Flux or luminous flux is the measure of the total amount of energy radiated per second from a light source in all directions. This parameter can be adjusted to reflect the changes in the sensitivity of the human eye with respect to different wavelengths of light. It is measured in lumens and used for describing the brightness of a projector. One lumen is defined as the luminous flux of the uniform point light source having a luminous intensity of 1 candela. Projectors will have lumen values typically ranging from 200 to 27000 lumens.
Luminous flux is the sum of power at all wavelengths in the visible band. The ratio of total luminous flux to the radiant flux is known as luminous efficacy. Flux is used as an objective measure of energy emitted by a light source in light bulb packaging applications.
For a monochromatic source producing light at a single wavelength, flux can be easily determined using the following relation:
Φv = Φ x Vλ x (683 lm/W)
where Φv is the luminous flux and Vλ is the luminous efficacy.
It is difficult to determine the luminous flux for a source that radiates over a spectrum. The process involves calculation of spectral power distribution for a particular source, followed by calculating the luminous flux at each wavelength. The addition of flux at each wavelength gives a total flux produced the light source in the visible region.