How can I ensure my gray card is color-accurate?

Question

I recently bought a gray card set (Zeikos ZE-DGC Digital Grey Card Set with Lanyard from Amazon) and although customers had no complaints about it, I belatedly became worried because the manufacturer's website contains no precise color information on the cards. I took a series of test pictures which turned out okay, but without any precise color information for any object in my house, I have no way to determine whether the card set is color-accurate.

I tried taking direct flash photos in a dark room with my Nikon SB600, assuming my flash would convey accurate color data to my camera, but the gray card still turned out a little off (RGB 78.9, 82.1, 84.6) and I don't know whether to blame the card or the flash or ambient reflections. I tried putting a magazine in the shot with a black-and-white photo on the cover, and it did seem to get balanced with the card close to gray (62, 62, 60) but I don't know for sure the printer intended that color. What other options do I have?

Answer 1

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.

Answer 2

1. What Stan said (of course :-) ).

2. The grey card is likely to be "close enough" to spectrally neutral to be useful. The exact reflectance level is uncertain but also not crucially important - see below.

Many people sell this card set with a range of descriptions. Some say

• The Zeikos ZE-DGC Digital Grey Card Set features a digital grey color reference card, a pure white color reference card, a true black color reference card, a user's guide and a lanyard with a detachable clip. Get perfect exposures every time with this accessory!

One such example here along with a range of competing products.

While that does not tell you what it IS it does tell you that they think that they are trying to be "real".

Exposure: If you were using the grey card for exposure (which you aren't) it will ideally have a known percentage reflectance (12% or 12.5% or 13% or 14% or 18% or ... agh - the exact value is the subject of an ongoing many decades old holy battle some details here and the exact level does not matter as long as you know what results it gives you.

Color balance: For color balance the important aspect is that the card is spectrally neutral. If illuminated with "white" light from a perfect black body source it should reflect equal red, green and blue components. As the % grey in the previous paragraph varies the exact levels of R, G & B reflectance will vary, but they should be the same for each component. So, in your tests, if you were using a perfect white light source (we all should be so lucky :-) ), you would hope your grey card to return levels of (78, 78, 78) or (90, 90, 90) or (63, 63, 63) or ... . It is extremely likely that the people who planned the Zeikos grey card were trying to get spectrally neutral reflectance. It is almost certain that when making a ~= $10 3-card set with the grey "painted" on white plastic that they were not wholly successful in this attempt. It's also quite likely that the result will be "close enough" to be useful and that over time you will be able to make allowances for any slight bias that you may detect OR that you will find that any bias is small enough to not matter.

A number of user reviews of this specific card set are here. Of potential use. Users seem quite happy with the results.