LED Glossary

Light | Lumen | Candela | Candlepower | Light Distribution | Illuminance | Luminance | Glare | Correlated Color Temperature (CCT) | Color Rendering Index (CRI) | Lamp | Lamp Life | Efficacy | Luminaire | Luminous Efficiency | Spacing Criteria


Light is a form of electromagnetic energy. Electric light sources convert electrical energy to “visible” radiant electromagnetic energy, which initiates the seeing process.

Light is reflected or transmitted by an object, received by the rods and cones of the eye and then interpreted by the brain as a visual image.

The IESNA defines light as a radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to about 780 nanometers.

Return to Top ^


The lumen (lm) is a measure of the total power of a source perceived by the human eye. While 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 total light output of a lamp. Ratings are determined and published by the lamp manufacturer.

Light output depreciates through time due to the deterioration of the lamp and components as well as the blackening of the interior surface of the bulb.

    Therefore, lamp manufacturers provide two lumen values:

  • Initial lumens, also referred to as rated lumens, which are lumens measured before depreciation occurs.
  • Mean lumens, also referred to as design lumens, which are the lumens the lamp will most likely emit at 40% of the lamps’ life.

Return to Top ^

Candela (Luminous Intensity)

Luminous intensity, also referred to as candlepower, is light emitted in a particular direction. The unit of measure for luminous intensity of a light source in a specific direction is in candelas (cd). Since luminous intensity is a property of the source itself, the candlepower for a specified direction remains the same, regardless of distance from the source.

Return to Top ^

Candlepower (Luminous Intensity) Distribution

Luminous intensity measured in candelas. The candlepower at various angles from a lamp or light fixture can be shown in a numerical table and/or a graph, both of which can be found in photometric reports. A candlepower summary table provides luminous intensity values at different angles. A candlepower graph shows a curve of plotted luminous intensity values, which allows an individual to visualize the light distribution of a particular lamp or fixture.

Return to Top ^

Light Distribution

Light distribution can be defined as the pattern of light produced by a lamp or a luminaire. The light distribution of a luminaire depends on its photometric characteristics represented by a luminous intensity distribution curve. The International Commission on Illumination (CIE) has classified the light distribution of indoor light fixtures as the percentage of light directed upward and downward as follows:

  • Direct: 90-100% downward
  • Semi-direct: 60-90% downward; 10-40% upward
  • Semi-indirect: 10-40% downward; 60-90% upward
  • Indirect lighting: 90-100% upward
  • General diffuse: 40-60% downward; 40-60% upward

Return to Top ^


Illuminance is calculated as the number of lumens (lm) per unit area. The two common units used to measure illuminance are: footcandles (fc) = lm/ft2 and lux (lx) = lm/m2.

    For conversion purposes:

  • 1 fc = 10.76 lx
  • 1 lx = 0.0929 fc

The IESNA 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 small pieces of machinery).

Return to Top ^


Luminance is the photometric quantity associated with the 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).

When evaluating conditions with excessive high or low contrast, luminance should be considered, rather than illuminance. Luminance meters and luminance imaging photometer systems provide for the evaluation of the visual environment in units of luminous intensity per unit area (cd/m2).

Return to Top ^


The IESNA defines glare as the sensation produced by luminances within the visual field area sufficiently greater than the luminance to which the eyes are adopted (IESNA Lighting Handbook Ninth Edition). This causes annoyance, discomfort or loss in visual performance and visibility. The magnitude of the sensation of glare depends upon such factors as the size of the task, relative position of the light and the viewer, the luminance of a source, the number of sources and the luminance to which the eyes are adopted.

    The four most common types of glare are:

  • Direct glare
  • Indirect (reflected) glare
  • Disability glare
  • Discomfort glare

Bright areas, such as luminaires, ceilings and windows that are directly in the field of view cause direct glare. Indirect glare is caused by light that is reflected to the eye from surfaces that are in the field of view often in the task area. Disability glare reduces visual performance and visibility. Discomfort glare produces physical discomfort. It is possible to experience disability glare without discomfort and conversely, discomfort without disability; however, one often accompanies the other.

Return to Top ^

Correlated Color Temperature (CCT)

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. In contrast to the temperature scale, lamps with a CCT rating below 3200K are usually considered “warm” sources, while those with a CCT above 4000K are usually considered “cool” in appearance.

Return to Top ^

Color Rendering Index (CRI)

CRI is a measure of the degree of color shift objects undergo when illuminated by the light source as compared with 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”. CRI expresses the color rendering capability of a lamp on a scale of 0 to 100.

CRI is a general indicator of how “natural” object colors will appear when illuminated by a particular light source. Generally, a CRI of 70 and above will be required for most lighting applications.

Return to Top ^


    The broad categories of electric lamps are:

  • LED – Solid state lamp that uses light-emitting diodes as the source of light. An LED emits light of a single primary color, but in combination with other diodes can produce colors of any hue.
  • Incandescent – a filament, usually of coiled tungsten wire, is heated to incandescence by the flow of current.
  • Fluorescent- low pressure mercury is ionized inside the lamp, producing primarily ultraviolet radiant energy which causes phosphors to fluoresce.

High Intensity Discharge- pressurized gases inside an arc tube are ionized by current flow between electrodes, emitting light.

    For each of the three types of HID sources, the gases in the arc tube are different:

  • Mercury vapor-mercury, plus small quantities of argon, neon, and krypton
  • Metal halide-same as vapor lamps, plus iodides of sodium and scandium; other metal halide lamps may have iodides of rare-earth elements
  • High pressure sodium-sodium, mercury amalgam and xenon

Return to Top ^

Lamp Life

A traditional incandescent light bulb has reached the end of its life when the filament breaks and the light output drops to zero. With an LED, the light output slowly and continuously decreases as the LED lamp loses efficiency. The LED has reached the end of its life when its light output reaches 50% of the rated luminous output, not when it reaches zero.

Return to Top ^


Efficacy for light sources and lighting systems is expressed in lumens per watt. Lamp efficacy is calculated by dividing lamp lumens by lamp watts.

Return to Top ^


An LED luminaire, commonly called a light fixture, contains a housing, a driver, LEDs, and electrical wiring. In addition to these parts, a luminaire may also contain one or more of the following components: reflector, lens, diffuser, gasket, latch, decorative trim, or mounting hardware-to provide protection; improve efficiency, appearance, or service; control glare; or affect distribution.

Return to Top ^

Luminous Efficiency

Luminous efficiency expresses the percentage of initial lamp lumens that are ultimately emitted by the light fixture. The efficiency of a light fixture does not necessarily indicate its effectiveness in delivering lumens to the work plane, or its appropriateness for the application.

Return to Top ^

Spacing Criteria

Luminaire manufacturers provide spacing criteria (SC) or spacing to mounting height ratios (S/MH), for specific light fixtures with direct lighting distribution. These light fixtures include downlights, troffers, high and low-bay fixtures. These ratios are used to calculate the maximum recommended installation spacing to obtain an even pattern of light on the surface below the light fixtures. SC ratios help ensure that a space is evenly lighted by slightly overlapping the light distribution from each light fixture. SC typically range from 0.9 to 1.7, but can be as low as 0.5 or higher than 2.

Return to Top ^