Re: Opacity vs transparency (Was: Re: pigment for gum )

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From: Sandy King (sanking@clemson.edu)
Date: 09/12/03-08:27:12 AM Z


Thanks Greg, you are absolutely right. This exchange has been very
hard to follow because of the obvious confusion of terms. Density is
the common log of opacity, which merely means that we are using a
numerical shorthand to express opacity. Some may be confusing opacity
with transmittance, but these are different, though related, in that
opacity is the reciprocal of transmittance.

For example, a material that has a transmittance of 20% has an
opacity of 5 and a density of log 0.7. A material that has a
transmittance of 50% has an opacity of 2 and a density of log 0.3.

Sandy

>Sorry, but I can't keep my fingers off the keys. There is no
>difference between "density" and opacity. Go back and have a look at
>your textbooks. I think somebody already pointed out the relationship
>of opacity to density. Varying degrees of "opacity" are equivalent to
>varying degrees of density they are just expressed (mathematically) in
>a different fashion, but only if you're dealing with densitometry. Do
>you really thing about "density" when you photograph a tree?
>
>I think the confusion may stem from confusing methods used to read
>reflected density (reflected light actually) and transmitted density?
>It is possible that no light is making its' way to the paper, but
>rather that light which strikes the pigment is being reflected back
>from the pigment (hint - stop thinking in terms of gelatin/silver
>processes). Correct me if I'm wrong, but I think what Katherine means
>when she uses the term opaque is that no light is being reflected from
>the paper base, but rather the light is being reflected back from the
>surface of the pigment. A warning to those folks versed in
>Sensitometry - color densitometry is much different than B&W and
>colorimetry even mores so (IMHO :*).
>
>-greg schmitz <gws1@columbia.edu>
>
>
>
>
>==================
>
>All of our current environmental problems are unanticipated harmful
>consequences of our existing technology. There is no basis for
>believing that technology will miraculously stop causing new and
>unanticipated problems while it is solving the problems that it
>previously produced.
>
> Prof. Jared Diamond, UCLA
> Harper's Magazine, 2003 June


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