From: Garet Denise (garet@rmi.net)
Date: 02/29/00-01:33:49 PM Z
Hi,
I've recently gained access to a reflection densitometer (Thanks Larry!)
and am in the process of calibrating digital negatives from and Epson
printer to use for gum. I'm following the guidelines from Dan Burkholder's
book. The first thing I did was to read the reflective density of the
example print of Ole-No-Moire included with the book. I found that the
reflectance values of the print are not linear (although the curve is
smooth). That is, the reflective density of the 50% patch (0.74) is less
than half of the 100% patch (2.28). The difference between 80% and 90%
(1.82-1.46=0.36) is greater than the difference between 30% and 40%
(0.57-0.46=0.16), etc, etc, etc. I also printed a positive (normal) print
of the Ole-No-Moire grayscale file supplied with the book on my Epson.
This file has a grayscale along side of it. Reflection densities of this
print follow closely with the curve described above.
I've checked the machine in transmission mode on a Stouffer scale and also
in reflection mode on a Kodak reflection step wedge and it appears to be
linear. So, the question: Is the machine in need of calibration (doesn't
appear so) or is this the way that densities need to be laid down on paper
in order to "look right" to the eye? Hopefully the answer to this question
does not involve logarithms (even though I have an engineering degree I
never got along that well with logs). For now I am attempting to make the
reflection curve from my prints match the non-linear curve from the print
supplied with the book. Any insight would be appreciated.
For those interested I list the reflection densities I read from the print
supplied with the book. It's tough to supply tabular data in e-mails, but
here goes. The first number is the theoretical value (i.e. value in the
digital file), the second number is the actual reflective density reading,
the third number is obtained by subtracting the density for 0% from the
second number and dividing by the density for 100%.
Theoretical Value -- Density Reading -- Relative Density
0% -- 0.06 -- 0.0%
5% -- 0.10 -- 1.8%
10% -- 0.15 -- 4.1%
20% -- 0.26 -- 9.0%
30% -- 0.41 -- 15.8%
40% -- 0.57 -- 23.0%
50% -- 0.74 -- 30.6%
60% -- 0.93 -- 39.2%
70% -- 1.17 -- 50.0%
80% -- 1.46 -- 63.1%
90% -- 1.82 -- 79.3%
95% -- 2.03 -- 88.7%
100% -- 2.28 -- 100%
Garet Denise
garet@rmi.net
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