From: David J. Romano (romano@agfa.com)
Date: 02/29/00-02:58:33 PM Z
Garet,
I had some trouble interpreting what you've calculated, but let me make a few
comments. Density is a logarithmic function. You can't just divide densities to
get dot area. You have to first convert the densities into reflectances, a
linear function. Densities which differ by .30 differ in reflectance by a
factor of two. Also, if your densitometer does not calculate dot area for you,
follow the Murray-Davies formula for determining dot area.
1-(10^-(Density of the tint-Density of the paper))
divided by
1-(10^-(Density of the solid-Density of the paper))
times 100
= percent dot area
Example using your data.
Ds = 2.28 (100% dot)
Dp = .06 (0% dot)
Dt = .74 (your 50% patch)
1-(10^-(.74-.06)) = .791
divided by
1-(10^-(2.28-.06)) = .994
times 100
= 79.6% dot area
In the graphic arts world, we would refer to this output as having 29.6% dot
gain.
The correct dot areas are as follows:
Input% Density Output%
0 .06 0
5 .10 8.9
10 .15 18.8
20 .26 37.1
30 .41 55.7
40 .57 69.5
50 .74 79.6
60 .93 87.0
70 1.17 92.8
80 1.46 96.6
90 1.82 98.9
95 2.03 99.5
100 2.28 100
You should plot these values to determine a linearization curve For example,
you need to request about a 27% dot to get a 50% dot on the output. However, it
would be better if you would make a print from this negative and measure the
dot area of that, and plot those values against the entered values. It's more
difficult to explain that to do.
To simplify it for you, you could remove the second half of the equation, which
in this case, resulted in .994 if your density range is above 2.0. The error
introduced by this will be less than 1% dot area. Here the density range is
2.22, so the error would only be about 0.5%. As you can see this in the
difference between 79.1% and 79.6%. The error bars on the densitometer
measurements themselves is +/- 2% depending on the quality of the sample.
Hope this helps.
Dave
Garet Denise wrote:
> 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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