optical density

[Guy Villard]

From: Guy Villard

I cannot talk 100 % about the digital system, except that I believe that
the numbers you refer (255) corresponds to the amount of grey level that
can be presented. That means that going from white to black, you have in
that case 255 discrete level of greys, from brighter to darker one.
In terms of conventional, all the values can be reached. In that case,
looking through a film, we talk about light transmitted :
     if all the light of a source is passing through the film, than the
transmission is 100% (absorbance is 0%) : this is the case when there is no
filmat all or an extreme clear film.
     if the absorption is 90% , then the light transmitted is only 10 %
     if the light absorbed is 99%, then the light transmitted is only 1%,
...
We consider the optical density, which is the decimal logarithm of the
inverse of the transmitted light
OD = log (light emitted / light transmitted through the film)
In the first exemple, light emitted = light transmitted ===> OD = log(1) =
0
In the second exemple, light transmitted = 10% of light emitted ===> OD =
log (1/10%) = log (10) = 1
In the third exemple, light transmitted = 1% of light emitted ===> OD = log
(1/1%) = log 100 = 2
And so on.
The limitations are : the measure system, and the maximum covering power
(DMax) of the film (depending on the type of film, its silver content,
...). Generally for a medical film it is in the range from 3 up to 5, but a
film used for industrial applications can reach levels superior to OD = 8
Hope it helps




"Dr. Ackermann Gabor" <acker at KONFOG.SOTE.HU> on 11/08/99 04:01:36

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Subject:  optical density




Dear oradlisters,
could anyone help me? My question is what is the relation between the
face values of optical densities in digital and conventional
radiography? In the case of digital radiography the range is from 0
to 255, in conventional we can talk about 0 (100%), 1 (10%) etc.
My question is, how mutch is the 10%, 1% etc. in the digital terms.
I guess conventional 100% is equivalent to 255 in digital terms. But
what about te others?

Thank you for your help
Gabor Ackermann