Understanding Luminance Values

V-Ray V-Ray for 3ds Max General
1

I am trying to simulate the brightness (luminance) of a TV or monitor with a high degree of physical accuracy. The standard unit for screen brightness is nits. 1 nit = 1 cd/m^2.

I have a VrayLight (plane) set to a luminance value of 1.0 lm/m^2/sr. I could be wrong, but my understanding is this equals 1.0 cd/m^2, which equals 1.0 nits.

When I check the luminance value of the light in the frame buffer using the VrayLightingAnalysis render element, it is multiplied by a value of π (3.14) in units (cd).

Can someone connect the dots mathematically for me?

2

After further research, I may have found a clue. Luminous exitance (aka luminous emittance) is a lesser-used quantity, measured in lm/m^2. This is not to be confused with illuminance (lux), which is also measured in lm/m^2. Candela - Wikipedia

For a perfect Lambert emitter, Luminous exitance = Luminance (nits) * π. https://en.wikipedia.org/wiki/Lamber…_luminous_flux

So is the luminance mode of the VrayLightingAnalysis reporting Luminous exitance (lm/m^2), not Luminous intensity (cd) as the label in the render element dialogue box indicates?