Lighting Research Center Lighting Research Center
    Volume 7 Issue 2
March 2003 (revised February 2007)    
candela - The Systeme International d'Unities (SI) of luminous intensity. One candela is one lumen per steradian. Formerly, candle. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. zenith - In the lighting discipline, zenith is the angle pointing directly upward from the luminaire, or 180. Zenith is opposite nadir. In astronomical usage, zenith is the highest point in the sky, directly above the observation point. disability glare - A type of glare that causes a loss of visibility from stray light being scattered within the eye. discomfort glare - The sensation of annoyance or even pain induced by overly bright sources. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. luminous flux - Luminous radiant power, measured in lumens. The overall light output of a lamp or luminaire. light trespass - A undesirable condition in which exterior light is cast where it is not wanted. luminous intensity - The luminous flux on a small surface centered on and normal to the direction divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity can be expressed in candelas or in lumens per steradian. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0. Nadir is opposite the zenith. noncutoff luminaire - IESNA classification that describes a luminaire light distribution in which there is no candela limitation in the zone above maximum candela. (See also cutoff classification and cutoff angle.) semicutoff luminaire - IESNA classification that describes a luminaire light distribution in which the candela per 1000 lamp lumens does not numerically exceed 50 (5%) at or above an angle of 90 above nadir, and 200 (20%) at or above a vertical angle of 80 above nadir. This applies to all lateral angles around the luminaire. sky glow - Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors. spill light - Light that falls outside of the area intended to be lighted. steradian (sr) - A unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius. uplight - Light directed upward at greater than 90 above nadir. The source of uplight can be from a combination of direct uplight and reflected light. cutoff angle - The angle of light distribution from a luminaire, measured upward from nadir, between the vertical axis and the first line at which the bare source (lamp) is not visible. direct uplight - Light emitted upward by a luminaire. cutoff classification - The classification system of the Illuminating Engineering Society of North America (IESNA) that describes the light distribution of anoutdoor luminiare. Cutoff classifications define the luminous intensity limits in two illumination zones that occur within the range of 80° to 180° above nadir. North America (IESNA) that describes the light distribution of an outdoor luminaire. Cutoff classifications define the luminous intensity limits in two illumination zones that occur within the range of 80 to 180 above nadir. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. fully shielded luminaire - A luminaire that emits no direct uplight, but which has no limitation on the intensity in the region between 80 and 90. cutoff luminaire - IESNA classification that describes a luminaire having a light distribution in which the candela per 1000 lamp lumens does not numerically exceed 25 (2.5%) at or above an angle of 90 above nadir, and 100 (10%) at or above a vertical angle of 80 above nadir. This applies to all lateral angles around the luminaire. full cutoff luminaire - IESNA classification that describes a luminaire having a light distribution in which zero candela intensity occurs at or above an angle of 90 above nadir. Additionally, the candela per 1000 lamp lumens does not numerically exceed 100 (10%) at or above a vertical angle of 80 above nadir. This applies to all lateral angles around the luminaire. fixture - A complete lighting unit consisting of lamp or lamps and the parts designed to distribute the light, position and protect the lamp(s), and connect the lamp(s) to the power supply. (Also referred to as luminaire.)
What is the difference between full cutoff and fully shielded?

The term full cutoff has and is being used to describe luminaires that have no direct uplight (no light emitted above horizontal). However, in addition to that limitation, the Illuminating Engineering Society of North America (IESNA) definition also requires luminaires to comply with the glare requirement limiting intensity of light from the luminaire in the region between 80° and 90°.

The term full cutoff is often substituted for the term fully shielded. The terms are not equivalent. Fully shielded luminaires emit no direct uplight, but have no limitation on the intensity in the region between 80° and 90°. Luminaires that fall under the IESNA full cutoff, cutoff, semicutoff, and noncutoff definitions, may also qualify as fully shielded. It may be obvious that a luminaire that is characterized as an IESNA full cutoff luminaire is fully shielded, but not as obvious when luminaires with other IESNA classifications may also qualify. Consider a semicutoff luminaire containing a 1000 lumen lamp that has no direct uplight but a candela value of 150 between 80° and 90°. This luminaire is considered to be fully shielded. However, if it were mistakenly labeled a full cutoff luminaire, this can become quite confusing. In 2002, the IESNA chartered a new committee to address the inconsistencies and confusion.

There is also a confusing assumption that a luminaire with a flat lens qualifies as a full cutoff luminaire. While this may be true sometimes, it is not always the case. Depending on the structure of the luminaire, reflections off the housing may result in some amount of direct uplight from the luminaire. Consider the hypothetical luminaire in Figure 14. Reflections from below the lens may result in some direct uplight from the luminaire. The IESNA full cutoff classification also has a limitation on light in the glare zone between 80° and 90°. A flat lens on a luminaire does not guarantee that this requirement is met.

Figure 14. Example of a flat lens luminaire that may have some uplight reflected from the mechanical structures below the lens

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