Beakman (beakman@netcom.com)
Sun, 23 May 1999 14:18:41 -0700 (PDT)
> David,
>
> You mentioned your paper and suggested that I read it. I have read it, not
> recently but I have read it about a couple of years ago, I think. I think it
> is an excellent paper and have recommended others to read it. My comment to
> others is that the paper shows a clear understanding of the process of
> calibration for the most part. I said for the most part because there is a
> small part that I think is a little not accurate.
>
> That part says something like make sure you argue, debate, fight, or do
> anything to convince your service bureau so that the optical density is the
> desired value, not just the % dot. To maintain the optical density is of
> course needed, but it should have been there if the imagesetter is calibrated
> consistently.
>
> This issue is, of course, related to what you just said recently.
>
> It has been a few years already, but I still remember when I read it the
> first time, what came to my mind was the explanation must have been given by
> some service bureau that does not calibrate the system regularly enough. That
> a patch can have the same percent dot but with different optical density is
> their business BS (sorry but I run out of vocab to describe it).
>
> << Next you measure the OPTICAL density of the 1% box and find that the
> reading is say, 1.8. O.K., now you crank up the lasers and run another
> neg. This time the optical density of the opaque square is 7.0, and the
> OPTICAL density of the 1% box is 3.1 EVEN THOUGH THE % DOT STILL MEASURES 1%
> >>
>
> You see, optical density measures opacity, which is 1/transmittance.
> Transmittance measure how much amount of light that passes through. In fact,
> it measure how many percent of light the film passes through!
>
> Let's take an example of 50% dot (I use 50% because it is half the amount of
> light, or one stop, so many are familiar with it already). Because the film
> passes 50% of light, then density MUST BE 0.30 (+ base + fog of course).
> Repeat, 50% dot MUST have optical density of 0.30. If it is not 0.30, it is
> NOT 50%.
Dave,
I have no desire to get into a debate here -- I've been down that path
too many times and I'm a bit weary of it. Let me put it this way...
First, I wasn't using a service bureau. I was running the machines
myself. Second, I measured the step tablets (which I include on every
sheet of film) as the film emerged from the processor and I can tell you
that what I described was accurate. Light of some intensity was getting
through those "opaque" dots and influencing the optical density
measurement, but NOT affecting the % dot measurement. Perhaps the
densitometer (a nice X-Rite model) has a different measurement mechanism
for measuring % dot -- a different cut-off threshold perhaps.
Regardless, the nice, theoretical relationship you describe between % dot
and optical density did not hold true in the real world. The opacity of
those dots is important in platinum printing, but relatively less so
in making offset printing plates.
best regards,
David Fokos
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