There are several types of heat absorbing filters. For enlargers and projectors, we must have the type that passes visible light but absorbs excess heat from the light passing through.
They’re made of glass of a special alloy containing an iron compound. The iron compound absorbs heat from the light passing through it and that increases the temperature of the filter. Since they can get hot, heat absorbing filters are tempered to prevent the filter from breaking due to thermal shock.
Even so, if the filter gets too hot it can still break. The specifications usually state a maximum operating temperature of 300F (149C). Most often the filter simply splits into two pieces.
If a heat absorbing filter is split, so long as the retainer holds the pieces edge-to-edge in the proper configuration, the filter will continue to absorb excess heat.
Since these filters are almost always outside of the optical path from negative to print the split in the filter won’t affect the image.
When you encounter a split heat absorbing filter, the break is almost always the result of thermal shock, not physical trauma.
Dichroic filters are glass filters with thin metallic elements deposited onto the glass in a precisely controlled THICKNESS. These operate by the mechanism of thin film interference. The coating material has little color of its own.
It’s only when deposited onto the glass in a precisely controlled thickness that we have the sharp cutting filtration so that only a specific color is passed: cyan, yellow, or magenta for color enlargers and blue or green for VC light sources.
OK, thanks Ian for correcting me, and explaining that crucial difference between Heat Absorbing and color Dichroic filters, and confirming the tempered nature of these filters.
Originally Posted by Ian C
The iron content is probably what Schott glass refers to as "Ionically colored glass" in this glass characteristic PDF of their KG5 HA glass:
Schott glass also writes in it's PDF about interference (dichroic) filters:
"Filters with hard thin film coatings form another part of our total production program.
SCHOTT employs two processes for producing such hard coatings:
1. Reactive ion plating
2. Ion assisted evaporation
Both processes produce very durable thin film coatings. Ion plating, however, gives rise to especially hard and compact coatings. These filters have a negligibly low degree of temperature dependence with respect to spectral transmittance. They are hence suitable for applications involving greater changes in temperature but where changes in the spectral characteristics are undesirable."
"These filters utilize the interference effect to transmit or reflect certain spectral ranges of the electromagnetic radiation. Hereto numerous thin layers with differing refractive indices are brought up to a substrate. The optical thicknesses of these layers are usually a quarter of a given design wavelength or multiple thereof.
When electromagnetic radiation encounters such a multilayer system, the incident beam is split at every interface between two layers of differing refractive indices into a transmitted and a reflected beam. This process is repeated at every successive interface, resulting in the formation of numerous superimposing secondary beams that give rise to interference, either in a constructive or a destructive manner. A wide variety of spectral characteristics with high transmission or high reflection ratings can be produced by varying the nature, number, thicknesses and order of the layers.
The coatings of the interference filters described in this catalog are manufactured by the process of vapor-deposition under high vacuum.
In the case of so-called “soft” coatings, additional measures are normally taken to protect the filters from damage by e.g. handling or from moisture. This is usually achieved by supplementary cementing with suitable glass. The upper temperature limit for these filters is essentially determined by the nature of the optical cement used.
Within certain areas of the UV spectrum it is not possible to use optical cements due to the inherent absorption involved. In such cases the coated substrates are fitted into appropriate mounts and protected by suitable glasses.
In the case of so-called “hard” coatings, the layers of which normally consist of very stable metal oxides, there is generally no need for additional protection. Depending on the substrate selected, interference filters with hard coatings can be operated at temperatures up to about 350°C."
The Edmund IR Cut-Off filter at about 1mm seems like a reasonable functional substitute. (May be too small at 25mm square, though) http://www.edmundoptics.com/onlineca...productID=1328
Thank Ian for the info on HA filters. I think that explains a lot. I just tried some temp measurements in my working head which has the 300w bulbs. I got one reading with the IR thermometer at over 400F - that was likely aimed directly at the lamp. With a Type K probe near or touching the metal frame the temp rose to over 350F within a minute (focus). It would seem that we are operating at or over the spec for glass. Your comment about the glass splitting and staying in place I think is very valid. The frame only holds the glass on two sides so depending on the break the glass might sit there for sometime, maybe only falling out when moved or turned on the side. ( Both Jeff and I noticed it was broken when it rattling around after shipping or moving. )
Since most or all HA glass is tempered it creates another problem. All I read indicates that the glass must be cut to size before it is tempered, so unless someone has a glass tempering oven at home, do-it-yourself is ruled out.
I think this gives us few options:
1) Find a source of the 15x30x2 glass from old stock
2) Find someone to make the required glass (with a min order less than 1000sq meters!)
3) Rig another piece of glass to the metal frame
4) find and buy another head.
5) operate without the HA glass and hope the Dichro filter holds up.
I got lucky and did #4 although I'd like to fix up the broken head I have.
Number 1 is looking very remote right now, after all it's been long time since these were made (10 years ?).
3,5 might be doable but results will vary - as they say.
the search continues,
I think option #3 is the most realistic. I saw your photo of the round omega glass - I'm sure some sort of fastener could be devised to hold it slung under the original HA glass location. Even a loose piece of oversize HA glass slipped between the lamp and the dichroics (not attached) might be workable.
Originally Posted by George Nova Scotia
Option #4 is a good idea in general. You never know when the gremlins are going to hit...it is always good to have a backup. The problem is that 2nd hand units are getting harder and harder to find. Several years ago they popped up on ebay at a fairly regular clip - now they are few and far between. To make things worse, the prices of 2nd hand 500 systems seem to have been climbing quite a bit as of late.
Option #5 - It might work, but I don't want to be the one to sacrifice the dichroics in my 500h to find out :)
I don't think so. Schott reports about 350 degrees Celsius, which is 662 degrees Fahrenheit, as the maximum allowable operating temperature for their tempered hard lined interference glass. If the same value is true for the tempered HA glass, than 350F (178C) is well below the maximum value.
Originally Posted by George Nova Scotia
I haven't had issues with the HA glass breaking, even when running the head multiple times in FOCUS mode close one after another for extended periods of time (minutes).
Remember also the bulbs and glass is force cooled through the fan and accompanying small ducts ending above the lamps, precisely to protect the bulbs and filters.
OK, sorry, I now see I missed Ian's remark about the maximum operating temperature of HA glass at 300F (149C), which is close to your reading...
Although I do see a thermal expansion factor for 300 C listed in the Schott glass HA PDFs, suggesting the glass may be operated at such high temperatures... The Surplus Shed also lists 300 C as the maximum for HA: http://www.surplusshed.com/pages/item/pm1070.html
Also nice reading is this Edmund Optic's Key Optical Filter Terminology FAQ explanation:
Mike at fpointinc.com may have a source, seeing as he is in the glass business.
Mike recommended Edmund Scientific (someone tried them already and failed?) and Sycamore Glass. I am awaiting reply from Sycamore still.
Originally Posted by ic-racer
Sycamore Glass doesn't have it in 2mm.