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.
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
I use Makrolon LD as it is specific to LED diffusion.
Tin Can
If you have one just show it to them.
"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.
(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.
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.
@Jose I did research on LED diffusion a couple years ago and some results can be found in DIY below.
http://www.largeformatphotography.in...=1#post1100925
Tin Can
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...
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.
Tin Can
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