Luminance contrast, not luminosity contrast

[svgeesus]

The FAST checklist uses the incorrect term "luminosity". This is a common mistake, but is incorrect. The correct term (and the one used in WCAG) is "luminance".

Wikipedia has an accurate definition of luminosity, which is a term used in astronomy:

Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time.[1][2] In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object.[3][4]

In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, L⊙.

Notice that this is total electromagnetic energy, unrelated to whether that is visible light or some other energy (infrared, ultraviolet, etc) and unrelated to human perception.

The Wikipedia definition of luminance is okay, but hides the relevant difference away under luminous intensity:

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

The crucial point here is that the value is weighted for human perception, so green and yellow light makes more contribution than orange, red, blue, and violet.

I suspect that the confusion comes from the term luminosity function in the definition? But the luminosity function is a graph of (human perceived) brightness vs. visible wavelength, and unrelated to the astronomical definition.

Anyway, please change

Luminosity and hue contrast can adapt to user requirements.

to

Luminance and hue contrast can adapt to user requirements.

and also change

The WCAG 2.0 luminosity contrast ratio

to

The WCAG 2.1 luminance contrast ratio

Notice that WCAG 2.0 and 2.1 both use the correct term, luminance:

L1 is the relative luminance of the lighter of the colors