在看《Advanced Lighting And Materials With Shaders》里看到一段关于radiance几个关键字很方便的记法：
Irradiance is the total amount of energy received per unit area of surface.
Illuminance measures the amount of visible light energy in photometric terms, not included heat, and radiation.
Radiance is the measure of energy that is reflected by the surface.
Luminance is the measure of photometrically weighted light energy that leaves the surface, and is therefore the more interesting term in a graphics context.