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Question about NC and fluorescent WB

Discussion in 'General Technical Discussion' started by Rich Gibson, Dec 20, 2005.

  1. I've been using NC a while now and understand how the names assigned to various WB settings actually relate to the Kelvin temperature of the available light. However, do the fluorescent settings involve some other mathematical manipulations? For example, with standard fluorescent lights we all know you get a yellow cast, and once in a gym I took some shots in which some of the sodium (?) vapor lights had a green cast while others had a red cast. I'm guessing it had to do with ageing.

    Anyway, I was curious if anyone knew.

    Thanks, Rich
  2. PJohnP


    Feb 5, 2005
    Rich :

    I can't comment on the essential mathematics for WB settings, but the different types of lights actually have a different distribution of light in their spectra. It's not a neat and tidy "normal" distribution.

    The fluorescent, neon, and incandecent light mechanisms are such that the spectrum isn't "natural", and that's not just an adjustment of the light curve to the right or left for equivalent temperatures. There's a reason that there are "full spectrum" light bulbs available, as electrical excitation of gases (e.g., neon or sodium vapour), fluorescence of coatings, or heating a filament until it glows visibly are not the same as sunlight. There are areas of the spectrum that we'd consider "normal" missing from each of the light types.

    Even sunlight shifts are a function of altitude and time of year (Thom Hogan does a marvellous job of explaining the arbitrary nature of "neutral" conditions in his books), let alone the time of day, clouds, etc.

    So. I'd strongly suspect that WB calculations are based on more than a simple temperature "slide" of the data.

    Or at least that's what I'd hope... :Smart: :eek: 

    John P.
  3. Iliah


    Jan 29, 2005
    Dear Rich,

    Color temperature in photography is often approximated by R/B ratio. That works acceptably well for sunlight, good flash, and studio quality incandescent light. If the spectral distribution of the light source is actually line spectrum, like with many fluorescent and mercury sodium vapour lights, all three channels are involved in white balancing. On top of that, some programs use channel mixer to compensate for zones lacking in the spectrum.
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