# Optimal aperture position?

Started 2 months ago | Discussions thread

E Dinkla wrote:

ProfHankD wrote:

fferreres wrote:

ProfHankD wrote:

None of this is particularly difficult, but the printed masks tend to be pretty miserable quality.

Prof, what do you think of this paper?

https://arxiv.org/pdf/1605.09737.pdf

Interesting. However, 3D printing of dots doesn't work very well. About 8 years ago, I wrote code that converts a continuous image into one or more layers of continuous extrusion along a space-filling curve (it modifies a Hilbert curve) to achieve the desired optical density... that could easily be modified to make spray masks.

I know you are very familiar with both optics and 3D printer. Could it be a poor mans approach to use spray paint with the right 3D shape (elevated)? The paper I point to is for more complex things, but gives an idea of ...the idea.

My motivation is that I really want to use material that does not produce diffraction, haze, so the filter may not be perfectly even, but I am not thinking that much of specular highlights, but of very smooth out of focus areas

Well, I guess I need to describe my homemade sputtering machine. I made it long ago (before I had a 3D printer), I never photographed it, and no longer have it, so this will be words without photos....

Basically, it starts with a cardboard box.

On the top of the box is a little (wood then, now, I'd 3D-print it) mount that a filter can screw into such that the filter is held horizontally under the mount. The mount has a motor on it (a hand drill would do) so it can spin the filter just above the top of the box.

Now the magic: there's a shaped slit in the top of the box under the filter.

Here's how it works. You mount a clear filter and start the motor spinning. Take a can of black spray paint, shake it, and start spraying into a small hole made in the side of the box. Slowly, the paint particles will start to fill the box as a mist with only the finest particles reaching the top... and depositing on the bottom of the filter through the slit.

Since the filter is spinning on center, the distribution of paint on the filter will be rotationally symmetric, and the fraction at any radius is a function of the slit width and radius from the center. Basically, it should be of width 0 in the center and gradually widen to wider than the radius grows. I originally built a simple widening slit, but now I'd 3D print the slit as a spiral shape -- or perhaps do a spray mask along the lines described above?

Anyway, it was touchy as heck and rather messy, but work it did. The one I built was just not precise enough to give a really smooth distribution and there were a bunch of parameters I never got around to tweaking. With 3D-printed parts it would probably work much better. Maybe I'll design and make a new one?

A fine piece of engineering, worth rebuilding again IMHO. There was no issue that the smaller droplets did not adhere well to the glass?

Thanks. The droplets stick well enough, but there can be some radial smearing because the velocity of the outer edge of the filter is much higher than near the center (the exact center ideally isn't moving at all). The optical density is also subject to unevenness due to turbulence in the air pulled along with the spinning filter... which is why I'm thinking a modified spiral opening might be better. If I build another, I'd definitely also make the spin speed better controlled and variable. BTW, if filters were larger, e.g., for a telescope, the crude box-with-a-slit would have been absolutely fine.

Incidentally, I am at a university that has a nanotechnology center with real sputtering equipment and everything else you could want for VLSI and solar-cell fab; they even have a molecular 3D printer. It would be quite easy for me get stuff done there if I wanted to do real production-quality stuff... it's just we'd quickly get back to the \$100+ build cost per filter range.

Reminds me of the resin box (Dutch: stuifkist) as used in the aquatint / heliogravure process. Powdered resin blown upwards within a box, take some time that the smaller particles are left in the air, insert copper or zinc plate for a given time. Remove plate and heat it evenly up so the resin adheres to the plate. Basic grain pattern mask to resist the etching fluid after further reservations steps are done like a photographic emulsion that is exposed with a continuous tone image and developed so a gradual penetration of the etching fluid will happen.

https://www.polymetaal.nl/contents/de/p4146_Stuifkist_SK-50_capaciteit_50x60_cm.html

A grand scale experiment I undertook was using a spray gun and black paint to make a black mist where the droplet size came near 50 micron size. Still had to sieve the powder to create the 50 micron particles I intended to use for a halftone masking ink for making gradation masks for silkscreen printing. In the end to messy so I returned to sieve (medicinal) activated carbon powder mixed in PVA. Used by my customers to paint the separation films for art prints.

Wow. I'm not sure which of us is more crazy.

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