Photographic colour temperature

NB. this is different to colour calibration.

Digital colour temperature adjustment:

• see also:
  • http://www.ronbigelow.com/articles/white/white_balance.htm 
• there are many methods to get accurate colour temperature in digital photographs or images:
  • use the camera color temperature "white balance":
    • can just set it to auto white balance and hope it is accurate
      • BUT many situations can trick the camera
      • A camera set to Auto-WB is always making changes. It's always adjusting the color as the framing changes, as the focus changes, and as the exposure changes. There's no consistency between photos, and there's no control over the quality of your images.
    • set it to a set colour temperature (eg. daylight or flash) and hope this is accurate
      • BUT colour temperature of daylight changes with time of day
      • AND even when using the apparently constant temperature of flash, the final temperature is determined by reflected flash light from surrounding materials such as painted walls
    • do a custom white balance by placing a grey or white card in front of the camera lens with it lit by the same light that will light your subject and follow the camera instructions to set the white balance.
      • a manual WB helps with accurate colors AND consistency between shots.
      • this is especially important if you are colour blind and thus have difficulty making adjustments later.
    • alternatively, to assist in colour temperature and exposure determinations before you take the photo, you may wish to consider purchasing either:
      • an 18% neutral gray card which can also be used for setting exposures.
      • expodisc but requires you to face the camera at the light source with it on, can also be used for checking the calibration of your camera's light meter
      • WarmCards system for $US45 to allow you to customise the effect you want, such as a warmer more pleasing image.
      • see also:
        • http://www.rawworkflow.com's WhiBal card and tutorials 
  • use RAW development software assuming you have shot in RAW file mode:
    • preferably need an image with a grey card in it so that the grey card area can be used to set the white balance as per the software's instructions, then this setting can be used for other images taken in the same lighting.
  • use Photoshop's levels setting:
    • ideally the colout remperature should be fixed before you get it into Photoshop, otherwise you should preferably have one image with a gray card in it which has been lit by the same lighting.
    • open the Image-Adjustment-Levels dialog box and select the middle eye dropper ("set gray point") and click on the gray card in the image. This will automatically adjust the image to remove any colour casts. Without doing any other adjustments, click save and then this adjustment can be used for any other image taken under the same lighting.
    • alternatively you can use the left eye dropper to select the darkest region, or the right eye dropper to select the whitest region that is not burnt out.
  • use Photoshop's curves setting:
    • this is a bit more difficult, so won't go into details here. 
    • one way is to know what some common colors should be then correct the image to achieve it, thus when working in CYMK mode:
      • magenta in sky blue should be half that of the cyan
      • skin tones in a Caucasian person should consist of roughly equal parts of magenta and yellow, and a dash of cyan (equal to 15-25% the value of the magenta and yellow). Darker skinned people will have more cyan, and lighter skinned people will have less. Oriental people will have little cyan, and a smidgen more yellow than magenta.
      • neutral gray colors should have equal amounts of yellow and magenta, with 10-20% more cyan. (In theory, there should be equal amounts of each color, but cyan is a weak ink, and needs some help.)
      • see also: http://www.techcolor.com/help/correcting.html 
•  

Analog colour temperature conversion in detail:

• one way to work out which filter to use is to work out the mired difference between the film colour temperature and the lighting colour temperature via the table below
  • ie. daylight film 5500K = 182 mired, and thus when using tungsten light source at 3200K = 312 mired, this equates to a difference of 130 mired and the closest filter to this is the 80A filter which gives a mired shift of minus 131 mired
• if you have plenty of money, then you can purchase a colour temperature meter which will assist you in working out which filter to use by giving you a reading of the ambient colour temperature.

Colour temperature of light sources:

• candle flame	1500°K	"cool white" flourescent	4500°K
  40W tungsten	2760	"cool white" flourescent	4500
  60W tungsten	2790	blue photoflood	4800
  100W tungsten	2860	noon daylight, electronic flash	5500
  500W tungsten	2950	sunrise	2000
  1000W tungsten	3000	direct sunlight 10am-3pm avg.	5500-6000
  studio lamps	3200	"daylight" flourescent; heavy overcast	6500
  photofloods	3400	light overcast sky	6800-7500
  "warm white" flourescent	3500	hazy blue sky	7500-8400
  clear flash lamp / sunlight early morning	4200	clear blue sky	10000-27000

Colour temperature to mired value table:

Mired shift values of common colour conversion filters:

• values are approximate & for critical work should be checked by practical test, especially if combinations are used.