What are full-spectrum light sources?

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Volume 7 Issue 5	September 2003 (revised March 2005)

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. 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. 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.

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