ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation.
ballast factor (BF) - The ratio of the light output of a fluorescent lamp or lamps operated on a ballast to the light output of the lamp(s) operated on a standard (reference) ballast. Ballast factor depends on both the ballast and the lamp type; a single ballast can have several ballast factors depending on lamp type.
color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural.
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.
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.
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.
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.
lamp lumen depreciation (LLD) - The reduction in lamp light output that progressively occurs during lamp life.
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.
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 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.
flicker - A rapid and continuous change in light levels caused by the modulation of the light output from fluorescent lamps.
correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT.
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.
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.
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).
phosphors - Materials used in a light source to produce or modify its spectral emission distribution. In fluorescent and high intensity discharge lamps, the phosphors fluoresce (emit visible light) when excited by ultraviolet radiation produced by mercury vapor inside the lamp when energized by an electric arc. In a light emitting diode, phosphors convert short-wavelength light or ultraviolet radiation produced by a semiconductor die into longer-wavelength light, usually with the goal of producing white illumination.
dimming ballast - A device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic.
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.
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.
power factor (PF) - The ratio of active power (in watts) to apparent power (in rms volt-amperes), power factor is a measure of how effectively an electric load converts power into useful work. Power factor (PF) is calculated using the equation
PF = (active power) / [(rms voltage) x (rms current)].
Phase displacement and current distortion both reduce power factor. A power factor of 0.9 or greater indicates a high power factor ballast.
medium bi-pin - A type of connector commonly used on T-8 and T-12 fluorescent lamps. Two small pins protrude from the lamp ends, which are inserted into a socket in the fixture.
miniature bi-pin - A type of connector commonly used on T-5 lamps. Similar in design to but smaller than medium bi-pin connectors, it uses two small pins that protrude from the lamp ends and are inserted into a fixture socket.
photosensor - A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient.
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.
programmed start - Refers to a type of rapid start ballast that optimizes the starting process by waiting until the lamp’s electrodes have been heated to apply the starting voltage, thus easing the load to the electrode and extending lamp life. Standard rapid start ballasts heat the electrodes during the starting process to allow quicker starting without flicker.
lamp efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW).
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.
luminous flux - Luminous radiant power, measured in lumens. The overall light output of a lamp or luminaire.
direct luminaire - A luminaire that emits light in the general direction of the task to be illuminated. The term usually refers to luminaires that emit light in a downward direction.
color rendering - A general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source.
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.
ambient temperature - The temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp.
preheating time - Also referred to as preheat time and lamp preheat time. The length of time that a ballast heats a lamp’s electrodes before initiating the lamp arc. Rapid start ballasts preheat a lamp before initiating the arc in order to ease starting. Too short or too l
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.
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).
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.
lumen maintenance - The ability of a lamp to retain its light 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.
uniformity - The degree of variation of illuminance over a given plane. Greater uniformity means less variation of illuminance. The uniformity ratio of illuminance is a measure of that variation expressed as either the ratio of the minimum to the maximum illuminance or the ratio of the minimum to the average illuminance.
wall-washing - The practice of illuminating vertical surfaces, such as walls. Wall-washer luminaries are designed to illuminate vertical surfaces.
Do T5 lamps have better lumen maintenance than T8 or T12 lamps?
Lumen maintenance refers to how a lamp maintains its light output over
its lifetime. Both T5 and T5 high output (T5 HO) lamps maintain a higher
light output than T12 and most T8 lamps. Manufacturers claim that T5
and T5 HO lamps retain more than 95% of light output at 8,000 burning
hours (40% of rated average life). An improved phosphor coating reduces
mercury absorption, which is one of the main causes of lamp
lumen depreciation, leading to a higher lumen maintenance value.
Also, because the T5 is smaller in diameter, it has a much smaller surface
area. Manufacturers use only tri-phosphors
in the T5, compared to the T8 and T12, in which there may be a mix of
tri-phosphors with halo-phosphors.
Figure 9 illustrates lumen maintenance curves for T5, T8, and T12 lamps
(see How long do T5 lamps last before they burn out?
). In Figure 9, T12 lamps are labeled as "CW/HO," which
stands for cool white high output.
Figure 9. Lumen Maintenance Curves of T5, T8, and T12 Lamps as a Function of Burning Time
This diagram is based on OSRAM SYLVANIA's catalog, "Product Information Bulletin-PENTRON & PENTRON HO LINEAR T5."