Smoothest yet not specular metal or plastic sheets?

[nwytg]

I'll get into the details why I need this for below but first I'll explain what I'm looking for.

I basically need a flat sheet of aluminum or any plastic in grey or black color that doesn't provide specular reflections, yet is smooth enough to not have any noticeable texture when looking with the naked eye from the closest possible distance.

I've checked some catalogues for banners, video projection screens, matte plexiglass and matte aluminum, sandblasted glass and acrylic and the off-the-shelf ones I've seen so far have a pretty noticeable texture from up close. I've also gotten some silver nano sized particle powder but haven't managed to succuessfully evenly adhere it to a surface. Haven't been able to test etching yet as well.

The reason I need as smooth as possible is I am building a stereoscope ( https://en.wikipedia.org/wiki/Stereoscope ) but the images projected by a pico projector rather than printed on the screen. This poses a problem as any texture on the screen becomes much more noticeable, more than when looking from very close distance. However if I use a specular screen such as smooth plastic or metal then the issue is the lens of the projector becomes visible as well which even though is AR coated still has some of the projected light scattering inside it or projecting on it as well so a very visible glare is seen with the projected image. So I think a specular screen is out of the question.

With diffuse screens though the off-the-shelf screen materials I've tested so far are too textury.

Any ideas what I can use given this info?

PS. I am aware of VR headsets, I'm working on something different for which HMDs aren't suitable.

 

[Tom.G]

Try https://www.google.com/search?&q=microtexture+projection+screen
9,930 hits.
Not cheap, but it sounds like you need just a tiny piece for your project.

 

[nwytg]

"9 times finer" isn't fine enough I believe and that's fine for video projection screens that are meant to display at most 4-8 times more pixels than 1080p ("future-proof" "up to 8k" as second result mentions) and be viewed from at least a meter away and need the ability to be rolled up and stored. For this though what I said still stands and when I say off-the-shelf screen materials I've tested so far I also mean the catalogues some of these projection screen companies sent me.
You have to realize that even 8000 pixels over 100 inches is nothing compared to 1920 pixels over just few inches viewed through a magnifying lens like in a stereoscope.

What I need basically doesn't have a market so it's fine there isn't an off the shelf product just for what I need, but I was hoping we could find an off the shelf item that I could repurpose or modify or build my own somehow.

 

[Tom.G]

Quoting from pg 12 of https://patents.google.com/patent/US7457036

2. A projection screen according to claim 1 wherein the
anamorphic microlenses are about 0.5um and about 500 um
in size.

Small enough?

 

[nwytg]

Thank you but I'm sorry, Tom, it seems like you're just Googling stuff without checking the results. I can use google too, but I need helpful links or advice, not just links. Sorry for being blunt but this is what I mean:

quote from the patent:
"Projection screens according to other exemplary embodiments of the present invention comprise a substrate including a reflective metal face having a textured surface, to provide reflective microstructures of about 0.5 μm to about 500 μm in size"

"Projection screens may be fabricated, according to exemplary embodiments of the invention, by texturing a surface of a reflective metal substrate..."

Well that's what I am basically asking, how to get a metal with small enough textures. The patent talks about more stuff to improve gain while retaining maximum possible FOV and I don't even need that because the eyebox in a stereoscope provides a very tiny FOV, I just need a metal (or plastic if that is possible) sheet that is smooth enough (small enough texture) yet not so smooth to be a completely specular mirror.

 

[Tom.G]

OK, sorry.

I was operating under the apparently false assumption that the granularity size was the important characteristic and other things, scattering angle, etc., were secondary as long as they didn't get in the way.
I do have some vertical blinds in the bedroom with no specular reflection and no visible texture using a magnifying glass of 2in. focal length. Trouble is they are an off-white plastic and I have no idea what brand they are, but that is at least an existence proof that such characteristics are available.

Hopefully others here can find better options for you.

 

[nwytg]

I was operating under the apparently false assumption that the granularity size was the important characteristic

Tom, what is this in response to?

I do have some vertical blinds in the bedroom with no specular reflection and no visible texture using a magnifying glass of 2in. focal length. Trouble is they are an off-white plastic and I have no idea what brand they are, but that is at least an existence proof that such characteristics are available.

That does sound like something I could use. Haven't seen such matte blinds myself.

 

[Tom.G]

Tom, what is this in response to?

In response to a continuing theme in you posts:

This poses a problem as any texture on the screen becomes much more noticeable,

that doesn't provide specular reflections, yet is smooth enough to not have any noticeable texture when looking with the naked eye from the closest possible distance.

"9 times finer" isn't fine enough

Well that's what I am basically asking, how to get a metal with small enough textures.

I was hoping we could find an off the shelf item that I could repurpose or modify

and the quote from the patent...

by texturing a surface of a reflective metal substrate...

 

[nwytg]

I'm still not following, sorry. What do you mean by "I was operating under the apparently false assumption that the granularity size was the important characteristic and other things, scattering angle, etc., were secondary as long as they didn't get in the way."?

I've said few times the textures (granularity) I currently achieve are too noticeable. So it is the important characteristic.

So what do you mean?

 

[Tom.G]

I'm still not following, sorry. What do you mean by "I was operating under the apparently false assumption that the granularity size was the important characteristic and other things, scattering angle, etc., were secondary as long as they didn't get in the way."?

I've said few times the textures (granularity) I currently achieve are too noticeable. So it is the important characteristic.

So what do you mean?

I mean that:

• The referred-to patent described feature sizes at or well below the resolution of the human eye, which I believe met your requirement in post #1 of "not have any noticeable texture when looking with the naked eye from the closest possible distance."
• And that the existence of the patent may give you a lead to a product as you requested in post #3 for "I was hoping we could find an off the shelf item that I could repurpose..."
• And your post #5 "The patent talks about more stuff to improve gain while retaining maximum possible FOV and I don't even need that... led me to believe that scattering angle and FOV were detrimental to your requirements rather than don't-cares.
• Also in your post #5 you stated "I need helpful links". OK, I was mistaken, the patent link turned out to be not helpful, for which I apologized.

It's getting late here, we can continue this another day if need be. Goodnight.

 

[nwytg]

I wasn't saying the patent was not helpful because of what it achieved and I said it achieved more than what I needed. I was just saying it doesn't mention how the tiny granularity size/texture size on a reflective surface is produced because that wasn't the point of the patent.

As for product lead, sadly first time I hear of such a projection screen and I've researched them all so this probably never became a product. Probably too expensive to produce and can't be folded up so has to stay in the size range of OLED/LCD screens at which point those are better objectively as don't require a projector .

Still, thanks for the time and ideas.
Those vertical blinds sound very interesting and I'd never expect to find a smooth but matte material there (the ones I've had in offices have had decorative textures on them). Definitely could work. Any chance you could figure out the brand? You might be able to find some branding on the slats (the plastic weights at the bottom).

Some kind of rubbery material might also work.

 

[verty]

What about a gel and a mirror? Or car tints on a mirror? You don't need to mass produce it, right?

 

[nwytg]

Not sure how that can be made to work. Please tell me more.

 

[verty]

Well it's up to you to make it work. It's nothing to do with me, I just made a suggestion and it's for you to take it forward now. Bye.

 

[berkeman]

The reason I need as smooth as possible is I am building a stereoscope ( https://en.wikipedia.org/wiki/Stereoscope ) but the images projected by a pico projector rather than printed on the screen.

I've also gotten some silver nano sized particle powder but haven't managed to succuessfully evenly adhere it to a surface. Haven't been able to test etching yet as well.

Apolgies if this has been covered already in this thread (I just started reading it), but it seems like etching may be a good approach, as used in the semiconductor fabrication industry. What size do you need for this sheet? It sounds like it will be pretty small, right?

Using the semiconductor etching techniques of typical fab lines, you can put down all sorts of patterns of different colors at very small geometries. Do you have a university near you that has a small clean room and fab line as part of their EE program? If so, you may be able to ask for help in making the surface that you want. They are often looking for interesting projects for their students in the EE program. You could look at what sorts of materials you would use and what etching patterns and layers to use to make the surface that you want. Just an idea.

 

[nwytg]

Well it's up to you to make it work. It's nothing to do with me, I just made a suggestion and it's for you to take it forward now. Bye.

No. I'm trying to be polite here but this is not an advice. That's not how you give suggestions by throwing words and leaving. You can't give a hypothetical solution to a problem by only mentioning the "what" without the "how". You mentioned two materials and gave zero explanation how they would be used. That's not a suggestion. Both gels and tint films are specular. Unless you mean something else than what people understand by "gels" and "films", you should specify and I asked you to. Otherwise saying you gave a suggestion and saying bye after being asked to clarify is just plain rude and pollutes the thread with pointless posts.

 

[berkeman]

No. That's not how you give suggestions by throwing words and leaving. You mentioned two materials and gave zero explanation how they would be used. Both gels and tint films are specular. Unless you mean something else than what people understand by "gels" and "films", you should specify and I asked you to. Otherwise saying you gave a suggestion and saying bye after being asked to clarify is just plain rude and pollutes the thread with pointless posts.

I think there are frustrations brimming over from your other threads and posts. Let's all stay calm here and try to help you figure this out. Thanks.

 

[nwytg]

Berkeman, my supposed frustration comes from the two replies here, not internally or the single other thread I've made. I was given a suggestion with the poster refusing to clarify what he meant at all. I have zero interest in starting an online argument but the last response from the poster was uncalled for.
I wanted to clarify that, now I can move on and respond to your suggestion, where you actually explain what you have in mind (etching).

 

[nwytg]

Apolgies if this has been covered already in this thread (I just started reading it), but it seems like etching may be a good approach, as used in the semiconductor fabrication industry. What size do you need for this sheet? It sounds like it will be pretty small, right?

Using the semiconductor etching techniques of typical fab lines, you can put down all sorts of patterns of different colors at very small geometries. Do you have a university near you that has a small clean room and fab line as part of their EE program? If so, you may be able to ask for help in making the surface that you want. They are often looking for interesting projects for their students in the EE program. You could look at what sorts of materials you would use and what etching patterns and layers to use to make the surface that you want. Just an idea.

Definitely considering etching as a possible option at this point. My size requirement is only 50x50mm square space to be etched or otherwise textured.
Any ideas what's a typical etching resolution for a PCB? Technically I think with either laser cutter or a demagnified DLP projector the resolution can be as low as the wavelength of light itself so below my ~5 micron texture requirement but how well the physics and chemistry behind etching will allow such low res mask textures to remained adhered to the sheet during the etching process and prevent the reaction to dig under the mask is another story I have no idea about.

 

[berkeman]

I wasn't suggesting PCB copper etching -- that's probably around 0.1mm for typical PCB processes, I would guess. I was suggesting using silicon IC fabrication etching and deposition technologies. But I'm still not clear how big of a surface you need to have in your application. If it's bigger than about a 4" wafer, it may not be practical or low cost to use IC fab technology.

 

[nwytg]

I edited my post when you were responding, the size is only 50x50mm.

 

[berkeman]

I edited my post when you were responding, the size is only 50x50mm.

Thanks, that's right in the wheelhouse of low-cost silicon IC fab etching. You mentioned in your OP that you wanted the overall color to be black, with no specular reflection, is that right? There are likely some deposition materials and etch patterns that can make that happen in that size range...

EDIT / ADD -- And since you don't need the semiconductor properties of silicon, you can use any substrate material that works for the application. The key is to use the imaging and selective etching capabilities of low cost IC fab processes.

 

[nwytg]

You mentioned in your OP that you wanted the overall color to be black, with no specular reflection, is that right?

Correct.

that's right in the wheelhouse of low-cost silicon IC fab etching.

Yeah, I had no idea there were low cost options for silicon etching. Short of university where else can I get this done?

Thanks.

 

[berkeman]

Short of university where else can I get this done?

That's a good question. I'll do some searching. Since you can probably use a substrate material other than silicon, it may only take some reasonable optics for the imaging of the mask(s) and some chemicals for the deposition and etching. Do you have any of those resources available? If not, we can try to figure out what industry near you has similar capabilities for low-volume production...

 

[nwytg]

It doesn't even have to be near me, I'm perfectly fine with paying for shipping if there are online services or similar.

As for DIY, well I do have access to a blue multimode laser CNC and an IR 80W laser CNC, but I doubt the motors can move in nanometer steps or focus the beam to a few nanometer spot because (correct me if I'm wrong) the spot can't be smaller than the emitter diameter.
DLP projector can be used but don't have access to a specialty lens that can project the size of the DLP chip itself or just slightly bigger. DLP documentation states it's possible but I'm not aware of projectors for sale fitted with such lenses.
So DIY seems a bit problematic unless I'm wrong and/or there's other ways to go.

 

[berkeman]

It doesn't even have to be near me, I'm perfectly fine with paying for shipping if there are online services or similar.

As for DIY, well I do have access to a blue multimode laser CNC and an IR 80W laser CNC, but I doubt the motors can move in nanometer steps or focus the beam to a few nanometer spot because (correct me if I'm wrong) the spot can't be smaller than the emitter diameter.
DLP projector can be used but don't have access to a specialty lens that can project the size of the DLP chip itself or just slightly bigger. DLP documentation states it's possible but I'm not aware of projectors for sale fitted with such lenses.
So DIY seems a bit problematic unless I'm wrong and/or there's other ways to go.

Yeah, it does sound like having it done somewhere and shipping it may be the best choice, if we can figure out what will work best for you. I'll do some searching...

 

[berkeman]

If we started with some black substrate, and deposited a thin SiO2 layer on it, do you have any idea what pattern would give you the non-specular reflective properties you want? Maybe a pattern of dots or squares 10um in size and spaced 10um apart? Will the light source be white, or will it be from LEDs with a mixed monochromatic content?

 

[nwytg]

I'm not sure I was thinking a simple noise or checkerboard pattern would do the trick

LEDs with a mixed monochromatic content

It will be like this. The LEDs have a peak wavelength with most generated light and with a narrow wavelength range for the rest of the generated light, so its not pure monochromatic like lasers with a single wavelength with coherence and speckle artifact, in case that's why you're asking.

 

[256bits]

However if I use a specular screen such as smooth plastic or metal then the issue is the lens of the projector becomes visible

This I don't understand.
Is it something to do with the setup of the optics?

If one projects an image onto a screen or wall, what one is seeing is the reflection of light from the screen.
The image should not show an artifact of the lens
As the screen imperfections become more finer, it turns more and more into a mirror.
If one now sees an artifact of the lens, it stands to reason it was always there in the first place, perhaps just washed out from the image luminosity, and just less noticeable.

 

[256bits]

so below my ~5 micron texture requirement

Where did this requirement come from?
The human eye can resolve around 1 arc minute.
The texture the eye can discern becomes a function of distance to the object.

If this guys calcs are correct( you might want to check it out since it is a blog, and blogs do have errors ), at a distance of 4 inch, a 1 arc minute will allow the eye to resolve nothing less than 29 microns.
https://wolfcrow.com/blog/notes-by-dr-optoglass-the-resolution-of-the-human-eye/

 

[nwytg]

Where did this requirement come from?

Because the screen viewed through a stereoscope or head mounted display is viewed through a magnifying lens, that's quite different than viewing something from up close because basically the image becomes a virtual image the size of a large wall appearing meters away from you and any texture on it will get scaled up accordingly. 50mm is stretched over 90 degrees of your eye's field of view.

This I don't understand.
Is it something to do with the setup of the optics?

If one projects an image onto a screen or wall, what one is seeing is the reflection of light from the screen

Screens and most walls cause diffuse reeflection, not specular. With specular reflection you either will see the projection lens only or a bit of diffuse reflection mixed with that. "Artifact of the lens" is you seeing the lens with light scattering inside the lens barrel and illuminating it, projecting slightly on the lens itself because no AR coating is perfect. This appears to our eyes as glare as is blurry and slightly bright. If we had perfect optics with 100% pass AR coating and light not scattering inside the lens assembly we would still see something, the DLP micromirror chip itself illuminated. That's how the Avegant Glyph device works.

As the screen imperfections become more finer, it turns more and more into a mirror.

True, the question is how fine we can get without seeing the reflection of the projection lens and our eyes on the screen itself. I see no way to know this without testing etching or somehow texturing the screen such as micron sized glass beads on a surface acting as retroreflectors.

Silver screens made of silver have been used in cinema and still metal-containing screens are used, but they definitely are not pure mirrors although they have that glare from the lens (called a "hotspot") which can be reduced in software or a painter or somehow applied gradient mask.
Some cinema screens use simple white or grey matte or slightly glossy 1.0-1.2 gain screens which has no hotspot or limited viewing angle before it gets t0o dim but that in turn requires brighter projector to achieve same brightness as silver screens.

What I'm basically trying to do is figure out how much the textures on cinema silver or matte screens can be reduced without ending up with a pure specular mirror. Although I don't need to use silver and can use black color to improve contrast because the screen on my stereoscope is 100-1000x smaller than video projector screens so brightness doesn't matter which is one of the reasons silver screens are used in cinemas, to improve gain (brightness) in cost of viewable FOV. The pico projector I have can produce 300 ANSI lumens which is too bright as is for two 50x50mm screens.

 

[256bits]

Because the screen viewed through a stereoscope...

Ok. Thanks for the explanation. I think I get it now.
You need something with imperfection size a factor smaller than the pixel size of the image.

 

[nwytg]

You need something with imperfection size a factor smaller than the pixel size of the image.

I'm not sure it needs to be if the pixels were very small. But it definitely needs to be smaller than the human eye can detect. So for a 20/20 vision which is 60 pixels per degree roughly 50mm stretched to 90 degrees gives 1/108th mm or about 9 micron (50mm / 90 deg x 60 pixels) grain or scratch(?) size I think? (this is of course ignoring lens distortion messing with the pixels per degre value).

If at 9 micron the individual pixels can be seen I figured 5um is a safe bet to make them blend together. However 9 or slightly bigger may trick the brain into blending them as well. I don't know and there doesn't seem to be readily available research on exactly this so best bet is probably building such a screen and testing.

 

[CWatters]

I might be out of the ball park but some tablet and mobile phone cases have a smooth matt finish. The one on my phone has some texture but I can't see any on the one protecting my tablet. It's a plastic with a slightly rubbery feeling to it.

 

[nwytg]

I might be out of the ball park but some tablet and mobile phone cases have a smooth matt finish. The one on my phone has some texture but I can't see any on the one protecting my tablet. It's a plastic with a slightly rubbery feeling to it.

Thanks for the idea
I checked 4 matte silicone cases for iphones. Thw white matte one almost didn't appear to have any apparent texture after projected onto, same with gray, the two black ones have some speckle shine on them. My guess is all of them do just the white shine on the brighter ones is less apparent. Maybe something to do with size of colorant dye particles and/or some of them acting as retroreflectors.
But it's pretty darn close to what I hoped for.

I cut one up and it has a rubbery layer adhered to a more rigid plastic layer. Don't know if it really is silicone or that's just a marketing term. I've used elastic silicone sheets before for decorative uses and also some silicone covered frame resistant components in 3d printers, both cases silicone was pretty glossy.
Any ideas what's the actual material uses in these or how to find out? Maybe I can find a black sheet with a different colorant with less glow to it if I know what material it is.