[filmscanners] RE: Topic: Callier Effect

["Cliff Rames" <crameslist@worldnet.att.net>]

Hi Dean,

I scan the Stouffer tablet in raw mode to get linear data from the SS4000,
then use Excel to plot the average RGB values for each step vs. the expected
values. The results are not a straight line but a curve that is fit very
well with a power function with an exponent (gamma) around 1.3 - 1.4. So the
Q factor I am measuring is relative to the densities specified by Stouffer
(which appear to be diffuse densities).

"Photographic Sensitometry" by Todd & Zakia defines Q factor as the ratio:
(specular density)/(diffuse density)
They also show a Q factor of 1.40 - 1.41 for a photographic silver neutral
density filter, which is consistent with my results.

The Callier effect depends not only on the film but also the geometry of the
source illumination and the geometry of the sensor. Even though the SS4000
has a relatively diffuse illuminant, the lens and sensor assembly collects
over a narrow angle making it dominantly specular. I would guess that most
of the film scanners discussed in this list have a similar sensor geometry
and Q factor when scanning silver emulsions. (Drum scanners are probably
different.)

Incidentally an engineer at Polaroid confirmed that the SS4000 is in fact
linear with respect to dye-based films.

Regards,
Cliff Rames

>Topic: Callier Effect
>================
>Date: Wed, 09 Oct 2002 07:35:46 -0700
>From: "Shough, Dean" <dean.shough@lmco.com>
>----------------------------------------
>>Another consideration when scanning silver B&W film is the Callier
>>effect.
>>
>>On my SS4000 I measured a factor of about 1.4 with a Stouffer step
>>tablet, so a density of 2.7 effectively becomes 1.4 X 2.7 = 3.8, which
>>is certainly more than this and other scanners can handle.
>>
>>Regards,
>>Cliff Rames
>>
>
>How did you measure the Callier effect?
>As I understand the Callier effect, it is a property of the film (in this
>case, the Stouffer tablet) that measures the relative transmission between
a
>diffuse and a collimated optical system.  Where or how did you do this?
>And, does this mean that you can measure the degree of
diffusion/collimation
>of the SS4000? Or other scanners?
>
>--------
>Dean Shough
>dean.shough@lmco.com

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