The light emitted by a more typical fluorescent tube consists of several prominent spectral lines. Daylight consists of a much more even spread of wavelengths. Lamp manufacturers often aim to make fluorescent lamps which reproduce this distribution in the energy they emit. In the following section we will look at some of the theory behind photometry. Radiant flux is usually measured in watts.
Luminous flux is useful for describing the total light output of light sources.
However, to describe the distribution of light from a source, luminous intensity is used. Luminous intensity - the luminous flux emitted per unit solid angle in a specified direction. Both these have AREA measures associated with them. Illuminance of any surface varies as cosine of angle of incidence. Inverse square law and cosine law can be combined.