High Dynamic Range (HDR) imaging techniques capture greater ranges of scene information, and attempt to convey that information with HDR displays. HDR imaging can improve the rendering of most scenes. Is the improved appearance caused by the increased luminance range and increased accuracy of display luminances? This paper examines the effect of increased luminance range on appearance in uniform color spaces. Our experiments show that increasing the luminance range of a transparent display by a factor of 500 has minimal effect on appearance. Two important image-dependent mechanisms are responsible for the small amount of change. First, intraocular scattered light, or veiling glare, limits the range of luminances on the retina. Second, human spatial processing, as seen in simultaneous contrast experiments, makes scatter limited retinal images appear to have higher contrast. These two image-dependent mechanisms work to counteract each other. Scatter acts to decreased the stimulus range on the retina, and spatial comparisons heighten apparent contrast. Both mechanisms are responsible for the observed small changes in the appearance range with large changes in luminance range.