Measuring the impact of flare light on Dynamic Range

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.