R lamp

A common reflector lamp. An incandescent filament or electric discharge lamp in which the sides of the outer blown-glass bulb are coated with a reflecting material so as to direct the light. The light-transmitting region may be clear, frosted, or patterned.

Rapid start

A method of starting fluorescent lamps in which the electrodes are heated prior to starting, using a starter that is an integral part of the ballast. Heating the electrodes before starting the lamps reduces the voltage required to strike the electric arc between the electrodes. A rapid-start system starts smoothly, without flashing.

Rare-earth phosphors

A group of phosphors containing rare-earth elements. Rare-earth phosphors are used in fluorescent lamps to achieve high efficacy and better color rendering. They produce light in very narrow wavelength bands.

Rated average lamp life

Also referred to as lamp rated life. Lamps are tested in controlled settings and the point at which 50% of a given sample burns out is listed as the lamps' rated average lamp life.

Rated lamp life

The number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp or group of lamps may vary from the published rated life. Rated life is based on standard test conditions.

Rated lamp life

See lamp rated life.

Rated light output

The sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire.

Rated light output from lamp(s)

The sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire.

Rated lumen

Also referred to as rated light output from lamp in lumens. Lumen refers to 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. Manufacturers rate their lamps' initial light output after 100 hours of operation.

Reactive power

Power that creates no useful work. It results when current is not in phase with voltage. It is calculated using the equation reactive power = V x A x sin(q) where q is the phase displacement angle.

Relative beam diameter (manufacturer)

The normalized beam diameter based on manufacturer-supplied beam angles.

Relative beam diameter (NLPIP)

The normalized beam diameter based on NLPIP-measured values.

Relative CBCP (manufacturer)

The normalized center beam candlepower based on manufacturer-supplied values.

Relative system efficacy

The ratio of relative light output (RLO) to system active power. For each lamp type, relative system efficacy is normalized to the highest value at the maximum light output level, which is assigned a relative system efficacy value of 100%.

Restrike time

The time required for a lamp to restrike, or start, and to return to 90% of its initial light output after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted.

Root-mean-square (rms)

The effective average value of a periodic quantity such as an alternating current or voltage wave, calculated by averaging the squared values of the amplitude over one period and taking the square root of that average.

Semiconductor

A material whose electrical conductivity is between that of a conductor and an insulator; the conductivity of most semiconductors is temperature dependent.

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.

Sensitivity adjustment

A trim potentiometer (sometimes called a "trim pot") or a set of dip switches used to refine the response function of a photosensor. Some photosensors include a remote trim pot that allows for adjustment at a distance from the photosensor housing.

Shielding

Blocking an electric or magnetic field with a metallic substance. The incident field induces currents in the metallic substance, and these currents induce a field that opposes the incident field. Shielding reduces radiated electromagnetic waves. Electronic components, wires, lamps, and devices can all be shielded.

Sky glow

Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors.

Skylight

A device similar to a window that is placed in a roof, allowing sunlight to enter a structure, thus reducing the need for electric lighting. Skylights can be used to reduce peak load demand by taking advantage of sunlight during the peak demand time of the day.

Sound Rating

Magnetic ballasts sometimes produce a humming noise caused by vibration of the magnetic core. Electronic ballasts operate at high frequencies and are usually less noisy. Ballasts are rated from "A" to "F" based on their noise levels. Ratings define the range of ambient sound levels in which people will not notice the ballast noise. The higher the rating, the more noise that will be required to mask the ballast hum.

Spectral power distribution (SPD)

A representation of the radiant power emitted by a light source as a function of wavelength.

Specular angle

The reflected angle of light striking a surface, which is equal to and in the same plane as the incident angle.

Specular reflection

Light incident on a surface that is redirected at the specular angle. Glossy or shiny surfaces exhibit a high degree of specular reflection.

Spill light

Light that falls outside of the area intended to be lighted.

Starting method

The method a ballast uses to start a lamp. For compact fluorescent lamps (CFLs), ballasts use one of three methods: preheat, instant start, or rapid start. Dimming electronic ballasts use one of these starting methods: rapid start, programmed start, or controlled rapid start.

Starting time

The time it takes the lamp to start from the point at which voltage is applied to the lamp until stable operation.

Starting voltage

The voltage applied across the lamp during starting.

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.

Substrate

For light emitting diodes, the material on which the devices are constructed.

Supply voltage

The voltage, usually direct, applied by an external source to the circuit of an electrode.

System efficacy

Also referred to as relative system efficacy, system efficacy is a measurement of a system''s ability to convert electricity into light. Measured in lumens per watt (LPW), system efficacy is the ratio of the light output (in lumens) to the active power (in watts).

Time delay range

For motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps.

Total harmonic distortion (THD)

A measure of the degree to which a sinusoidal wave shape is distorted by harmonics, with higher values of THD indicating greater distortion.

Track head diameter

The size of the luminaire used in a track lighting system.

Track luminaire options

Accessories available for track luminaires.

Track mounting

For track luminaires, the method by which the track is attached to the ceiling.

Transformer

Transformers are electrical devices with no moving parts, which change distribution voltages to higher or lower levels. When used with incandescent or halogen lamps, they typically step 120-V distribution downward to 12V, although 5.5V and 24-V models are also offered.

Transients

For an alternating current circuit, a momentary voltage surge, often at amplitudes 10 to 20 times the normal voltage.

Tri-level switching

Control of light source intensity at three discrete levels in addition to off.

Tri-phosphor

A mixture of three phosphors to convert ultraviolet radiation to visible light in fluorescent lamps; each of the phosphors emits light that is blue, green or red in appearance with the combination producing white light.

Tri-Phosphors

Tri-phosphors are a blend of three narrow-band phosphors (red, blue, and green) that provide improved color rendition and higher light output versus some other types of phosphors.