candlepower [candela], hence the name (fig. 6-13). Acandle radiates light equally in all directions. If youimagine such a source surrounded by a transparentsphere of one foot [meter] radius (figure) than bydefinition, the amount of luminous energy (flux)emanating from one square foot [meter] of surface onthe sphere is one lumen [lumen], abbreviated lm.Since there are 40 square feet [meters] of surfacearea in such a sphere, it follows that a source of onecandlepower [candela] intensity produces 40 or 12.57lm (a lumen is a unit of light quantity), and in terms ofpower is equal to 0.0015 watt. It therefore also followsthat 1-cp [cd] source produces 12.57 times 0.0015 watt;that is, 0.0189 watt or approximately 1/50 watt ofluminous energy. The lumen, as luminous flux, orquantity of light, is comparable to the flow of current inan electrical circuitOne lumen of luminous energy occurrence on onesquare foot of area produces an illumination of onefootcandle (fc). When the area is expressed in squaremeters, the illumination is expressed in lux (lx). If youwere to consider a light bulb to be comparable to asprinkler head, then the amount of water released wouldbe the lumens and the amount of water per square foot(meter) of floor area would be the footcandles [lux].The metric unit, lux, is smaller than the correspondingorder to change footcandle to lux, you would multiplyby 10.764.Restating what you have just learnedmathematically, it would look like this:footcandles =lumenssquare feet of areaOrlux=lumenssquare meter of areaHigh Intensity Discharge LightingEfforts to improve the power efficiency and reducethe maintenance costs led to the development of a newfamily of lighting that has been generally categorized ashigh--intensity-discharge lamps (HID). These lampsall have a negative-resistance characteristic. Thismeans that the resistance decreases as the lamps heat up.As the resistance decreases, the current increases. Infact, the current will increase indefinitely unless acurrent--limiting device is provided. All gaseousconduction HID lamps, therefore, have current limiters,called "ballasts." Lamp life and more light per watt aretwo main advantages that HID lamps have overincandescent bulbs. The basic types of HID lamps usedunit, footcandles, by a ratio of approximately 10 to 1. Inin area lighting consist of three groups of lamps:Figure 6-13.—Relationship between a light source of one candlepower and the illumination produced.6-12