The ray showing 50% is at the angle 30° from the surface where cos 60° = 0.5. For an ideal lambertian surface the reflection will follow the cosine law. Reflection from a surface can be complicated by surface roughness. The intensity or pointance from such a surface in any direction is proportional to the cosine of the reflected angle. For example, an evenly illuminated flat surface such as a sheet of paper is approximately lambertian in that the luminance that you see from any direction is essentially the same. It behaves according to the inverse square law.Ī flat surface that reflects or emits equal luminance in every direction from the surface is said to be a lambertian surface. Unit illustrationįor a point source, the emitted light intensity or pointance is the same in all directions, or isotropic. In the luminous case it is measured in lumens/m 2 steradian which is equivalent to candela/m 2 = nit. In the radiant case it is measured in watts/m 2 steradian and is also called radiance. The power per unit area per unit solid angle is sometimes called sterance.

Otherwise, for a flat radiating surface, known as a lambertian, the intensity falls off as the cosine of the observation angle with respect to the surface normal. If the intensity ( I = dΦ/dω ) of a source is the same in all directions, the source is called isotropic. For visible light it is expressed in lumens per steradian = candela. In the case of radiant power, it is expressed in watts per steradian.

It expresses the directionality of the radiated energy and is appropriate for the description of point sources. The power (flux) per unit solid angle (sometimes called pointance) is the nearest precise terminology to the common term intensity.

Intensity of light equation power