Infra-red filter on dichroic head?

[Poco]

I recently got a Chromega E Dichroic II enlarger by way of an ugly Ebay transaction. The enlarger arrived with a smashed infra-red filter on the diffuser box, which I have temporarily replaced with plain glass. Is the Ifra-red filtering really necessary for the proper functioning of the Dichro head? I haven't had any experience with one before and haven't a clue. Initially, I plan only to enlarge B&W, but may give color a shot some day.

Any input appreciated...

-Michael

 

[clogz]

An ifra red filter is certainly necessary for colour printing. On the other hand it depends on the coating of your enlarging lens as this may protect against IR light waves. So I think you'd better replace the smashed-up filter.

Good luck

Hans

 

[Poco]

Thanks, Hans.

I suppose I'll have to hunt for the replacement part, or for some appropriate substitute.

 

[Donald Miller]

I have not encountered an IR filter on any of the three enlargers that I have owned and the countless others that I have seen or used. Are you talking about a red filter that is usually placed into the light path (most commonly beneath the lens)? If this is what you are asking about then you can very easily do without it.

 

[Donald Miller]

If, however, this is a filter that is pertinant to the E model, then I would check with the folks at Focal Point, (a search will locate them for you), since they deal with a number of different enlargers, parts, and accessories. Hope that this helps. Good luck.

 

[Poco]

The filter is basically a 1" diameter piece of clear looking glass that the light enters the diffusion box through. Originally I thought it was just meant to shield the interior of the box from the heat of the halogen bulb, but then I noticed the glass had a faint opalescence to it ...and then a note on the diffusion box itself that said it was equipped with a "infra-red filter." Presumably it's meant to filter out infra-red rather than meant to mimic the effects of infra-red film.

I will indeed do the search you suggest.

Thanks.

 

[Thilo Schmid]

Michael,

you will only need an IR Filter in your dichroic head, if you are processing color paper. The cyan forming layers of most color papers is sensitive down to ~750nm. Since almost all enlarger lenses are not corrected for IR, part of the paper "sees" an unsharp image. BTW: the same happens with the UV Portion of the light on all papers (including B&W). Fortunately, most of the UV Light is absobed by the lens (which is not the case with IR Light). Good dochoic heads do have both IR and UV filters.

 

[Ed Sukach]

A "Dichroic" filter is better described as a "heat glass", ... an Infrared filter for *really invisible* infrared radiation, otherwise known as "heat".

Its function is to absorb heat in the optical path and keep the negative stage relatively cool to prevent "popping" out of focus, or in extreme cases (and not ALL that extreme either), damage to the film.

You CAN get away without it. Personally - I wouldn't try.

 

[Thilo Schmid]

</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Apr 9 2003, 10:50 PM)</td></tr><tr><td id='QUOTE'>A "Dichroic" filter is better described as a "heat glass", ... an Infrared filter for *really invisible* infrared radiation, otherwise known as "heat".</td></tr></table><span class='postcolor'>
A Dichroic Filter is based on light inference instead of dyes. This is the reason why the don't wear out (except from getting dirty). Whether a Dichroic Filter absorbs, reflects or passes IR light, depends on the characteristics/specifications of the filter. Not all Dichroic Filters cancel IR nor are all "hot mirrors". Even if they would be, the effect would be dependent on the amount of filtration dialed in.

 

[Ed Sukach]

</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Apr 10 2003, 03:17 AM)</td></tr><tr><td id='QUOTE'></span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Apr 9 2003, 10:50 PM)</td></tr><tr><td id='QUOTE'>A "Dichroic" filter is better described as a "heat glass", ... an Infrared filter for *really invisible* infrared radiation, otherwise known as "heat".</td></tr></table><span class='postcolor'>
A Dichroic Filter is based on light inference instead of dyes. This is the reason why the don't wear out (except from getting dirty). Whether a Dichroic Filter absorbs, reflects or passes IR light, depends on the characteristics/specifications of the filter. Not all Dichroic Filters cancel IR nor are all "hot mirrors". Even if they would be, the effect would be dependent on the amount of filtration dialed in.</td></tr></table><span class='postcolor'>
"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.

I've checked the "Spectral sensitivity" information I have (I only have the data sheets from Agfa) and color paper does have a greater "range" than black and white. Agfa black and white papers drop off to *very little* after about 550 nm, a consideration for safelight use. With their color papers, Spectral and Color sensitivity of the cyan layer end at about 720 nm.

I think some of the confusion here may stem from the term "Infra Red" filter. The filters for use with IR film <on the camera> *PASS* IR and filter out everything else (yeh, I know - oversimplication), while IR filters in OPTICAL PATHS (sometimes referred to as "dichroic filters" or "heat glass, or "heat mirrors" are there to *REMOVE* Infra Red radiation.

I've printed color for some time, now, and the idea of controlling IR for some effect or other has never ben a consideration. I've never heard anyone critique one of my prints by saying "Needs more IR."

 

[Dave Mueller]

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.

 

[Thilo Schmid]

</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.&nbsp; I don't infer any "adjustablilty" from the original post.&nbsp; </td></tr></table><span class='postcolor'>
Ed,

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.

 

[docholliday]

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!

 

[Ed Sukach]

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...

 

[chrisl]

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)

 

[Thilo Schmid]

</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'>
Chrisl,

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.

 

[Thilo Schmid]

</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'>
Doc,

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.