Wayde Allen (wallen@boulder.nist.gov)
Wed, 16 Jun 1999 09:00:10 -0600 (MDT)
On Tue, 15 Jun 1999, Kathryn Garrison wrote:
> I've been looking at several books referenced on this list - several talk
> about negative density. What does that really mean (beyond a light
> transmission amount) and how does one test for it?
For film negatives it is a measure of how much light is absorbed as
opposed to transmitted expressed as a base 10 logarithm or
density = log (incident_light / transmitted_light).
For a print you measure the reflection density or the ratio of the light
reflected by the paper base divided by the light relected by the portion
of the image being tested.
(See http://www.infomaniak.ch/~bonavolt/sensito.htm)
The measurement is made using a densitometer. Conceptually a densitometer
consists of a light source that is placed on one side of the film and the
the transmitted light is measured with a photo detector on the other side.
A good densitometer is fairly expensive however, so many of us rely on
visual comparison to judge density.
For instance, if you have a step wedge (piece of film with regions of
known density) you can lay it on a light table next to the negative, and
simply see how the densities compare. To improve your accuracy, punch a
hole in two pieces of cardboard. You then put the hole in one of the
pieces of cardboard over the region of your negative that you want to
read, and "scan" the hole in the other piece over the step wedge until you
find a match.
You can order step wedges from Stouffers directly or if you want instant
gratification the Xpedex store on Walnut street has them for a couple
dollars more (they have alignment pins there too - mentioned only since we
were talking about them yesterday).
- Wayde
(wallen@boulder.nist.gov)
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