This optical rule dictates that the light reflected by a perfectly matte object scales with the cosine of the viewing angle relative to the perpendicular axis of the surface. Because the apparent area of the surface decreases at the exact same rate as the intensity, the surface appears equally bright from any viewing angle. It is the mathematical foundation for calculating illuminance on angled surfaces and modeling diffuse reflections.
Calculate the illuminance on that tilted solar array using Lambert's cosine law.
A perfect Lambertian surface is a theoretical ideal, but materials like Spectralon or fresh snow come very close.
Related terms
Inverse square law
The rule that illuminance from a point source falls with the square of distance, so double the distance gives a quarter of the light.
Illuminance
The amount of light falling on a surface per unit area, measured in lux or foot-candles.
Luminous intensity
How much light a source sends out in one specific direction, measured in candelas.