"Kenneth" <kdw### [at] earthlinknet> wrote:

> A Kodak "photographic gray card" -- neutral gray, in other words-- has an
> actual reflectance of only 18% (not 50% as would be assumed.) This
> corresponds to a PERCEPTUAL brightness of "half as bright as white,"
> whether viewed with the eye or by photographic film.  As Poynton says in
> his gamma discussion, "A source having a luminance of 18% of a reference
> luminance will appear half as bright." This is the human eye's PERCEPTUAL
> response to luminance....which isn't linear.  A POV-generated gray value of
> <.5,.5,.5>, applied to an object with finish{ambient 1 diffuse 0}, should,
> it seems to me, reproduce on screen in the same way...half as PERCEPTUALLY
> bright as (POV) white.  "Raw" monitor gamma/OS
> gamma/display_gamma/assumed_gamma all contribute to make that a reality, of
> course. So POV's assumed_gamma must be chosen to take that POV <.5,.5,.5>
> gray and place it squarely in the "middle" of our PERCEPTION of
> black-to-white.
>

The 18 % refers to the amount of light that is reflected from the grey card
in relation to the light that hits it. 100 % reflection is a rare thing in
the real world, a white sheet of paper will reflect about four times as
much as the grey reference, and a black sheet of paper will reflect about a
quarter as much as grey. This corresponds to two 'stops' to either side of
gray, in photographic terms.

This middle gray on a grey card is produced, I think, by mixing equal
amounts of black and white paint (assuming they are of equal strength), so
it is a 50 % gray in 'paint' terms. I have compared the tone of a grey card
to that of a grayscale chart that you can get in a paint store, and it
matches that 50 % tone exactly. So the paint industry and the photo
industry uses the same scale, it seems.

Perception is a difficult matter, but as far as I know our senses are
logarithmic in nature; doubling the amount of light is perceived as the
same change, no matter where you are on the scale (sorry if my english is
unclear).

H

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