Lighting Terminology

Special terms and concepts are used to define the characteristics of lamps and luminaries and to standardize the units of measurement. The most important of these are described here.

Light and radiation
Light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nanometers to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.

Luminous flux £X
The radiant power is the total radiated power in watts, also called radiant flux. This power must be factored by the sensitivity of the human eye to determine luminous flux in lumens.

Luminous intensity I

Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian . A steradian is the standard unit solid angle; a sphere encloses 4 π (approximately 12.57) steradians.

Luminance E

In photometry, luminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. In SI derived units these are measured in lux (lx) or lumens per square metre (cd•sr•m−2). In the CGS system, the unit of luminance is the phot, which is equal to 10,000 lux. The foot-candle is a non-metric unit of luminance that is used in photography.

Luminance L

The physical measure of brightness. Luminous intensity per unit projected area of any surface, as measured from a specific direction. Luminance (usually 'L' in formulas) is the amount of visible light leaving a point on a surface in a given direction. This "surface" can be a physical surface or an imaginary plane, and the light leaving the surface can be due to reflection, transmission, and/or emission. Standard unit of luminance is candela per square meter (cd/m²).

Color Temperature
Color temperature is a characteristic of visible light that has important applications in lighting, photography, ideography, publishing, manufacturing, astrophysics, horticulture, and other fields. The color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source. Color temperatures over 5,000K are called cool colors (bluish white), while lower color temperatures (2,700–3,000 K) are called warm colors (yellowish white through red).

DIN 5033 Chromaticity diagram

The CIE 1931 x,y chromaticity space, also showing the chromaticity of black body light sources of various temperatures (Planckian locus), and lines of constant correlated color temperature.

Light color
The light color of a lamp can be neatly defined in terms of color temperature. There are four main categories here:

  1. 1. Warm White:2700K ~ 3300K
  2. Natural White:3300K~5000K
  3. Daylight White:5000K~7000K
  4. Cool White: >7000K

Despite having the same light color, lamps may have very different color rendering properties owing to the spectral composition of their light.

Color rendering
The color rendering index (CRI), sometimes called color rendition index, is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography. It is defined by the International Commission on Illumination as follows:

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant The CRI of a light source does not indicate the apparent color of the light source; that information is under the rubric of the correlated color temperature (CCT).

CRI's ability to predict color appearance has been criticized in favor of measures based on color appearance models, such as CIECAM02 and, for daylight simulators, the CIE Metamerism Index. CRI is not a good indicator for use in visual assessment, especially for sources below 5000 kelvin (K).

Luminary efficiency
Luminary efficiency is the ratio of light output emitted by the luminaries to the light output emitted by its lamps. Another way of looking at it: Luminary efficiency is the percentage of light output produced by the lamps that are in turn emitted by the luminaries.

Lumina ire efficacy describes the efficacy of the entire luminaries, including the light source, ballast and luminaries losses. The Luminaries Efficacy Rating (LER) provides a metric for comparing the relative energy efficiency of fluorescent luminaries. Initiated in response to the Energy Policy Act of 1992, LER offers a voluntary rating standard for several categories of commercial and industrial fluorescent luminaries such as 2×4 recessed lenses and louvered luminaries, plastic wrap around and strip lights (NEMA LE 5-2001).

LER is expressed:
LER = [Lumina ire Efficiency (EFF) x Total Rated Lamp Lumens (TTL) x Ballast Factor (BF)] ÷ [Lumina ire Watts Input]

光譜 Spectrum
Range of colors observed when white light was dispersed through a prism. Spectrum, referred to a plot of light intensity or power as a function of frequency or wavelength, also known as a spectral density. Mass spectrum of a peptide showing the isotopic distribution The term now applies to any signal that can be measured or decomposed along a continuous variable such as energy in electron spectroscopy or mass to charge ratio in mass spectrometry.

SAMPLE Spectrum of Cool White LED:

Incandescent lamp
Incandescent light bulb, incandescent lamp or incandescent light globe is an electric light which produces light with a filament wire heated to a high temperature by an electric current passing through it, until it glows (see Incandescence). Incandescent bulbs are less efficient than several other modern types of light bulbs; most incandescent bulbs convert less than 5% of the energy they use into visible light (with the remaining energy being converted into heat)

Gas discharge lamp
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, plasma. The character of the gas discharge critically depends on the frequency or modulation of the current.

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. An LED is often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching.