Correlated Color Temperature
Originally, a term used to describe the "whiteness" of incandescent lamp light. Color temperature is directly related to the physical temperature of the filament in incandescent lamps so the Kelvin (absolute) temperature scale is used to describe color temperature. For discharge lamps where no hot filament is involved, the term "correlated color temperature" is used to indicate that the light appears "as if" the discharge is operating at a given color temperature. Chromaticity is expressed either in Kelvins (K) or as "x" and "y" coordinates on the CIE standard Chromaticity Diagram. Although it may not seem sensible, a higher temperature color (K) describes a visually cooler, bluer light source. Typical color temperatures are 2800K (incandescent), 3000K (halogen), 4100K (cool white or SP41 Fluorescent), and 5000K (daylight-simulating fluorescent colors such as Chroma 50 and SPX 50).
http://www.gelighting.com/na/institu...y_deg_tmpr_clr
Excellent point Yardboy, see thats another gimmie we take for granted so much so I didnt mention it. K is for Kelvin not thousand
Woops!
I got the below info from
http://www.interiorsmag.com/technology/ALM026.html
I usually like to use osram or GE but they dont have this type of detailed info up on their site.
COLOR TEMPERATURE
Because the apparent color of a blackbody depends upon its temperature, the idea evolved of using color temperature to describe the color of light sources. Color temperature refers to the absolute temperature of the laboratory blackbody when its visible radiation closely matches the color of the light source. That is, it is an approximate way of identifying the source as appearing warm or cool.
The Kelvin scale (at the left) is used to identify the color of light. It is the closely matching temperature and corresponding color of a laboratory blackbody heated through the various stages of incandescence from red to blue-white and referred to as the correlated color temperature (CCT). The color rendering index (CRI) for each source is shown in parentheses at the right.
The Kelvin scale is used to identify the temperature with a zero point at absolute zero (minus 459 degrees Fahrenheit) and intervals similar to those of the Celsius scale -- with the freezing and boiling points of water 100 degrees apart -- at 273K and 373K. A theoretical blackbody becomes red at 3000K, white at 5000K, blue-white at 8000K, and brilliant blue at 60,000K.
Because the light source color may not exactly match that of a blackbody, the term correlated color temperature (CCT) is used and expressed in Kelvins (K). Technically, the color temperature designation should apply only to sources with a continuous spectrum, such as incandescent lamps and natural light. It is, however, often used empirically to describe the degree of whiteness for other lamps, such as fluorescent, mercury, metal halide, and
high pressure sodium, where the spectral distribution is predominantly made up of peaks of energy rather than a continuous range. An accompanying diagram gives the correlated color temperatures for some of the more commonly used light sources, along with some comparative natural sunlight and skylight values.
COLOR RENDERING
Further characterization of electric light sources is sometimes referred to as the quality of light, or the color rendering capabilities of the source. The color rendering index (CRI) is expressed in numerical values based on comparing thr spectral energy content of a light source with that of a full-spectrum reference source. Based on a maximum value of 100 for full-spectrum natural light -- and values very close to 100 for incandescent lamps -- the index numbers are always less than 100. They typically range between 20 and 80 for most common light sources. CRI values appear in parentheses on the color temperature chart.
The development of CRI values is a complicated process and far from perfect. We are cautioned as designers that these values are intended only as a guide and that certain restrictions must be observed for optimum accuracy.
The must important limitation is said to be that color rendering values should be used only to compare light sources with similar color temperatures (6000K daylight fluorescent and clear mercury, for example). Common sense tells us, however, that a 5000K fluorescent lamp with a CRI of 90 will render colors better than a 2100K high pressure sodium lamp with a CRI of 22. Experience also tells us that 3000K and 4100K fluorescent lamps, with identical CRIs of 70 to 75, will make colors appear somewhat different -- even though both lamps will elicit a similar perceived level of quality or lack of color distortion.
Also, most observers can distinguish the difference between a tungsten halogen incandescent lamp with a color temperature of 3400K and a CRI of 99 and an ordinary incandescent lamp with a color temperature of 2800K and a CRI of 92. The tungsten halogen light will be whiter and render colors slightly more vividly, even though the difference in numerical values is small.
Good color rendition is also often associated with our subjective impressions and what seems familiar to us, such as the appearance of objects under incandescent lighting. Even though it is deficient in the blue range of wavelengths, incandescent has become accepted as normal through decades of use.
Similar to other decisions involved in the lighting design process, the selection of light source color must be based in large part on previous experience. However, the numerical references for color temperature and color rendering give us useful comparative guidelines to support our intuitive judgments.
The next Lighting Graphics column will show how different color sources can be combined to produce white light -- with interesting color shadows and accents.
November 1987, Architectural Lighting Magazine
