Headlight strength is measured by the color output and the lumen. Color output is measured in degrees Kelvin, whereas lumen is a measure of the actual light output, or intensity, of the headlight.
The hue, or color, of a light source is measured in Kelvin temperature. The spectrum of measurement was named for British physicist William Kelvin. The actual spectrum of colors, represented in "degrees Kelvin." contains reds at the lower end of the spectrum and changes to blue at the upper end.
The hue of headlights with a light source hue of 3200K is approximately the same as that produced by quartz lighting. It has less red light and more blue light than household incandescent bulbs, but not quite the blue-white light produced by fluorescent bulbs.
Headlights with a 4000K rating will produce a light with a more blueish tint than 3200K lights. Headlights with this hue will produce a broader range of color reflection than 3200K lighting due to the closer approximation of natural sunlight.
A headlight that produces a higher Kelvin rating may actually be less intense depending on the brightness of the bulb. Generally, most aftermarket headlights have a greater brightness over stock headlights, regardless of color temperature.
