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

[Richard Knoppow <dickburk >]

At 02:34 PM 12/10/2001 -0800, you wrote:
>"Q.G. de Bakker" wrote:
>> 
>> Mark Rabiner wrote:
>> 
>> > I always wonder about all this because sooner or later the light is
>> > going to be focused on the negative and the heat issue is right there as
>> > to if the neg is going to pop or not.
>> 
>> The light is not going to be focussed on the negative.
>> If it were focussed on the film, the negative would not pop but melt away
>> real quick, despite dichroïc mirrors. ;-)
>
>You're right, not focused on the negative unless it is a condenser
>system. You could call it that on that one.
>
>But it's gotta PASS THROUGH the neg sooner or later
>	or i really am missing something! :)
>
>Mark Rabiner
>Portland, Oregon USA
>http://www.markrabiner.com
>
 In a condenser system the light is focused on the entrance pupil of the
lens. 
 Common condenser enlargeres are actually partially diffusing. The light
source is a large diffuse lamp. This has the advantage of making condenser
focus much less critical than it would be where a point source of some sort
was used. Further, the effects of the light are somewhere between those of
a true diffusing source and a truely collimated light source. 
  True point source condenser systems are made for enlargers. They are used
for special purposes where the utmost in edge contrast and resolution are
needed, such as enlarging from microfilm or making photo murals. They bring
out negative blemishes too much for routine use without offering any real
advantage. 
  A true colimated source requires that the condenser be focused rather
precisely every time focus or magnification is changed. Also, because the
source is focused inside the lens, the iris no longer functions to control
the brightness of the image. Stopping down only increases diffraction. In a
true collimated source system the lens is used wide open and intensity
varied in some other way. 
  So called cold light lamps are actually fluorescent lamps. They like to
run hot. The light output of a gas discharge lamp varies with its
temperature so where the lamp is run intermittantly, as it is in most
cold-light enlargers, a heater is supplied to keep the lamp temperature up.
The heater may cause poping but it will stabilize the negative position
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  

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