What is Color Temperature?
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Color temperature is measured in degrees Kelvin (K). The color temperature of a lamp (bulb) describes how the light appears when the human eye looks directly at the illuminated bulb. Color temperature is measured by a unit called the Kelvin (K). The Kelvin thermodynamic temperature scale is defined so that absolute zero is 0 kelvins (K). (Note: The Celsius and Fahrenheit scales are defined so that absolute zero is −273.15 °C or −459.67 °F). Imagine heating a bar of steel and observing the color of the bar at increasing temperatures. At some point the bar will appear to glow a dull red. As heat is added, the dull red turns to yellow, then to white, then to bluish white, finally to blue.
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A light bulb that produces light perceived as yellowish white will have a color temperature of around 2700K. As the color temperature increases to 3000K - 3500K, the color of the light appears less yellow and more white. When the color temperature is 5000K or higher the light produced appears bluish white. The color temperature of daylight varies, but is often in the 5000K to 7000K range.
When the desired lighting effect is "warm", use light sources in the 2700K - 2800K range. Most common incandescent light bulbs will produce light in this color temperature range. An exception is the incandescent light bulb with a neodymium coating. The neodymium filters out the yellow and red wavelengths of the visible spectrum leaving predominately blue wavelengths. So even though neodymium light bulbs have a 2800K color temperature, the light they produce appears to be bluer - similar to daylight and to other light bulbs that product light with color temperatures in the 5000K or higher range.
When the desired effect is neutral or bright white, use light sources in the 3000K - 3500K range. For a slightly bluer, "cooler" effect use 4000K.
What Color Temperature Simulates Daylight?
To give the perception of daylight (bluish white light), use light sources with a color temperature of 5000K or higher.
It is important to note that color temperature is not the same as color rendering. The color temperature of a light source does not describe or predict the ability of that light source to render color accurately. For more on this topic, go to the explanation of color rendering.