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.

Abstract

Lighting Answers:Manufacturers of full-spectrum light sources have claimed a variety of benefits for their products, including better visibility, improved color rendering, better health, and greater productivity. Light sources promoted as full-spectrum can cost over ten times as much as nearly identical products that do not bear the full-spectrum claim. This report addresses questions about full-spectrum light sources: What are full-spectrum light sources? How valid are the claimed benefits? Are these products worth the extra cost? This report also proposes a convenient definition for full-spectrum light sources that can be used to quantify the extent to which a given light source deviates from a full-spectrum light source. This revision presents an improved calculation method for this new full-spectrum metric.

Introduction

The term full-spectrum was coined in the 1960s by photobiologist Dr. John Ott to describe electric light sources that simulate the visible and ultraviolet (UV) spectrum of natural light. There are now dozens of electric lighting products marketed as full-spectrum, some promising that they closely simulate daylight and can therefore provide benefits such as better visibility, improved health, and greater productivity. Among the claims:

  • "…the closest replication of natural sunlight available." (Verilux, 2003)
  • "…simulates the full-color and balanced ultraviolet spectrum of natural outdoor light." (NaturalLighting.com, 2003)
  • "True daylight-balanced, full-spectrum light provides highest levels of see-ability; improves performance of color matching, close detail work and all visual tasks." (True Sun, 2003)
  • "Slows aging of the retina; reduces chance of skin cancer; improves general overall mood and feeling of well-being." (Full Spectrum Solutions, 2003)
  • "… perfect for plants, pets, and the treatment of seasonal affective disorder (SAD)." (Duro-Test, 2003)
  • "Helps reduce eye fatigue, and are especially effective over computer screens and other visually demanding tasks." (Duro-Test, 2003)
  • "Recommended for applications where high color discrimination is a must and to enhance people's sense of comfort and well being." (Lumiram, 2003)

Different companies have different ideas about what constitutes a full-spectrum light source, and what it is about full-spectrum light that yields the claimed benefits. Some insist that invisible-to-the-eye UV radiation is a necessary ingredient in full-spectrum light. Recently, several lighting products have emerged that reduce radiation in a small part of the visible spectrum in an effort to improve visibility—and these products are also called full-spectrum.

Most full-spectrum light sources are marketed at a premium price over other light sources, and they generally produce fewer lumens per watt than comparable light sources. If valid, the benefits claimed for full-spectrum light sources would seem to be well worth the additional expense and the loss in efficacy. But with each manufacturer making up its own definition of full-spectrum lighting, consumers have no way to know exactly what they are getting or what benefits to actually expect.

This Lighting Answers explores consumer perceptions about full-spectrum light sources and assesses the validity of various manufacturers' claims. This report also offers a definition of full-spectrum light sources.

 

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