Ignitor

A device, either by itself or in association with other components, that generates voltage pulses to start discharge lamps.

Illuminance

The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent.

Illumination

The process of using light to see objects at a particular location.

Impedance

A measure of the total opposition to current flow in an alternating current circuit. The unit of impedance is the ohm Ω .

Incident angle

The angle between a ray of light reaching a surface and a line normal (perpendicular) to that surface.

Indication

The process of using a light source as something to be seen as in signaling.

Indirect lighting

Light arriving at a surface after reflecting from one or more surfaces (usually walls and/or ceilings) that are not part of the luminaire.

Infrared radiation

Any radiant energy within the wavelength range of 770 to 106 nanometers is considered infrared energy. (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m).

Initial light output

A lamp's light output, in lumens, after 100 hours of seasoning.

Instant start

A method of starting fluorescent lamps in which the voltage that is applied across the electrodes to strike the electric arc is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting. This method starts the lamps without flashing. It is more energy efficient than rapid or preheat starting, but results in greater wear on the electrodes during starting. The life of instant-start lamps that are switched on and off frequently may be reduced by as much as 25 percent relative to rapid-start operation. However, for longer burning cycles (such as 12 hours per start), there may be no difference in lamp life for different starting methods.

Intensity (luminous intensity)

Total luminous flux within a given solid angle, in units of candelas, or lumens per steradian.

Interoperability

The ability to communicate such information as temperature, illuminance levels, status of security devices, and occupancy among building systems and their controls.

Isotemperature

A set of coordinates within which all points have the same temperature. In a color space diagram, isotemperature lines represent lights with identical correlated color temperatures.

Junction temperature

For light emitting diodes, the temperature of the light-emitting portion of the device (see PN junction), which is inversely correlated with its light output.

Lamp

A radiant light source.

Lamp base position

The location of the lamp socket, either in the center of the top of the ballast or on the side of the ballast. Modular ballasts for circular compact fluorescent lamps (CFLs) have a lamp socket located at the end of a wiring harness.

Lamp current

The current flowing between a lamp's electrodes during operation.

Lamp efficacy

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

Lamp electrode voltage

Voltage to the electrodes to operate a lamp.

Lamp envelope

The shape of either the bare lamp or the capsule surrounding the lamp. Common shapes include quad, triple tube, four-tube, coiled tube, A-line, circular, square, globe, capsule (bullet), reflector, and decorative.

Lamp life

The median life span of a very large number of lamps (also known as the average rated life). Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started.

Lamp lumen depreciation (LLD)

The reduction in lamp light output that progressively occurs during lamp life.

Lamp operating current

Current flowing through a lamp during normal operation.

Lamp quantity and type

The number of lamps (in parentheses) used by the luminaire, followed by a generic designation indicating the type.

Lamp rated 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.

Lamp shield type

The material used in a luminaire to shield the lamp from the environment. Lamp shields are required by Underwriters Laboratories for some lamp types.

Lamp starting current

Current flowing through a lamp during starting operation.

Light loss

The reduced light output caused by a circuit-level power reducer expressed as a percentage of the light output without the circuit-level power reducer. (Full system output minus reduced output with a lighting-circuit power reducer divided by the full system output times 100.)

Light power density (LPD)

Sometimes referred to as power density. A measurement of the ratio of light output in an area and the electric power used to produce that light. LPD is determined by dividing the total light output by the total wattage consumed and is measured in lumens per watt.

Light trespass

A undesirable condition in which spill light is cast where it is not wanted.

Line voltage

The 110-120-volt household current, generally standard in North America.

Load capacity

The maximum total power that can be connected to an occupancy sensor.

Load shedding

The practice of turning off electrical devices during peak energy demand hours to reduce building energy use.

Louver

A fixed shield, usually divided into small cells, that is attached to the face of a luminaire to reduce direct glare.

Low battery voltage disconnect

Indicates whether or not an exit sign has a circuit that is designed to disconnect the battery after it is discharged. This circuit prevents damage to the battery. Lead acid and lead calcium batteries need this circuit, but nickel cadmium batteries do not.

Low-voltage circuit protection

Protection for a ballast's low-voltage control circuit from high voltage spikes. Does not apply to high-voltage controls.

Lumen (lm)

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. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation.

Lumen depreciation

The decrease in lumen output that occurs as a lamp is operated, until failure. Also referred to as lamp lumen depreciation (LLD).

Lumen maintenance

The ability of a lamp to retain its lumen output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1.

Lumen maintenance

The lumens produced by a light source at any given time during its operating life as a percentage of its lumens at the beginning of life.

Luminaire

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

Luminaire angle

The vertical (altitude) angle used in luminaire photometry to express the direction of the light output being measured. Light coming straight down is at 0° (the nadir).

Luminaire efficacy

The ratio of the measured light output of a luminaire to its active power, expressed in lumens per watt (LPW).

Luminaire efficiency

The ratio, expressed as a percentage, of the light output of a luminaire to the light output of the luminaire's lamp(s). Luminaire efficiency accounts for the optical and thermal effects that occur within the luminaire under standard test conditions.

Luminance

The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter).

Luminance contrast

Luminance contrast quantifies the relative brightness of an object against its background. It can range from zero and one. The closer the luminance contrast is to one, the greater the relative brightness of the object against its background.

Luminous flux

The rate of flow of light, measured in lumens. The overall light output of a lamp

Luminous intensity

The luminous flux on a small surface centered on and normal to the direction divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity can be expressed in candelas or in lumens per steradian.

Lux (lx)

A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle.