The quantity of light produced by any lamp may be expressed in three ways: In lumens, foot-candles, and in candle-power.
Lumens are the units used to measure light quantity as the light leaves the lamp. In incandescent lamps, if one would screw a plain incandescent lamp into a plain socket with no reflector, the light would be emitted at 360 degrees. If one would place a lumen meter at any point around the lamp, the meter would measure the amount of light in that direction. A lumen can be described as “the amount of light passing a given point at a given time, irrespective of direction”.
Initial lumens – the amount of light in lumens (for a fluorescent lamp), after about 100 hours of operation.
Mean lumens – The amount of light the lamp produces after it has operated for approximately 40 percent of its rated life. Lamp light reduces as the lamp continues to be used, hence mean lumens have a lower value than initial lumens.
Foot-candles - not all the light a lamp produces reaches the work plane. Some light is lost in the fixture and some is absorbed by room surfaces before it reaches the work surface. (This action is known as coefficient of utilization for which there is a standard formula for calculating this measurement). The light that reaches the work surface is measured in foot-candles, (by a standard light meter). The Illuminating Engineering Society, (IES), a group of professional lighting engineers and designers, establishes the standards for the amount of light (foot-candles), which should be provided for certain activities to be performed in a given area.
Candle-power – The term candle-power is given to the measurement of light when one puts a light bulb into a reflector, or uses a reflector lamp. Candle-power is a “directed lumen”, or light that is forced, (by a reflector), in a certain direction. This measurement usually records the “maximum beam candle-power”, which means the amount of light directly in the center of the beam. This “beam” can be a “spot”, a “narrow beam”, or a “flood”.
- Mercury Vapor, (the first of this type used for commercial purposes.)
- Metal Halide
- High Pressure sodium
- Ceramic Metal Halide
- Xenon short arc lamps
All of the listed lamps emit light in basically the same manner. We start with an outer glass envelope. Within this glass envelope is an arc tube that contains a gas, usually argon, krypton, or a mixture of the two. On each end of the arc tube are tungsten electrodes. There are also metal salts within the tube. When the internal temperature and internal pressure build to a predetermined level, an arc will strike across the electrodes. As the arc builds up the heat and pressure will melt the metal salts to form plasma which increases the intensity of the light and reduces the power consumption.
This chart shows how much you can save by switching a 75 watt incandescent to a 20 watt H&H Twistee.
A Tale of Two Light Bulbs
Life Cycle Costs of Two Lighting Choices
|Life Cycle Cost Factors||Incandescent Light Bulb||Compact Fluorescent Bulb|
|Watts Consumed||75 WattIncandescent Bulb||20 WattH&H Twistee|
|Number of Lamps Used Over 10,000 Hours||13||1|
|Rated Lamp Life (Hours)||750||10,000|
|KWH Used Over 10,000 Hours||750||200|
|Cost per KWH (average)||$0.085||$0.085|
|Electricity Costper 10,000 Hours||$63.75||$17.00|
|Cost Per Bulb||$0.75||$20.00|
|Total Life Cycle Cost||$73.50||$37.00|
|Total Savings From One Compact Fluorescent Bulb||$36.50|