Well, I had presented my Reflection Disc solution, which had been tested against (R) Spectralon when a forum member came up with the remark, that there are commercially available gray cards available, claiming to also work equally well in UV. So of course I got one and run tests against the industry standard (R) Spectralon as well as against my Reflection Disc (approx 75% reflection). That WhiBal Card has a white, a black area also, as well as a large gray area - all have been measured.
Long story short, here are the results.
[click on image to see a larger one]
SPECTRALON (100=rose, 50=violet, 25=teal, 12.5%=green); ReflectionDisc=blue, Whibal Card (white = yellow, gray=pink, black=red)
[confidence area: approx. 360 - 775nm]
That WhiBal card is certainly useful for VIS + NIR, but not for UV.
The ReflectionDisc certainly is not perfect, but works quite well in practice and delivers jpgs directly out of the camera with acceptable whitebalance for UV, VIS and NIR, also for a quick exposure check.
Here the photographic proof of the above - reflected UV using Baader-U filter and Xenon flash.
The card should be "white" or light gray, which it isn't. The cubes are Spectralon laying on my ReflectivityDisc.
It does work well in NIR though - here the card appears gray as it should be.
The cubes are Spectralon and my ReflectivityDisc stay the same color.
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos