Diffusion material for UV?

ProfessorC1983 · Aug 26, 2020

I'm working on a new design for an alt-process exposure unit (EDIT: actually an enlarger!) and trying to find an appropriate diffuser material. I have a need to even out light from relatively high-wattage UV LED bulbs across the full negative area, while passing through as much UV as possible to minimize exposure time. I wasn't able to find any obvious information on how common diffusion materials handle UV, and there are so many options (e.g. Rosco) that I'd prefer not to resort to trial and error if possible. Material shouldn't matter since I am dealing with relatively low heat from LEDs. Can anyone recommend a good starting place for a UV diffuser?

wiltw · Aug 27, 2020

There is a class of plastics that has high UV transmission, so it can be used in applications such as healthcare or chemistry.

https://topas.com/uv-transparency?g...2eI5kg6VKfhSqH6XUETgz6iT_H64d_YRoC8QsQAvD_BwE

fgorga · Aug 27, 2020

wiltw said:
There is a class of plastics that has high UV transmission, so it can be used in applications such as healthcare or chemistry.

https://topas.com/uv-transparency?g...2eI5kg6VKfhSqH6XUETgz6iT_H64d_YRoC8QsQAvD_BwE

Interesting material, but not likely a good solution unless it is inexpensive and you can design to make the diffuser easily replaceable. Even then, this would be problematic as the UV transparency will likely be changing over time and thus the exposure time as well.

See the last section at the link you posted. I quote, the emphasis is mine:

Application Guidance
While TOPAS COC is highly transparent to UV, it is not particularly resistant to long term UV exposure, because UV stabilizers reduce ultraviolet transmission. We have optimized our medical polymers for the best available UV transparency by eliminating UV stabilizers, and thus we do not recommend use in repeat sterilization or other long term UV exposure applications. Single UV cycle applications such as diagnostics and analytical testing are our specialty and TOPAS COC medical grade polymer is the healthcare industry's top choice for these uses.

koraks · Aug 27, 2020

ProfessorC1983 said:
I have a need to even out light from relatively high-wattage UV LED bulbs across the full negative area

Why do you have this need? Fairly tightly spaced leds would need only an inch or two of distance to the paper to be perfectly even. Are you sure you're not trying to a solve a problem that doesn't exist?

fgorga · Aug 27, 2020

ProfessorC1983 said:
I'm working on a new design for an alt-process exposure unit and trying to find an appropriate diffuser material. I have a need to even out light from relatively high-wattage UV LED bulbs across the full negative area, while passing through as much UV as possible to minimize exposure time. I wasn't able to find any obvious information on how common diffusion materials handle UV, and there are so many options (e.g. Rosco) that I'd prefer not to resort to trial and error if possible. Material shouldn't matter since I am dealing with relatively low heat from LEDs. Can anyone recommend a good starting place for a UV diffuser?

I really doubt that you will find an organic material that is both UV stable and transparent. The problem is that organic molecules begin to absorb light at about 400 mm and the absorption drops rapidly below than. Here are absorption curves for some common plastics to illustrate the point: https://www.gsoptics.com/transmission-curves/

When high UV transparency is needed for scientific instrumentation, the usual solution is quartz. Optical grade quartz is very expensive, relatively fragile and, as far as I know, not available in very large pieces. Not likely a practical solution.

All of that said, I would experiment with a thin(ish) ground glass.

The reason I say this is that the wavelength needed to expose most alternative process materials can not be too far into the UV. After all we use glass in our exposure frames.... thus common glass must be transparent enough at the required wavelength. Adding another layer of glass would certainly impact the exposure time some, but as is often the case in life... you don't get something for nothing or there's not such thing as a free lunch... pick you favorite saying!

As for the suitably of the Rosco materials, do they still offer a book of small swatches of the materials they offer? If so you could easily and inexpensively test their suitably just by exposing a sheet of sensitizer paper through the swatches as you would a negative.

nmp · Aug 27, 2020

If not thin glass, try thin acrylic (without any fancy coatings or additives to block UV) which I think is more transparent in the UVA region.

koraks · Aug 27, 2020

Acrylic, as far as I'm aware, is a *very* effective UB-blocker at the wavelengths of interest to us. At least, that was what I noted when I had the (not so) 'bright' idea of trying to construct a no-break contact printing frame with allegedly UV-transparent acrylic (turned out it was a rather unfortunate formulation in the manufacturer's datasheet...they meant 'UV-resistant'...)

Fraunhofer · Aug 27, 2020

I am not sure if I understand the need for this. There are oodles of plans for UV exposure units out there (and commercial ones) and I have not seen one which has a diffuser. I have built a LED based one and fluorescent tube version and they both do totally fine w/o the diffuser. The problem I see, is that a diffuser will cost you at least two stops in light intensity (even if fully UV transparent) and I think this would lead to unacceptably long exposure times.

For an imaging system like an enlarger, this is a completely different story, though.

If you still think you need one, quartz glass is highly UV transparent and you probably can make it into a diffuser using fine grit (1500 or so) wet sand paper and patience.

fgorga · Aug 27, 2020

nmp said:
If not thin glass, try thin acrylic (without any fancy coatings or additives to block UV) which I think is more transparent in the UVA region.

The transmission of light by acrylic polymers drops off very steeply after 400 nm... see the link in my original response. Also see Koraks report below... we now have actual data!!!.

The "fancy coatings" added to acrylic on expensive glazing for art may well reduce this further. But in my view this is mostly marketing as the plastic itself is pretty opaque in the UV. Thus a little more absorption by the coating in unlikely to be significant. That said, there is very little actual data available on these coatings, so this is most informed speculation on my part!

Some acrylic (and and almost all polycarbonate) meant as glazing as used in say greenhouses have a coating that absorbs significant UV. These are added to protect the plastic from the effects of UV which is why they come with instructions to install the coated side facing out. These are needed because the absorption of solar UV by the polymers themselves causes them to degrade. Again, though, I have never found data about exactly which wavelengths of light are involved in this.

For that matter, I have never seen data on exactly what wavelengths of light are involved in the photochemistry of the cyanotype (or any other alt process for that matter). We can make adequately short exposures under glass and by using "black light" LEDs. Both of these facts suggest that we are making use of wavelengths greater than 300 nm so, as I suggested in my earlier post a thin piece of plain ground glass should be adequate.

fgorga · Aug 27, 2020

koraks said:
Acrylic, as far as I'm aware, is a *very* effective UB-blocker at the wavelengths of interest to us. At least, that was what I noted when I had the (not so) 'bright' idea of trying to construct a no-break contact printing frame with allegedly UV-transparent acrylic (turned out it was a rather unfortunate formulation in the manufacturer's datasheet...they meant 'UV-resistant'...)

Thanks for a real data point!

This is exactly the result I would expect after a career of working in the lab where spectrophotometry (quantitative measurements of light absorption) is a regularly used method.

fgorga · Aug 27, 2020

Fraunhofer said:
I am not sure if I understand the need for this. There are oodles of plans for UV exposure units out there (and commercial ones) and I have not seen one which has a diffuser. I have built a LED based one and fluorescent tube version and they both do totally fine w/o the diffuser. The problem I see, is that a diffuser will cost you at least two stops in light intensity (even if fully UV transparent) and I think this would lead to unacceptably long exposure times.

For an imaging system like an enlarger, this is a completely different story, though.

If you still think you need one, quartz glass is highly UV transparent and you probably can make it into a diffuser using fine grit (1500 or so) wet sand paper and patience.

I agree with most of this... I have never seen either a home built or a commercial UV exposure unit that uses a diffuser no matter what the light course is LED, fluorescent tune or HID bulb. (Not that I've seem them all!)

As for the use of quartz glass, the optical properties are, as you suggest, appropriate.

However, the original poster will need very deep pockets as this material is very expensive. I also wonder if one can even find quartz glass that is large enough for a practical exposure unit. In my experience with scientific instruments I have never seen a piece of optical quartz larger than a few square cm.

ProfessorC1983 · Aug 27, 2020

Thanks for the input, everyone! And for forcing me to admit that there was a bit of sleight-of-hand in my original post... as my ultimate goal is actually to build a UV LED diffuser enlarger, but I wanted to avoid the "can't be done" type of responses, so I demurred and asked in the context of an exposure unit. Point taken that they're actually very different questions!

I've been quite successfully exposing cyanotype, gum dichromate, VDB and salted paper under my modified Omega B600 condenser enlarger that I've rigged with a 100W UV LED light source. But I've done measurements with a UV meter and know I'm losing a lot of light through the thick condenser lenses and could improve exposure times potentially by 1-2 stops or more by using a diffuser material instead of the condensers, which would allow me to build a new housing altogether and enlarge bigger negatives, add a fan to prevent warping on long exposures, etc.

Lesson learned that it's always better to ask the real question at hand rather than hide behind generalities!

All of that said, I think I'll give the ground glass idea a try since it's cheap and plentiful, while keeping my eye out for a piece of optical quartz in the right size.

Thanks again!

nmp · Aug 27, 2020

koraks said:
Acrylic, as far as I'm aware, is a *very* effective UB-blocker at the wavelengths of interest to us. At least, that was what I noted when I had the (not so) 'bright' idea of trying to construct a no-break contact printing frame with allegedly UV-transparent acrylic (turned out it was a rather unfortunate formulation in the manufacturer's datasheet...they meant 'UV-resistant'...)

I guess I was wrong about acrylic being more UV transparent than glass. The traditional general purpose acrylic does indeed absorb more UV than the window glass - however it seems that is mostly due to the additives that are present as UV stabilizers which are themselves UV absorbers (kind of how they do their stabilization.) Even the basic Plexiglas contains these additives. If they are touted as "UV resistant" you can bet they do not have high UV transmittance.

https://www.hydrosight.com/acrylic-and-ultraviolet-light

A good review of various grades of Plexiglas with their transmission spectra:

https://www.plexiglas.com/export/si...-optical-and-transmission-characteristics.pdf

For the OP, this G-UVT grade of Plexiglas which is designed for low UV absorbance application might be worth looking at:

https://www.plexiglas.com/export/si...dias/downloads/sheet-docs/plexiglas-g-uvt.pdf

The other thing to consider is that an acrylic sheeting can be obtained in thicknesses at low as 1/25" compared to glass at 1/4" so you can save some UV by going real thin with the traditional acrylic.

Good luck! This sounds like a breakthru project....

:Niranjan.

koraks · Aug 28, 2020

Ok, for an enlarger light source, the glass can be smaller than for a typical contact frame. If I were to do this, I'd just start with a bit of plain old window glass and grind it. Perhaps you'll need two layers with a bit of space between them to get perfectly even diffusion. Window glass is fairly transparent to the kind of wavelengths you need. A greater concern is the loss of light due to the diffusion itself; a lot is going to be wasted in the housing, but there's not much you can do except minimizing the distance between the light source and the film.

reddesert · Aug 28, 2020

If you can use use something that has decent transmission longward of about 340-350 nm, then optical suppliers like Edmund Optics can sell you opal or white diffusing glass. Opal glass was a standard choice for diffusing elements when you need more even diffusion than a ground glass, Edmunds has some stock but they say it's out of production, and it is commercial grade and fairly expensive. The white diffusing glass they suggest as an alternative is only mildly expensive. Either of these is likely to be better for UV stability and transmission than most plastics.
Opal glass up to 125x125mm: https://www.edmundoptics.com/f/opal-diffusing-glass/11914/
White glass up to 200x200mm: https://www.edmundoptics.com/f/white-diffusing-glass/15084/
These are a little small for a contact printer, but if you really want an enlarger then maybe it would work. Look for the transmission curve under "technical images."

Diffusion material for UV?

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