Originally posted by falconeye Because the spectrum of the black body radiation is defined for all frequencies, one can compute the temperature from the ratio of intensities for any two frequencies. If one is adding a third frequency (color) then one ends up with having 2 temperatures from two adjacent intensity ratios.
If the light source's spectrum is discrete or continous has nothing to do with all of this.
I don't think that is quite right -
Although any light source a color temperature can be calculated -
that is somewhat less useful than a CCT for a light source that approximates to a good/perfect radiating black body -
A CCT for any light source that falls far away from the incandescent black body locus - the
Planckian Locus (Wikipedia) - is almost meaningless in terms of any practical usage (LEDs especially colored ones have very narrow bandwidth/frequencies and are almost monochromatic so do not approximate to an incandescent black body therefore would fall very far away from the Planckian locus)
Planckian Locus - ie: the line that traces a incandescent black body radiator at any given
CCT (Corrolated Color Temperature)
Take a position way off the locus of approx x=0.425, y=0.3 would have more blue than a light on or above the locus and still have approx the same CCT.
Planckian locus (by CIE)
So looking at this graph one can see this seems a light with more blue but off (below) the locus/line could have a low CCT.
In fact looking at both diagrams since extrapolating those Tc lines which tend to converge -
ultimately it would get to an area off (below) the locus where tiny shifts could result in big differences in CCT/Tc?
So even though in theory one might be able to calculate a CCT for discontinuous spectrum - it is less than useful and can be very misleading - especially one attempts to apply that color temperature to RAW processing
Originally posted by falconeye Because image processing software does use three colors indeed (RGB), we get a "temperature" and a "hue" measure. Where a "hue" means that the light didn't have a defined temperature in the first place.
You know I had never thought of it this way -
but that logic does not quite hold water either.
Our eyes see by RGB receptors as well, but when a light source is separated to its color components (like using a prism - that famous "Darkside of the Moon" logo
) one can see that a continous spectrum has many color components. Whereas "white" or any other color made up from separate Red, Green and Blue LEDs do NOT - they separate out to Red, Green and Blue only - the Bayer matrix sensor can detect this and it makes a difference to the photo.
Try it and see for yourself - shoot a magenta scene
1) lit by gel'd/filtered tungsten/halogen
and a separate one
2) lit with just red and blue LEDs
There's a huge difference when one processes it -
why? because the filtered tungsten light has still a lot of color components to work with (albeit a bit low) which the Bayer RGB matrix captures.
Whereas the LED version only has Red and Blue to work with......
For more detailed discussion re: color temperature and how the Planckian Locus is relevant please see:
page 2 of this CPF thread
Last edited by UnknownVT; 03-19-2010 at 10:42 AM.