Point vs diffuse UV light for polymer plates?

MurrayMinchin · Feb 6, 2023

In the back of Clay Harmon's book, Polymer Photogravure, there is a section where different artists explain their methods. Both point (single bulb) and diffuse (tubes or LED) light sources are used to obvious great effect.

What I'm having trouble grasping is how a diffuse light source can give a 'sharp' image using either stochastic screens or DTP (direct to plate).

With a diffuse source, does light from anything but near vertical become shadowed by the adjacent texture on the screen?

Vaughn · Feb 6, 2023

I believe the the LEDs will be halfway between a point and a fully diffuse light sources.

As a carbon printer, I find the thicker my emulsion, the greater the softening of the image due to the use of a diffuse light source. Many would have a some difficulty telling the difference in sharpness. I can and prefer the sharper method. So it might depend on the thickness of the polymer layer one is exposing down into, and what one finds as acceptable sharpness.

MurrayMinchin · Feb 6, 2023

Vaughn said:
I believe the the LEDs will be halfway between a point and a fully diffuse light sources.

As a carbon printer, I find the thicker my emulsion, the greater the softening of the image due to the use of a diffuse light source. Many would have a some difficulty telling the difference in sharpness. I can and prefer the sharper method. So it might depend on the thickness of the polymer layer one is exposing down into, and what one finds as acceptable sharpness.

Thanks. Any info at this point (in a navel gazing & thumb twiddling pre-etching press purchase commitment phase for about 5 years) is good.

I have a DIY LED UV light strip light source and have tracked down a Canadian source for Toyobo Print Tight KM73 plates...so according to your trained eye (tip my hat your way on that one) I should have a 'good enough' setup to get started...if I actually get an etching press...

koraks · Feb 7, 2023

MurrayMinchin said:
What I'm having trouble grasping is how a diffuse light source can give a 'sharp' image using either stochastic screens or DTP (direct to plate).

It doesn't. Diffuse light will result in considerable dot gain, which is also bound to be of an irregular nature across the plate. Photopolymer is just too thick/high to work well with a diffuse light source. Especially those toyobo plates that are even considerably thicker (the emulsion, that is) than the adhesive polymer film. You really want a collimated light source. LED strips at some distance may work. Be sure to test the distance bit; you probably need 2ft at the very least, probably more. A point source may be a little easier to get to work properly.

This is one of the reasons I quit photopolymer a couple of years ago. Back then I ran into a number of issues and in hindsight this was a more major one than I realized back then.

When you start working on this, make sure to inspect your dots really, really well. Calibration is going to be hell if your process doesn't reliably image dots (or rather, pits). You really want them to be of a consistent geometry. Especially the deep shadows (many pits close to each other) requires good process control. Watch out for mottling and reduced density which are signs of foul biting and can result from lack of light source collimation.

MurrayMinchin · Feb 7, 2023

koraks said:
It doesn't. Diffuse light will result in considerable dot gain, which is also bound to be of an irregular nature across the plate. Photopolymer is just too thick/high to work well with a diffuse light source. Especially those toyobo plates that are even considerably thicker (the emulsion, that is) than the adhesive polymer film. You really want a collimated light source. LED strips at some distance may work. Be sure to test the distance bit; you probably need 2ft at the very least, probably more. A point source may be a little easier to get to work properly.

This is one of the reasons I quit photopolymer a couple of years ago. Back then I ran into a number of issues and in hindsight this was a more major one than I realized back then.

When you start working on this, make sure to inspect your dots really, really well. Calibration is going to be hell if your process doesn't reliably image dots (or rather, pits). You really want them to be of a consistent geometry. Especially the deep shadows (many pits close to each other) requires good process control. Watch out for mottling and reduced density which are signs of foul biting and can result from lack of light source collimation.

Thanks, there's a few nuggets of information worth tracking down in your post.

Will definitely look into single bulb UV light sources and ways to control exposure time.

Getting into large format was one thing, trying out alternative hand coated printing was another, but the plate/ink/paper thing is a completely different world.

koraks · Feb 8, 2023

MurrayMinchin said:
but the plate/ink/paper thing is a completely different world.

It is - for better or worse It's a beautiful process, though, in all seriousness. What appealed to me was the freedom of choice in terms of hue and the possibilities for layering prints. I'm doing all this now with carbon transfer, which appears to suit me better. I guess I'm more of a photochemical guy than a photomechanical one!

MurrayMinchin said:
Will definitely look into single bulb UV light sources and ways to control exposure time.

Any old timer should do. Exposures for polymer plates are generally quite short. When I was doing this, they were around 40 seconds or so with the light source I used back then. I'm still using the same film occasionally for making PCB's, btw. I never actually tried the Toyobo plates (there are several alternative brands btw; you could shop around), but as said, they're nice because the polymer layer is thicker and as a consequence, each well can hold more ink. They also save you the time of having to laminate your own plates, although with a little practice, this wasn't all that hard to do.

For UV light sources, I'd recommend starting out with the simplest UV LED floodlight you can find. A unit around 50W should get you pretty far, I estimate. Make sure to get the type with a single COB light source in the middle, not the type that consists of a large array of smaller LEDs across a larger surface area.

MurrayMinchin · Feb 8, 2023

koraks said:
KIt is - for better or worse It's a beautiful process, though, in all seriousness. What appealed to me was the freedom of choice in terms of hue and the possibilities for layering prints. I'm doing all this now with carbon transfer, which appears to suit me better. I guess I'm more of a photochemical guy than a photomechanical one!

Any old timer should do. Exposures for polymer plates are generally quite short. When I was doing this, they were around 40 seconds or so with the light source I used back then. I'm still using the same film occasionally for making PCB's, btw. I never actually tried the Toyobo plates (there are several alternative brands btw; you could shop around), but as said, they're nice because the polymer layer is thicker and as a consequence, each well can hold more ink. They also save you the time of having to laminate your own plates, although with a little practice, this wasn't all that hard to do.

For UV light sources, I'd recommend starting out with the simplest UV LED floodlight you can find. A unit around 50W should get you pretty far, I estimate. Make sure to get the type with a single COB light source in the middle, not the type that consists of a large array of smaller LEDs across a larger surface area.

Right now I'm exploring around to see what methodolagy fits me and my work the best.

Pretty sure it was in a David Kachel article about arriving at a 'fine print' where he described how important Going Too Far is in that process. His point was that if the last step on a test strip looked good, then you should make another one because the next step might be even better...this also went for dodging, burning, cropping, etc.

If you don't push the boundaries, how do you know how far you can go?

Playing with salt prints right now, but polymer photogravures keep whispering my name so will definitely give it a go, one day.

Thanks for taking the time to share.

koraks · Feb 8, 2023

MurrayMinchin said:
Pretty sure it was in a David Kachel article about arriving at a 'fine print' where he described how important Going Too Far is in that process.

Indeed. If I may add - if you're going to overshoot the target, do it with gusto. It's sometimes hard to tell if you've gone too far if the steps taken are cautious and subtle. Be bold!

MurrayMinchin · Feb 8, 2023

For those following along...found these hard won words of wisdom from Clay Harmon,

"I'll add some additional thoughts about using print frames - I just gave a two day class at Basho on polymergravure, and we did not have access to a vacuum frame. We spent 3/4 of the first day fighting contact problems with the split back printing frames. For instance, did you know that glass that looks flat is sometimes not?

The issue is not sharpness. The issue is that anything short of perfect contact between either the aquatint screen or the positive will allow light to hit the highly reflective plate and bounce sideways onto other areas of the plate. And what you end up with are overexposed light patchy areas running through your plate. It is maddening.

I strongly advise against starting this process until you have a good point source light and a vacuum frame. It will be an exercise in frustration otherwise. You'll spend loads of money on plates that end up in the trash. I speak from painful experience on this."

Found in this thread:

Vacuum Frame - Photogravure work

Hi there all, This is my first post to Hybrid so I hope it isn't a daft one. I want to do some photogravure work using photopolymer plates and digital negatives. Although maybe not the most popular process (maybe becasue of the need for an etching press, plates, inks etc) I want to give it...

www.photrio.com

Point vs diffuse UV light for polymer plates?

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