Flux Density Irradiance is a measure of radiometric flux per unit area.
Illuminance is a measure of photometric flux per unit area.
In the diagram shown above, the lightbulb is producing 1 candela . A steradian is defined as the solid angle which, having its vertex at the center of the sphere, cuts off an area equal to the square of its radius. Note that as the beam of light projects farther from the source, it expands, becoming less dense. The Inverse Square Law defines the relationship between irradiance from a point source and the distance to the measurement surface. It states that the intensity per unit area varies in inverse proportion to the square of the distance between the source and the surface.