How can I tell if a reflector is pure white?

Question

I want to use a piece of fabric as an inexpensive reflector.

Is there any way to find out if it is pure white, so that I can be sure it will not add a colour cast to the light it reflects?

Asked by AJ Finch

Answer

There are a few things you could do:

• Compare it with a reflector that is known to be pure white (a purpose made white balance card would be good for this).
• Take a photograph of it with a light source of (approximately) known colour temperature (e.g. the sun) and spectrum and do a raw conversion with this colour temp and look at the RGB values.

The first method will probably be a lot more reliable as it won't succumb to lighting colour shifts or the vagaries of Raw processing. However, provided it's not massively off (in which case you'd be able to see that clearly) it probably doesn't matter if it's pure white.

Colour temp is relative so the only time it matters is if you're using it alongside other light modifiers which are pure white, so my suggestion, if this is the case, would be to try the first option and compare it to your other modifiers either by eye, or better by shooting images of them under controlled lighting.

Answered by Matt Grum

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