Beakman (beakman@netcom.com)
Mon, 24 May 1999 18:01:20 -0700 (PDT)
Tadeuz,
Thank you for researching this and posting it. I think this helps
explain the phenomena that I saw in the lab.
Best regards,
David Fokos
> 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
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