Without knowing it, they've asked a question which has an technical answer, though not one they're expecting, and an unexpected practical answer too.
Let's go back to Part I of the series and review what white balance is:
"White balance is the process of adjusting color casts, so that objects which appear white to human eyes/brains are rendered white in the photograph by the camera."To put it more simply, photographers set a digital camera's white balance to get the colors in images “right,” where the images' colors are what photographers' brains tell them their eyes are seeing.
Digital cameras calculate white balance via their RGB exposure meter. The exposure meter reads color temperature based on the ratio of the amount of blue light to the amount of red light striking the meter's sensor, with green light ignored, and then calculates white balance.
Digital camera manufacturers apply a standard calibration for their light meters, in order to permit its use to properly set exposure and white balance. Otherwise making well exposed images would be unacceptably "hit or miss."
The standard chosen was gray. Why gray? Gray because it's in between black and white, and therefore considered “neutral.” If you measure to average, then you've compensated for both ends and thereby through the full range of tones (colors).
Of course there are many shades of gray, so the question is which one. In the professional world of printing, it's claimed the half way point between black and white reflects 18% of the light, hence 18% gray. That number has been ingrained in the minds of photographers, but is it right for digital photography?
There are potential problems with that. For example, if you use a Nikon DSLR and take a photo of an 18% gray card, I think you'll find it's about 1/2 f/stop off. Some believe Nikon calibrates its light meters to an ANSI gray standard which would mean it's at 12% gray. I'm inclined to believe Nikon calibrates its light meters to ISO standards, which would be 12.7% gray. Looking at this data, we still don't know what gray to use for white balance.
Plus, these grays are primarily for setting exposure. Are these the right grays for white balance?
White light is the “sum” of the light spectrum of our solar system's sun, which might infer that white should reflect the light of a scene back to the camera without changing it's color, which is what is needed, a “neutral” way of getting the scene's color temperature read by the camera. Other colors, red or green for instance, we know, will “reflect” the scene's color temperature tainted by the color of the card itself and can't be used.
Bob Johnson, of “Earthbound Light” says, “In practice though, white itself isn't generally your best bet since any color sufficiently overexposed will yield pure white. It isn't really the brightness we are interested in measuring anyway but rather the color.”
I understand the point, but “overexposure” isn't a problem with the RGB light meter which is actually measuring the white balance. If this was a problem, photographers using white cards would get wildly inconsistent white balance settings, but they don't.
Additionally there is the problem of what white to use. For example, did you ever purchase copy machine paper? If you have, you know there are different “shades” of white.
Did anyone think the choice would be easy?White cards will work because practically, it virtually “neutrally” reflects the scene's light back to the sensor without making a meaningful change in the color tone of the light reflecting from it, but white isn't the best choice because you can get closer to actual “neutral” reflectivity.
With gray, we're more in the “neutral range,” but what gray is the best gray.
If you have a Nikon camera to set a custom white balance you could choose a 12% gray card, but they are virtually impossible to find, and not exactly “neutral” anyway. An 18% card is “neutral” for the print industry, but not necessarily “neutral” light reflectors for digital camera white balance measurement.
In the practical world, each of the grays above would get you close, but they aren't the best of the gray choices.
Enter the WhiBal light gray card, in which the percent gray is irrelevant, because the card has been designed in direct consideration of the way digital cameras measure color temperature, so that it reflects the light from the scene to the camera more “neutrally” than white cards, or other gray cards, so that digital cameras can determine color temperature and the white balance setting as accurately as possible.
Of all the cards available to set white balance, the WhiBal will do the best job, in my opinion.
What do I actually use?
I normally carry around an Expo Disc everywhere I go, and use it to set my custom white balance. It doesn't depend on reflecting the light in a scene to the camera. Instead, it properly filters the light for the camera directly. It's as accurate a product as I've found, and doesn't have the problems of using a “card” as explained in Part II of the series.
The Expo Disc doesn't take up much room in my shooting bag, but if per chance I'm trying to carry almost nothing, I always have a WhiBal card.