Efficacy

The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt.

Electrode preheat current

The current flowing through a lamp's electrodes to heat them during starting.

Electrodes

The structure that serves as the electric terminals at each end of electric discharge lamps.

Electromagnetic interference (EMI)

The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on fluorescent lighting systems in FCC Part 18.

Electromagnetic wave

A wave composed of perpendicular electric and magnetic fields. The wave propagates in a direction perpendicular to both fields.

Electronic ballast

A ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz.

Emergency options

Refers to options available when exit signs are operated on a non-utility power supply such as a generator, a central battery unit that operates several exit signs, or an individual rechargeable battery. Options include whether or not the exit sign increases the brightness of the light source if the utility-supplied power fails.

Field of view

The area covered by an occupancy sensor, often reported (for wall-mounted sensors) as a horizontal field of view or (for ceiling-mounted sensors) as the solid angle of the cone-shaped coverage area.

Filter

A device that allows currents at certain frequencies to pass while those at other frequencies are blocked. Filters reduce conducted electromagnetic waves by grounding the current or by increasing the impedance to a specific frequency.

Fixture

A complete lighting unit consisting of lamp or lamps and the parts designed to distribute the light, position and protect the lamp(s), and connect the lamp(s) to the power supply. (Also referred to as luminaire.)

Flicker

A rapid and continuous change in light levels caused by the modulation of the light output from fluorescent lamps.

Footcandle (fc)

A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent.

Frequency

The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz).

Full cutoff luminaire

IESNA classification that describes a luminaire having a light distribution in which zero candela intensity occurs at or above an angle of 90° above nadir. Additionally, the candela per 1000 lamp lumens does not numerically exceed 100 (10%) at or above a vertical angle of 80° above nadir. This applies to all lateral angles around the luminaire.

Full-spectrum color index (FSCI)

A mathematical transformation of full-spectrum index into a zero to 100 scale, where the resulting values are directly comparable to color rendering index. An equal energy spectrum is defined as having an FSCI value of 100, a "standard warm white" fluorescent lamp has an FSCI value of 50, and a monochromatic light source (e.g., low pressure sodium) has an FSCI value of 0.

Full-spectrum index (FSI)

A mathematical measure of how much a light source's spectrum deviates from an equal energy spectrum, based on the slope of its cumulative spectrum.

Fully shielded luminaire

A luminaire that emits no direct uplight, but which has no limitation on the intensity in the region between 80° and 90°.

Fundamental

The component of a periodic wave that has the lowest frequency. It is also called the first-order harmonic.

Gamut area

A measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions.

Gas-discharge lamps

An electric lamp that produces light from gas atoms excited by an electric current.

Glare

The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility.

Glow current

The flow of electrons away from a rapid-start lamp's electrodes during preheating. The higher the glow current, the faster the electrodes' emissive coating degrades, increasing lamp-end darkening and reducing lamp life.

Grounded

A circuit or metal object that is connected to the earth at one or more points. Done mostly for safety, grounding also reduces electromagnetic waves.

Halo-phosphors

Also referred to as halophosphates. Phosphors are the white powder inside fluorescent lamps that fluoresces (emits visible light) when excited by the ultraviolet radiation produced by the mercury vapor that is energized by the electric arc sustained inside the lamp. Phosphors are used to achieve high efficacy, good color rendering, and low lamp lumen depreciation. Halo-phosphors, however, are limited in their ability to provide a high color rendering index without sacrificing light output and are often mixed with other phosphors.

Halogen cycle

Halogen incandescent lamps are in the same family as standard incandescent lamps. The basic operating principle is the same, except that chemicals called halogens are introduced in the gas fill. When electricity passes through the lamp's filament, it is heated until it glows and emits light. In this process, tungsten from the filament evaporates and, over the life of the lamp, causes the glass bulb wall to slowly blacken and the filament to disintegrate until the lamp fails. Halogens remove evaporated tungsten from the glass wall and redeposit it back onto the filament. As a result, tungsten does not build up on the bulb, so the light output does not degrade as rapidly.

Halogen lamp

An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps.

Halophosphates

The class of phosphors commonly used in fluorescent lamps. Halophosphates are limited in their ability to provide a high color rendering index without sacrificing light output. Standard T12 lamps containing halophosphates are the most common and least expensive fluorescent lamps, but United States federal regulations require that all fluorescent lamps must meet minimum efficacy and CRI standards, and 40-watt T12 halophosphate lamps do not meet these standards. T8 lamps usually contain both halophosphates and rare-earth phosphors.

Harmonic

For a distorted waveform, a component of the wave with a frequency that is an integer multiple of the fundamental

Harmonic distortion

Distorted waveshapes contain components with frequencies that are multiples of the fundamental frequency. These higher frequency components are known as harmonics.

Harmonics

Distortions of a periodic sinusoidal waveform represented as a harmonic series of sinusoidal waveforms of different amplitude and phase. A harmonic series is a group of different frequency waveforms that are multiples of the lowest or fundamental frequency.

Heat sinking

Adding a material, usually metal, adjacent to an object in order to cool it through conduction.

High-intensity discharge (HID)

An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps.

High-pressure sodium (HPS)

A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a correlated color temperature (CCT) of approximately 2000 kelvins, although CCTs for lamps having higher CRI values range from 2200 to 2700 kelvins. Standard lamps have a CRI value of 22; others have CRI values from 60 to 80. HPS lamps are among the most efficacious light sources, with efficacies as high as 150 lumens per watt, although those with higher CRI values have efficacies as low as 25 lumens per watt.

Horizontal illuminance

The average density of luminous flux incident on a horizontal surface, measured in footcandles (fc) or lux (lx). One fc equals 10.76 lx.

Horizontal rotation range

The total angular horizontal rotation of the lamp-reflector assembly.