Lighting Terms and Terminology

Light and Optics

The electromagnetic spectrum includes all radiant energy from gamma rays and x-rays, through the ultraviolet and the visible to the infrared and radio. Electromagnetic radiation has both wave properties (wavelength, frequency) and particle properties (photons)

The higher the energy, the `bluer' the photon, the shorter the wavelength, and the higher the frequency.

LIGHT is the radiation capable of producing visual sensation, or brightness. It is characterized by wavelengths from 380 nm (violet) to 770 nm (red).(atmospheric cutoff is ~320 nm.)

All solid objects radiate a continuous spectrum according to their temperature - the hotter it is the bluer (but speak of "colder") it appears. This kind of temperature from a `blackbody' radiator is called the color temperature

Lamp color is also rated by the color rendering index (CRI); it is derived by noting any changes in appearance in eight standard color samples when illuminated by the lamp as compared with a blackbody of the same color temperature. The highest CRI rating is 100.

Heated gases produce bright line or emission line spectra as electrons around the atoms in the gas spit out photons of specific wavelengths.

Light spreads out with distance from a point source in such a way that when you double the distance, the intensity decreases by four. (The inverse r-squared law.) Light is further diluted if the surface it strikes is not perpendicular to the direction of light flow.

Light `rays' may be reflected (angle of incidence = angle of reflection) and so redirected; this principle is used in the design of reflectors in lighting fixtures.

Reflectance is the ratio of the reflected to the incident light (usually depends on the geometry and wavelength).

Light rays are refracted when passing from one medium into another, by an angle that depends on the optical properties of the medium (and the wavelength of the light). This principle is also used in lighting fixture optical elements.

Light Measurement Parameters

Light `flow' from a lamp is measured in lumens. A uniform point source of one candela (luminous intensity in any direction [defined for 555 nm radiant intensity of 1/683 watt per steradian] ~ one dinner candle) at the center of sphere of 1 foot radius gives 1 lumen per square foot. This is also one lumen per steradian. Area of the one foot sphere is 12.57 sq. ft. so the 1 cd. source produces 12.57 lumens. The quantity of light shining on a surface is the illuminance. It is measured in lumens per square foot ( footcandles, horizontal or vertical), or lumens per square meter (lux). 1 fc = 10.76 lx.

A few examples of illumination levels

Luminance is the technical name for what we see coming from an illuminated surface (e.g. emitted at the surface of a lamp, or reflected from the ground) with units of candelas per square meter. Lighting efficacy is indicated by the lumens produced in relation to power consumed as lumens/watt.

Light Sources

There are six families of conventional lamps commonly used in outdoor lighting applications: incandescent, fluorescent, mercury vapor (MV), metal halide (MH), high-pressure sodium (HPS), and low-pressure sodium (LPS). Except for incandescent, they are all gas discharge sources, i.e. light is emitted when an electric current passes through the gas.

MV, MH, and HPS are high-intensity discharge (HID) lamps: there is an inner quartz envelope (arc tube) tolerant of high temperature (300 - 400 deg F) and pressure (1/4-4 atm.) and containing a mixture of gases (argon and mercury, argon and sodium, et al..

All gas discharge lamps require BALLASTS

Incandescent Lamps Fluorescent Mercury Vapor Metal Halide High Pressure Sodium Low Pressure Sodium Lamp Manufacturers; Philips, Osram/Sylvania, GE, Iwasaki, Ushio, et al.

Lamp vendors:;

Lamp Lumen Depreciation: the output of most lamps drops over the life of the lamp, especially MV, but not for LPS.

Lighting Organizations

Web Resources

Luminaire Types

Lamp Distribution Patterns

The distribution of light on the ground and around the luminaire depends on the design of the reflector/refracting elements. There are several standard distributions (Type I, Type II, etc); this information is provided by the manufacturers for their luminaires.

The Coefficient of Utilization (CU) is the ratio of the amount of light that falls onto the intended area to the total produced by the lamp.

The cutoff angle is the angle, measured up from the nadir, between the vertical axis and the line of sight at which the bare source is not visible.

Glare: when you can see the source directly.

Common names usually reflect something about the design style.

Examples include wall packs, shoe boxes, hockey pucks, drop-refractor or drop-dish cobra heads, acorns, post tops, bed-pan shields, bollards.

p. a. ianna, 1 November 1997