How can I verify whether a gray card is truly color neutral?

Question

I bought an inexpensive gray card set and realized the manufacturer does not provide any color accuracy or neutrality specs. My test shots look reasonable, but I have no reliable reference at home to confirm whether the gray card is actually neutral.

I tried photographing it with direct flash in a dark room, but the RGB values were slightly different across channels, so I’m not sure whether the issue is the card, the flash, or reflections. I also compared it to a printed black-and-white image, but that is not a trustworthy color reference either.

What is a practical way to check whether a gray card is color-accurate enough for white balance use?

user2719 · Accepted Answer

If you know somebody with an X-Rite (ColorChecker) or DataColor (SpyderCheckr) reference card, you can check with a single exposure including both your card and the known-good reference. Both X-Rite and DataColor products are time-sensitive and should be good (±1 or 2 in all channels) if they are stored properly in their light-tight cases and are within their expiry date. DataColor includes a fugitive-dye fade swatch on their card that will indicate excess exposure to light.

Failing that, you can always take it to your local paint emporium. Seriously—they use calibrated photospectrometers (like the Pantone CAPSURE) to match colors, and even if their equipment is fully integrated into a paint-mixing recipe machine, you can look at the recommended mixing recipe to see if they need to add, say, a red or blue pigment to match your card.

How can I verify whether a gray card is truly color neutral?

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