Lighting Research Center

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    Volume 7 Issue 5
September 2003 (revised March 2005)    
Full-Spectrum Light Sources
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. 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. minimal erythema dose (MED) - The quantity of ultraviolet radiation (expressed in Joules per square meter) required to produce the first perceptible, redness reaction on human skin with clearly defined borders. MED can vary significantly depending on factors such as skin pigmentation. x-bar - Color matching function x-bar, y-bar, z-bar are used to define the color-matching properties of the CIE 1931 standard observer. In 1931, CIE defined the color-matching functions x-bar, y-bar, z-bar in the wavelength range from 380nm to 780 nm at wavelength intervals of 5nm. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. positive affect - Relatively mild shifts in current mood in a positive direction.
What are full-spectrum light sources?

The term full-spectrum is not a technical term, but rather a marketing term implying a smooth and continuous spectral power distribution (SPD) without the spikes and troughs in radiant energy common with most discharge light sources (e.g., fluorescent and metal halide). Full-spectrum products are usually marketed as electric light sources that emulate natural daylight; the explicit or implicit message is that "natural" daylight is always better than "artificial" electric light. Some full-spectrum light sources are also marketed as emitting ultraviolet (UV) radiation, as well as visible light.

Figure 1 shows the SPDs of two electric light sources claimed to be full-spectrum, the Duro-Test Vita-Lite 5500 fluorescent lamp and GE Reveal 60W incandescent lamp, as well as daylight at 5500K correlated color temperature (CCT). Daylight has a smooth SPD without sharp spikes or dips, whereas the claimed full-spectrum lamps have SPDs that differ from daylight and from each other.

Figure 1. Spectral power distribution comparison


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