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UltravioletPhotography

Protective UV pass glass for thin filters?


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Hello. I wanted to ask owners of expensive thin filters such as Schott S8612 or BG40 if they need protective glass cover. I'm looking to get either 1mm or similar thickness filter so I imagine at such thickness filter is going to be brittle but thinner filters offer better UV transmission.

I don't want expensive filter to be scratched or broken during long use since I plan to use S8612 for color photography and take on trips as well. There is going to be some light loss due to reflection but I think it's worth to have. Cementing such glass could prevent reflection. Or maybe Schott filter include already such glass or not?

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For UV, 1mm S8612 is too thin I think. Too much IR will get through. Probably 1.5mm is sturdier and better. For visible light photography, get BG38 2mm. I don’t know what kind of glass cover you plan to use but most glass absorbs UV so it would need to be Borofloat or similar.
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I wouldn't clad the S8612 with clear glass cladding. It can be done, using Schott N-WG280 or Hoya BK-7, which would usually be made very thin and glued.

Just get the S8612, if you have to clean it, use cerium oxide.

Thing is, S8612 1mm thick is basically the same as BG40 2mm, and I would not personally use S8612 for visual, I would use BG40 2mm or BG38 2mm for visual.

For UV stacking you need to know what thickness of U glass you are starting with in order to calculate what thickness of S8612 to use.

Seldom would a UV-only stack require S8612 1mm, unless you are starting with very thick U glass.

If you are just wanting to use the S8612 for visual, then don't, instead use BG40 or BG38, 2mm either one.

Don't worry about the filters getting scratched. Even if you use clear cladding it can get scratched also.

A scratch wouldn't show up in a photo anyway.

You would never use BG40 12mm 1mm for anything, maybe visual, but it would be like using S8612 0.5mm thick, with a slightly different visual color balance.

The only reason to get S8612 is for UV only stacking. You 'can' use it for visual, in a pinch... but why? Only use it for UV stacks.

Here are some typical UV stacks

U-360 2mm (or UG1) + S8612 2mm (S8612 can be 1.5/1.75 in this stack, but OD will suffer)

UG11/U-340 1-2mm + S8612 1.5mm-2mm.

These are rough, best to know what you are starting with, and make a precise calculation, but the point being here that using thin S8612 is not usually done.

On the other hand, IF you were to use, lets say, U-340 2.5mm thick, then you 'could' get away with using S9612 1mm thick, but that is still slightly above OD4,

so I would still use S8612 1.5mm thick for that.

Let me know if you have any questions or want stack calculations.

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I agree with Cadmium's post above.

It might be a bit confusing due to a few typos.

 

Cladding is seldom made if not really needed and is making the cost of the filter much higher, especially in smaller production volumes.

It is mainly done to protect some filter glass types that might oxidise when in a bad environment over long time.

Some glass types like the S8612 are more sensitive to this.

I have had my S8612 almost two years and have not seen any signs of that problem yet.

 

I can imagine a cladding could be good on an extremely thin filter to make it mechanically tougher too, but there is no reason to make either S8612 or BG40 / BG38 that thin for normal photography.

 

My idea of best alternatives of general purpose IR-rejecting filters are:

S8612, 2mm as a stack component with any U-glass.

BG38 or BG40, 2mm alone as a VIS filter

 

S8612, 2mm is very good and the best alternative as a component for UV-stacking due to the good IR-rejection and good UV + VIS transmission.

2mm give enough IR-rejection to work well with any normal U-glass types.

The small improvement of transmission in UV + VIS by making the S8612 thinner than 2mm is seldom worth the loss of IR-attenuation, that changes fast with different thickness.

 

However when thick enough to reject IR well S8612 also cut too much of the red in visual.

 

BG40 or BG38, 2mm are good alternatives, used alone for VIS on a full spectrum converted camera, to reject IR and give a good color balance including red image-components.

 

When designing filter components in a stack to be glued together the thickness of each of them can be optimised further.

However the main gain of a glued stack is to get fewer air-glass transitions.

Each such transition costs ca 4.6% of the light passing through the filter-stack.

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Only found 1 typo

I thought it was 2 or 3 typos, but it might have been my dyslexia too. Sorry.

Cannot see them now.

 

The important thing is that all you wrote is technically correct. :)

 

-----

Found the second typo again:

"I wouldn't clad the S612 with clear glass cladding."

:) :)

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Cadmium

I would be interested in knowing more about your "stack calciulations" please. I am still struggling to get my head around the effects of filter thickness on spectral transmission

Thank you

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Cadmium

I would be interested in knowing more about your "stack calciulations" please. I am still struggling to get my head around the effects of filter thickness on spectral transmission

Thank you

 

Depends on what glass you want to start with, how thick, then use enough S8612 thickness to suppress the Red/IR 700nm range down to whatever OD you want.

Assuming you want the typical UV-only stack, then simplified, it goes like this:

1) decide between UG11/U-340 type (which cuts below 400nm) or UG1/U-360 type (which cuts at or above 400nm depending on thickness.

2) decide where you want the peak UV to be, 360nm for example, or lower, or higher.

If you want peak UV to be lower nm, then thicker U glass, if you want it higher nm then thinner.

Thickness of the U glass will,

A - change the peak % (amplitude) of the UV transmission,

B - shift the peak lower or higher nm (when stacked),

C - shift the 400nm cutoff point (lower/higher).

Keep those aspects in mind when picking the glass type in step 1.

3) Lets say you choose UG11 1.2mm thick, then run that in Schott Filter program with S8612 1.5mm, see where the OD suppression of the 700nm range drops down to,

not enough? Add more thickness to S8612 until you get the OD you want.

Adding S8612 thickness will also reduce the peak UV % (amplitude) transmission, and UV band width.

OD at or above 1E-03 is not enough.

OD in the 1E-03 to 1E-04 range may work OK.

OD below 1E-04 is enough.

OD below 1E-05 is very strong and more will only rob you of UV peak strength, UV pass width, and make exposure time longer.

It is a balancing act of sorts.

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Adrian, That probably didn't answer your question...

I start off simple, then I keep thinking of other things.

If you have a U filter, a dual band U filter, one that has red/ir 700nm, and if you want me to make you a graph, let me know the glass type and thickness.

Or if you just want me to make some graph that comes as close as I can to your specifications, let me know.

I guess I obviously like making graphs... so no problem.

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