I never thought I would see the day when Bill was wrong, but the
gray scale is indeed an 18% reflectance card, meant to mimic the
average reflectance of an average scene. The best thing about it is
that most camera meters make the same assumption; therefore if
exposure is set to a gray card, you have the best chance of matching
image to lighting. In my many years in the color darkroom I often
used the gray card (along with other color checker cards) to
establish good color match with a color meter, reference charts or
just my eye. You can see these cards at bhphotovideo.com, search
for gray card. My favorite is the Kodak card, 18% gray on one side,
90% white on the other. Marlin in cold but warming Denver