Light Sources and Ballasts

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Incandescent Lamps

Incandescent lamps are filament lamps that generate light by passing current through a tungsten filament, which heats the tungsten to a high temperature that causes it to glow (emit light). The hotter the filament, the higher the color temperature of the light. This method of generating light is inherently inefficient, however, as significantly more heat is generated than light.

The color quality of light generated by a glowing filament is generally quite good, with a smooth spectral power distribution that has increased output with increased wavelength, creating warm light (CCT of approximately 2800 for standard incandescent lamps operated at rated voltage) with high CRI.

Because the filament can become exceedingly bright, some incandescent lamps are frosted to diffuse the emitted light over the surface area of the bulb. Clear bulbs permit more optical control, since reflectors and other optical devices can be designed to carefully control light from a small filament.

Both the Energy Policy Act of 1992 (EPACT) and recent legislation (Energy Independence Security Act, 2007) have effectively outlawed a number of commonly used incandescent lamps, with the 2007 legislation outlawing many of the standard lamps used in residences in phases between 2012 and 2014. After these dates, incandescent lamps that are sold in the U.S. will be required to meet specific energy efficiency and lamp life targets. The goal is for consumers to switch to comparable compact fluorescent lamps, which offer significant energy savings. Some incandescent products will be converted to EISA compliant halogen products.

Incandescent lamps are easily dimmed by lowering the supplied voltage to the lamp. Dimming results in a reduction in both efficacy and lamp color temperature.

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Lead Author(s): Rick Mistrick

Halogen Lamps

Halogen lamps are a special form of filament lamp where halogens, elements such as iodine or bromine, are added to the gas surrounding the filament. The construction of these lamps involves a fused quartz capsule that closely surrounds a small spiral/wound filament. The halogen and inert gasses contained within this capsule are at much higher pressure than used in standard incandescent lamps. In some lamps, this capsule may be placed within a second and larger hard glass envelope, such as with larger reflector lamps. The small capsule is designed to produce higher gas and bulb wall temperatures than are present in a standard incandescent lamp. At these temperatures, the halogen particles capture any tungsten molecules that have evaporated from the filament and redeposit them back onto the hot filament. This increases lamp life and reduces lumen losses due to the bulb wall blackening (caused by the evaporated tungsten). The high operating temperatures also increase lamp efficacy (lumens per watt) and lamp color temperature over that of a standard incandescent lamp.

Halogen lamps are available in both line and low voltage varieties, with low voltage lamps typically being operated at 12 V, which requires a transformer. Low voltage lamps permit the use of smaller filaments, which provide the opportunity to achieve tight optical control. Halogen lamps, like standard incandescent lamps are easily dimmed, however the low voltage varieties require a dimmer that is compatible with the type of transformer being used (which can be either magnetic or electronic). Dimming results in a reduction in lamp efficacy and color temperature and will also temporarily halt the halogen cycle that returns tungsten to the lamp filament.

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Lead Author(s): Rick Mistrick

Fluorescent Lamps

Fluorescent lamps are a discharge lamp and produce light by passing an arc across a gas. In the case of fluorescent lamps, the gas is low-pressure mercury with another inert gas such as argon, xenon, neon, or krypton. The plasma arc causes the mercury molecules to emit a significant amount of ultraviolet (UV) radiation as well as some visible light. In a fluorescent lamp, the inside wall of the glass bulb is coated with a fine phosphor dust. The phosphors absorb the UV radiation and re-radiate visible wavelengths. Fluorescent lamps come in a wide range of CCTs with different CRI values based on the composition of phosphors that have been applied in the lamp.

Fluorescent lamps require a ballast to energize and maintain the arc. Today, most ballasts are electronic, as opposed to the older magnetic versions which consumed more energy. Electronic ballasts also operate at much higher frequencies than magnetic ballasts. Dimming of fluorescent lamps is possible, but requires a dimming ballast.

Common sizes of linear fluorescent lamps are T5 and T8, where the number indicates the diameter of the bulb in eighths of an inch. Older model T12 lamps are prime candidates for retrofit to T8 models, which utilize the same socket, but require different ballasts. T5 lamps are metric lamps, and are shorter than comparable T8 and T12 versions.

Fluorescent lamps are commonly used in building interiors due to their relatively high system efficacy (as high as 80-100 lumens per watt), which is roughly 4 times that of an incandescent lamp. Because of the size of the lamp, fluorescent lamps do not generally allow for tight beam control, except perhaps for some applications requiring a continous, well controlled wash of light, where a T5 lamp and linear reflector can perform quite well.

Fluorescent lamp output will vary with temperature. Most lamps reach their maximum output in ambient conditions of 25 C, although this occurs at 35 C for T5 and some compact fluorescent lamps. Higher or lower temperatures result in reduced lamp output.

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Lead Author(s): Rick Mistrick

High intensity discharge lamps

High Intensity Discharge (HID) lamps include a variety of different lamp types, including mercury, which was the first type of HID lamp but is now virtually obsolete due to its low efficacy and relatively poor color rendering; metal halide (MH); and high pressure sodium (HPS. As discharge lamps, each of these requires a ballast to start and operate the lamp.

All HID lamps are constructed with an arc tube that is centered within a larger outer bulb. A variety of different outer bulb shapes are available. When the optical situation requires a small point-like source for better optical control, the outer bulb is clear. When direct view of the arc is likely to cause excessive glare, and precise light control is not required, an HID lamp with a frosted outer bulb may be applied in a luminaire.

The advantages of HID lamps include their ability to generate large amounts of light with relatively high efficacy from a relatively small source. Disadvantages include warm-up time, restrike time if power to the lamp is temporarily interrupted, and a few other lamp-specific qualities that will be discussed below.

Metal Halide

Metal halide lamps were developed in the mid 1960s. They involve a mercury arc that includes metal halide salts in the arc tube to emit light at a variety of wavelengths. The spectral makeup of metal halide lamps provides good color rendering while also permitting the color temperature to be tuned to either a warm or cool source. Metal halide lamps are available with CCTs ranging from 3000 to 4200K and CRIs ranging from 65 to 95.

MH lamps are available in a broad range of wattages and shapes, from 20W through 1500W (and sometimes higher) that include both reflector and standard types with screw- base, pin-base and double-ended sockets. Metal Halide lamps are available in a variety of different configurations, including standard probe start, pulse start and ceramic metal halide.

Standard MH lamps

Standard MH lamps are most likely probe start lamps with an arc tube constructed of fused quartz glass. Inherent problems with MH lamps include color shift that occurs across time and in some cases variations in lamp color when new, resulting in lamps of small, but noticeable color difference – slight tints of pink, purple, green, etc. that differ by lamp. Another potential drawback of MH lamps is their susceptibility to arc tube rupture, which can break the outer bulb and cause hot glass to fall from open luminaires. Luminaires over potentially combustible materials should have enclosed housings that safeguard against this by containing any ruptured lamp material. To help prevent ruptures, MH lamps should not be operated continuously, and must be turned off for at least 15 minutes at least once per week.

Pulse Start MH lamps

Pulse start MH lamps offer improved performance over standard MH lamps. Probe starting helps to maintain higher lamp lumen output over time with reduced color shift, longer life and faster warm-up and restrike times. The improvement in Lamp Lumen Depreciation ( LLD) is important since conventional MH lamps have perhaps the greatest light loss all lamp types over their lifetime.

Ceramic MH Lamps

Ceramic metal halide lamps (CMH) utilize an arc tube of similar material to that used in the High Pressure Sodium lamp. This ceramic material helps to stabilize the material within the arc tube resulting in more uniform color appearance over time. CMH lamps also have higher efficacies than standard metal halide lamps. They were primarily developed for indoor applications and are available in many of the lower wattage sizes and shapes. High Pressure Sodium (HPS) Lamps

HPS lamps have long been used for exterior lighting due to their high efficacy. They emit light that is quite warm at 2000-2200K, but of relatively poor CRI (22). Due to the highly reactive nature of sodium, the arc tube in a HPS lamp is constructed of a translucent ceramic material, and is characteristically long and thin. HPS lamps will typically cycle on and off at the end of their useful life. Non-cycling lamps are available that are shut down once their end of life is reached.

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Lead Author(s): Rick Mistrick


Lighting Emitting Diodes (LEDs) belong to a group of light sources that are classified as solid state lighting (SSL). LEDs are semiconductor diodes that emit light when an electrical charge is applied to them. Being diodes, these electronic devices only permit current to pass in one direction. LEDs are constructed of two dissimilar materials in a very thin layer to form a PN junction (PN stands for positive-negative). When a charge is applied to this junction, the semiconductor material emits light as positively and negatively charged particles are combined. The type of semiconductor material from which the LED is constructed determines the color of the emitted light. LEDs are available in a wide variety of colors. White LEDs are also available, where white light is created by the addition of phosphors to the LED to create white light. White light can also be produced by the integration of three or more colored LEDs.

LEDs have the advantage of a very long life. The life of an LED is based on the projected point at which light output will have depreciated to 70% (most common) or 50% of the initial output. For this reason, when designing with LEDs, the lamp lumen depreciation factor should be the fraction of the initial light output that will eventually be reached before the lamp is replaced (such as the 0.7 value). LEDs are susceptible to variations in both light output and operating life with changes in junction temperature during operation. The PN junction temperature is critical and should be kept as low as possible. This is why most LED lamps and luminaires have fins and other heat dissipation devices to cool the lamps. LEDs are excellent sources for use in refrigeration equipment due to their excellent performance under cold conditions.

LEDs operate primarily on DC current at low voltages. The electrical device that converts typical AC power to DC for the purpose of operating LEDs is referred to as a driver. Some LED drivers are also capable of dimming the LED lamp. In most cases, LED dimming is performed using phase dimming, where the current is switched on and off at a very high rate to dim the LED. Different LEDs have different forward currents. LED drivers typically are designed to deliver constant current to the LEDs.

LEDs are available in a variety of different configurations. In most cases, the are produced on printed circuit boards. They have been integrated into lamp housings to create the equivalent of PAR lamps, A-lamps and even fluorescent-like tubes. These lamps typically have fins and other heat dissipation devices to minimize the pn junction temperature.

LEDs are very small in size and therefore very bright. A refractor or other light diffusing element is typically placed between the LED and a space to avoid having occupants directly view the light emitting element. Many LEDs are fitted with an epoxy cover, the shape of which determines the distribution of the emitted light.

LEDs are current-driven devices, with the amount of current affecting the junction temperature, which will cause a deterioration in both lumen output and lamp life if it becomes too high.

LEDs are most commonly applied today in outdoor lighting, signage, downlighting, wall-washing and task lighting.

When LEDs are manufactured, they are not created with very precise spectral power distributions, but deviate slightly in color from one LED to another. For this reason, the LEDs are tested and “binned” based on their color properties. The LEDs with similar performance are then sold together to provide more consistent color across different LED sources.

LEDs are more difficult to measure photometrically than other more traditional light sources. Once reason is that a luminaire’s performance is inherently tied to the thermal management provided by the luminaire. It makes little sense to quote lumen output values for LEDs that are used within luminaires since the LEDs are generally not interchangeable and have been selected specifically for use within the luminaire. Hence, the photometry used for LED luminaires is most commonly absolute photometry, as opposed to the relative photometry that is used for most other lighting equipment (where the luminaire performance is based on the manufacturer’s rated lumens for the bare lamp). Because absolute photometry is used, LED luminaires will typically be listed as having luminaire efficiencies of 100%, since there is no bare lamp data against which to compare the total emitted luminaire lumens. In comparing LED and other luminaires, it is important to assess only the emitted lumens and the luminaire candlepower distribution, and not apply luminaire efficiency ratings.

Section Key Resources

Lead Author(s): Rick Mistrick

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