Interesting Test Image - impossible to white balance

[xpatUSA]

I'm comparing two different RAW converters with the aim of favoring the one versus the other. With the intention of comparing color rendition, I took the card ("don't leave home without it") and a slant-edge target (to focus on) out into the Real World, laid them on a tree stump under a clear blue sky, and took the shot.

Here's a crop:



I found that the hues of the neutral colors in the pic were different comparing the card patches with the target paper. Considerably different. That is to say, if I clicked on a gray patch on the card the paper went bluish but the white patch did not.

Card gray: 102 102 102
Paper: 210 210 244 (which is a HSB of 240 deg 14% sat and 96% value)

Of course, clicking on the paper puts all the card colors off. While realizing that this can happen, I was surprised at it's severity under a single homogenous source of light - i.e. blue sky with no direct sunlight.

[Graystar]

What is the source of your white paper?

Robin Myers (of Robin Myers Imaging, who makes the Digital Gray Card and is also the creator of Apple's ColorSync) has tested some color charts and has found that some contain UV Brighteners (or OBAs...Optical Brightening Agents.) However they're called, they take invisible UV light and convert it into visible blue light (that's how you can have those "ultrabright" copy papers with 108% reflectance.) While OBAs are great for making white things look "whiter" to human eyes, they wreak havoc with white balance settings. The fact that your white paper went blue is an indicator that you might have OBAs.

Here's a page with some test results...see if your target is in the list. He has others on his website...you have to look around.
http://www.rmimaging.com/information/color_charts.html

OBAs present the same problem as a scene with both a sunlit area and an area of open shade. The sunlit area has one white balance setting, and the open shade area, being illuminated by the blue sky, has a different white balance setting. Our eyes perform partial processing to a scene on the fly, correcting luminance and color so that we see more accurately (well...so that we see what we think we should be seeing...not quite the same thing.) So the only solution is to post process by creating a mask and applying different white balance settings to different areas of the scene.

That's just life with imperfect imaging systems (well...the imaging systems are darn near perfect...it's our eyes that change what's really there.)

[dan marchant]

"Would you exchange two cartons of your old printer paper for that one carton of new Daz blue whitener printer paper?"

[xpatUSA]

Thanks Gents,

Yes, the paper was a likely culprit - not that I remember what I used - it was printed a while back. I did have a pack of HP "Bright White" - that could have been what I used. The card itself is a Macbeth (X-rite) mini-card which I trust well.

I don't normally print images to keep or sell, so I know little about that end of the hobby. And, for my purpose, I white-balanced on the card and cropped out the rest.