Image contrast and dynamic range in a projector or display refer to the ratio of brightest white to deepest black. These parameters can be a useful measure of image quality, but are not the only factor for determining quality.
The two images below are examples of good contrast and low contrast. A histogram is included with each photo to bring attention to the visual impact of high black level in low dynamic range displays. (The low end of the “X” axis represents black level.) The images underscore the importance of deep blacks in addition to peak whites in displays. Simply increasing brightness of a low dynamic range projector or display will wash out the image, a concern that is particularly important when displaying High Dynamic Range (HDR) content.
Figure P-4. High Contrast
Figure P-5. Low Contrast
Contrast is measured in multiple ways. Sequential contrast is defined as the ratio of peak white to black, and tends to be limited by the image formation technology of the projector or display. Intra-frame contrast is measured using a checkerboard pattern, and can be impacted by numerous optical anamolies, such as lens flare or even port glass flare. SMPTE RP 431-2 Reference Projector calls for a nominal ratio of 2000:1 for sequential contrast in projectors, with a minimum 1200:1 sequential contrast for cinemas. The SMPTE Recommended Practice also calls for a nominal intra-frame contrast of 150:1, with a minimum 100:1 intra-frame contrast for projectors in cinemas.
SMPTE RP 431-2 Reference Projector is a useful reference for many parameters concerning picture presentation, including the cinema reference white level of 48 nits and reference white chromaticity coordinates.
It should be mentioned that the contrast numbers quoted above target standard cinema picture presentation, not HDR Cinema picture presentation. HDR standards for cinema have not yet emerged at the time of this writing. But HDR standards for home entertainment do exist. The UHD Alliance HDR10 specification sets two standards: (1) more than 1000 nits peak white and less than .05 nits black, and (2) more than 540 nits peak white and less than .0005 nits black. Specification (1) has a sequential dynamic range of 20,000:1, while specification (2) has a dynamic range of 1,080,000:1.
As a step towards establishing appropriate light levels for HDR Cinema, the American Society of Cinematographers (ASC) released its Cinema Display Evaluation Plan & Test Protocol to guide future research.