The cosine law, also known as Lambert’s cosine law or the cosine emission law, is a fundamental principle in the lighting industry. It states that the radiant intensity from an ideal diffusely reflecting surface is directly proportional to the cosine of the angle between the direction of incident light and the surface normal. This law, named after Johann Heinrich Lambert, is studied in optics and plays a crucial role in understanding how light is distributed and how it affects the illumination of a surface.