Cracked dichroic filter

Joel_L · Mar 8, 2022

Hi,

I've been thinking about setting up a darkroom again. I pulled out my two enlargers to clean them up. One of my enlargers is a Patterson ( Philips ) PCS2000. As I was cleaning it up I noticed that the red filter has a crack in it. When I light up the red at full intensity I do not notice the crack in the projection. What I worry about is through all the heat up/cool down cycles that it will start chipping. Is there a good source for filters if I decide to change it. I would also expect there are some specs behind the filter I need to know, possibly measure on the old filter.

Any thoughts?

Thanks
Joerl

ic-racer · Mar 8, 2022

Very nice enlarger! Not many people have these.
I use the condenser version, Philips PCS150 which has a similar additive light source. For B&W printing the red light is only used for focusing (to make 'white' light in conjunction with the other two). If you remove the filter, focusing will be even brighter. Since yours is the diffusion version, if you leave the filter in place, any irregularity of the filters projection would be removed in the diffusion box.
(picture shows PCS2000 with diffusion box removed)

AgX · Mar 8, 2022

Wait until there is chipping of the coating. So far I have not yet seen such, with the exception of dichroic-reflector coatings at lamps themselves.

Joel_L · Mar 8, 2022

I will be doing color, so don't want to remove it. But you are right, the diffusion box will take care of any artifact. I guess as long as it doesn't crumble to where white light is getting through, it should be fine. But if I can find a replacement, I would put it in.

AgX · Mar 8, 2022

It is not only about any irregularity in the illumination as such, but a chipping coating reduces at best the maximum density, at worst the hue changes.

ic-racer · Mar 8, 2022

Ok for color, in general, dichroic filters are very expensive.
However, the Rosco brand "Permacolor" filters are said to be dichroic.
I have not tried these but looks like it might work, 2" primary red for $16 at B&H:

Dirb9 · Mar 8, 2022

Edmunds optics makes high quality dichroic filters, should be more than good enough for an enlarger. You'll have to measure to figure out what size fits. They've been pretty responsive when I've had questions about other filters; they might be able to cut a filter to fit your specific enlarger. https://www.edmundoptics.com/c/dich...60_s:QWRkaXRpdmUgQ29sb3I1&27563=27563_s:UmVk0

AgX · Mar 9, 2022

Are you sure the original filter is a dichroic one? And even if so, why must the substitute one be a dichroic one too? (I see the benefits of a dichroic ones, but seen the discussion so those likely are not needed.)

Joel_L · Mar 9, 2022

I found Thor Labs who does additive and subtractive Dichroic filters. I might see if they can cut to shape.
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=986

I see Edmunds has one in 12.5mm square, that seems real close to the right size. I'll have to measure and see.

I'm also wondering if there is an advantage to replacing all three as a set. I think maybe not because I believe on of the advantages of Dichroic filters is they don't fade.

@AgX - I think they are Dichroic, both because I think I read it somewhere and also by appearance.

AgX · Mar 9, 2022

But why have a filter custom made, when yours is still working properly ? (I assume it still is mechanically stable.)

AgX · Mar 9, 2022

Joel_L said:
I'm also wondering if there is an advantage to replacing all three as a set. I think maybe not because I believe on of the advantages of Dichroic filters is they don't fade.

The classic glass filters do not fade. Actually they are more stable than dichroic ones.

So far I see this all as trying to solve an issue there is not.
You now know of the alternatives you got, forget about them now and start instead using your enlarger.

Joel_L · Mar 9, 2022

This is the best answer!

AgX said:
You now know of the alternatives you got, forget about them now and start instead using your enlarger.

But to complete, both Thor Labs and Edmund filters are thicker. I would likely replace all if it ends up I ever need to.

Now to find a place to set things up.

AgX · Mar 9, 2022

My point was that even if ckracking over the full diameter, the resting two halves will likely stay in place. If not or if you want to be prepared for such consider stabilizing, maybe by a ring insert from metal wire or so. It depends on how the filter is retained now.

Joel_L · Mar 9, 2022

They are in a frame, so likely to stay together. The filters are not so expensive that I wouldn't mind have them on hand just in case. It's always annoying to go start a session then find out you now have a problem.....

I don't know if glass filters have the same edge and bandpass characteristics as Dichroic.

AgX · Mar 9, 2022

Typical for mineral dyed glass is that it got no steep edges spectrum-wise. Whereas dichroic coatings form steep edges, that deliberately can be moved by design, much more than with the former.

BMbikerider · Mar 9, 2022

AgX said:
Are you sure the original filter is a dichroic one? And even if so, why must the substitute one be a dichroic one too? (I see the benefits of a dichroic ones, but seen the discussion so those likely are not needed.)

As far as I am aware all modern and reasonably modern colour enlargers use dichroic filters, so the Philips one will be as well. It is almost impossible for a filter to crack when in the carrier, in situ, they can stand a lot of heat. I can only think that there is a degree of clumsiness on behalf of a previous owner. Don't worry about the crack if it is still held in the carrier, it shouldn't fall out and will not affect the filtration one little bit.

Cracked dichroic filter

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