Westminster fountain at sunset

by Jorge Córdoba

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I know that one can photograph a color-neutral card (white, gray, black) and use it for white balance, either in-camera or in post-processing.

Using an image of a color-neutral card taken under given lighting conditions, is there a way that the color temperature number can be measured by examining the color values in the image file? Presumably one knows or assumes the tone of the card, e.g. all-white, 18% gray, or all black.

It'd be nice to develop a feel for color temperatures by measuring various sources.

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With my camera it's possible to set the white balance to a particular color temperature, so a weak technique would be to set a number, shoot a white sheet, and adjust until the sheet looks white in the LCD, and then note what the final setting was. However, that sounds like a lot of fiddling, and I don't trust the color judgment of my eyes. –  jfklein13 Aug 4 '10 at 21:25
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Do note that the LCD is not (on any camera I've seen so far) color calibrated. Never trust the colors (or contrast and saturation for that matter) you see on the back of the camera. –  RBerteig Aug 5 '10 at 7:49

2 Answers 2

up vote 12 down vote accepted

White balance tools (like those in Lightroom or Photoshop) will tell you the color temperature of the neutral card essentially directly, based on the color temperature data in the RAW file.

More theoretically, if you're shooting with a known color temperature (even approximately known), then the color of the neutral target is directly related to the color temperature of the lighting. Doing this precisely has several big catches (e.g., it doesn't really apply to fluorescent or mixed lighting, and understanding color theory is a prerequisite), but doing it relatively is eminently practical:

  • if the card is bluish, it's lit with a higher color temperature than your setting.
  • if the card is reddish, it's lit with a lower color temperature than your setting.

With practice, the card isn't needed; you can see the relevant color casts in any neutral object, and particularly in shadows.

Personally, I think the best advice is to try to memorize the approximate color temperatures of typical situations, like this chart from Cambridge in Colour (the full article is worth a read):

  • 1000-2000K – Candlelight
  • 2500-3500K – Tungsten Bulb (household variety)
  • 3000-4000K – Sunrise/Sunset (clear sky)
  • 4000-5000K – Fluorescent Lamps1
  • 5000-5500K – Electronic Flash
  • 5000-6500K – Daylight with Clear Sky (sun overhead)2
  • 6500-8000K – Moderately Overcast Sky
  • 9000-10000K – Shade or Heavily Overcast Sky

1 Fluorescent lamps aren't standard light sources, and many variations exist, so this should be considered a very approximate figure.
2 There's an old joke about the noon sky outside of Kodak's offices being exactly 5500K.

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+1 Good joke! :D –  jrista Aug 5 '10 at 5:23
    
Regarding your statements above- "if the card is bluish, ... Should they be "If the card is bluish, then the source light colour temperature less than the setting. And if the card is reddish, its lit with a higher colour temperature than the setting. –  goldenmean Nov 30 '10 at 17:38
    
Warm sources (orange) have lower color temperature. Cool sources (blue) have higher temperatures. If the card is bluish the source light is a higher temperature than the setting. When you raise the temperature setting the resulting picture will become warmer to compensate for the cooler light. When you lower the temperature setting the picture will be rendered cooler to compensate for the warmer light. –  Michael Clark Feb 21 '13 at 9:05

You can use the RAW conversion software, this works for example:

  • Take a photo of a color neutral object in RAW format.
  • Open it in Photoshow Camera Raw.
  • Select the White Balance Tool.
  • Click on the neutral object in the image.
  • The Temperature setting changes to make the object neutral, so it shows the color temperature.
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