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



  From Jose R.:

> ....in the Durst is a cold or regular mirror because I don't know
> the difference between them, but I assume that is a regular one, not
> a white reflector anyway.

Here is a supplier of flat cold mirrors. 

http://www.edmundoptics.com/IOD/DisplayProduct.cfm?Productid=1900

The reflection/transmission diagrams there are self-explanatory.
"Normal incidence" cold mirror exist also. To my opinion a 45 degree
cold mirror looks more attractive than a conventional "anti-heat"
filter since the heat is not retained inside the cold mirror, whereas
it is absorbed in a regular "anti-heat" ("anti-IR") filter ; in a
slide projector this "anti-caloric glass" (this is the French
terminology ;-)) should not be clamped too firmly, and should be
allowed to expand freely under the effect of a high temperature raise.
So-called "Cold Reflector" tungsten bulbs are based on a similar
pinciple since most of the light (even if the reflector is actually
spherical or parabolic) is reflected at about 90 degrees from the
direction of emission like in a flat 45 degree mirror illuminated by a
perfectly parallel beam.

I'm considering (well, sort of dreaming about ;-) to rebuilt one day
the illumination system of my 6x6 slide prjector (an old, manual but
rock-solid, French-made SFOM, ~1960s) to replace the old-style 220V
"high voltage" bulb (like in the Rollei P11 projector) by a modern low
voltage bulb. Adding either a separate cold mirror or using a bulb
with a built-in reflector or combining both (like in the top-class
Leica RT projector series) is what sounds the most reasonable approach
taking into account the easy availability of cold mirror technology
now.

> If you are interested I can send a jpeg of the diagrams of both
> heads.

Sure I'd appreciate that but please do send the files off RUG list of
course; my e-mail is <bigler  .

Thanks,
- -- 
Emmanuel BIGLER         
<bigler  

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