All light measurements can be traced back to the Candela, which is the unit of luminous intensity. The Candela is also sometimes called candlepower, and is similar to an older unit, the candle, which was based upon an actual candle of specified dimensions and construction.
There are two basic methods used by various flashlight manufacturers to rate the light output of their products.
Total Luminous Output may be expressed in candela or in lumens. This is a measure of the entire light output of the flashlight regardless of beam focus. It is almost solely a function of the lamp, and for practical purposes is equal to the lamp output which lamp manufacturers rate in Mean Spherical Candela (Sometimes called Mean Spherical Candlepower), or in lumens. Multiply candela by 12.57 to convert to lumens. For example, the 20 watt lamp used in the SL-35X™ is rated at 450 lumens or 35.8 Mean Spherical Candela.
Peak Beam Candlepower is a measure of the brightest spot in the focused beam. It is a function of both the output of the lamp and the efficiency of the reflector. The focused spot of light has the same intensity that a bare source of unfocused light of the same candlepower would produce on the same area from the same distance. For example, a flashlight of 20,000 beam candlepower would project, within its “hot spot,” the same amount of light on a wall as would a bare lamp of 20,000 candela at the same distance. Typical values of beam candlepower will run into the tens of thousands for powerful rechargeable flashlights.
Since the Total Output system does not take into account the focusing efficiency of the reflector, there is no way to convert between the two rating systems.
Two other units often encountered are the Footcandle and its metric counterpart the Lux. These are measures of illumination on areas of one square foot and one square meter respectively. Photometers (light meters) are generally calibrated in either footcandles or lux. Because of the mathematics of the definitions and calculations, a source measured in beam candlepower will produce the same number of footcandles at a distance of one foot and an equal number of lux at one meter. For example, a lightsource which produces 1,000 footcandles at one foot will produce 1,000 lux at one meter.
Additionally, all light rating systems depend on the perception of the human eye and are therefore subjective. Another important factor besides absolute intensity is the whiteness (technically the Color Temperature) of the light source. If two equally intense light sources differ in color temperature, the eye will perceive the whiter source (higher color temperature) to be brighter than the more yellow one. Color temperature is expressed in degrees on the Kelvin scale. A very white flashlight lamp will be rated around 3200 K.