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jose angel
7-Sep-2016, 07:02
Hi,
Does anybody know the technical name of the white opal translucent plastic material used in diffusion heads? It is just "acrylic"?
I mean the one placed close above the film holder, like in Durst and many other heads. It is quite common, about 3mm thickness, I need it to finish a new custom LED head I made for my 5x7". It must be the right one to avoid hot spots.
I`m going to buy it at the plastics shop and want to name it properly. Thanks.

LabRat
7-Sep-2016, 07:17
Just white acrylic is OK... Some are slightly denser white...

They probably have a bunch in the scrap bin, you can buy for peanuts... They use it a lot for sign making... Or try a sign shop...

Steve K

Randy Moe
7-Sep-2016, 07:34
I use Makrolon LD as it is specific to LED diffusion.

Bob Salomon
7-Sep-2016, 08:01
If you have one just show it to them.

Drew Wiley
7-Sep-2016, 08:37
"Opal" refers to true glass with a very high Lambertian light scatter coefficient. Acrylics differ. I recommend a compromise between optimal scatter and a reasonable degree of translucency. This kind of product is typically marketed as Sign White acrylic. It has more diffusion than the kind of diffusion plastic typically
used for fluorescent tubes, but is much less dense than ordinary white acrylic. And don't accept white styrene as a substitute for acrylic - it yellows and becomes
brittle over time. Sign White is a common item, and most retail plastic shops have bargain scrap bins.

Drew Wiley
7-Sep-2016, 08:51
(Interrupted). Avoiding hot spots is a more complicated subject. Ideally, you must match your light head to the falloff of a specific enlarging lens. This means having a thicker piece of plastic and grinding it thinner in areas. Then you carefully measure and test your progress. A big headache if you don't know how to
do it or don't have the right tools. Hopefully your LED head is even and oversized, so you can accomodate any edge and corner falloff simply by burning in a bit
when you print.

jose angel
7-Sep-2016, 09:48
Yes, the head works right... I hope. I already made and used a few and applied all the changes to this one. It has the intensity, color and size I was looking for (Drew, the previous one had to be modified for the fall off issue, so I did this one oversized). I have tested it with a smaller acrylic from a Trinobox? by Durst and results are fine.
Thank you all for the info. I`ll ask for the Sign White thing or for something closer to the Trinobox material. Randy, I was not aware of specialized plastics for LED diffusion, thanks. I`ll post the results.

Randy Moe
7-Sep-2016, 10:15
@Jose I did research on LED diffusion a couple years ago and some results can be found in DIY below.

http://www.largeformatphotography.info/forum/showthread.php?110376-Hacking-8x10-and-11x14-enlarger-LED-head&p=1100925&viewfull=1#post1100925

jose angel
8-Sep-2016, 08:37
Well, I finally bought some material and made a few fast tests.
Looks like the acrylic material here is usually called methacrylate, they had it in the so called frosted and opal finish. They also offered me some exotic and branded versions, but only by order and in very large sheets, so I ended buying two pieces of opal material (3 and 4mm) they cut from the scrap bin (total cost was 7.00 euro). The material looks almost identical to the one on the Durst, maybe a very very slightly yellowish -if any-.

I made some contrast test prints that came out as expected.

After that I made a couple prints (without a negative on the stage) looking for the fall off. I compared both acrylic sheets, and found the 3mm one to be a better choice. Looks like fall off is less pronounced, and it is almost two stops faster than the 4mm sheet.

Drew, the LED plate is so even, sized to 8-1/2x8-1/2" for a 5x7" holder. I use to burn almost all my LF prints, so fall off is not an issue to me at all. But I now wonder if I can minimize fall off, -not by grinding-, but maybe changing the LED spread (adding an extra row or increasing diode density at the short sides of the frame). Illumination power is almost perfect to me (a bit on excess), so I really don`t want to add more diodes.

I also wonder about the LED plate to diffuser distance (right now a bit less than 2"), don`t know if any change could improve something. Randy, tonight I`ll carefully read the thread you linked, thanks.

I`m very excited with the new head, so what I really want is to definitely close the box and start printing... :D

Randy Moe
8-Sep-2016, 08:49
I just got an old model Aristo 1212 head which is one piece.

I haven't printed with it yet, but it is bright and I can see the tube coils right through what I assume is the OE diffuser...

I would love to see what you have made. I plan to copy this guy's plans. http://jbhphoto.com/blog/2016/07/02/leds-vc-printing/

I also plan to make an LED point source for large condensers. Heat reduction and longevity are a goal after print quality.

jose angel
8-Sep-2016, 10:30
Well, I think I`m not qualified to talk about enlarging heads... I try to keep it simply and just working. Mine were made to fit the original head base, it is simply a plate with some LED lines (9 in total), that act as a top cover like a hat. The head base has a filter drawer that can keep a diffuser at the bottom. The previous one was an adapted OLED plate, which I much preferred, but that I modified a couple times to get the right illumination and coverage. I never found the perfect diodes to fit that mount. The enlarger is a Omega E6 type one I bought long time ago.
FWIW below is the fall off test print with a Componon-S 180 at working aperture, first with the 3mm sheet (left), then with the 4mm sheet (right) which loose almost two stops and with a more obvious spot:

154792154793

And a fast straight print,

154794

BTW, although heat is much lower that tungsten bulbs, they also get hot if abused and their working temperature is limited. The "hat" I made is intended to dissipate heat, but much hotter that the LED bulbs I have used in other enlargers.

The acrylic sheet I bought, at a distance of 2" doesn`t show the LED spots which are quite bright. Same as with the Durst diffuser I used for testing.

Randy Moe
8-Sep-2016, 11:47
Very few 'qualified' experts survive. Now we try and learn.

There are few sacred cows.

Drew Wiley
8-Sep-2016, 12:48
There are more efficient methods of diffusion, with wider scatter and less light loss. But expect to pay a hundred bucks for a piece of material this size instead of fifty cents from the plastic shop scrap bin. I can provide info to anyone who really needs an alternative; but it's trickier material to fabricate. You can also double-diffuse with the ordinary acrylic sheet, with spacing in between, and of course, a bit of equivalent illumination loss.

Randy Moe
8-Sep-2016, 12:50
Double layers of Makrolon LD works for me. https://www.curbellplastics.com/Research-Solutions/Technical-Resources/Technical-Resources/Makrolon-LD-Flyer

brucetaylor
8-Sep-2016, 18:04
I don't have any experience with the diffusion plastics, but I did make simple LED head for my Omega F. There is a thread here on the site somewhere about it. The LED panel I used was for transparency backlighting so it was fairly even to start with, but I had some unevenness. I used a sheet of diffusion (Roscoe I think) for movie/theatrical lighting. I spaced it off from the light source about an inch and used a picture frame with glass to support it. It works well and was very easy to make.

Randy Moe
8-Sep-2016, 18:30
There are many ways to do anything. I use an enlarger without a lens for contact prints. I saw my diffusion was center hot. I took a piece of translucent plastic and made a center filter with a pencil.

Anybody here learn to do shading and gradients in drawing class?

I checked results with spotmeter and prints. No reason to improve it now...

jose angel
9-Sep-2016, 01:13
Drew and all, do you think the light loss I`m experiencing can be improved with a better diffusion material? I understand it`s a lens issue, so even with a perfectly flat light source there will be always the light fall off issue at the corners.

I think to have a perfectly even print, either the Randy`s center filter, or a "circular variable diode density/intensity" source design, or as you mentioned, an "aspherical" shaped diffusion element is needed. At a first sight, the complexity of the make and the easiness of a simple burning makes me think it could be not worth it. At the end, this level of fall off is what I have gotten with almost all the enlargers I have used.

LabRat
9-Sep-2016, 05:40
I have heard of someone making a center filter for a diffuser by using a very slow process film that was in contact under the diffuser and given a VERY brief exposure (possible with LED's or ND's over the lightsource or flashed), and developed to a low density, and mounted over the diffuser... The center of the film received slightly more light and built more density then the edges, so formed a mask... Worked well I'm told, but I never tried it (I went for using an oversized diffuser + mixing box, or put a mixing box over a condenser...)

Sometimes the problem is that the mixing box white has dulled or yellowed, and the edges reflect less light than the center area... Re-painting the mixing box interior with a flat white spray paint (give many VERY short/fast dusting coats over styrofoam to prevent melting) will bounce light inside more evenly... Sometimes reflective foil near inside edges of diffuser will help, but sometimes too much...

Steve K

Drew Wiley
9-Sep-2016, 08:49
Trying to inkjet print or airbrush what amounts to a center grad filter applied to a diffusion sheet has some inherent problems. First of all, is it going to be permanent or will it discolor and fade? Second (speaking in principle), are you contemplating color enlargement or merely typical black and white printing? It
would be hard to design something like this that is truly color neutral. Similarly with mixing boxes. An ideal paint for these is sold as "densitometer white", composed of 98% barium sulfate (significantly whiter and more stable than titanium dioxide), but around $200 per half pint. I found an art supply substitute that
that was 95% and a tenth that price. So far it hasn't yellowed in my 1500W halogen colorhead. Ideally, I recommend a polished aluminum or stainless steel "mirror box" instead of foam; but it's harder to design these correctly.

jose angel
9-Sep-2016, 10:04
Right, I tested a polished metal box with diode light and reflections made the thing useless... it was literally impossible to avoid them.
I wonder if a black housing in the LED option is a much better choice than white... Doesn`t an oversized diode plate will project the light beams without reflections over the diffuser? I made mine now this way.

LabRat`s idea seem quite good; maybe I can instead to scan and laser print a digital negative to be used over the diffusion sheet. I`ll work on it. Anyway, looking at the fall off test print above I find the resulting effect quite similar to any good condenser enlarger.

Out of curiosity, I want to check if "my" fall off is due to the lens or to the head (or a mix of both). I think it`s just matter of measuring the light at different points over the diffusion sheet (with a spot meter?). And I still have to compare it to my dichroic LPL.

Drew Wiley
9-Sep-2016, 10:08
Falloff is easier to detect if you exaggerate it. In other words, print the light only (no neg in the carrier) using a very hard grade VC paper, and strongly develop it.
This should be done for each enlarging lens you intend to use, at the your most typical intended f-stop setting.