How can you have a scene linear reflectance greater than 100%?

Started 3 months ago | Questions thread
spider-mario Contributing Member • Posts: 958
Re: 100% of what?

alanr0 wrote:


There may be a standard, but I suspect the reference point corresponding to 100% scene luminance is somewhat arbitrary, and chosen by the camera manufacturer or the photographer to accommodate scenes which include parts which are much brighter than the intended average.

Perhaps the "reflectance" tag in your screen shot is simply sloppy terminology - but for all I know, it could be accepted usage in the video community.

As far as I know, the video community does generally use reflectance for reference levels. See for example page 6 of BT.2408-4:

It is important to know the reflectance of greyscale charts and white cards, to ensure that cameras are aligned to deliver the appropriate signal level and consistency in production.

An 18% grey card is commonly used for camera set-up in non-live workflows as it is the closest standard reflectance card to skin tones. It may also be useful when trying to match SDR and HDR cameras as the 18% grey should not be affected by any SDR camera “knee”.

A 75%HLG or 58%PQ marker on a waveform monitor, representing the reference level, will help the camera shader ensure that objects placed at the centre of interest within a scene are placed within the appropriate signal range, and that sufficient headroom is reserved for specular highlights.

And page 5:

The reference level, HDR Reference White, is defined in this Report as the nominal signal level obtained from an HDR camera and a 100% reflectance white card resulting in a nominal luminance of 203 cd/m² on a PQ display or on an HLG display that has a nominal peak luminance capability of 1000 cd/m². That is the signal level that would result from a 100% Lambertian reflector placed at the centre of interest within a scene under controlled lighting, commonly referred to as diffuse white¹. There may be brighter whites captured by the camera that are not at the centre of interest, and may therefore be brighter than the HDR Reference White.

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