Making a UV projector for alt-process prints

[AndrewBurns]

A truly spectacular achievement and great photo looking! A 350W UV LED, wow...

I've done a little work related to DLP UV projection, so I'm curious about that have you tested the cyanotype process for different wavelengths like 405 nm? Increasing the UV wavelength from 365-385 nm to 405 nm means that many devices designed for visible light can operate much better, and sometimes the increased transmission efficiency can compensate for the decrease in photon energy. This article here suggests that the exposure efficiency of 405nm is about half that of 365nm: https://pubs.rsc.org/en/content/getauthorversionpdf/C4PP00166D

Thanks! Yes previously I've used a 405nm UV source and an LCD screen for contact printing (placing the LCD screen directly onto the paper and shining UV light through the screen to form the exposure) and it worked perfectly fine for cyanotype. I was able to get very short exposure times with this system, however it was limited to the size of the LCD screen which is why I decided to make the projector.

You're right that shorter wavelengths are less optically efficient, both in that lenses and the LCD screen absorb it more strongly and also the LEDs themselves are less efficient, and so although the process might have a peak sensitivity at a short wavelength you might get your best exposure time at a longer wavelength. The reason I dropped from 405 to 380nm for my projector is because I wanted to expose gelatine sensitised with DAS which barely responds at all to 405nm but is reasonably sensitive to 380nm. Although I've recently found that even with the shorter wavelength I'm struggling to get a good exposure because DAS has a minimum amount of energy required to activate and my projector is struggling to generate that much energy despite how powerful it is.

 

[Yezishu]

Thanks! Yes previously I've used a 405nm UV source and an LCD screen for contact printing (placing the LCD screen directly onto the paper and shining UV light through the screen to form the exposure) and it worked perfectly fine for cyanotype. I was able to get very short exposure times with this system, however it was limited to the size of the LCD screen which is why I decided to make the projector.

You're right that shorter wavelengths are less optically efficient, both in that lenses and the LCD screen absorb it more strongly and also the LEDs themselves are less efficient, and so although the process might have a peak sensitivity at a short wavelength you might get your best exposure time at a longer wavelength. The reason I dropped from 405 to 380nm for my projector is because I wanted to expose gelatine sensitised with DAS which barely responds at all to 405nm but is reasonably sensitive to 380nm. Although I've recently found that even with the shorter wavelength I'm struggling to get a good exposure because DAS has a minimum amount of energy required to activate and my projector is struggling to generate that much energy despite how powerful it is.

Thank you for the reply! I have met the similar UV power issue when using DLP for photochemical reactions. Normally we use 405nm LEDs because everything can be much cheaper. However, if the materials are not sensitive to 405nm(as you say), there's not much else to be done—we have to switch to 365nm, even if it means without warranty and rapid degeneration of the DMD chip......

I also saw your attempts at splicing, and it is already fantastic with flexible substrates, they are usually difficult due to substrate deformation. Paper shrinks and deforms when wet, and the results of splicing on paper have never been better than on a glass plate (for us). I think this is much enough for hard substrates like acrylic sheets/gelatin?

 

[AndrewBurns]

Thank you for the reply! I have met the similar UV power issue when using DLP for photochemical reactions. Normally we use 405nm LEDs because everything can be much cheaper. However, if the materials are not sensitive to 405nm(as you say), there's not much else to be done—we have to switch to 365nm, even if it means without warranty and rapid degeneration of the DMD chip......

I also saw your attempts at splicing, and it is already fantastic with flexible substrates, they are usually difficult due to substrate deformation. Paper shrinks and deforms when wet, and the results of splicing on paper have never been better than on a glass plate (for us). I think this is much enough for hard substrates like acrylic sheets/gelatin?

Before I designed this projector I did look at DLP systems as a way of avoiding the huge energy loss of LCD screens, but as you say none were rated for UV operation (unless you wanted to spend a huge amount of money), the amount of power they could handle at UV wavelengths was a tiny fraction of their visible light capability and their useful lifetime at UV wavelengths was also stated to be much shorter.

I'm not sure what you mean by splicing?

 

[Yezishu]

Before I designed this projector I did look at DLP systems as a way of avoiding the huge energy loss of LCD screens, but as you say none were rated for UV operation (unless you wanted to spend a huge amount of money), the amount of power they could handle at UV wavelengths was a tiny fraction of their visible light capability and their useful lifetime at UV wavelengths was also stated to be much shorter.

I'm not sure what you mean by splicing?

DMD has a limited number of total pixels, so the area exposed in a single exposure is limited while maintaining a certain image resolution. We need to use splicing/multiple exposures to obtain a larger image (mainly on the glass). The work of handling the image boundaries is basically what you've already done.

 

[AndrewBurns]

DMD has a limited number of total pixels, so the area exposed in a single exposure is limited while maintaining a certain image resolution. We need to use splicing/multiple exposures to obtain a larger image (mainly on the glass). The work of handling the image boundaries is basically what you've already done.

Ahh yeah I tried stitching together multiple exposures with my LCD contact printing system and could never get it to work reliably. The problem with photo printing vs. lithography is for photo printing you want to display a wide range of tonal values by modulating the UV light intensity, and the human eye/brain is very good at picking up unnatural looking discontinuities, like for example a line of different density at the boundary between two exposures. My understanding of lithography is that you only want to expose or not expose the photo mask, and so stitching together multiple exposures is easier.

I could see that stitching might work if you were doing halftone type exposures, because again each dot is either fully exposed or not at all, and so there would be less visual discontinuity if the boundary between tiles didn't get exactly equal exposure, but I think it would still be a big job.

 

[Yezishu]

Ahh yeah I tried stitching together multiple exposures with my LCD contact printing system and could never get it to work reliably. The problem with photo printing vs. lithography is for photo printing you want to display a wide range of tonal values by modulating the UV light intensity, and the human eye/brain is very good at picking up unnatural looking discontinuities, like for example a line of different density at the boundary between two exposures. My understanding of lithography is that you only want to expose or not expose the photo mask, and so stitching together multiple exposures is easier.

I could see that stitching might work if you were doing halftone type exposures, because again each dot is either fully exposed or not at all, and so there would be less visual discontinuity if the boundary between tiles didn't get exactly equal exposure, but I think it would still be a big job.

We also work with gradient exposures (utilizing 256 or more distinct exposure doses). In my opinion, while studying the characteristic curves of photochemical reactions to ensure overlapping areas perform as expected is a difficult task, the main challenge remains precise mechanical alignment. If there are random variations in the stitching position for each exposure, it becomes nearly impossible to manage. This is partly why overlapping results on paper are far inferior to those on silicon or glass; for us, the shrinkage of the paper during the slow drying process causes significant issues.

For the first question, our approach is to build a database, test the parameters required for each light intensity, and then use software to automatically process the splicing. For example, given two images with an overlap length of A, the light intensity on one side of the overlap area gradually decreases to 0 via curve B, while the light intensity on the other side increases from 0 to its normal value via curve C. Different combinations of B, and C are tested until the effect of the overlap area is the same as a normal exposure. Due to the non-linearity of the response, curves B and C may be asymmetrical, and they may differ under low and high light intensities (as you observed some reaction requires an initial energy). However, this is based on the premise that the mechanical motion is precise enough to accurately reproduce A each time.