Why are gray cards used for white balance instead of pure white cards?

Question

I understand that an 18% gray card provides a neutral target for setting custom white balance. But why is gray commonly used instead of a pure white target?

Is it because gray is easier to manufacture consistently, less likely to stain or yellow, or because white can cause exposure/clipping issues when the camera measures the reference?

user4390 · Accepted Answer

When you have already set your exposure parameters, white could be clipped in some single color channel (but not all, so your camera won't show it as blown), therefore not being very good basis for color balance adjustment.

Also, paper will turn yellow during time. And just looking at the sheets currently on my desk, there's three different tones of white papers already.

That said, I've used white paper many times and it will get you close enough in all but very critical studio shoots. Very often you'll have multiple different light sources, reflections from colored surfaces and/or shadows vs. lit areas, all having much more impact on colors than slight imperfection in metering the white balance. Just make sure you don't clip any color channels (e.g. by spot metering without compensation from the paper, or verifying exposure parameters using color channel histograms).

Why are gray cards used for white balance instead of pure white cards?

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