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.
Is ultraviolet radiation production important?

Some full-spectrum fluorescent lamps are promoted as producing ultraviolet (UV) radiation. This is peculiar, since in general, UV radiation should be avoided. UV radiation fades and deteriorates architectural materials and works of art. Even though full-spectrum lamps might improve the color appearance of artwork, museums specifically require all radiation shorter than 400 nm to be filtered completely from light sources illuminating environmentally sensitive pieces, such as watercolor paintings, and historical artifacts (Rea, 2000). Except in certain unusual cases, it is also undesirable for people to expose the eye or the skin to UV radiation. Adverse effects of excessive UV radiation include sunburn (erythema), cataracts, and skin cancer (Rea, 2000). Several organizations, including the Illuminating Engineering Society of North America (IESNA), the American Conference of Governmental Industrial Hygienists (ACGIH), and the National Institute for Occupational Safety and Health (NIOSH), have specified acceptable limits for occupational ultraviolet exposure (IESNA, 1996; ACGIH, 1991; NIOSH, 1972).

However, skin exposure to a fairly narrow band of UV radiation, UVB between 290 and 315 nm, can promote the synthesis of vitamin D, which is necessary for proper bone development and maintenance (Holick, 1985). Dietary sources of vitamin D, including dairy and fish products, provide sufficient vitamin D to have eliminated the incidence of bone-related problems such as rickets, in modern society (Jablonski and Chaplin, 2002.). These dietary supplements therefore minimize the importance of UVB radiation exposure for most people.

Full-spectrum fluorescent lamps that produce UV radiation use special phosphors with peak emissions in the UVA band (315 nm-400 nm), typically at 355 nm. Although the relative amount of UV radiation emitted by these lamps may be the same as a particular phase of daylight, the absolute amount of UV radiation they emit is quite small. For comparison, approximately 22 minutes of sunlight exposure near midday will produce 1.5 minimal erythema doses (MEDs) of UVB radiation exposure, enough to induce a pronounced temporary increase in vitamin D concentration (Holick, 1985). One MED is the amount of exposure that produces noticeable skin redness, so the exposure needed to affect vitamin D levels is substantial. Based upon the published data from one manufacturer of fluorescent lamps emulating the UVB content of daylight, it would take at least 30 hours of constant exposure to these lamps when operated at ceiling height to provide 1.5 MEDs (see the CASE STUDY Case Study). Based on UVB intensity data from another study (Ball, 2002), eight hours in an office or classroom under a claimed full-spectrum lamp will produce a smaller ultraviolet dose than one minute spent outdoors in bright sunlight.

Indoor environments such as offices and schools further reduce UV exposure because most lighting fixtures and architectural materials absorb UV radiation. UVB radiation that does not strike the skin directly is unlikely to reflect off objects, floors, and walls back to the skin. The resulting exposure level will be well below the threshold for measurable vitamin D production. Therefore, fluorescent lamps claiming to emulate the relative UV content in daylight can be disregarded as a viable source of UVB radiation for humans. Since there are no known benefits to human health from UVA radiation, it can be further concluded that the modest amounts of UV radiation produced by these lamps have no beneficial impact on human health.

Ironically, even small but constant amounts of UV (UVA and UVB) radiation will eventually degrade a wide variety of architectural materials such as carpet and cloth, wood products, and printed matter.

One claim occasionally cited as a benefit of fluorescent light sources emulating the UV content of daylight is the enhanced brightness of paper and clothing treated with whitening agents. Fluorescent whitening agents are used to counteract the otherwise yellow appearance of paper and cloth, making them appear whiter and brighter.

To assess this claim, NLPIP compared the relative luminance of white paper and of white cloth illuminated alternately by two fluorescent lamps of identical correlated color temperature (CCT), one claiming to emulate the relative UV content of daylight and one without the UV phosphor. By causing the whitening in the cloth or paper to fluoresce, the UV radiation from these lamps should produce higher luminance for the same given illuminance. Indeed, the measured luminance of a white paper sample and of a white cloth sample were 1.7% and 2.3% higher, respectively, when illuminated by the full-spectrum fluorescent lamp with more UV radiation. These effects were also perceptible when alternatively viewed, but any assumed benefits of these relatively small brightness-enhancing effects have never been documented. It should also be noted that lamps emulating the UV content of daylight have about 30% to 40% lower lamp efficacy (lumens per watt) than conventional fluorescent lamps of the same CCT, partly because additional electric power is required to generate the invisible UV radiation.

In summary, there are no known health effects from the UV radiation generated from these lamps, but the UV radiation from these lamps can be harmful to many of the materials commonly found in architectural spaces. Further, although the UV radiation generated from these lamps can induce relatively higher luminance in white paper and clothing, the loss in lamp efficacy needed to produce the invisible UV radiation is substantially greater than the fluorescence-induced luminance resulting from UV radiation.

 

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