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Kelvin is a unit of measurement used to describe the hue of a specific light source. This is not necessarily related to the heat output of the light source but rather the color of the light output. The higher the Kelvin value of the light source, the closer the light's color output will be to actual sunlight.
Bulbs with an output of 3500K or lower on the scale will have an amber hue. Bulbs in the mid-range of 3500K-4100K will have a white hue. Bulbs in the higher 4200K+ range will tend to have more of a blue hue closer to, or exceeding, that of sunlight. Most modern HID lamp systems have a bluish hue because they operate within that higher end of the scale.
Today's high tech bulbs vary the Kelvin value by altering the composition and materials of the thin glass crystal that encloses the bulb. This enables them to increase the light output over older technology lamps that used a thick, colored lens on the lamp housing to filter the light output.
The higher the Kelvin value, the more visible light will reflect back to the driver, increasing visibility at night. The light output from these higher Kelvin value bulbs is made up of a wide range of the visible light spectrum. White light contains all colors of the spectrum and the whiter the light produced, the more it will reflect off a wider range of objects independent of their color.
Bulbs with an output in the lower, amber Kelvin range work best in fog, snow or rain conditions to highlight objects and enhance contrast. The light output of these bulbs is made up of a much longer wavelength band of the visible light spectrum. White light that is made up of all colors of the spectrum tends to scatter or disperse differently off fog droplets, rain droplets and snow causing a blurry, white out or dispersion effect. Most drivers have experienced this when they turn on their high beams in fog and everything seems to blend together in front of them. Using a single color, longer wavelength band of the spectrum reduces this dispersion. Human eyes are most sensitive to amber and green colors that have a longer wavelength. These longer wavelength colors scatter the least and reflect the most visible light back to the driver under these conditions. That is why amber color output is still the color of choice for most fog lamps.
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