Design Nomenclature / Light & Optics
HDR
Also called high dynamic range, HDR10, PQ, HLG, SDR
As imaging, storing luminance in floating point so nothing clips. As a display standard, a higher peak luminance, a perceptual transfer function — PQ or HLG — in place of a gamma curve, and a wider gamut, with values carried in absolute nits rather than relative to a reference white.
In practice
The phone camera's HDR button means neither: it is a multi-exposure merge, tone-mapped straight back down to SDR. Three unrelated meanings wearing one badge.
Not to be confused with
- Tone mappingThe transform fitting a scene's luminance range into what a display can show — compressing highlights, holding mid-tone contrast, and deciding where things clip. It is neither a colour correction nor optional: some curve is always applied, and choosing none means choosing a hard clip.
- Dynamic rangeThe ratio between the largest and smallest luminance a scene, sensor, or display can carry, usually quoted in stops of doubling. A real scene may span twenty; a good sensor fourteen; an SDR display under ten. Everything difficult about photography lives in that gap.
- HDRIA photographic capture of the whole sphere of incident radiance at a point, stored in floating point so the sun stays thousands of times brighter than the sky rather than clipping to white. Used as a source, it supplies direction, colour, softness, and reflection from a real location at once.
- GammaThe power-law transfer function between encoded code values and light — historically the CRT's own response near 2.2, now kept deliberately because it spends limited bits where the eye discriminates most finely. sRGB's is piecewise, with a linear toe, averaging roughly 2.2 overall. It is also one of the three things that fix an RGB space, with primaries and white point.