RE: Color spectrum Vs Degree Kelvin??
First of all thanks Miles, I've been lurking and learning for several months,
and finally someone asked a question that falls in my area of expertise
(Imaging and photo sci.)
>>I was wondering if any or our resident technophiles could help me
understand the relationship between the color spectrum of light as
measured by nanometers vs. degrees Kelvin. <<
The Kelvin rating (the term degrees is not used) is a color index that is
derived by heating a black body (a Carbon instrument). As the black body is
heated it glows and emits light. EG. at 3,000 K the black body emits a very
red orange light (similar the color of your standard houshold tungsten
filament bulb). At 9,000 K the black body is emitting a very blue light
(similar to the light reflected off of a blue sky). And by the way it's very
The color spectrum refers to the content of the emitted light accross all the
visible light wavelengths. Many light sources are full spectrum. They are
emiting light at every wavelength in the visible light spectrum. For example
a tungsten bulb is full spectrum, but it emits more in the red area of the
spectrum than in the blue or green, thus the lower Kelvin rating. A Xennon
arc (electronic flash tube) is full spectrum but emits more light in the blue
and green than the red, and usually has a high Kelvin rating, 9,000 -10,000
Kelvin if they are not filtered.
Many light sources emit only a band, spikes or a narrow range in the visible
light spectrum. Helium Lasers emit a very narrow spectrum (620nm???). In a
graph of the visible light spectrum the Helium laser registers as a single
line or wavelength.
Flourescent Bulbs produce light by exciting phosphors. There are many
phosphors with a varying spectral charateristics. The standard office cool
white emits little to no light in the red region. The majority of the light
is emitted in the green spectrum because our eyes are more sensitive to
green. Hey they look brighter! If you were to look at a graph of the visible
light spectrum the cool white bulbs have a large spike in the green region
and some smaller spikes in the blue and only negligible stuff in the red. Our
eyes (brain) compensates for the color shift. Nor does we notice the missing
spectrum since we only have single wavelength red, green and blue receptors.
Manufactures can pick phospors to create different color ratings. For full
spectrum bulbs they add red emitting phosphors to the mix, but the bulbs
still have missing some wavelengths and they have spikes in other areas (Red,
Green & Blue).
>> Is it possible to say I have a 5000k light which corresponds to a certain
wavelength of light as measured in nanometers or is degrees Kelvin a
combination of spectrums as I would assume it would have to be if a 5000k
bulb is considered a full spectrum bulb.<<
No, the 5,000 Kelvin does not refer to a certain wavelength but the color
resulting from the mix of wavelengths. For example a Tungsten light can be
filtered and or burned hotter to create the 5,000 K color and it is full
spectrum. A flourescent bulb could have just three phosphors a red, green and
blue one to acheive a 5,000 K color. You can evalutuate the quality of the
flourescent bulbs by their CRI (Color Rendering Index). A bulb that has a
high CRI generally has a good number of and mixture of phosphors.
FYI, Infrared and Ultraviolet are not classified as light on the
Hope this helps.