07 March 2011

Measure CFL color temperature with a picture

Say you are lighting a room with two pendants. One day you replace one pendant's CFL bulb and find out that, despite being both "warm light", their color rendering is different. You may notice this effect looking at their light spot on the ceiling, or observing usual room objects under the two bulbs.

That was my case, with a brand new "warm light" 2700K CFL looking colder than a Philips CFL. There my curiosity stroke: how to find out the actual color temperature of CFL bulbs?

Having played with digital pictures I remembered that it is possible to control while balance (that usually sits in "AWB" mode) in post-production, that is with a picture editing software. One condition: the camera must be able to save pictures in RAW format, not JPEG. Most recent point-and-shoot cameras should be able to save uncompressed pictures ("RAW").

The white balance is controlled by adjusting a slider to the light source's actual color temperature: colors will look natural only when picture settings match light source's value.

So, here's the procedure I tried:
  • take a picture of the object/room lighted with the suspect bulb, it MUST be in RAW
  • colors might look wrong, it doesn't matter
  • download the picture to your computer and open it up with your camera producer's RAW processing software (Canon's is called Digital Photo Professional)
  • edit the RAW picture settings and find out how to control the white balance
  • select "Color temperature" and move the slider until picture colors match real ones: that WB value is your actual CFL's color temperature
Canon's slider goes from 2500 to 10000 Kelvin in 100 K steps. Probably you will not be able to find the exact value, but a non-trained human eye can recognize a range of +-/ 200 K around the actual, true setting.

Few words of caution. Wait for the CFL to warm up before taking the picture. Mind that your computer screen might modify colors too. More?