在看《Advanced Lighting And Materials With Shaders》里看到一段關于radiance幾個關鍵字很方便的記法：
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.
前兩者是關于表面接收的光的屬性，后兩者則是表面反射出的光的屬性。
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.