Friday, 14 November 2008 00:43
All lamps undergo some lumen depreciation or decrease in light output, as a lamp is operated. The lower the light loss over time, the higher a lamp's lumen maintenance will be.
Some of the causes: Supply Voltage Variation High or low voltages fed to lamps (incandescent) or ballasts (fluorescent and HID) from the building's power distribution can result in an increase or decrease of a lamp's lumen output. Electronic ballasts are not as sensitive to small variations in supply voltage as magnetic ballasts. Some models provide constant light output at +10% variation.
Optical Factor
The amount of space lamps take up serves as an obstruction to light leaving the fixture that is reflected internally. Since lamps absorb mass, they absorb some of this light output. The result is what is called the Optical Factor. T12 lamps have an Optical Factor of 1. Removing lamps, or installing thinner-diameter T10 or T8 lamps, can result in a higher Optical Factor.
Ambient Fixture Temperature
This factor deals with fluorescent systems. Deviations above or below the ideal fixture operating temperature can affect the amount of light leaving the lamp.
Fixture Surface Depreciation
As a fixture ages, its surfaces begin deteriorate. Blemishes absorb light instead of reflecting it; shielding materials may begin to discolor due to constant exposure to heat. This light loss factor is difficult to predict.
Fixture Surface Depreciation
As a fixture ages, its surfaces begin deteriorate. Blemishes absorb light instead of reflecting it; shielding materials may begin to discolor due to constant exposure to heat. This light loss factor is difficult to predict.
Lamp Aging
As a lamp ages and nears end of life, it produces less and less light on a predictable curve, the extent of which depending on the type of lamp. When a fluorescent lamp is first lighted, its output is abnormally high. During the first 100 hours of burning it will drop by about 5% (burn in). Deterioration during the remaining life of the lamp is slower The published "Initial Lumens" for fluorescent lamps is the value obtained after the first 100 hours of burning. T8 lamps typically last 24,000 hours at 95% of their after initial burn in light output.
The depreciation in light output is due to a gradual deterioration of the phosphor powders and a blackening of the inside of the tube. This is brought about primarily by two factors:
1. First, a bombardment of the electrodes by the arc stream that causes a deposit of mercury and electrode material on the lamp wall.
2. Second, the ultraviolet energy generated in the arc stream causes a photochemical effect on the glass tubing. Essentially, the ultraviolet "boils" impurities out of the tubing which contaminate the phosphor.
Fixture (Luminaire) Dirt Depreciation
Dirt and dust present in all ambient environments are ultimately attracted to and trapped in electrical equipment. The extent of dust collecting on the lamps depends on the environment, what type of fixture is in use, whether it is ventilated or not, and the type of work performed in the area. The extent of LDD depends on these conditions and also how often the fixtures will be cleaned.
