One of the nice things about how we see is that our brains are adept at adjusting our perception of what we see according to the lighting conditions. If we know that something is white we perceive it to be white, and can generally ignore the actual lighting conditions. For example, if you put on a pair of tinted sunglasses for a few minutes everything appears to take on the tint of the glasses. And then your brain adjusts, and everything appears to be the normal colour again.
Cameras don't do this.
For example, shoot a photo in daylight and everything has a blueish tint. Shoot under indoor lighting and everything looks different. Shoot with a flash and things change again. Most cameras do a reasonable job of adapting to this. Either with a White Balance setting of "auto", or by allowing you to specify the lighting conditions.
However, "reasonable" does not equal perfect. Which is where the WhiBal card comes in.
The WhiBal is an example of a "grey card". It's a very specific, known shade of grey, and many post-processing tools, such as Photoshop or Lightroom, have a feature whereby you can select a part of the image that you know to be neutral, and it can automatically correct the image's white balance. All you need to do is find a part of the image that's supposed to be neutral.
That's not always easy. So what many applications support is the ability to make an exposure change in one image, and then synchronise that change to one or more other images.
A worked example may make this easier to follow. Yesterday I was at Big Basin Redwoods State Park. Each time I walked to a different location where the light had changed (because of the varying amount of tree canopy cover) I took a photo of the WhiBal card, and then carried on shooting whatever caught my eye.
After returning, and importing the images in to Lightroom, one of the first tasks is to correct the white balance. Here's one of the photos of the WhiBal card before adjustment.
Correcting the white balance for this picture is easy. Just press "w", which brings up an eye-dropper, and click anywhere in the grey face of the card. That colour corrects the image to this.
Synchronising this colour correction is easy in Lightroom. Select the first photo (the one that contains the WhiBal card). It's important to select this one first, as Lightroom uses it as the master from which the settings are copied. Then select all the other photos taken under the same conditions. Make sure you're in the Develop module, and hit the Synchronise button in the bottom right of the interface.
A dialog pops up asking which information you want to sync. Make sure that only White Balance is selected.
And that takes this:
This isn't a perfect adjustment yet, but it's a good start. One of the things this would benefit from is adjusting the white and black point in the image -- a feature that Photoshop supports but Lightroom doesn't. As you can see from the shot of the card, it also includes black and white reference targets which makes these sorts of adjustments very easy too.
Grey cards aren't new inventions. But the WhiBal is a very handy one to carry around, especially in the credit-card sized form factor.