High Dynamic Range imaging: is the game worth the candle?

High dynamic range imaging (HDRi) is a technology concerned with representing a range of luminances larger than state of the art displays and closer to luminance ranges occuring in natural scenes. We investigate whether the approach of evaluating a dynamic range by the number of just noticeable differences contained within does make sense in a HDRi workflow. We found that disturbing effects of neighboring luminances were hardly perceivable on a standard dynamic range display if the background luminance exceeds 5 cd/m2. Introduction Representation in imaging media is limited by the spatial resolution, the temporal resolution and the sets of available colors, named gamut. Since the human visual system has a higher ability to distinguish luminance differences than chromatic differences on a spatial scale [1, 2, 3], the luminance information transfers the structure of an image representing a scene [4, 5]. The luminance range of an image is defined by the difference of the highest to the lowest luminance contained in it, while luminance contrast is defined by ratios of them. Contemporary standard displays achieve peak luminances between 80 and 400 cd/m2. In print, the reflectance of the paper can be reduced to 0.5% to 10% , depending on the paper type. Within this dynamic range, a number of luminance steps can be made out by the human visual system. The number of these luminance steps is the dynamic resolution of a visual system. The number of representable levels in an imaging system should be higher than the dynamic resolution, levels are often represented in the binary system, e.g. 256 representable levels are expressed as log2(256) = 8bit (Figure 1). Distinction of gray levels is of particular interest in medical applications, since many medical imaging methods (tomography, mammography) deliver achromatic pictures only [6]. Taking the data on which the DICOM standard is based and assuming a peak luminance of 4000 cd/m2, approximately 210 = 1024 gray levels can be simultaneously differentiated under optimal conditions [7]. However, in images, neighboring high intensity regions can deteriorate the contrast detection performance. This is due to glare effects in the eye [8, 9]. Reflections of ambient light at a display surface can significantly influence contrasts in low intensity regions [10]. Since contrasts in low intensity regions of an image tend to be of importance for the diagnostic purpose, the studies of medical displays therefore often use very low luminance levels of background intensity and room illumination [9]. Glare effects have been quantitatively investigated in the first half of the 20th century with the equivalent background technique [11, pg. 578]. The Stiles-Holladay equation states that the influence of a point glare source E at eccentricity θ on an increment contrast threshold for a background luminance L leads to an equivalent increment contrast threshold as a background luminance β : β = L+10 E θ 2 , θ > 0.5deg . (1) Assuming independent contributions, glare sources are additive. The Stiles–Holladay equation has later been extended for age effects, influence of ocular pigmentation and eccentricity angles larger than 30◦ [8, 12]. We were not interested in adding to this theory, but to test whether glare effects can be reproduced on displays when background luminance exceeds 5 cd/m2. For this, we simulated a standard dynamic range display with a peak luminance below 200 cd/m2 on a high dynamic range display with a peak luminance exceeding 2000 cd/m2. Figure 1. A particular spatial arrangement of the 256 luminance steps available in 8bit sRGB. While one should hardly see any horizontal borders between adjacent squares represented in print or on standard displays, almost each border is clearly visible on a display with a high luminance range when the maximal luminance is chosen as a white point. 21st Color and Imaging Conference Final Program and Proceedings 231