What are the important performance characteristics of 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 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. 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 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. 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 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. 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.

There are seven important performance characteristics to consider: lamp life, correlated color temperature (CCT), color rendering index (CRI), lumen maintenance, beam angle and center beam candlepower, lumen output, and luminous efficacy.

Lamp life:

According to manufacturers' catalogs, the average rated life of MR16 lamps used in architectural lighting applications ranges between 2000 and 10,000 hours. This lifespan is equivalent to 8 months to 3 years and 4 months, respectively, assuming the lamps are on 8 hours per day. Lowering the supply voltage slightly from its specified operating voltage can extend the life of an MR16 lamp.

Correlated color temperature (CCT):

An MR16 lamp produces light by the incandescence of its tungsten filament. This method of light production, along with the reflector characteristics, determines the lamp’s color characteristics. MR16 lamps have a CCT of between 2800 K and 3200 K depending on the manufacturer and type of lamp. The CCT of MR16 lamps is higher than that of general incandescent lamps because their filament temperature is higher due to a more compact filament size made for low-voltage use. In addition, the dichroic coatings on the reflectors of MR16 lamps remove some long-wavelength light, resulting in higher CCTs. A handful of manufacturers have designed their dichroic reflectors to remove additional long-wavelength light to obtain MR16 lamps with CCTs of up to 4700 K (see “What are MR16 lamps?").

Color rendering index (CRI):

The CRI of MR16 lamps ranges between 95 and 100. The color of the light may change as the lamps age. The most common reason for this change is the degradation of the reflector coating over time. Degradation occurs because of the decomposition of the dichroic coating or oxidation of the metallic (aluminum) coating materials. All coating materials undergo this process, but some of them withstand decomposition longer than others. The quality of reflector coatings differs from manufacturer to manufacturer and even between products by the same manufacturer.

Lumen maintenance:

There are two lamp components that determine the lumen maintenance of MR16 lamps: the halogen filament capsule within the lamp and the reflector. The lumen maintenance of the filament capsule is excellent because the regenerative halogen cycle that occurs within the filament capsule keeps the bulb wall from blackening. The halogen gas removes evaporated tungsten from the bulb wall, preventing bulb wall blackening, and in turn, keeping the lumen output relatively constant over time. This process results in higher lumen maintenance than that of non-halogen incandescent lamps. The lumen maintenance of halogen filament capsules varies depending on the quality of components; at 40% of the rated life, it may be as high as 95% (Rea 2000). The reflector, on the other hand, can affect lumen maintenance negatively over time because of degradation of the coating material or dirt accumulation.

Beam angle and center beam candlepower:

Beam angle and center beam candlepower (CBCP) are performance parameters that characterize the beam appearance and the maximum beam intensity of a directional lamp. According to manufacturers' catalogs, the beam angles of MR16 lamps range from 7 to 60 degrees, and their CBCP may range from about 500 up to 15,000 candelas, depending on different wattage and beam angle combinations.

Lumen output:

Most lamp manufacturers do not publish lumen output ratings for MR16 lamps or other reflectorized lamps in their catalogs. Instead, they publish beam angle and CBCP, which provide more accurate information about the performance characteristics of the lamp. NLPIP tested several 50-watt MR16 samples of the same type (EXN) to determine their lumen output, which ranged between 560 lumens to 710 lumens, and averaged 625 lumens.

Luminous efficacy:

In general, low-voltage halogen lamps have higher efficacies than common incandescent lamps because the low-voltage filament is more compact than a 120-volt filament. The low-voltage filament does not need as much electric power to keep it hot. When a low-voltage lamp uses a reflector, its efficacy decreases because of light losses associated with light absorption. NLPIP tested several MR16 samples to determine their efficacy. Efficacies ranged from 12 to 15 lumens per watt. These results account for the light losses from the reflector but not the losses of the transformer, which are 10%, as a rule of thumb. MR16 lamps that use an infrared (IR) coating on their filament capsules have higher efficacies than non IR-coated lamps. The IR coating used in MR16 lamps is a relatively new technology. It is a multi-layer dielectric film used on the outside surface of the filament capsule to reflect the IR radiation from the filament back to re-heat the filament. This reheating causes the luminous efficacy of the tungsten incandescence radiation to increase because less electrical power is required to heat the filament.

Although reflector lamps, in general, may have lower luminous efficacies than non-directional sources such as compact fluorescent lamps (CFLs), their light distribution makes them more effective for directing light in a specific direction without wasting much light elsewhere. Figure 3-1 shows the lighting effects produced on a painting by a 20-watt reflector CFL and a 20-watt MR16 lamp. Figure 3-2 shows how the MR16 lamp provides higher illuminances on the painting compared to the reflector CFL. The light from the MR16 lamp is concentrated within its beam spread with little light spill outside the painting. MR16 lamps have much better optical control and can produce much narrower beam angles while CFLs, although having higher efficacies (lumens per watt), cannot be easily controlled from an optical point of view because the light source is larger. Figure 3-3 shows a plot of the illuminance measurements across the horizontal centerline of the painting for both lamps. As this graph shows, the illuminances measured at the grid points in the painting area are higher for the MR16 lamp than for the reflector CFL of the same wattage. The average illuminance on the painting is 680 lux (63.2 footcandles) for the 20-watt MR16, versus 375 lux (34.9 footcandles) for the 20-watt reflector CFL. To achieve the same illuminances on the painting using a CFL, one would have to increase the wattage of the CFL lamp by 80%. In this case, the MR16 lamp is more effective in delivering the light where it is needed, which could result in energy savings.

Figure 3-1. Lighting effects of a reflector CFL and an MR16 lamp

20-watt R40 CFL	20-watt 40° MR16 lamp

Figure 3-2. Illuminances (lux) on and around the painting from a reflector CFL and an MR16 lamp

20-watt R40 CFL	20-watt 40° MR16 lamp

Figure 3-3. Horizontal centerline illuminance plot of a reflector CFL and an MR16 lamp

*1 lux = 0.0929 footcandles

20-watt R40 CFL vs. 20-watt 40° MR16 lamp