monitors white backgrounds. The surfaces have the same reflec­ (Agostini & Proffitt, 1993; Arend & Goldstein, 1987a, tance, but they appear to be different. The surface 1987b; Arend & Reeves, 1986; Arend & Spehar, 1993a, surrounded by black appears to be lighter; that sur­ 1993b; Bruno, 1994; Kingdom & Moulden, 1991; Para­ rounded by white, darker. In most studies ofcontrast, the diso & Nakayama, 1991; Schirillo, Reeves, & Arend, size ofthis difference (the contrast effect) is reportedly 1990; Schirillo & Shevell, 1993a, 1993b; Shevell, Hol­ rather small. A number of paper-and-illuminant studies liday, & Whittle, 1992; Whittle, 1992). Using computer of the standard contrast arrangement have reported ef­ simulations involves careful control ofthe spatial distri­ fects on the order ofabout 1 Munsell unit (Gibbs & Law­ bution of monitor intensities to evoke visual responses son, 1974; Gilchrist, 1988; Gilchrist, Delman, & Jacob­ equivalent to those evoked by surfaces and illuminant. sen, 1983; White, 1981), In studies ofcontrast involving However, perfect equivalence is hard to achieve in prac­ a greater degree of spatial articulation, even weaker ef­ tice. This difficulty has consequences for the compara­ fects have sometimes been reported. 1 For instance, Agos­ bility of CRT data with data obtained with paper-and­ tini and Gerbino (1989) placed middle gray surfaces in a illuminant methods. What differences in the overall context made of disks on a homogeneous background. intensity distributions can be allowed without affecting The disks were either white on a black background or comparability? What characteristics of CRT monitors black on a white background. The whole display was and ofpaper-and-illuminant displays make it difficult to mounted on a curved surface and placed in a viewing equate corresponding spatial distributions ofluminance? box. Observers viewed it through an aperture so that the These are empirical questions, and answers to them are display filled their field of view. Observed contrast ef­ critical to the development ofthe field. Yet, to this date fects were on the order of0.5 Munsell units. Only a few no published work has addressed them. In this paper, a paper-and-illuminant studies reported somewhat larger first step is made in this direction by measuring one of contrast effects. For instance, Burgh and Grindley (1962) the most basic effects oflightness perception, simultane­ found average contrast effects of about 2 Munsell units ous contrast, in several comparable CRT and paper-and­ in the traditional bipartite arrangement. illuminant displays. How do these results compare to CRT measures of contrast? A first answer may be found in CRT studies that can be compared to corresponding paper-and­ illuminant studies either because they used displays that The authors are indebted to M. Braunstein, S. Shevell, and two were similar spatially or because ofcomparable levels of anonymous reviewers for helpful comments on an earlier draft, and to articulation. For instance, Bruno (1994) replicated pre­ A. Gilchrist for suggesting that we perform the present experiments. vious paper-and-illuminant experiments by Gilchrist This work was supported in part by CNR Grant 92.01065.PF67. Cor­ respondence should be addressed to T. Agostini, Department of Psy­ (1988) using a CRT simulation. The CRT display was chology, University of Trieste, via Universita 7, 34123 Trieste, Italy equivalent to Gilchrist's in terms ofluminances and spa­ (e-mail: [email protected]). tial relationships, but it subtended a smaller visual angle Copyright 1996 Psychonomic Society, Inc. 250 CRT VS. PAPER-AND-ILLUMINANT 251 than Gilchrist's and it was observed in an otherwise dark but are difficult to separate in practice. Ofchiefinterest room. Bruno found contrast effects as large as 2.5 Mun­ are the observation conditions for a monitor display. In sell units under conditions comparable to those where typical studies, CRT-simulated displays are presented in Gilchrist found effects of about 1 unit. In a number of a dark or dimly lit room. Thus, the monitor becomes an CRT studies, Arend and his collaborators placed center­ area of relatively high mean luminance, surrounded by surround displays similar to those used to demonstrate darkness or near darkness. Having the monitor as the SLC within the context oflarger achromatic Mondrians only source of light in the room is desirable for a num­ (for a summary of all these studies, see Arend, 1994). ber ofreasons. First ofall, turning offother sources ofil­ Typical displays for these studies did not fill the entire lumination eliminates the possibility of veiling light, field of view. However, because of the spatial articula­ allowing more precise control of display luminances. tion provided by the Mondrian surround, they could be Second, it prevents reflections from forming on the taken as somewhat comparable to those ofAgostini and screen, again allowing better control of luminance and Gerbino (1989). Arend reported overall deviations from increasing display realism. On the other hand, in many correct Munsell matches to be of the order of 1.0-1.5 paper-and-illuminant studies, observers are able to see units. In conclusion, it seems that CRT studies of con­ both the experimental display and the surfaces surround­ trast report somewhat larger effects than comparable ing it. Thus, once the whole field of view is taken into paper-and-illuminant studies. Candidate explanations account, paper-and-illuminant displays may involve a for this unexpected difference are naturally grouped into rather different overall spatial distribution ofluminance three categories. than CRT displays even if the luminances of the SLC The first category consists of factors that cannot be stimulus are carefully equated. separated in principle from the method of presentation. There are reasons to suspect that observation condi­ For instance, a monitor is a light-emitting device, whereas tions may be an important factor in accounting for the surfaces reflect light from the illuminant. However, this observed difference in SLC effects between CRT and difference should be irrelevant as long as the spatial dis­ paper-and-illuminant displays. For one, it is instructive tribution of light at the eye is kept the same. More con­ that Burgh and Grindley (1962) reported unusually large cretely, most monitors have intensity ranges of about contrast effects despite their using a paper-and-illuminant 2 logarithmic units and resolution of 256 discrete gray method. In their experimental setup, the display was sur­ levels. It is possible that range, small imperfections in rounded by an otherwise dark field so that their observa­ monitor calibration, or both, contribute in some unknown tion conditions closely mimicked those that are typical way to increasing contrast effects. With paper-and­ ofCRT studies. Another factor may be lightness effects illuminant displays, on the other hand, it is often quite observed on surfaces placed under a strong beam oflight hard to achieve complete control ofthe spatial distribu­ but otherwise surrounded by darkness. Gelb (1929) tion ofthe illumination and to measure it accurately. Un­ showed that a black surface appears white ifit is illumi­ wanted and unnoticed spatial variation of the illumina­ nated in such a way that the border of the illumination tion, or failures to measure it to a satisfactory degree of coincides with the border ofthe surface and the rest of accuracy, may also affect measures of contrast in some the field is in darkness. That the surface appears white is unknown way. However, the effect ofthese factors should perhaps not too surprising, for the system has informa­ not be systematic across different studies. tion concerning only the relationship ofthis area ofhigh The second category consists offactors that are read­ luminance to the rest of the field, which is dark. How­ ily separated from the method of presentation, but tend ever, anybody who has tried demonstrating the Gelb ef­ to be correlated with it for reasons of convenience. For fect knows that these conditions are rarely attained in full. instance, monitor studies often tend to employ displays In most cases, one is able to cast a strong beam oflight that subtend smaller visual angles than do displays used on a black surface suspended in front of a wall, but the in paper-and-illuminant studies. Given that edge dis­ wall is also illuminated in part. Even in these conditions, tances are known to have a small but measurable effect however, the black surface appears almost white to most (Freeman, 1967; lund & Armington, 1975), it could be observers, a fact consistent with the speculation that the that this provokes a detectable increase in contrast.
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