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Light Introduction

Lighting Update Manual

US EPA Office of Air and Radiation 6202J
EPA 430-B-95-003, January 1995
U.S. EPA Green Lights Program

Illumination

Quantity of Illumination.

Light Output

The most common measure of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. For example, a T12 40-watt fluorescent lamp may have a rating of 3050 lumens. Similarly, a light fixture's output can be expressed in lumens. As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when the lamp is new).

Light Level

Light intensity measured on a plane at a specific location is called illuminance. Illuminance is measured in footcandles, which are workplane lumens per square foot. You can measure illuminance using a light meter located on the work surface where tasks are performed. Using simple arithmetic and manufacturers' photometric data, you can predict illuminance for a defined space. (Lux is the metric unit for illuminance, measured in lumens per square meter. To convert footcandles to lux, multiply footcandles by 10.76.)

Brightness

Another measurement of light is luminance, sometimes called brightness. This measures light "leaving" a surface in a particular direction, and considers the illuminance on the surface and the reflectance of the surface.

The human eye does not see illuminance; it sees luminance. Therefore, the amount of light delivered into the space and the reflectance of the surfaces in the space affects your ability to see.

Quantity Measures

Luminous flux is commonly called light output and is measured in lumens (lm). Illuminance is called light level and is measured in footcandles (fc). Luminance is referred to as brightness and is measured in footlamberts (fL) or candelas/m2 (cd/m2).

Determining Target Light Levels

The Illuminating Engineering Society of North America has developed a procedure for determining the appropriate average light level for a particular space. This procedure ( used extensively by designers and engineers ( recommends a target light level by considering the following:

  • The task(s) being performed (contrast, size, etc.)
  • The ages of the occupants
  • The importance of speed and accuracy

    Then, the appropriate type and quantity of lamps and light fixtures may be selected based on the following:

  • Fixture efficiency
  • Lamp lumen output
  • The reflectance of surrounding surfaces
  • The effects of light losses from lamp lumen depreciation and dirt accumulation
  • Room size and shape
  • Availability of natural light (daylight)

    When designing a new or upgraded lighting system, one must be careful to avoid overlighting a space. In the past, spaces were designed for as much as 200 footcandles in places where 50 footcandles may not only be adequate, but superior. This was partly due to the misconception that the more light in a space, the higher the quality. Not only does overlighting waste energy, but it can also reduce lighting quality. Refer to Exhibit 2 for light levels recommended by the Illuminating Engineering Society of North America. Within a listed range of illuminance, three factors dictate the proper level: age of the occupant(s), speed and accuracy requirements, and background contrast.

    For example, to light a space that uses computers, the overhead light fixtures should provide up to 30 fc of ambient lighting. The task lights should provide the additional footcandles needed to achieve a total illuminance of up to 50 fc for reading and writing. For illuminance recommendations for specific visual tasks, refer to the IES Lighting Handbook, 1993, or to the IES Recommended Practice No. 24 (for VDT lighting).

    Quality Measures

    Visual comfort probability (VCP) indicates the percent of people who are comfortable with the glare from a fixture. Spacing criteria (SC) refers to the maximum recommended distance between fixtures to ensure uniformity. Color rendering index (CRI) indicates the color appearance of an object under a source as compared to a reference source.

    Quality of Illumination

    Improvements in lighting quality can yield high dividends for US businesses. Gains in worker productivity may result by providing corrected light levels with reduced glare. Although the cost of energy for lighting is substantial, it is small compared with the cost of labor. Therefore, these gains in productivity may be even more valuable than the energy savings associated with new lighting technologies. In retail spaces, attractive and comfortable lighting designs can attract clientele and enhance sales.

    Three quality issues are addressed in this section:

  • Glare
  • Uniformity of illuminance
  • Color rendition

    Glare

    Perhaps the most important factor with respect to lighting quality is glare. Glare is a sensation caused by luminances in the visual field that are too bright. Discomfort, annoyance, or reduced productivity can result.

    A bright object alone does not necessarily cause glare, but a bright object in front of a dark background, however, usually will cause glare. Contrast is the relationship between the luminance of an object and its background. Although the visual task generally becomes easier with increased contrast, too much contrast causes glare and makes the visual task much more difficult.

    You can reduce glare or luminance ratios by not exceeding suggested light levels and by using lighting equipment designed to reduce glare. A louver or lens is commonly used to block direct viewing of a light source. Indirect lighting, or uplighting, can create a low glare environment by uniformly lighting the ceiling. Also, proper fixture placement can reduce reflected glare on work surfaces or computer screens. Standard data now provided with luminaire specifications include tables of its visual comfort probability (VCP) ratings for various room geometries. The VCP index provides an indication of the percentage of people in a given space that would find the glare from a fixture to be acceptable. A minimum VCP of 70 is recommended for commercial interiors, while luminaires with VCPs exceeding 80 are recommended in computer areas.

    Uniformity of Illuminance on Tasks

    The uniformity of illuminance is a quality issue that addresses how evenly light spreads over a task area. Although a room's average illuminance may be appropriate, two factors may compromise uniformity.

    Improper fixture placement based on the luminaire's spacing criteria (ratio of maxim recommended fixture spacing distance to mounting height above task height) fixtures that are retrofit with reflectors that narrow the light distribution

    Non-uniform illuminance causes several problems:

  • Inadequate light levels in some areas
  • Visual discomfort when tasks require frequent shifting of view from underlit to overlit areas
  • Bright spots and patches of light on floors and walls that cause distraction and generate a low quality appearance

    Color Rendition

    The ability to see colors properly is another aspect of lighting quality. Light sources vary in their ability to accurately reflect the true colors of people and objects. The color rendering index (CRI) scale is used to compare the effect of a light source on the color appearance of its surroundings.

    A scale of 0 to 100 defines the CRI. A higher CRI means better color rendering, or less color shift. CRIs in the range of 75-100 are considered excellent, while 65-75 are good. The range of 55-65 is fair, and 0-55 is poor. Under higher CRI sources, surface colors appear brighter, improving the aesthetics of the space. Sometimes, higher CRI sources create the illusion of higher illuminance levels.
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