densitometers, what are they measuring for Alt processes

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From: ken watson (watsok@frii.com)
Date: 03/08/01-10:51:40 AM Z


If this equipment is not reliable for measuring pyro negatives for alt work
it is most likely a combination of light source and sensor design problems.
Since this combination is almost unheard of, except in the art community ie
not Kodak supplied materials, I doubt any consideration was made to make
accurate measurements of the combination of the pyro stain vs the real
wavelength and sensitivity of the Alt process.

It might be better to get a more actinic light source to illuminate the
sensor / negative. The original light source may have a very small to no UV
component in it's spectrum. If the process is sensitive to UV spectrum and
you throw away ( by not having) this measurement it is understandable that
there would be an error in the measurement.

I doubt that it would be financially feasible to try and get a sensor that
has its sensitivity curve adjusted for alt processes. If the sensor is
silicon based, it seems, they are notoriously poor in the blue to UV region
and very good in the IR to red spectrum. So it may be necessary to provide
a lot more UV than "normal" to allow for this insensitivity of the sensor vs
the process. The sensor may have a glass window vs a quartz "glass" window.
Glass is a good UV cut filter, quartz allows UV to pass. Glass passes IR and
visible spectrum very well.

Just putting a blue filter in the sensing path may have little effect as the
wavelength that a lot of the Alt process are sensitive to are very close or
into the ultra violet. This may allow to much "low frequency blue" light in
to cause a false reading. And if the blue filter is common glass see above
re UV cut filter.

What you need is a "high pass" filter or one that cuts all the light form a
certain frequency down ( towards IR ) and passes all the light from a
certain ( blue) frequency up. This may be the best hope of getting a
densitometer to come close to being an effective tool for Pyro negatives in
the Alt process arena.

..-----Original Message-----
..From: Joe Portale [mailto:jportale@gci-net.com]
..Sent: Thursday, March 08, 2001 7:06 AM
..To: alt-photo-process-l@skyway.usask.ca
..Subject: Re: range of densitometers
..
..
..Lukas,
..
..The range will depend on the spoteter you are useing. To get an
..idea, every
..three EV's equals 1.0 density. If you meter goes from zero to 10
..EV's, you
..will have a range from 0.0 to 3.0 I have never had any luck reading a pyro
..neg with a B&W densitometer and a blue filter. Even with a color
..densitometer, the readings are suspect. When reading for an alt process,
..you are concerned with over all printing desnity and contrast, use your
..setup without a filter.
..
..Good luck,
..
..Joe Portale
..Tucson, AZ
..
..
..
..----- Original Message -----
..From: "Lukas Werth" <lukas.werth@rz.hu-berlin.de>
..To: <alt-photo-process-l@sask.usask.ca>
..Sent: Thursday, March 08, 2001 5:33 AM
..Subject: range of densitometers
..
..
..> The question about densitometers brought another one to my mind:
..>
..> Since some time, I am playing with the idea to build my own densitometer
..by
..> using my spotmeter according to the plans of David Fokos in "Beyond the
..> Zone System".
..>
..> However, what range does such a densitometer have? My negs are developed
..in
..> pyro - can they be metered by such a self-built device with a
..blue filter
..> attached?
..>
..> Lukas
..>
..


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