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Light Guide

Light Loss Factors

When a light fixture is activated, it produces light which must leave the lamp, then the fixture, then reach the workplane where it is needed. Along the way, a number of operating and environmental conditions interfere with the transmission of light, resulting in wasted lumens. The lighting designer must provide a system that will take into account these conditions so that despite them the lighting system will provide proper quantity of light over time.

These conditions are captured as metrics called the light loss factors. Metrics are used to perform how something behaves. Light loss factors are captured as percentages or decimals (example: 0.95), which are then multiplied to result in a final Light Loss Factor in lighting calculations. There are two types of light loss factors, non-recoverable and recoverable.

Non-Recoverable Light Loss Factors

Some light loss factors are called "non-recoverable" because preventative maintenance generally does not affect the extent of the light loss. These include ballast factor, ambient fixture temperature, supply voltage variation, optical factor and fixture surface depreciation.

Ballast Factor

Lamps and ballasts experience losses when operating together as a system. The percentage of a lamp's initial rated lumens produced by a given ballast is called the Ballast 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.

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. The IESNA Lighting Handbook contains supply voltage variation data for various generic lamps; another source of information is the manufacturer's literature.

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.

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.

Recoverable Light Loss Factors

Some light loss factors are called "recoverable" because preventative maintenance can reduce the extent of the light loss. These include lamp burnouts, lamp lumen depreciation (LLD), fixture (luminaire) dirt depreciation (LLD) and room surface dirt depreciation (RSDD).

Lamp Burnouts

When a lamp expires, it becomes a "burnout." Lighting designers usually assume that the burnout will be replaced immediately. However, if it is known that a percentage of the lamps are burnouts at any given time, then a light loss factor must be reckoned with. For example, if 5% of the lamps are burnouts at any given time, then this light loss factor would be 0.95. Remember that 100% rated life is defined when 50% of the lamps in a large sample of lamps have failed.

Lamp Lumen Depreciation

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. If group relamping is employed as a planned maintenance strategy, then take the LLD factor for the point in life at which the lamps are replaced en masse. Otherwise, use an average, which is at 40% of life. See the Table below for typical LLD values for typical lamps.

Table. Typical LLD factors for several lamp types. Note that additional phosphor coatings to improve CRI in fluorescent lamps improves lumen maintenance.

F32T8, 85 CRI


F96T12/CW "Slimline"


F96T12 "Slimline," 85 CRI




F96T12/HO, 85 CRI


Compact fluorescent


Mercury vapor


Metal halide


High pressure sodium



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. To determine this factor, first identify the fixture type's maintenance category (I through VI) in ascending order of imperviousness to dirt and dust intrusion (see the IESNA Lighting Handbook for more information).

An industrial strip fixture with no top or bottom enclosure is an example of a Maintenance Category I fixture. A direct-indirect fixture is a Category II; an industrial strip fixture with an apertured top and bottom is a Category III. Deep-celled parabolic fluorescent fixtures are Category IV fixtures. A lensed fluorescent troffer is a Maintenance Category V fixture. A pure indirect fixture is a Category VI. See the IESNA Lighting Handbook for evaluating dirt conditions.

Adding It All UP

Once all light loss factors are determined, multiply one against the next (A x B x C …) until a final Light Loss Factor (LLF) results that can be used in lighting design calculations.


Additional References:

Lumen Maintenance and Light Loss Factors - U.S. Department of Energy

LED Lumen Maintenance and Light Loss Factors - U.S. Department of Energy


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