Luminance is luminous intensity per unit area or equivalently illuminance per solid angle, and is measured in nits (or, to use standard SI terminology, candelas per square meter - the two terms are equivalent). Luminance is often used to describe the brightness of a particular surface as viewed from a particular direction; for example, a computer monitor might be described as having a brightness of 300 nits (but that would likely only be true when viewing straight on instead of at an angle). See here for some common approximate luminance values.
Luminance
Luminance.Nits
nits : Basics.Float -> Luminance
Construct a luminance value from a number of nits. One nit is equal to one candela per square meter, or equivalently one lux per steradian.
inNits : Luminance -> Basics.Float
Convert a luminance value to a number of nits.
footLamberts : Basics.Float -> Luminance
Construct a luminance value from a number of foot-lamberts.
inFootLamberts : Luminance -> Basics.Float
Convert a luminance value to a number of foot-lamberts.