It's typically referred to in enlarger and projector manuals as the "heat absorbing glass". It absorbs IR radiation (and gets hot!). It is there to prevent (actually it only reduces) heating of the negative. For replacements, try www.classic-enlargers.com.
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Apr 10 2003, 02:21 PM)</td></tr><tr><td id='QUOTE'>"Poco" described the filter in question to be "on the diffuser box", which would indicate to me that it had, in fact, a heat attenuation purpose. I don't infer any "adjustablilty" from the original post. </td></tr></table><span class='postcolor'>
I thought you were arguing that the dichroic filters inside the color head work like infrared filters, too. I must have misunderstood you.
Thermal radiation is usually not a problem within an enlarger and a simple piece of glass would stop it (that's why a Greenhouse functions in practice). The dedicated IR filters within an enlarger head are used to eleminate near IR which might lead to an unsharp overlay on color papers.
The IR filters usually used within enlarger heads are dichroic (inference) filters, too.
Hell, just put an APO enlargin lens on the 'larger and focus the IR, UV, etc. It's like shooting with the Zeiss Superachromat lenses, it adds to the print!
It might be of interest to consider just what is meant by "infra-red" and "ultraviolet" - and "color temperature".
"Color Temperature" is determined by what is known as "black body radiation" ... As an object - any object - otherwise known as a "black body" - is heated, it starts to radiate energy- and the color spectrum of that radiation is identified by the temperature of the body - usually given as degrees on the "Kelvin" scale, where zero is equal to absolute zero ( -273 deg. C).
A low temperature will produce a band of light wavelengths centered around a "long" midpoint - otherwise known as "red" - a high temperature will produce a spectrum centered around a "short" wavelength - "blue". The human eye can only detect light of certain wavelengths -- between about 400 nanometers and 700 nanometers. Light outside 400nm and 700nm is not visible - hence, "Infra" and "Ultra".
Infrared is common - we experience it, mainly as heat. A clothes iron will generate long-wave infra-red. Somewhere in this mess of photograhy information, I have a description of a process using "clothes iron" radiation to produce images.
Ultraviolet, is another story - the "black body" has to be heated to a *high temperature* to radiate UV -- the sun does - at a temperature of 5500 - 6000 degrees Kelvin. The problem there is finding a material that will withstand that amount of heat -- tungsten and platinum will make 3400 - 3500K (something like that - what IS the melting temperature of platinum?), so incandescent lamps (as in the majority of enlargers) will not produce *much* (note I did not say "none") ultraviolet - if they were to operate at temperatures that high, their life would be short, to say the least.
Now - Black and White enlarging papers (at least the ones I know of) are not sensitive to Infra-Red radiation - longer than ~ 700nm. They ARE sensitive to ultra-violet - shorter than ~ 400nm, but there is not much of that from the incndescent lamps common to photography. Color paper is generally only sensitive to light in the visible range.
So ... It is usually a "good thing" to remove - or transport to another area - heat in an optical system - due to thermal expansion and thermal stress.
BTW - "Black body radiation" is really only valid for incandescent sources - Flourescent is another bag altogether. Excited gas discharge does not produce a normal spectrum - there are lots of "funny" peak bursts in pulsating flourescent light... and ultraviolet radiation. That is why studio flash discharge lamps are coated - to control UV radiation.
Anyhow - I would replace the "Dichroic" filer. The engineers that designed that system probably duked it out with the cost cutters in Accounting - and proved to THEM it was necessary - so who would I be to argue...
Doc, so you got a new APO enlarging lens? Is it better than the regular Componon S/Rodagons?
And Tschmid, dichroic filters do go bad. I too bought a used Dichroic head and the filters are spotty and not consistent. I bought an Analyser Pro at the same time and couldn't get it to work properly. Switched to under the lens filtration, and it worked great! I think after awhile, they loose the consistent filtration/colors. B&H sells replacements for like $36 and are held in place with superglue (got direct info from SimmonsOmega)
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (chrisl @ Apr 11 2003, 03:07 PM)</td></tr><tr><td id='QUOTE'>And Tschmid, dichroic filters do go bad... I think after awhile, they loose the consistent filtration/colors.</td></tr></table><span class='postcolor'>
Dichroic filters may get dirty and may wear due to thermal stress (get broken, burned in dust, etc). But they do not fade like a dye-based filter.
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (docholliday @ Apr 11 2003, 12:11 PM)</td></tr><tr><td id='QUOTE'>Hell, just put an APO enlargin lens on the 'larger and focus the IR, UV, etc. It's like shooting with the Zeiss Superachromat lenses, it adds to the print!</td></tr></table><span class='postcolor'>
you might be right with the Zeiss Superachromat (and if ultimate sharpness adds to the print - which is not always the case), but there is no one available as enlarger lens. None of the EL-APO-lenses (except perhaps the $4k APO EL-Nikkor - but I doubt) is corrected from IR to UV. And since IR is useless for printing purposes, filtration makes a lot of sense.