Lighting Research Center Lighting Research Center
    Volume 11 Issue 1
May 2010    
LED Residential Under-cabinet Luminaires
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. lamp rated life - The number of operating hours at which half of a large group of product samples are expected to fail. The rated life is a median value of life expectancy; individual lamp life may vary considerably from the published rated life and operating conditions (e.g., temperature, hours per start) may affect actual life because rated life is based on standard test conditions. In addition, the way a product fails can vary by technology. For example, incandescent lamps abruptly stop producing any light while LEDs are considered to have failed when their light output drops below a certain fraction of the initial level. 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. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. luminous flux - Luminous radiant power, measured in lumens. The overall light output of a lamp or luminaire. 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. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. efficacy - The ratio of light output (in lumens) to input power (in watts), expressed as lumens per watt (LPW). illuminance - The density of luminous flux incident upon a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux. Models are listed first by light source, then alphabetically by brand, then by model number.
What other considerations are there when selecting an under-cabinet luminaire?

Additional factors to consider when purchasing an under-cabinet luminaire are the color of the light produced, warranty, the ability to electrically link multiple luminaires, and the shadows they create.

Light color

Correlated color temperature (CCT) indicates the “warmth” or “coolness” of the light, as shown in Figure 4. Lower CCTs provide a warmer appearance with a more yellow tone. Higher CCTs provide a cooler look with a bluer tone.

Figure 4. Color appearance.


Color appearance chart

ASSIST recommends Volume 2, Issue 1: A Homeowner's Guide to Residential Under-cabinet Lighting: Getting Good Lighting for Your Kitchen Counters suggests that for wood tones, warm-colored tiles, copper-toned metal backsplashes or warm-toned walls, look for warm CCTs in the 2700–3500 kelvin (K) range. For decorative glass tiles, glass shelves, cool-toned tiles or gray-toned metal colors, look for cool CCTs in the 3500–5000 K range. Also, consumers may want to match the color temperature of the overhead lighting in the kitchen. There is no good or bad color temperature; the consumer’s preference, taking into account the above guidelines, should determine this specification. Table 7 shows the CCT values of the under-cabinet luminaires tested by NLPIP. The values listed are the weighted means for each luminaire calculated by measuring the CCT of the light falling on a grid of measurement points on the horizontal surface and weighted by the illuminance at each point.

Color rendering index (CRI) is a measure of a lamp’s color rendering quality. Higher CRI values indicate that objects will appear more natural and viewers will be able to distinguish between different colors on those objects more easily. Table 7 shows the CRI of the under-cabinet luminaires tested by NLPIP. NLPIP staff measured the CRI of the LED under-cabinet luminaires using an integrating sphere following the IESNA LM-79 test standard. The CRI of the fluorescent luminaires was determined from the codes printed on the lamps by the manufacturers. The CRI of the xenon lamps is assumed to be 100 based upon the spectral power distribution of an incandescent filament.

Table 7. CCT of tested luminaires.
Light source Brand Model CCT on horizontal surface (K) CRI
LED GE 10408 2852 68
GE 10409 2872 68
Utilitech 283278 3111 70
Utilitech 283520 3864 73
Fluorescent GE 10113 3090 80-90
Utilitech 069486 3735 60-70
Xenon (incandescent) GE 10136 2617 100
Utilitech 283542 2525 100

The results of NLPIP testing shown in Table 7 indicate that the LED and fluorescent luminaires are available with either cool or warm light, while the xenon luminaires provide warm light.

Warranty

The under-cabinet LED luminaires identified for this study have no user-replaceable parts. Instead, the entire luminaire must be replaced. Despite the fact that manufacturers sometimes claim long lifetimes of LED products (e.g., the packaging of the Utilitech 283278 lists its lifetime as 50,000 hours) and that the cost of ownership is highly dependent on the lifetime, the warranty provided by manufacturers is similar to that of the fluorescent and xenon under-cabinet luminaires, as shown in Table 6. The packaging materials of only two of the luminaires provided details of the warranty, the Utilitech 283520 and Utilitech 283542; these luminaires are warranted “to be free from defects in materials and workmanship for a period of (1) year from the date of original purchase by the consumer… [Good Earth Lighting] will repair or replace (at [their] option) the unit in the original color, and style if available, or in a similar color and style if the original item has been discontinued, without charge, exclusive of bulbs.”

Table 8. Under-cabinet luminaire warranties.
Light source Brand Model Warranty length*
LED GE 10408 3 years
GE 10409 3 years
Utilitech 283278 NS
Utilitech 283520 1 year (exclusive of bulb)
Fluorescent GE 10113 3 years (exclusive of bulb)
Utilitech 069486 1 year (exclusive of bulb)
Xenon (incandescent) GE 10136 3 years (exclusive of bulb)
Utilitech 283542 1 year (exclusive of bulb)
* NS - Information not supplied on package materials
Linking

Some luminaires offer the feature of allowing “linking” or wiring from one luminaire to the next, which can be useful when installing a row of luminaires under long cabinets. The fluorescent models come with wire leads that provide the flexibility to be wired to either a junction box, a lead with a plug, or to an adjacent luminaire.

Table 9. Under-cabinet luminaires' ability to electrically link to one another.
Light source Brand Model Linking ability?
LED GE 10408 Yes, built-in socket
GE 10409 Yes, built-in socket
Utilitech 283278 No
Utilitech 283520 No
Fluorescent GE 10113 Yes, direct wire
Utilitech 069486 Yes, direct wire
Xenon (incandescent) GE 10136 Yes, built-in socket
Utilitech 283542 Yes, built-in socket
Dimming

None of the luminaires tested come with continuous dimming controls built in. The Utilitech 283542 can be turned on to a high or low setting.

NLPIP recommends using under-cabinet luminaires with a built-in dimmer switch or bilevel switch in order to give the user the flexibility to find a pleasing light level. Also, operating the luminaire at lower illumination levels when possible will save energy.

Aiming

The Utilitech 283542 is capable of tilting forward or backward to aim the light. The light distribution of other luminaires has a fixed direction — directly down toward the horizontal application area (countertop).

Shadows

Objects illuminated by under-cabinet luminaires with multiple lamps create multiple shadows on the counter surface, as shown in Figure 5, which some may find to be aesthetically displeasing. These discrete multiple shadows (left) are created by all of the LED models and the GE 10136 xenon model. The two fluorescent luminaires have lower-luminance light sources and a diffuser, so they create softer, graded shadows (right). The Utilitech 283542 is an intermediate case; sandblasted glass over the three halogen lamps provides a moderate amount of diffusion.

Figure 5. Discrete vs. soft shadows.

Discrete vs. soft shadows (click for larger image)
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