Dichroic Glass

[focus_on_infinity]

Does dichroic glass age or fade? Does it ever need to be replaced? I have an Omega dichroic head that I'm about to mount to my D3. Just wondering if that's something that's part of maintenence.

 

[AgX]

It does not age by light (light-fading)

It does not age by time (age-fading)

But as a surface coating it is susceptible to chemical environmental influence. (See the corrosion of AR caotings at front lenses which are similar).

Also I have experienced a lot of dichroic layers having flipped off at halogen lamps with integrated reflector.

 

[focus_on_infinity]

It does not age by light (light-fading)

It does not age by time (age-fading)

But as a surface coating it is susceptible to chemical environmental influence. (See the corrosion of AR caotings at front lenses which are similar).

Also I have experienced a lot of dichroic layers having flipped off at halogen lamps with integrated reflector.

Interesting. What do you mean by flipped off? Do you mean some bulbs work and others don't, due to the reflector? How do you know which is which?

 

[AgX]

Better wording "flaked off". The visible-light-reflection coating was ruptured and falls off in flakes, leaving clear glass. I still am not sure about the reason, but saw this at lamps only so far anyway.

So far I have not seen anything like this at mixing-chamber filters.

 

[Dirb9]

I'm pretty sure AGx is referring to the thin film coating; bits of it can flake off. Generally caused by improper cooling in the head. Most color enlarging lamps have infrared reflectors built in, so much of the heat goes out the back of the bulb, not towards the filters. I suppose using a bulb designed for say a track light instead of a photographic purpose could reflect all of the heat onto the filters and overheat them. Used to be more common when color heads had external fans and people forgot to turn them on before printing. As long as the fans run on your Chromega it shouldn't be an issue.

The other color issue to check out/be aware of is the mixing box; over the years the foam 'reflectors' inside can start crumbling and yellowing (easy enough to peek inside and see), it can be fixed by relining the mixing box with new foam.

 

[MattKing]

Dichroic filters can also get dirty.

 

[AgX]

I'm pretty sure AGx is referring to the thin film coating; bits of it can flake off. Generally caused by improper cooling in the head.

Yes, though being extremely thin these flakes reminds one of flakes of laquer.

 

[focus_on_infinity]

The other color issue to check out/be aware of is the mixing box; over the years the foam 'reflectors' inside can start crumbling and yellowing (easy enough to peek inside and see), it can be fixed by relining the mixing box with new foam.

OK, cool. Yeah, that was my next question was about the foam. I saw some instructions on replacing the foam, but I was concerned about the flammability of the material in such close proximity to a halogen bulb. Would it be correct to expect that a proper photographic enlarging bulb and a working fan would keep the temperature in range?

 

[AgX]

I do not know of the original polystyrene foam plates used in the respective mixing chamber, but at least over here those PS foam plates used for building isolation are made from PS to which a flame retarder has been added.

 

[DREW WILEY]

So yes, dichroic filters can indeed lose efficiency over time, and often do, due to either bits of coating spalling off (excess heat), or gradual grime build up inside a colorhead as less than ideal gasket, paint, or diffusion materials slowly decompose. Big commercial color enlargers are designed to tolerate higher internal heat levels than amateur enlargers; but if the needed light output level was high enough, their dichroic filters and reflectors needed to be periodically replaced. If you do open up an older colorhead to inspect and clean filters, just realize that old coatings can be quite fragile, and must be cleaned with extreme care.

An Omega D3 does NOT have a high-output characteristics running hot enough to worry about as long as the fan is working. It's still a good idea to inspect the general condition of the filters, and to make sure the mixing box hasn't yellowed. But if the filters look bad, it would probably be cheaper and easier to replace the entire colorhead than just the filters themselves. Full units, chassis and all, sell for relatively little these days.

 

[ic-racer]

As mentioned, Dichroic filters are mirrors and even though they are just glass and metallic coating, they could deteriorate with time like a common reflecting mirror, if not cared for.
Personally I have not ever encountered one with severe deterioration. If someone ever finds one deteriorated like this, they should post it. Most commonly I see broken ones. Sometimes broken from an attempt at cleaning.