The dynamic range of real world scenes may vary from around 102 to greater than 107, whilst the dynamic range of monitors may vary from 102 to 105. In this paper, we investigate the impact of the dynamic range ratio (DRratio) between the captured scene and the displayed image, upon the value of system gamma preferred by subjects (a simple global power law transformation applied to the image). To do so, we present an image dataset with a broad distribution of dynamic ranges upon various sub-ranges of a SIM2 monitor. The full dynamic range of the monitor is 105 and we present images using either the full range, 75% or 50% of this, while maintaining a fixed mid-luminance level. We find that the preferred system gamma is inversely correlated with the DRratio and importantly, is one (linear) when the DRratio is one. This strongly suggests that the visual system is optimized for processing images only when the dynamic range is presented correctly. The DRratio is not the only factor. By using 50% of the monitor dynamic range and using either the lower, middle or upper portion of the monitor, we show that increasing the overall luminance level also increases the preferred system gamma, although to a lesser extent than the DRratio.
The dynamic range (DR; defined as the range of exposure between saturation and 0 dB SNR) of recent High Dynamic Range (HDR) image sensors, can be extremely high: 120 dB or more. But the dynamic range of real imaging systems that include lenses is limited by veiling glare (susceptibility to flare light from reflections inside the lens), and hence rarely approaches this level. Standard veiling glare measurements, such as ISO 18844, made from charts with black cavities on white fields, yield numbers (expressed as a percentage of the pixel level in nearby light areas) that are much worse than expected for actual camera dynamic range. Camera dynamic range is typically measured from grayscale charts, and is strongly affected by veiling glare, which is a function of the lens, chart design, and the surrounding field. Many HDR systems employ tone mapping— which enables HDR scenes to be rendered in displays with limited dynamic range by compressing (flattening) tones over large areas while attempting to maintain local contrast in small areas. Measurements of tone-mapped images from standard grayscale charts often show low contrast over a wide tonal range, and give no indication of local contrast, which is especially important for the automotive and security industries, where lighting is uncontrolled and the visibility of low contrast features in shadow regions is critical. We discuss the interaction between veiling glare and dynamic range measurements and we propose a new transmissive test chart and dynamic range definition that directly indicates the visibility of low contrast features over a wide range of scene brightness.