Definition of Luminance

[chrisn]

Originally posted by @svgeesus in #346:

Luminance is a photometric measurement of intensity per area, brightness is the perception of luminance.

Right (for the first part. The second is misleading unless we mention cube roots). Currently the definition in the spec is a bit weak:

luminance
perceived brightness of a colour
Note

Luminance and chromaticity together fully define a perceived colour. A formal definition of luminance is found at [COLORIMETRY].

Of course, if we define luminance correctly this then adds the problem that we need to define the terms photometric, luminous intensity, and photopic observer.

In terms of the average readership (who will not have purchased CIE 15, so isn't helped by our current reference) perhaps we could point to something like

https://colorusage.arc.nasa.gov/lum_and_chrom.php

and give a better, but still informal, definition? Something like

Luminance:
an objective measurement of the emitted light intensity, taking into account the sensitivity of the human eye to different wavelengths. See Luminance and Chromaticity or, for a formal definition, [[Colorimetry]].

@Myndex replied with:

I think the simplest definition:

• Luminance $L$ — a physical measure of visible light intensity.

Works for most purposes. If we add in the weighting for spectral sensitivity, then we need to futher clarify that lightness, darkness, and brightness are not part of that weighting.

A more complete, mini-definition:

• Luminance $L$ — a physical measure of visible light intensity.

  • Luminance is measured from a surface, expressed as candela per square meter (cd·m²).
  • Luminance is weighted per the spectral sensitivity of human vision.
  • Luminance is mathematically linear, as is physical light.
    • Therefore, Luminance is not proportional to the human visual perception of lightness, darkness, or brightness.

• Relative luminance $Y$ is proportional to physical values of luminance.

  • $Y$ is normalized to a range, such a 0 (black) to 100 (white).
  • The symbol $Y$ relates to CIE 1931 $XYZ$ and should not be confused with $Y^\prime$

• Luma $Y^\prime$ (Y prime) is a gamma encoded signal, used in some broadcast video systems.

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See also: https://cie.co.at/eilvterm/17-21-050