Flux, expressed in watts, is a measure of the rate of energy flow, in joules per second. Since photon energy is inversely proportional to wavelength, ultraviolet photons are more powerful than visible or infrared. Luminous flux is a measure of the power of visible light. Photopic flux, expressed in lumens, is weighted to match the responsivity of the human eye, which is most sensitive to yellow-green. Scotopic flux is weighted to the sensitivity of the human eye in the dark adapted state.
Illuminance is a measure of photometric flux per unit area, or visible flux density.
In figure 7.4, 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. The number of steradians in a beam is equal to the projected area divided by the square of the distance. Note that as the beam of light projects farther from the source, it expands, becoming less dense. Cosine Law Irradiance measurements should be made facing the source, if possible.