Philips Lighting Pakistan - The cosine law

The cosine law

The illuminance at a point in a plane not perpendicular to the direction of the luminous intensity is equal to the luminous intensity in the direction of the point, divided by the square of the distance between the light source. It has to be multiplied by the cosine of the angle g that the direction of light incidence makes with the perpendicular to the plane (Fig. 8). Thus :

This is called the Cosine law.

For horizontal surface, it will often be more practical to modify the above formula by replacing the distance (d) between the light source above the surface, as a function of its height h.

If the ray of light is not reaching perpendicular to the plane but at an angle g, the result is the formula.

Vertical illuminance

On similar reasoning, the vertical illuminance at that point will be