Light Loss Factors

When a light fixture is turned on, it emits light that must pass through the lamp, the fixture, and finally reach the workplane where it is needed. During this journey, various operational and environmental conditions can hinder light transmission, leading to wasted lumens. The lighting designer must account for these conditions to ensure the lighting system delivers the appropriate amount of light over time.

These conditions are quantified as metrics known as light loss factors. These metrics assess performance and are expressed as percentages or decimals (e.g., 0.95), which are multiplied to obtain 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 termed "non-recoverable" because routine maintenance does not significantly affect the degree of light loss. These include ballast factor, ambient fixture temperature, supply voltage variation, optical factor, and fixture surface depreciation.

Ballast Factor

Lamps and ballasts incur losses when operating together as a system. The percentage of a lamp's initial rated lumens produced by a specific ballast is known as the Ballast Factor.

Ambient Fixture Temperature

This factor pertains to fluorescent systems. Deviations from the optimal fixture operating temperature can impact the amount of light emitted from the lamp.

Supply Voltage Variation

Variations in voltage supplied to lamps (incandescent) or ballasts (fluorescent and HID) from the building's power distribution can cause an increase or decrease in a lamp's lumen output. Electronic ballasts are less sensitive to minor voltage variations compared to magnetic ballasts. Some models maintain constant light output with a +10% variation. The IESNA Lighting Handbook provides supply voltage variation data for various generic lamps; additional information can be found in the manufacturer's literature.

Optical Factor

The space occupied by lamps obstructs the light leaving the fixture that is reflected internally. Since lamps absorb mass, they also absorb some light output, resulting in the Optical Factor. T12 lamps have an Optical Factor of 1. Removing lamps or installing thinner-diameter T10 or T8 lamps can increase the Optical Factor.

Fixture Surface Depreciation

As a fixture ages, its surfaces deteriorate. Blemishes absorb light instead of reflecting it, and shielding materials may discolor due to heat exposure. This light loss factor is challenging to predict.

Recoverable Light Loss Factors

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

Lamp Burnouts

When a lamp fails, it is considered a "burnout." Lighting designers generally assume immediate replacement of burnouts. However, if a percentage of lamps are known to be burnouts at any time, a light loss factor must be considered. For instance, if 5% of lamps are burnouts at any time, the light loss factor would be 0.95. Remember that 100% rated life is defined when 50% of lamps in a large sample have failed.

Lamp Lumen Depreciation

As a lamp ages and approaches the end of its life, it emits progressively less light on a predictable curve, depending on the lamp type. If group relamping is used as a maintenance strategy, use the LLD factor for the point when lamps are replaced en masse. Otherwise, use an average at 40% of life. Refer to the Table below for typical LLD values for various lamps.

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

How Light Fixtures Affect Light Loss

Light fixture housings play a crucial role in the efficiency and effectiveness of lighting systems. Here are some key factors on how they affect light loss:

1.Reflectivity of Materials

The materials used in the housing can significantly influence light loss. Highly reflective materials can help direct more light into the desired area, while dull or dark surfaces can absorb light, reducing overall output.

2. Design and Shape

The design and shape of the fixture can impact how light is distributed. Fixtures that are designed to focus light (like recessed can lights) can minimize loss, whereas poorly designed fixtures may scatter light inefficiently.

3. Lens and Diffuser Quality

If a fixture includes a lens or diffuser, its quality and clarity can affect light transmission. Scratched, dirty, or frosted lenses can reduce light output by scattering or absorbing light.

4. Heat Management

Effective heat management in a light fixture housing can influence light loss. Excessive heat can lead to decreased efficiency in light sources, particularly in LEDs, which may result in diminished light output over time.

5. Installation and Positioning

Improper installation or positioning of light fixtures can lead to increased light loss. Fixtures that are angled incorrectly or obstructed by furniture or other objects can diminish the effectiveness of the light.

6. Type of Light Source

Different light sources (LEDs, incandescent, fluorescent) have varying efficiencies, and the housing must be compatible with the type of light source used. Mismatched fixtures can lead to increased light loss.

7. Maintenance

Regular maintenance is essential to minimize light loss. Dust, dirt, and other obstructions on the fixture or bulbs can significantly reduce light output.

Fixture Housing Types

Different types of light fixture housings can significantly influence the amount of light lost during distribution. Here are some common types:

• Recessed Housings: These fixtures are installed into the ceiling, which can lead to light loss if not properly shielded or if the housing is poorly designed.

• Surface-Mounted Fixtures: These fixtures are mounted directly on the surface and can have varying degrees of light loss depending on their design and lens materials.

• Track Lighting: Track fixtures can direct light effectively, but the housing design can cause some light loss if the fixtures are not angled correctly.

• Pendant Lights: The design and materials of pendant housings can affect light output, with some designs causing more light loss than others.

• Wall Sconces: These fixtures can enhance ambient lighting but may also lead to light loss due to their positioning and the direction of the light emitted.

• Outdoor Fixtures: Weatherproof housings can sometimes trap light, reducing efficiency, especially if they have a frosted or opaque finish.

• LED Troffers: Common in commercial settings, the housing design can either enhance or diminish light output depending on the reflectors used.

When selecting light fixtures, it's essential to consider how the housing design will impact overall light efficiency and distribution.

Fixture (Luminaire) Dirt Depreciation

Dirt and dust present in all environments are attracted to and trapped in electrical equipment. The extent of dust accumulation on lamps depends on the environment, fixture type, ventilation, and work performed in the area. The extent of LDD depends on these conditions and how often fixtures are cleaned. To determine this factor, identify the fixture type's maintenance category (I through VI) in ascending order of resistance to dirt and dust intrusion (refer to 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 Category II; an industrial strip fixture with an apertured top and bottom is Category III. Deep-celled parabolic fluorescent fixtures are Category IV. A lensed fluorescent troffer is Maintenance Category V. A pure indirect fixture is Category VI. Refer to the IESNA Lighting Handbook for evaluating dirt conditions.

Adding It All Up

Once all light loss factors are identified, multiply them together (A x B x C …) to obtain a final Light Loss Factor (LLF) for use in lighting design calculations.