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An incandescent lamp is made with a wire filament enclosed in a bulb without oxygen and glows as the filament is heated. Less than 10 percent of the electrical power into an incandescent light bulb is converted into light, and the rest is converted into heat. Lamps of this type are still used, but they are being replaced with fluorescent lights or light emitting diodes. The incandescent lamp therefore is a resistor that just happens to give out light. But what type of light? White light is measured by its color temperature in degrees Kelvin (K). Typically, when we look outside on a sunny clear day, the Sun along with the blue sky provides a color temperature of about 4,500 to 5,500 degrees Kelvin. As the sun starts to go down in the afternoon, the color temperature drops to about 3,000 to 4,000 degrees Kelvin. Finally as the sun sets, we can clearly perceive the sunlight with a yellow to red tint, which means the sun’s color temperature has dropped below 3,000 degrees Kelvin. Human eyes adapt to the color temperature for the most part from about 3,000 to 5,000 degrees Kelvin and perceive light in this range as “white,” albeit at 3,000 degrees Kelvin, it has a warm tone. A standard incandescent bulb for room lighting such as a 100 watt bulb provides light at about 2,700 degrees Kelvin, which provides warm white light. For studio or movie lighting, generally the color temperature is a bit whiter (between 3,200 and 3,500 degrees Kelvin, and sometimes up to 4,000 degrees Kelvin). Halogen lamps or white photoflood lamps provide light in this color temperature range. Incandescent lamps exceeding 4,000 degrees usually are specially made and they are often coated in blue. For standard low-power lamps such as flashlight bulbs or indicator lights, the color temperature is somewhere between 2,000 and 3,000 degrees Kelvin.
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Ronald Quan (Electronics from the Ground Up: Learn by Hacking, Designing, and Inventing)