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Re: [Rollei] OT cold mirrors in color enlarger heads, DURST

> since the poping will happen before exposure. Obviously, the same thing
> will happen in any enlarger where the lamp is allowed to burn continuously
> and exposurel controlled by a shutter. The old Saltzman enlargers were
> arranged this way since they used Cooper-Hewett mercury lamps as a source.
> This type of lamp can not be operated intermittantly so a shutter, a
> variation of a Packard shutter, was mounted on a post in front of the lens.
>   This is not a bad technique where one wants to avoid blurring from
> negative motion but doesn't have or want to use a glass sandwich carrier.
> The negative is allowed to heat up and pop before the exposure.
>   Dichroic filters on the lamp prevent excessive heat from reaching the
> negative and may also remove enough IR to avoid problems with some color
> materials which are sensitive to it (it can throw off color balance) but
> probably do not remove enough heat to prevent negative popping. The only
> sure cure for that is a glass carrier.
> ----
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk  

Richard i used to use and still have my 5x7 Beseler cold light from the
50's which was not "quickstart" so had to be left on all the time and
gave off quite a cold colored light which worked with only some papers.
But I'd have it off a split second for a second before i hit my timer,
then turn it back on again.
It was slightly easier then pre popping negs with my condenser head and
just as eventually automatic - Or Autonomic.
And there were situations i recall in which a neg could pop but i think
long after not before the exposure on this one.
And that's because the bulb would get quite hot on for hours almost continously.
There are actually systems in which a florescent grid gets focused
though condensers onto the negs which I'd love to fool with some day.
But I'd think the condensers would absorb much of the UV which is what
ends up doing much of the exposing.
A simple piece of glass does. Plexiglas is much "faster" as a diffusion
device in front of the grid by a full stop according to my test.

I now have a sheet of glass in my Omega D2 as a quick and dirty neg
carrior as I'm printing full frame black border 11x14's from 4x5 negs.
With my Aristo VC 4500 head which has a green florescent grid and blue
one. The corners are just cut off but they look OK.
The negs just sit on the sheet of glass i had cut last week.

Mark Rabiner
Portland, Oregon USA