THE INVERSE SQUARE In Figure 7.5, the source of illumination has a luminous intensity of
LAW /candela. The level of illumination at surface A
is due to / lumens
on 1 square metre since the solid angle is 1 steradian.
The light rays are all meant to be at right angles to the surface being
illuminated. If the surface A
is removed so that all the light rays
which illuminated it now travel two metres to illuminate surface A
the luminous flux has spread over four square metres. There are
now /lumens on 4 square metres.
E = —