Tadeuz Jalocha (tjalocha@puc.cl)
Mon, 24 May 1999 14:55:39 -0300
David and Dave,
Sorry to jump into your thread, but i might help you some.
Indeed, both are right, but for different applications and media.
Let's look at the Formulas given for Dot Area given by Macbeth:
Transmission Dot Area:
%Dot Area Transmission = %Transmittance for negative dot area
or
%Dot Area = 100 - %Transmittance for positive dot area
As far, Dave is right, supposing a high enough D-max.
Now let's look at the reflection Dot Area formula, given too by Macbeth.
1 - 10^(-Dt)
Apparent Dot Area = ------------ * 100
1 - 10^(-Ds)
where: Dt = density of tint minus density of substrate
Ds = density of solid of the same color.
The booklet from where i took these equations states:
" THe dot area numbers that are computed by the densitometer using this
equation are representative of the way the HUMAN EYE sees the dot afer
it is printed, wich includes both physical and optical dot area."
The question is, why are reflective and transmissive dot area calculated
differently?
First, if Ds were more than 2, could i use the formula for transmissive
dot area instead?
The answer is no. Why? Simply because Ds of say 2,5 is not totally black
for our eyes. Effectively, like the booklet states, reflective materials
are intented to be seen by the human eye, so, we calculate what it sees.
But, what has this to do with this Alt-Photo-Process-List thread?
Well, the equations given for transmissive dot area are intendet for
photo-mechanical processes like plate engraving etc. These processes are
of very high contrast, so, a D-max of 2, like Dave says, can be
interpreted as total absense of light. This is an approximation that is
only valid if the sensitive layer is contrasty enough. If not, the last,
more precise, formula has to be used.
So, for example, for Pd/Pt prints, a D-max of even 4 may not be opaque
enough to be interpreted as total absense of light. So i recommend to
use the precise formula instead.
So, David get's right.
At this point i want to remember you, that the measures have to be taken
in the UV-channel of your densitometer to get meaningful readouts for
Alt. processes. Measuring in the UV-range gives even less contrast than
in the visible range.
It could be of interest, to determine the D-max needed for the different
Alt. processes, to allow the simplification given above.
I suppose, that for processes like Gum - Dichromate a D-max of 2 may
prove sufficient, but never for self masking processes.
I hope, it helped some.
Tadeuz Jalocha
FotoDave@aol.com wrote:
>
> 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%.
>
> In your example of cranking up the laser and you get, say, a density of 0.60,
> it is not correct to say that the percent is still 50% but the optical
> density is 0.60. It simply is not 50% anymore, and it is the job of the
> service bureau to maintain the value. It is accurate, however, to say "what
> is intended to be 50% (or what is exposed as 50%) now becomes 75%," or 25%,
> depending on whether we are talking about positive or negative.
>
> That is because simply by definition, an patch with optical density of 0.60
> does NOT pass 50% light that shine through it. It passes less light, so it is
> not, it cannot be called 50%.
>
> This is also the same as copying a patch that has 50% tone. Suppose you
> calibrate the whole thing so that after the copying, the copy also has 50%
> tone, or optical density of 0.30. Now, you overexpose by 1 stop or 2, and the
> dots grow, so the optical density is no longer 0.30. You do not say that it
> is still 50% dot but with different optical density. The fact is, it is no
> longer 50% tone (because the dots has grown due to overexposure).
>
> There is a direct, one-to-one conversion from percent dot to optical density.
> You can find a table from any graphic arts or printing book. There isn't such
> thing as 50% dot with different optical density. The only exception is when
> Dmax is less than 2.0. Then the opague area must also be included in the
> calculation of optical density because in this case, both the clear area and
> the opague "Dmax" area pass light through.
>
> Thus one can control the process by either dot measurement or by density
> measurement as Adam is trying to do. They are the same thing!
>
> What the service bureau was saying is that he still expose the same thing
> (expose 10% as 10%, 50% as 50% etc.), but the fact that the optical density
> changes show that the calibration is not maintained.
>
> Of course we are not living in a theoretical world, so process/normal
> variation and margin of error must be allowed, and we cannot have
> unreasonable expectation from the service bureau, but assuming that in offset
> printing, the usual range is 10% to 90%, we should reasonably expect 5% and
> 95% to be reasonably accurate... probably 2% margin on the low end and maybe
> 5% on the high end, but this of course depends on the printing requirement.
>
> Or it is exactly the same thing to require some repeatibility (with error
> tolerance) with optical reading on the high and low density.
>
> >> As it turned out, for me anyway, the OPTICAL density was important.
>
> Of course, I have no doubt about it; but again, the optical density measures
> the percent light that passes through.
>
> >> If you like, you can read all about it in a paper I wrote which is posted
> on the Bostick & Sullivan web page.
>
> As mentioned before, I have and probably have downloaded it somewhere. Again,
> I think it is an excellent excellent excellent paper.
>
> Best Regards,
>
> Dave S
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