Measurements of light refer to the intensity. These measurements are obtained by dividing either a power or a luminous flux by a solid angle, a planar area or a combination of the two. Intensity is the power transferred per unit area. When intensity is measuring waves the average power transfer over one period of the wave is used.
Intensity is found by taking the energy density at an arbitrary point in space and multiplying it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area. In the SI system, the unit is watts per meter squared.
For a point source of radiating energy in three dimensions where there is no energy lost to the medium, the intensity decreases in proportion to the distance from the object squared. Applying the law of conservation of energy we obtain the following equation
P = ∫I * dA
P is the net power radiated
I is the intensity as a function of position
In photometry and radiometry, intensity has different meanings. It is the luminous or radiant power per unit solid angle. This can cause confusion in optics where intensity can mean any of radiant intensity, luminous intensity or irradiant. This depends on the background of the person using the term.