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.
Summary

LEDs have strengths and weaknesses in comparison with other light sources. Figure 6 is a radar chart showing the strengths of each under-cabinet luminaire tested. The top-right axis shows the measured or estimated CRI of the light produced. The top-left axis shows the total luminous flux on the application area in lumens. The right axis is a measure of the uniformity. The value presented is the minimum illuminance on the horizontal application area divided by the average illumination on the horizontal application area (the inverse of the uniformity ratio shown in Table 4). The bottom-right axis shows the application efficacy in lumens per watt. The bottom-left axis shows the cost savings of the luminaire, represented as difference between the lifetime cost of the luminaire and the lifetime cost of the most expensive model. On all axes, the further the line is from the origin, the better.

In general, the LED models produce less light than the others but can result in a somewhat lower cost of ownership and better uniformity than the fluorescent models. The xenon models have the highest CRI and produce a similar amount of light as the fluorescent models but have low efficacy and are relatively expensive to own. The two fluorescent models tested produce about as much light as the xenon luminaires and are balanced on uniformity, cost savings, and application efficacy, in that their results do not tend to any extremes. There was a large difference in the CRI of the two fluorescent luminaires because they use different phosphor technologies.

Figure 6. Radar chart of LED under-cabinet luminaires showing
luminous flux, CRI, uniformity, application efficacy,
and cost savings.

radar chart

 

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