BCPS is the common measurement of light output of a flash unit. This reading is taken with the reflector pointing directly at the specially calibrated laboratory flash meter, generally at a distance of 10 feet or 3 meters.

Since the coverage of the reflector is not considered, it is incumbent upon the manufacturer to state the angle of coverage along with the BCPS rating. Otherwise, the BCPS rating could be misleading. Narrowing the angle of coverage generally results in a higher BCPS output.

Flash units are generally rated in BCPS rather than f-stops because BCPS is a definite light output rating whereas, f-stops are subject to other variables including light-to-subject distance, film speed and diffusion.

BCPS can be easily converted into workable f-stops. The industry has generally adopted ISO 100 and 10 feet as unofficial standards when rating electronic flash units in terms of f-stops.

Generally speaking, when a lamphead reflector or attachment is made to produce a softer (more diffused) quality of light, its BCPS output is lowered. Therefore, maximizing BCPS is not always the goal of the manufacturer. The goal is to maximize the efficiency of the conversion of energy (watt-seconds) stored in the power supply. It is also important to provide an assortment of reflectors and accessories that yield the efficient transfer of this light, to best meet the objective of various photographic lighting techniques.