The terms "daylight lamp" and "full-spectrum lamp" are often used interchangeably, and as of publication there are no regulations regarding the specific use of the terms. While both terms may be used to represent the same lamp, sometimes called a "daylight full-spectrum lamp," each term focuses on a different aspect of the lamp's characteristics.
"Full-spectrum" describes the ability of a lamp to put out light that contains all colors of the lighting spectrum. However, it is a marketing term rather than a technical description, so each manufacturer decides exactly what constitutes a full spectrum. The term was first used in the '60s by Dr. John Ott. Under his definition, a full-spectrum light source contained both visible and invisible color wavelengths of light. Daylight lamps were developed based on the need to simulate daylight for health reasons in people who were denied exposure to the sun, such as servicemen on Navy submarines and astronauts in the space program.
Fluorescent full-spectrum lamps--the type originally developed--do not present the different wavelengths of the spectrum evenly. Graphs of their outputs show spikes at various color temperatures, while natural daylight shows more of a bell curve on a graph. Lamp manufacturers have focused on recreating that even spectrum representation in their lamps, marketing them as daylight, rather than full-spectrum, lamps. One of the primary methods for doing this was moving from fluorescent lighting fixtures toward tungsten-based fixtures, and later, halogen fixtures.
Color temperature is a measurement of a light's color based on the way heat affects color. It is measured in degrees Kelvin. This system can be confusing because the higher the temperature in degrees Kelvin, the cooler the color of the light. Lower numbers on the scale represent yellows and oranges, while higher numbers represent blues. Natural daylight is in the blue end of the spectrum at approximately 5,500 degrees Kelvin, and manufactured daylight lamps have color temperatures in the 5,500 to 6,500 degrees Kelvin range. Full-spectrum lamps may or may not use this color temperature range. When they do, they are often referred to as "daylight balanced."
In addition to the visible color spectrum that runs from the warm yellows and reds to the cool blues and purples, there are known invisible wavelengths of light at the warm end, called infrared (IR), and at the cool end, called ultraviolet (UV). Both of these wavelengths can cause damage in the form of medical problems, including skin cancer, but they also have benefits, including possible strengthening of the immune system and helping the body absorb vitamin D. Full-spectrum lamp manufacturers often try to manipulate their products' outputs at these extremes of the spectrum, either eliminating IR and UV elements completely or incorporating only the beneficial ranges of those wavelengths. Daylight lamps are less likely to have the invisible wavelengths modified and may give off both beneficial and harmful UV and IR wavelengths.
Lamps marketed as full-spectrum and daylight are both more expensive than regular light bulbs, even bulbs that share the same light output properties but simply don't use the full-spectrum or daylight marketing terms. Full-spectrum bulbs, when bought as single units, can cost up to 10 times more than a standard tungsten bulb. Daylight bulbs are about half the price of full-spectrum bulbs.