Eiji Ogawa

Appearance matching of radiographic images using lightness index

Appearances of images are closely related with the luminance dependence of human visual characteristics. Radiographic images are displayed on the CRTs with various luminance as well as on high luminance light-boxes. We studied a tone scale that can improve consistency in appearance among various devices with different luminance. It is likely that radiologists diagnose images based on the relation between the brightness of region of interest and that of surrounding area. Lightness is defined as a relative brightness of region of interest compared with the maximum luminance level of the image. We think the lightness index can be applied for realizing the appearance matching of radiographic images. Lightness matching can be realized by displaying images with the tone scale which gets agreement of the gradients of the display tone scale, on the logarithm of output luminance vs. input data level plane, among display systems. In this paper we call it a lightness-equivalent characteristic. We evaluated the appearance consistency of images displayed with the log-luminance linear tone scale, as realizing the lightness equivalent characteristic, compared with those displayed with the perceptual-linear tone scale. In evaluation the log-luminance linear tone scale gave almost the same appearance among devices with different luminance. On the other hand, the perceptual-liner tone scale gave lower visual contrast for images on the lower luminance device than the higher luminance device, which might have lead to observers perceiving as different appearances.