Lighting Terminology:
The” Illumination Engineering Society of North America” (IESNA) is the recognized leading authority on lighting. The lighting terms listed below can be found in the IESNA Ready Reference Handbook, 4th Edition, which is a compendium of materials from the IESNA Lighting Handbook, 9th Edition. Some of the terms have been simplified and are hopefully a little easier to understand for those with less knowledge of the lighting world. These terms represent just a few of the lighting terms that are associated with the LED lighting world.
B.U.G Rating
BUG stands for “Backlight”, “Uplight” and “Glare”. the acronym describes the types of stray light escaping from an outdoor lighting luminaire. “B” stands for backlight, or the light directed in back of the mounting pole. “U” stands for uplight, or the light directed above the horizontal plane of the luminaire, and “G” stands for glare, or the amount of light emitted from the luminaire at angles known to cause glare.
Candela
IESNA Definition: the Standard International unit of luminous intensity. One candela is one lumen per steradian. Formerly, candle.
The intensity of a light source in a specific direction is expressed in candelas (cd). Any given light source will have many different intensities, depending upon the direction considered. Since intensity is a property of the source itself, the candlepower (luminous intensity expressed in candelas) for a specified direction remains the same, regardless of distance from the source .
Circadian Rythem
A circadian rhythm is a roughly 24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria. The term "circadian", coined by Franz Halberg,[1] comes from the Latin circa, "around," and diem or dies, "day", meaning literally "approximately one day." The formal study of biological temporal rhythms such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology.Circadian rhythms are endogenously generated, and can be entrained by external cues, called zeitgebers, the primary one of which is daylight.
Correlated Color Temperature, CCT
IESNA Definition: the absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.
The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source when heated to a particular temperature, measured in degrees Kelvin (K). The CCT rating for a lamp is a general "warmth" or "coolness" measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below 3200 K are usually considered "warm" sources, while those with a CCT above 4000 K are usually considered "cool" in appearance.
Color Rendering Index, CRI
IESNA Definition: measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source, of comparable color temperature.
Light sources differ in their ability to render the color of objects "correctly." The color rendering capability of a lamp is expressed by the Color Rendering Index (CRI). To determine the CRI rating of a specific lamp, testing eight standard color samples are illuminated by a reference light source defined as having a CRI of 100, which very closely matches the test lamp in color temperature. The chromaticity of the samples under the reference source is calculated. The test lamp is then used to illuminate the same samples. All "shifts" in chromaticity between the two tests are tabulated, and the results are averaged to arrive at a single CRI number for the test lamp. Since the CRI of a lamp is related to the reference source of similar color temperature, comparing the CRIs of lamps with different correlated color temperatures (CCT) provides no useful information.
Efficacy
IESNA Definition: the quotient of the total luminous flux emitted by the total lamp power input. It is expressed in lumens per watt (lm/W).
Efficacy for light sources and lighting systems is expressed in lumens per watt. For fluorescent and high intensity discharge (HID) sources, the associated ballast wattage should be included in determining the system efficacy, as should any reductions in lumen output associated with the lamp-ballast combination.
Lamp efficacy is calculated by dividing lamp lumens by lamp watts:
- 100-W A19 incandescent lamp produces 1740 lm
- 1740 lm ÷ 100 W = 17.4 lm/watt
System efficacy (for ballasted sources) is derived by multiplying rated lamp lumens by the ballast factor and dividing the result by total input watts:
Footcandle
IESNA definition: a unit of illuminace equal to 1 lm/ft.2 or 10.76 lx.
Isolux (isofootcandle) line
IESNA definition: a line plotted on any appropriate set of coordinates to show all the points on a surface where the illuminance is the same. A series of such lines for various values is called an Isolux (Iso-footcandle) diagram.
Illuminance
IESNA Definition: the areal density of the luminous flux incident at a point on a surface.
Illuminance is calculated as the number of lumens (lm) per unit area.
Two common units used to
measure illuminance are: |
|
For conversion
purposes: |
footcandles (fc) = lm/ft2 |
lux (lx) = lm/m2 |
|
|
1 lx = .0929 fc |
1 fc = 10.76 lx |
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The "IESNA Lighting Handbook, 2000", recommends illuminance values for a variety of lighting applications. These recommendations are categorized according to the level of complexity of the visual task being performed. Visual tasks can range from simple, where visual performance is not as important (walking through a corridor), to very specialized, where visual performance is of critical importance (assembling very small pieces of machinery).
Kelvin
IESNA definition: the unit of temperature used to designate the color temperature of a light source. A temperature scale where each degree is the same size as a centigrade degree, but the Kelvin scale has its zero at 273 Degrees C.
LM-79
The
Illuminating Engineering Society of North America (IESNA) published a documentary standard LM-79, which describes the methods for testing solid-state lighting products for their light output (lumens), energy efficiency (lumens per watt) and chromaticity. Click here to view LM-79 PDF Document
http://www.techstreet.com/cgi-bin/pdf/preview/2326631/previews/IES_LM-79-08_pre.pdf"
Lumen
IESNA Definition: Standard International (SI) unit of luminous flux. Radiometrically, it is determined from the radiant power. Photometrically, it is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.
The lumen is the time rate flow of light. Whereas a lamp will have many candela values, depending upon the direction of interest, it will have only one lumen output rating. The lumen rating can be considered as the measure of the summation of light output of a lamp. Ratings are determined and published by the lamp manufacturer.
Lumen Depreciation
IESNA definition: the decrease in lumen output that occurs as a lamp is operated, until failure.
Luminaire Efficiency
IESNA definition: the ratio of luminous flux (Lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.
Luminance
IESNA Definition: the quotient of the luminous flux at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction.
Luminance is the photometric quantity most closely associated with one's perception of brightness. It usually refers to the amount of light that reaches the eye of the observer measured in units of luminous intensity (candelas) per unit area (m2).
Mesopic Vision
IESNA definition: vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, that is, between about 3.4 and 0.034 cd/m squared.
Photometry
IESNA definition: the measurement of quantities associated with light.
Note: Photometry can be either visual, in which the eye is used to make a comparison, or physical, in which measurements are made by means of physical receptors.
Photopic Vision
IESNA definition: vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m squared. See scotopic vision.
Reaction Time
IESNA definition: the interval between the beginning of a stimulus and the beginning of a response of an observer.
Scotopic Vision
IESNA definition: vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m squared. See photopic vision.
Visual Acuity
IESNA definition: a measure of the ability to distinguish fine details, measured with a set of opotypes (test types for determining visual acuity) of different sizes. Quantitatively, it is the reciprocal of the minimum angular size in minutes of the critical detail on an object that can just be seen.
