Lighting Color Temperature in Basic Terms and Applications|
Article- January 2012 By Larson Electronics.com
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Very often, when we are about to purchase some form of lighting, whether it be for the home, garage or job site, we almost instinctively seek out lamps rated as being “warm white” in color. It’s not often though that we even give this choice much thought. For instance, just what do terms like “warm white” or “cool white” actually mean? Does it matter which we use, and are some color ratings more important or useful than others? For the most part, the average person chooses warm white simply because it is what they are accustomed to, and they really aren’t sure if another type might be better suited to their needs. The fact of the matter is that different color ratings do indeed have an impact on how effectively a chosen lamp will perform under specific conditions, and better performance can be had by matching a lamp to a particular application. However, in order to understand why this, it is first important to understand just what these color distinctions mean.
When we see lamps labeled as “warm white” or “cool white”, what we are looking at is the lamps color rating according to temperature. As you can guess, color temperature relates to heat, and denotes the temperature in Kelvins of a black body (piece of metal) as it heats and radiates light energy. For example, the wire filament in an incandescent lamp when no current is present is cold, and thus no heat or light energy is being radiated, thus it is “black”. If we apply current and cause the filament to heat up due to electrical resistance, the filament begins to radiate energy as light as well as heat, and we can then begin to note the temperature of the filament at the point where it begins radiating light. A small amount of current will cause the filament to heat up a little and glow reddish orange, higher current will heat it more and cause the radiated light color to shift to yellow, and still higher current will cause the filament to become even hotter and begin radiating yellowish white light.
Now, while simply applying more current is one way to make the filament hotter and thus change the color temperature, the more practical way this is done in real world applications is by changing the properties of the filament and thus altering how well it can conduct electrical current. Either way, making the filament hotter either by increasing current or modifying the filament, raises the color temperature of the light emitted. Regardless of how it is done however, the temperature at which light of a certain perceived color is radiated generally remains the same. The following chart provides a general example of how light color shifts as temperature increases. As can be seen, lower temperatures produce light in the yellowish-orange to red part of the light spectrum, while mid-range temperatures produce yellowish white and eventually bluish white light at very high temperatures.
Temperature Ranges and Application:
Lamps with color temps in the lower ranges of 2500 to 3000 Kelvin are generally considered warm and preferred for common residential uses due to the soft and subdued properties of the yellowish white light they produce. Light with a temperature of 5000 Kelvin and above is considered cool because it tends to be bluish white in color. This might initially seem somewhat contradictory since hotter temps are considered “cool” and vice versa, but the terminology is more intended to refer to perceptual appearances rather than actual temperature since how light is perceived is far more relevant in real world applications and easier to associate with appearance than actual temperatures.
Cool White and Warm White:
Lamps considered warm white are most commonly used in residential settings where a subdued and more relaxing type of illumination is desired. Warm white lighting has moderate to poor color rendering, but produces a softer and more diffused quality of light that is pleasing to the eye.
Commercial applications most often make use of lamps producing cool white light in the 4000 to 6500 Kelvin range because their light is sharper and more intense, with better color rendering and contrasting properties. Objects illuminated by these lamps appear brighter, more sharply defined, making these lamps well suited for offices, retail stores and commercial areas where detail and sharpness is important.
Lamps with temperatures around 5000-5500 Kelvin are usually considered to produce natural or “daylight” quality illumination and used where it is important to closely mimic how objects would appear under natural sunlight conditions. These lamps are typically used in automotive paint booths, retail showrooms, and in agricultural applications where the most natural appearing light possible is desired.
Lamps in the 6000 Kelvin range and up are considered cold and generally preferred for commercial and industrial applications due to the high intensity and sharp contrasting produced by their strong shift towards the blue-white spectrum. Lamps in these ranges are commonly found in warehouses, construction sites, and in large scale lighting applications like stadium or arena lighting where sharp detail and extended range is needed.
The chart below shows some of the average color temps for common light sources. It must be noted, that each type of lamp generally adheres to a somewhat broad temperature range and they often vary within their specific types according to minor differences in design. Manufacturers routinely modify lighting type designs to give them a broader application range, shifting their color temperature to some degree either up or down in order to fulfill a specific application. However, most types of lights are limited by their design and thus remain well within a set temperature range
Correlated Color Temperature:
At the basic level, color temperature is based on the temperature of a non radiating black body as we noted earlier. This is usually assumed to be a metal filament that when heated radiates light energy of differing perceived colors according to the temperature of the filament. However, incandescent lights are becoming obsolete, and there are many varieties of lamps which do not rely on heating a black body to produce light energy. As a result, it is necessary to relate perceived color temperature of these non-filament lamps to that of a black body radiator which to the human eye most closely matches the light from the lamp. As a result, non-filament lighting like fluorescents and LEDs are assigned a correlated color temperature in order to provide homogeny across lamp types.