Lighting Research Center Lighting Research Center
    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.
How valid are the claims regarding full-spectrum light sources?

Full-spectrum light sources and color perception. Full-spectrum light sources will probably provide excellent color rendering. Color rendering index (CRI) values for full-spectrum lighting sources are typically greater than 90. Color is a human perception constructed from the combination of the spectral power distribution (SPD) of the light source, the spectral reflectance of the materials being illuminated, and the tri-chromatic nature of the human visual system. If there are gaps or large variations in the SPD of a light source, there is a potential for confusion between the apparent colors of objects. Since full-spectrum light sources usually provide radiant power throughout the visible spectrum, subtle differences in the spectral reflectance characteristics of different objects are discernable. So, when color identification is part of the visual task, such as for graphic arts, museums and color printing applications, full-spectrum light sources will ensure good color discrimination.

Full-spectrum light sources and visual performance. Full-spectrum light sources will not provide better visual performance than other light sources under most circumstances. Visual performance is the speed and accuracy of processing achromatic information (e.g., black print on white paper) by the human visual system. At the relatively high light levels typically found in schools and offices, visual performance is essentially unaffected by the spectral power distribution of the light source, so full-spectrum light sources are, lumen for lumen, no better than any other light source.

Lighting produced by full-spectrum lamps may be, however, perceived as providing brighter architectural spaces than other lamps (Boyce, 2002; Berman, 1990). Three factors may contribute to this effect. First, full-spectrum light sources typically have a high correlated color temperature (CCT) of 5000K - 7500K. Lamps with higher CCT values produce greater brightness perception than lamps with lower CCT of the same luminance. Second, most full-spectrum light sources have high color rendering properties, meaning that surface colors will appear more saturated. Greater saturation will also give the impression of greater brightness (Boyce, 1977). Third, the ultraviolet (UV) radiation produced by some full-spectrum fluorescent lamps has a fluorescing, brightening effect on textiles and paper that have been treated with whitening agents. These combined effects on brightness perception may indeed have positive impact on building occupants, but greater perceived brightness can also be a liability, depending upon the expectations of the space's occupants (Veitch and McColl, 2001).

Full-spectrum light sources and health. Full-spectrum light sources will not provide better health than most other electric light sources. Recent research has shown that human daily activities are strongly influenced by the solar light/dark cycle. The most notable of these daily, or circadian, cycles is the sleep/wake cycle; but other activities including mental awareness, mood, and perhaps even the effectiveness of the immune system go through regular daily patterns. Light is the most important environmental stimulus for regulating these circadian cycles and synchronizing them to the solar day. Short wavelength (blue) light is particularly effective at regulating the circadian system; long wavelength (red) light is apparently inconsequential to the circadian system. Thus, to maximize efficiency in affecting the circadian system, a light source should not mimic a full spectrum, but instead should maximize only short wavelengths. Even if a full-spectrum light source includes short wavelength light in its spectrum, it will not necessarily ensure proper circadian regulation because, in addition, the proper intensity, timing, and duration of the light exposure are all equally important for satisfactory circadian regulation (Rea et. al, 2002).

Light therapy treatment of seasonal affective disorder (SAD) usually involves regulated exposure to a white light source, commonly 10,000 lux at the eye for 30 minutes per day (Partonen and Lönnqvist, 1998). Any white light source will be effective at these levels (Lam and Levitt, 1999), so full-spectrum light source is in no way special for treatment of SAD.

Full-spectrum light sources have no demonstrable benefit for dental health. These claims have no scientific merit (McColl and Veitch, 2001). The section, "Is ultraviolet radiation production important?" gives more detail.

Full-spectrum light sources and psychological benefits. Full-spectrum light sources may have psychological benefits, particularly in societies that place value on "natural" environments. One of the claims often associated with full-spectrum light sources is that they are most like natural daylight. Unlike full-spectrum electric light sources, however, daylight does not have a fixed spectrum. Rather, natural light varies with latitude, time of day, season, cloud cover, air pollution, ground reflectance, and, if a person is indoors, window tinting. Nevertheless, it cannot be denied that people consistently prefer natural lighting from windows and skylights to electrical lights. These preferences are robust and may reflect psychological associations with the natural environment that produce positive affect in many people. Positive affect induced by daylight may, in fact, help improve mood and motivation and thus increase productivity and retail sales. Full-spectrum light sources offer this positive association with daylight. Although positive psychological benefits from full-spectrum light sources may have been observed in some circumstances, there appears to be no biophysical explanation for those observations (Heschong, Wright & Okura, 2000). Still, the power of psychological associations cannot be denied and it is certainly conceivable that cleverly marketed full-spectrum light sources may provide beneficial effects to some people susceptible to that marketing. As NLPIP's survey demonstrated, there appears to be a strong positive association with full-spectrum light sources that has resulted from marketing, presumably because of the association between full-spectrum lighting and "natural" light.

 

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