Consistancy of output

Constant light output and quality are necessary characteristics of photographic light sources, especially for color work. Daylight, although an intense and form is free lighting. Still by no means at all is daylight, outdoor or available light is by no means constant.

The intensity and quality of outdoor lighting obviously vary with the season, time of day and weather. It's like planning an outdoor wedding without knowing what the weather will be that day. You need a back-up plan.

Artificial light sources are more reliable. Much effort goes into arranging lighting set-ups to simulate the desirable directional qualities of sunlight and diffuse daylight.

Electric light sources need a reliable power supply in order to maintain a constant output. If the frequency and/or voltage of the mains supply fluctuates, appreciable variation in light intensity and quality may result.

Incandescent lamps give a reliable output if used with some form of constant voltage control; but inevitably with age they darken, causing a lowering of both output and color temperature. Sources using the tungsten halogen cycle largely circumvent these problems. Fluorescent lamps have a long life, but they too gradually decrease in light output as they age.

An advantage of flashbulbs and electronic flash is the reliability of light output and quality that can be obtained. Flashbulbs in particular have an output which is dependent only on manufacturing tolerances.

Electronic flash gives an acceptably constant output provided adequate recharging time is allowed between successive flashes. The ready-light indicators fitted to many units glow when only about 80 percent of the charging voltage is reached; at this point the energy available for the flash is only some two- thirds of full charge. A further time must be allowed to elapse before discharge, to ensure full capacity is available.