What are MR16 Lamps?

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Volume 6 Issue 2	September 2002

Average rated life - The number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life. Beam angle - The angle at which luminous intensity is 50 percent of the maximum intensity. Center beam candlepower (CBCP) - Center beam candlepower is the luminous intensity at the center of a beam, expressed in candelas (cd). Color rendering index (CRI) - A measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (CCT). A CRI of 100 represents the maximum value. A lower CRI value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. The cool white fluorescent lamp has a CRI of 62; fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above. Compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. 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). PAR lamp - An incandescent or tungsten-halogen incandescent lamp with a hard glass bulb and an interior reflecting surface, a precisely placed filament, and a lens to control beam spread. The lens is hermetically sealed to the reflector. Metal halide PAR-lamps are also now available. 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. 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. Ultraviolet - Any radiant energy within the wavelength range 100 to 400 nanometers is considered ultraviolet radiation (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m). Wavelength - The distance between two corresponding points of a given wave. Wavelengths of light are measured in nanometers (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m) Beam appearance - The description of the beam's image on a wall as determined by subjective visual evaluations of each lamp. The descriptive categories used are smooth, cloud, two-contour, ripple, and variegated. Candela - The Systeme International d'Unities (SI) of luminous intensity. One candela is one lumen per steradian. Formerly, candle. Dichroic coating (dichroic filter) - A multi-layer coating that transmits certain wavelengths and reflects those not transmitted. 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. Intensity (luminous intensity) - Total luminous flux within a given solid angle, in units of candelas, or lumens per steradian. 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. 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.) Correlated color temperature (CCT) - A specification of the apparent color of a light source relative to the color appearance of an ideal incandescent source held at a particular temperature and measured on the Kelvin (K) scale. The CCT rating for a lamp is a general indication of the warmth or coolness of its appearance. As CCT increases, the appearance of the source shifts from reddish white toward bluish white; therefore, the higher the color temperature, the cooler the color appearance. 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. Efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. 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. 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. 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. Beam spread - The width of a light beam, expressed in degrees. The beam of light from a reflector-type lamp (PAR, R, ER, or MR) can be thought of as a cone. The beam spread is the angular width of the cone. Common beam spreads are known as spot, narrow, narrow flood, and flood.

"MR" stands for multifaceted reflector, a pressed glass reflector with the inside (reflecting side) surface composed of facets and covered by a reflective coating. These facets provide optical control by gathering the light from the filament to create a concentrated beam of light. The reflectors of some MR lamps have a smooth inside surface instead of facets, but they are still called MR lamps by convention. Figure 2-1 shows MR16 lamps with different reflector types.

Figure 2-1. MR16 lamps with different reflector designs *

*Incandescent A-lamp on far right shown for size comparison

The light source of MR lamps is a single-ended quartz halogen filament capsule. The reflective coating of MR16 lamps can be either dichroic or aluminum. A dichroic coating is a thin, multi-layer dielectric (non-metallic film) that allows infrared radiation (heat) from the filament capsule to pass through the reflector while it reflects visible radiation (light) forward (see Figure 2-2). An aluminum coating is a thin film of aluminum that, unlike the dichroic coating, reflects both infrared and visible radiation. Some MR16 lamps have a cover glass on the front end of the reflector. This cover is a safety measure designed to contain any broken fragments in case the lamp shatters when it fails (see “What are the disadvantages of MR16 lamps?”).

Figure 2-2. Schematic diagram of how a dichroic coating works

MR lamps come in different sizes. The size is determined by the maximum diameter of the lamp in eighth-of-an-inch increments (1 inch equals 2.5 centimeters). The most common MR lamp, the MR16, is 16 eighths of an inch or 2 inches (5 centimeters) in diameter at its largest circumference, hence the name "MR16." Other sizes include MR8 (1 inch, or 2.5 centimeters, diameter) and MR11 (1-3/8 inch, or 3.5 centimeters, diameter). The power ratings of MR16 lamps used in architectural lighting applications range from 10 to 100 watts.

Most MR16 lamps are operated using voltages lower than 120 volts, typically 12 volts. Some MR16 lamps, however, operate using 6 or 24 volts. A transformer is needed to reduce the line voltage from 120 or 277 volts to the appropriate level for these lamps. MR16 lamps can be dimmed through commercially available dimmers for low-voltage loads. Most low-voltage lamps have 2-pin bases. Other bases are the bayonet and turn and lock. A new generation of MR16 lamps with an integrated transformer has been developed and marketed in the last few years. These lamps operate on 120 volts and have a screwbase to fit medium-base (Edison) sockets. Figure 2-3 shows samples of commercially available MR16 lamps with different base types.

Figure 2-3. Different MR16 lamp base types

(a) 2-pin
(b) medium screwbase with integral transformer
(c) intermediate screwbase with integral transformer
(d) turn and lock
(e) bayonet