Friday, 7 December 2007

Lightbulbs: the measurement

When I wrote a post about new lightbulbs I've installed, paul asked a question about colour temperature.

The spec on the bulbs I bought (these) is 6500K but the shades they're in are slightly warmer. I guessed in the 5500-6000K range, based on the match I'm seeing with the daylight through the windows.

How wrong I can be. I measured by shooting direct at the lights through an Expodisk. Here are the exposure shots:

Capture One, 4600K, +12 (magenta) tint

Capture One, 4600K, 0 tint (from same file).
Yes, I know the sensor needs cleaning.

As you can see, the new bulbs have a strong green cast. These were long exposure (0.25") so flicker is not an issue. This is surprising given the very cool, neutral white colour that they appear to give off.

These are compact fluorescent lamps, which may have some bearing. however, as it happens, I did a similar test with incandescent in the same lamp shades yielding quite a green cast too. Odd, and not obvious to the eye.

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