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candlepower [candela], hence the name (fig. 6-13). A candle radiates light equally in all directions. If you imagine such a source surrounded by a transparent sphere of one foot [meter] radius (figure) than by definition, the amount of luminous energy (flux) emanating from one square foot [meter] of surface on the sphere is one lumen [lumen], abbreviated lm.

Since there are 40 square feet [meters] of surface area in such a sphere, it follows that a source of one candlepower [candela] intensity produces 40 or 12.57 lm (a lumen is a unit of light quantity), and in terms of power is equal to 0.0015 watt. It therefore also follows that 1-cp [cd] source produces 12.57 times 0.0015 watt; that is, 0.0189 watt or approximately 1/50 watt of luminous energy. The lumen, as luminous flux, or quantity of light, is comparable to the flow of current in an electrical circuit

One lumen of luminous energy occurrence on one square foot of area produces an illumination of one footcandle (fc). When the area is expressed in square meters, the illumination is expressed in lux (lx). If you were to consider a light bulb to be comparable to a sprinkler head, then the amount of water released would be 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 corresponding order to change footcandle to lux, you would multiply by 10.764.

Restating what you have just learned mathematically, it would look like this:

footcandles = lumens / square feet of area

Or

lux= lumens / square meter of area

Efforts to improve the power efficiency and reduce the maintenance costs led to the development of a new family of lighting that has been generally categorized as high--intensity-discharge lamps (HID). These lamps all have a negative-resistance characteristic. This means that the resistance decreases as the lamps heat up. As the resistance decreases, the current increases. In fact, the current will increase indefinitely unless a current--limiting device is provided. All gaseous conduction HID lamps, therefore, have current limiters, called

"ballasts." Lamp life and more light per watt are two main advantages that HID lamps have over incandescent bulbs. The basic types of HID lamps used unit, footcandles, by a ratio of approximately 10 to 1. In in area lighting consist of three groups of lamps:

Figure 6-13. - Relationship between a light source of one candlepower and the illumination produced.

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