Source for Ilford 500 head heat absorbing glass

[George Nova Scotia]

Lightwave replied back - nothing in stock there. Well no HA glass for the 500 anyway....

 

[Jeff Bannow]

Shoot. We may have to have something custom cut.

Have you tried putting anything thicker in there?

 

[George Nova Scotia]

Nope, the slot looks pretty tight. If I get a chance I'll pull one out later - the broken head I have is missing a few bolts anyway.

 

[George Nova Scotia]

Okay I took it apart. The first picture shows the assembly from the point of view of the lamp. You can see cracked piece of HA glass still in place. The second shows one half of the assembly, normally two bolts hold it together. The HA glass slot measures 2.5mm the dichro glass slot is about 1mm. The third picture shows the other side with the dichro glass in place. The fourth picture shows a piece of HA glass 33.5mm in dia that I pulled from an Omega C760 Dichro head - it sort of fits on top maybe a couple clips could be fashioned to hold it there. The last it a group shot. The C760 glass is 2mm thick. It almost could be made to fit but I'm not sure if it's tempered. From what I have read tempered glass is very difficult to cut and cutting/shaping destroys the temper near the edges. Maybe someone who knows glass can comment on the cracks in the rectangular piece and whether they indicate tempering.

I don't know if this helps much, but if you need any measurements it's living in a box on my desk right now.

 

[Marco B]

The C760 glass is 2mm thick. It almost could be made to fit but I'm not sure if it's tempered. From what I have read tempered glass is very difficult to cut and cutting/shaping destroys the temper near the edges. Maybe someone who knows glass can comment on the cracks in the rectangular piece and whether they indicate tempering.

The Ilford 500 manual says about the dichroics:

"The cut off frequencies of dichroic filters are affected by temperature, causing inconsistent results. To reduce this variation in transmission with temperature, the dichroic filters in the ILFOSPEED MULTIGRADE 500H enlarger head are tempered prior to use"

Now, correct me if I am wrong, from reading these Schott glass pages, I have the strong feeling Heat Absorbing (HA) glass, is actually just another type of dichroic filter glass with another spectral transmission pattern. So likely, the HA filters are tempered as well...

 

[Ian C]

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.

http://en.wikipedia.org/wiki/Filter_(optics)

http://www.tpub.com/content/construction/14044/css/14044_148.htm

http://en.wikipedia.org/wiki/Dichroic_filter

 

[Marco B]

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.
...
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.

The iron content is probably what Schott glass refers to as "Ionically colored glass" in this glass characteristic PDF of their KG5 HA glass:

http://www.schott.com/advanced_optics/english/download/schott_shortpass_kg5_2008_e.pdf

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."

And:

"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."

 

[ic-racer]

The Edmund IR Cut-Off filter at about 1mm seems like a reasonable functional substitute. (May be too small at 25mm square, though) Dead Link Removed

 

[George Nova Scotia]

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,

George

 

[DAP]

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,

George

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.

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

 

[Marco B]

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.

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.

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.

 

[Marco B]

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:

Dead Link Removed

 

[ic-racer]

Mike at fpointinc.com may have a source, seeing as he is in the glass business.

 

[Jeff Bannow]

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.

 

[Jeff Bannow]

Sycamore Glass doesn't have it in 2mm.

 

[Marco B]

Here is a page with a bunch of companies specializing in glass manufacture including heat absorbing glass. Maybe there is a useful link from there:

http://www.thomasnet.com/products/glass-heat-absorbing-34850602-1.html

Also interesting, this text from this page:

"Heat-absorbing glass contains controlled quantities of a ferrous iron admixture that absorbs much of the energy of the sun. Heat-absorbing glass is available in plate, heavy plate, sheet, patterned, tempered, wired, and laminated types. Heat-absorbing glass dissipates much of the heat it absorbs, but some of the heat is retained. Thus, heat-absorbing glass may become much hotter than ordinary plate glass. Because of its higher rate of expansion, heat- absorbing glass requires careful cutting, handling, and glazing. Sudden heating or cooling may induce edge stresses, which can result in failure if edges are improperly cut or damaged. Large lights made of heat-absorbing glass that are partially shaded or heavily draped are subject to higher working stresses and require special design consideration."

 

[Jeff Bannow]

Here is a page with a bunch of companies specializing in glass manufacture including heat absorbing glass. Maybe there is a useful link from there:

http://www.thomasnet.com/products/glass-heat-absorbing-34850602-1.html

Interesting. I'm sure they have large minimum orders though.

Edmund Optics just responded - they can only get what is listed on their site. They claim you can cut it normally though. Unfortunately, it's all too thick.

 

[George Nova Scotia]

Just to keep track, I've just emailed Newport Glass to see what they can do.

I may have possible solution for my second head. Looks like it would be pretty easy to hold the round filters I have in place with a simple loop of wire. I have a couple C760 heads that would be donors.

 

[Jeff Bannow]

I've seen the Omega glass available for around $20. That might be a reasonable option.

 

[Marco B]

I may have possible solution for my second head. Looks like it would be pretty easy to hold the round filters I have in place with a simple loop of wire. I have a couple C760 heads that would be donors.

Good thinking! Looks like a viable solution how you fixed it. It nicely covers the entire frame. Only possible issue is the expansion of the glass during heating up, but I guess this is no real problem considering the setup.

 

[George Nova Scotia]

Interesting that Edmund said the HA glass could be cut - that would indicate it's not tempered, but it could be cut and mounted much like the round piece I have but better. Copper wire should be soft enough to hold a piece in place without putting much stress on it. The piece I have doesn't lay as flat as I'd like, it needs to be a tiny bit smaller.

I've received a response from Newport Glass - they've asked for a sample so they can check it on the spectrometer and get a good match. I'll send them what I can and see what happens. Might take a week or so for it to get to them. I'll update here when I hear back.

George

 

[Jeff Bannow]

The lady that emailed me could also have just been wrong about cutting it. Not sure there.

 

[Jeff Bannow]

Odyssey Sales in the UK has some parts still (probably pulled from working systems I would imagine):

"£35 for heat absorbing glass 15 x 30x 2mm + £35 for each of the colour filters."

I've also found someone in the US that has parts as well.

 

[Marco B]

Interesting that Edmund said the HA glass could be cut - that would indicate it's not tempered, but it could be cut and mounted much like the round piece I have but better.

The lady that emailed me could also have just been wrong about cutting it. Not sure there.

I have seen both tempered and non-tempered heat absorbing glass filters advertised on more than one page. For example, if you look at this Hoya page:

http://www.hoyaoptics.com/color_filter/heat_absorbing.htm

you will notice they talk about "'Strengthened glass', produced by tempering processes, may also be supplied", suggesting they have both types of products for sale.

Not sure how vital tempered glass is in a high powered enlarger head like the Ilford 500H, or if it could be swapped with non-tempered without major issues. Maybe Ian C can comment on that. Since the filter is very close to lamp, I can imagine issues.

 

[George Nova Scotia]

Odyssey Sales in the UK has some parts still (probably pulled from working systems I would imagine):

"£35 for heat absorbing glass 15 x 30x 2mm + £35 for each of the colour filters."

I've also found someone in the US that has parts as well.

Hmm, £35 each x 4 would end up being over $200CDN with shipping, taxes, duty could well be over $300. That is about what I paid for a complete system on a Focomat IIC.

I've moved that head to Beseler 45 - I'd like to fix the older one but at that cost it's going to be hard to justify. Guess I'm just spoiled with current prices on used equipment .