Optical finish for acrylic -- vapor polishing and other techniques
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- เผยแพร่เมื่อ 20 ก.ค. 2013
- I needed to polish two acrylic lenses that were made on a CNC lathe to the best possible finish (hopefully, optical quality). I made some test coupons and tried three different polishing techniques on three different surfaces to see which combination of techniques would yield the best results. The winner was clearly 2000 grit sandpaper followed by Novus No.2 polish.
Vapor polishing:
• Video - วิทยาศาสตร์และเทคโนโลยี
We do highly polished acrylic where I work. The secret is that you have to perform the mechanical smoothing to a very high degree before the flame or chemical polish and you cannot skip the in between grits (320 to get the saw marks, 400, 600, 800, 1500, 2400, 3200, 4000, 8000, 12000). 2000 grit is nowhere near good enough ending smoothness to achieve a good optically clear surface. If you took it to 8000 then polished with slurry compound charge on a cotton felt wheel that would be good or for even higher quality, you take it to 12000 grit, felt wheel polish then flame polish. Another issue is that trying to flame polish by hand is art you would have to learn by trial and error, consistent flame polish is best done by driving your acrylic at a consistent speed past the flame.
Rick R Hi Rick, thanks for the info. Where do you work please? (not company name, just what you do ie camera lenses, lab equipment etc...)
I would like to shine lasers through the perspex as well as using cameras to make measurements in a lab type setup and would appreciate any advice you might have.
Rick R.... Thanks for the additional info. I was going to ask what would happen if lapped to 5000. And I thought that would be the limit. Didn't know there are people that polish to 12000. You tube is really great for learning. Thanks again.
Thanks Rick!
i agrre 2000 is not optimal in my experience 3000/5000/7000 is enough sanding then i switch to polishing depeds on your needs but 3k is lower limit imho. Maybe because i dont do it by hand but instead on a lathe/with a straight grinder rotary tool.
it is SO rare to watch a youtube video so informative, clear, and concise. thank you so much for this! answered my acrylic polishing questions, which i've had rolling around in my head for awhile.
Thanks for the demonstration Ben! I've been wondering on techniques for awhile, and you just saved me from having to try it all myself!
Rest in Piece old friend
Looks like the scratch pattern from 400 grit or previous grit is still noticeable on 2000 grit scope. Try to avoid skipping grits, move in linear motions instead of circular, then perpendicular movement when switching grits. This is to help identify when the scratch pattern from the previous grit is removed. Only then can you step up to the next grit.
you know you're a good educator when you can do a video on polishing acrylic, something I don't see myself attempting any time soon, and hold my attention for over 8 minutes.
So glad to have people like Ben out there supplying some truly mind-forward entertainment and shop-talk. Great videos Ben!
I use vapour polishing for plastic, and the best way I've found to do it is to use a large metal pot (though I have also used a giant pyrex beaker on occasion). I normally use a stainless steel pot. Put a few tablespoons of solvent in, and heat. Watch the vapour head as it rises up to the top, remove from heat to prevent the vapour head from over flowing.
Simply suspend your plastic in the vapour head, I use wire to hang it from. Add more heat if the vapour head begins to drop too low in the pot.
This can be done for long periods of time if necessary, and means far less solvent exposure for the operator, and consumes only small amounts of solvent. All the best!
methylene5
*In my art 3D class taught by a famous artist in NYC when we worked with acrylic in the end we had a 2 step polishing process, both involving a rotatory fabric tool to which we had to apply special chalk, that chalk had 2 variants, one red and one was blue, this is what gave the final transparency, however it was also very easy to mess up the smooth surface, but this tool also evened out the imperfections, there are more standard tools for this*
Very infomative. Thanks for taking the time to do this. Also thanks to all the other helpful comments.
Car clear-coat buffing compound is actually FAR superior to Novus, and will give a perfect finish. I use 3m finesse-it 3000 #1 #2 and #3 and the results are incredible even under a microscope. Way more economical also. I'm not paid by 3M by the way.
i have the same product and we use it to clear opticals for cars and yes it's quality product
Thanks! This is all useful information since I work a lot with acrylic. I haven't needed to polish any yet, but I might someday.
Thanks a lot for this video. It is a really nice example of sharing your experience of polishing acrylic materials. Well done. I'm going to get 1200 and 2000 grid sandpaper as well.
I've been working with acrylics for a long time. This is good info. Only comments are sanding should be done in a figure 8 rather than single circle. The figure 8 is an old school lapping technique used to change approach angles of loose abrasives which will unevenly abrade the external edges of your surface. The other thing is that extruded acrylic sucks for glueing. The internal stresses blow the material apart once the material is chemically compromised by the glues/solvents.
As for polishing... agreed, mechanical polishing is best.... flaming and chemical are nice for appearance but not optics.
In my experience it is possible to solvent bond two pieces together with clamping but if multiple lenses are stacked a lense or optical oil makes a perfect transition. Avoid bonding if possible.
Lastly, again, I liked this video and to add to the solvent bonding phase.... polishing closes the pores of the acrylic so the unpolished (but still flat machined) surfaces glue/bond much better. Great stuff here.
In my country, almost every schoolchild knows GOI polishing paste (read as GOI - which means that the State Optical Institute). This paste is very cheap (cheaper than sold on ebay), and it is sold in almost every hardware store. This paste is polished mirror and lens telescopes. It is very easy to use, available and cheap :).
Great video! I have several vapor polished manifolds for a flow cytometer I work on and they never look right, this gives me some great insight.
Extremely useful video; almost exactly what i was looking for as I am looking to restore the window on a piece of vintage electronics, thanks!
Great project, great video and a thorough process to follow. Thanks.
Just a side note, if you flame polish and try to use any solvent on the acrylic you will likely get crazing (cracking). You may have already observed this, but I thought it worth mentioning. Great videos! Thank you for sharing!
Fantastic vid. You pretty much pointing out everything there is to know about doing this within the context of why you where doing it. I would suggest (if not already) that on a rounded surface for optical quality you would want to use liquid only means on a shaped die. For flatter surfaces (like head lights) you could go with sand paper (minimum 800+ grit finish). Also, it's best to not actually boil your chosen solvent unless you have some sort of "sprayer" end on the hose that will flatten out the direction of the fumes as the make contact with the surface. I personally use the double boiler method and try not to heat the water it's self over what would normally come out of the tap. This results in a dense enough fume but not so much that it effects the molecular structure of what i'm polishing beyond the surface.
One improvement over your demonstrated edge heat finish method is to scrape the side with a decent scraper before you apply the heat. This trick is used by special effects departments to make edges nearly perfectly transparent.. In the woodworking world, well done scraping on some type of wood can completely replace sanding. Great videos, by the way.
Nice comparisons and comments. Very useful. I'm doing something like that acrylic visors for my space Helmet. So polish will be important... Thanks for sharing!
It's amazing, everything you do a video on becomes interesting.
Instantly fell in love with your videos and subscribed. Please keep videos coming. Youre a great person to learn from.
Great info, thanks! Trying to produce transparent bores in acrylic/polycarb after taking it off the lathe, though the bore is 3mm. Will see if I can sand in there though seems like slower RPM is helping a bit.
You mentioned that extruded acrylic is best for laser-cutting. However, as an owner of a laser-engraving business, I am confident to say that extruded acrylic is far worse than cast acrylic for laser-cutting. I'm not sure why, but the fumes seem to be far less severe when using cast acrylic, the flame-polished edge is more consistently preserved, and engraving is dramatically better quality.
Extruded acrylic has built-in stresses. All acrylic plastic should be annealed after fabrication. This is a step that everyone seems to miss! It should be out of the cutter, and immediately into the annealing oven for a lengthy cycle.
@@bpark10001 hi Brian, is that the case even with cast acrylic ? , does cast acrylic also get stresses during laser cutting which need to be annealed later.
@@scienceinc.9453 The difference between extruded acrylic & cast acrylic is that extruded acrylic has built-in stress as delivered from the factory. Once you start cutting it, you build in more stress, in addition to that already there. The only difference is that the extruded acrylic may warp if the cuts made release some as the built-in stress. The cast acrylic won't warp, but either will crack from the laser cut, flame polished, or solvent-treated surface. Sharp inside corners will crack first.
I made a light fixture with a bunch of 1.5" diameter cast acrylic rods with slots in the ends. The slots cracked several weeks after cutting. I then re-made them, annealing them for 24 hours. They cracked after 2 years. Then I changed the design & drilled & tapped holes in the ends, screwing Delrin end-caps with the slots, & annealed those for 24 hours. THAT has not cracked in 30 years.
www.acrylite.co/resources/knowledge-base/article/what-is-annealing-and-what-are-the-procedures-for-annealing-acrylite-r-acrylic-sheet?category=working-with-acrylite-r
I have theory why extruded acrylic makes more fumes (cast & extruded acrylic is same chemistry). The extruded acrylic moves when heated because of the stresses, moving into the laser beam, causing more of the plastic to fume. Normally, the cut line is very thin. So the slightest movement would dramatically increase the vaporized plastic amount.
Ideal setup would be to have the laser table & the plastic preheated to annealing temperature. Make the cuts, then pull the plastic to an oven for slow cooling. You would have to account for the expansion & cut oversize.
@@bpark10001 Thanks for the knowledge Brian, I had always assumed that the stresses were developed during the laser cutting process because hot acrylic which tries to expand ( high thermal expansion coefficient) is constrained by rest of the cool acrylic sheet which cause it to stress at cut points, but good to know that pre heating the sheet and then cutting might yield better results .
"Ok, bye." The sudden ending always gets me.
I did flame polishing in the sign business for 37 yrs. After sanding down to 600 grit, we found the best results for transparent gloss finish was straight acetylene, and methylene chloride for Polycarbonite Lexan.
This is amazingly thorough! Do you know if cast polyester resin behaves in a similar way?
I know this is an older video, but for those coming across it years later, like me, I’m pretty sure cast acrylic is what’s recommended for laser cutting. I think extruded acrylic is often used for things like retail displays, etc.
Awesome! Just as was thinking of ideas on how to make lenses with a CNC, you upload this! But, I haven't even started metalshop yet... and I don't have any acrylic to machine... haha. I'm definitely archiving this on my hard drive for later, because I plan on experimenting with optics soon after I learn how to properly use the major tools in a machine shop. This is exactly the video I was looking for! Thanks!
I have joined acrylic with solvent for many years (as a craft teacher) and I have had only partial success. I can get an optically perfect joint about 4 times out of 5. You are right about the finish, if the surface is too flat the solvent can't wick all the way through. The other problem is that the solvent causes slight expansion of the plastic and a joint that was optically perfect at first, developed crazing inside the joint after a few years, caused I presume by locked in stresses. The solvent has to diffuse out through the full thickness of the plastic and this can take many months for thick plastic. Polishing joined pieces revealed that the joint line was still softer than the parent material after a month. Perhaps a period at elevated temperature might speed the process. I have had some success with super glues which are a different type of acrylic but still bond well.
As for polishing I have once, just the once got a perfect finish straight off the lathe, the feed, tool and speed must have been just right. Mostly just the application of a thumb to the rotating plastic is all that is needed.
Acrylic plastic absorbs a lot of water vapour and all machining and joining is best done on acrylic that has been oven dried at 100 C for a few hours. This also relieves internal stresses left after the casting process. HTH.
Thanks for making this video! I’m cutting back some acrylic shelves that are slightly too narrow and wanted to know the best way to make it crystal clear again after cutting it. I’m going to use the novus system with ultra-fine sandpaper. Thanks again!
In the laser optics world you use colloidal silica to polish lenses and for semiconductor wafers too. The tech is in the polishing pad prep and conditioning while polishing. As you polish, the pad will load with compound and start to change its polishing quality and by constant conditioning you can maintain a very high polishing process to achieve the best constant optical standards. Also, Evap carbon onto polished lenses to form sapphire coating for catch proof coating. Also all laser optics have what is called a stack, alternating dielectric and transparent metal layers to increase transmissivity through the lens. This well engineered stack is also applied using some other devices to improve deposition in the stack. And two, if you don't get all of the water vapor out of your vacuum chamber before deposition the stack will not get optimal adhesion and will blow of during use. good luck
Hi Ben: Another possible polishing method is to lap polish. Machine a concave surface of approximately the same shape, coat with RTV (or similar), cover with plastic wrap and press the convex lens into it and let harden (a few folds in the plastic probably won’t matter). Then lap your lens with Novus and the concave lap. Even with the problem of the aspherical surfaces gapping, with small rotating offsets you might get a good polish with minimal refiguring. Cheers, Mark
One issue for the "lenses" you made is the correct profile, on a standard CNC the centres of the machined part surface won't be bang on. On the inner surface, though a lathe cuts on one side making the other a mirror of it, if the tool does not reach the centre then there will be a slight flat at the centre and if the tool goes beyond the centre the two halves will have a pointed profile.
If you measure either half on a surface co-ordinate machine it will give you the points so you can correct the profile by altering the start and finish points in the program.
Someone mentioned using a formed polishing "dolly for the grit" this will help solve that problem but you would have to set the profile of the dolly in the same way i mentioned for the part profile earlier. Probably a bit of overkill for the occasional part you are making but i do production parts that are similar and use a high precision cnc lathe to get the profile correct to within a micron.
My final polish is done with 0.5 micron diamond paste (it needs acropol in the mix so specific to acrylic)
Did you get an effective finish for your project?
SCIENCE rules with that MAN.Many THANKS!
Nice work. Very thorough and informative. Do you know if it's possible to solvent 2 thin pieces of acrylic together to make a thicker piece without compromising too much clarity? I would think if you clamped the pieces together tightly it would eliminate any air bubbles.
Brilliant Ben! I have never made lenses but I have always polished acrylic using sand paper and polishing compound, Seems to work great and its just like working on your car, always move to a finer grit (ideally 1200-2000) before you polish off and you get the best results!
I made the adapter myself. Sure, a video on extreme macro photography would be a good idea.
Another good option for a very flat sanding surface is samples of polished granite worktops. Perfect for small parts as the samples are usually around 100x100mm and cost a quid. Ebay has lots.
This is such handy information! Thank you for posting this. :)
I’m trying to clear up the dust cover on my turntable. This looks like a viable solution!
my family operates a plastics fabrication company in detroit, we specialize in custom fabrication and work with acrylic alot when polishing any surface it is best to prepare it by sanding regardless of how you will be doing the end polishing for flame polishing we generally will sand a routed or saw cut edge with 220 grit sand paper untill all the marks from the tool are gone then we repeat this process with 320 grit or 00 steelwool this provides a extremely fine surface for the flame to polish... flaming a piece will put stresses in the plastic so you do not want to flame any surface that mar come into contact with weldon 3 or 4 as the solvents will cause the acrylic to craze in many cases. i have a bunch of info on how to work with diffrent types of plastics if you have any questions let me know and ill try to help
some of the things we do at our shop include custom fabrication, plastics welding, forming, heat bending, machining, and more. if you have any questions about how to work with acrylics, polyethylene, polypropylene, Hdpe, pvc, phenolic/micarta, polycarbonate, or another type of plastic me or my family probably have experience working with it and can offer some information to you
Madman1555. how do you take out burn marks. I've done plenty but when you get 3/4 and above leaves burn marks. i usuallt buff it out with some compound rub
@@shawnknoblauch5114 Don't flame anything over 1/2"; buff only.
Im using a electric drill and a buffing pad takes some technique..with dico compound. I prefer the blue 7100952.
believe it or not a headlight cover restoration kit is a good source of the polishes and pads.
I love your adjustable lamp base. :)
This is a very informational video. Thank you.
I've got a question about getting an optical finish with an acrylic/polycarbonate tube - it is just a case of trying to sand down the inside of the tube, or is there another method of getting rid of machining marks?
You should deposit an anti reflective coating on the surface of your lenses with you vacuum chamber. An optical stack of TiO2 and SiO2 works well.
I'm a recent sub and I just wanted try say that I'm really enjoying your channel!
What about using a jewellers polish wheel as a finish? I found that also worked very nice with plastic resin on the buffer....? It is much higher in fine finish than 2000 wet grit......Any thoughts? We used to use that method years ago on good plexi and wow,,,,that looked near perfect!
you can buy #3000, #5000 and #7000 sandpaper on ebay relative cheap too, i recently bought so sheets for acrylic
Thank you, it is the best basically method for me.
I'm making lenses out of polycarbonate with a belt sander,
Thinking about trying the upturned vapour bucket technique.
I dont know how he makes polishing plastic interesting to watch, but he did it.
Ben, I dont know what these lenses are for, but It made me think that you making a telescope would be really neat!
Hi, have you tried making some sort of sanding attachment for the CNC lathe? I would imagine that the sanding would be more consistent if you follow a cnc profile with that attachment. Come to think of it, all 3 methods would benefit from being attached to a CNC profile.
thanks for this video. i have a project where I need a clear finish after machining the component. I did not want to have another outisde process to polish it. since I do not need an optic finish, just visually clear, i can do it myself with a torch. thank you
I am guessing this is good info for polishing other plastics such as BC 408 which has a polyvinyltoluene base.
I just LOVE your video's!!
I just found your channel and there are quite a few of your videos that interest me.
Did you ever post a video on gluing two sheets of cast acrylic together without losing optical quality? Or can this be done by the DIYer? I want to glue two sheets of acrylic together while placing designs made of very small gauge wire between them. (30 -34 gauge) without causing the acrylic to lose it's clarity.
I looked through the videos but didn't find it but I may have missed it. Thanks in advance and thanks for helping keep science in our lives.
Old video, i know. But you could try micro-mesh. It's what they use to polish stuff like an aircrafts canopy. Though keep in mind, micro-mesh follows a different grit standards than regular sandpaper. 3200 grit micro-mesh is equivalent to 2000 grit sandpaper. But 4000 grit micro-mesh is finer than 5000 grit sandpaper. And micro-mesh goes up to 12000 grit.
Speaking in terms of microns, 12000 grit micro-mesh is 2 microns. 2000 grit sandpaper is 12 microns.
Micro-mesh is very gentle too. It doesn't abrade away much material. Just enough to polish. So you're not gonna have a big risk of changing lens geometry.
Great video... And indeed please do the solvent welding video to !
Thanks !
I have a boat with a wrap around acrylic window. The style is beautiful, but the window is really jacked up. It’s pitted really bad and foggy. Replacing it would be expensive, and I’d like to see if there’s a way to polish it. Any Thoughts?
Solvent welding is exactly what I need. Did you ever make one
Great vid! Please do a video on solvent welding acrylic - I frequently laser cut acrylic and join it with solvent glue, but for some reason the solvent fills the join nicely at first, but as it starts to try, bubbles start forming/seeping in and kinda ruin the join. Not sure if maybe the laser cut edges are maybe a little too "jagged" and the water thin solvent is just drying up instead of bonding.
TIP: Use a wide "figure '8' " pattern when polishing acrylics to avoid surface deformation. The circular motion used here, causes a buildup of material in the center of the workpiece.
A peak-reading IR thermometer, following on the flame would give a nice consistent result.
I found when I was just casually doing this to repair scratches in safety glasses that best results were obtained with a very strong flame passed over the surface much more rapidly. (using a butane/propane torch)
With the acrylic in the highest heat area of the flame.
Boss video! Thanks for the info.
Ben, may I suggest a topic to build microlens for plenoptic cameras.
I would love to see if these methods work to make acrylic fresnel lens
There is a thread over on homeshopmachinist net where one of the guys does vapor polishing. He does it in a tin can. The thread is titled "I like chemical polishing" Might be worth a look.
At work we make a yearly trophy for Nike and the top crown and bottom stand are both made from cnc machined acrylic. One of the guys flame polishes it with a hydrogen-oxygen torch with a pretty standard brazing tip. He is very quick with the flame using multiple passes never dwelling, like spraying paint.
Use powder abrasives, cast a tin die the same radius as your lenses, and use felt with powder abrasives + water or oil (oil is slower and more messy to clean up, but I think it gives better results). Its much easier than it seems, tin can be cast so easily and already with an embedded centered axe to use with a drill or something. The last abrasive would be rouge or cerium oxide. So with one tin cast and several felts and several grades of powder abrasive, you will make very decent lenses.
Do you think that the 2000 grit paper will help buff out the mistakes from weld on adhesive?
Couple of things
Doing a vapor bath generally your supposed to apply the vapor in a container so the vapor can rest on the part since the vapor is heaver then air.
I use this method for 3d printing with abs which uses acetone but I've experimented with other chemicals and it more or less the same.
I use a presto steamer to heat the chemical and enclose the part. It comes with a tempered lid.
You'll only need to do this for 30 seconds at a time about 3 times at 40 minute intervals and it's best to freeze the lid so the liquid condenses and reduces the potency.
For sanding they have sand paper all the way up to 12,000 grit which would give you a higher polish.
One thing you mentioned is that you jumped from 600 grit to 2000 girt. That's generally not a best practice as your spending more time for less results.
You should start with 600,800,1000,1200 elec
it will save you quite a bit time and remove the previous grit faster.
using a concave shaped sanding block and a random orbit sander may yield better results. I once saw a mirror being polished for a telescope that incorporated these methods.
Seems like micro-mesh would be a great solution to the sanding problem here. I wonder what the difference is.
Do these techniques hold true for polycarbonate? I would imagine that flame polishing would still work.
Could you possibly do a video on etching acrylic/plexiglass?
Hi, i have a plastic helmet that I got for a cosplay that i stupidly ruined. I ruined it by putting a clear coat acrylic on it didn't like the way that looked and used Acetone to remove it which clouded and melted the plastic then to try and get rid of that put more clear coat on it to try and make it clear. all of these were suggestions from people who obviously don't know how to deal with plastic, I dont either. Anyway, any suggestions on a good method of getting it actually clear again?
I have had good results with acetone, built a vapour bath and immersed the plastic in warm acetone vapour. Time of immersion is critical
Hi ben, do you think it's possible to make a telescope mirror this way? That would be quite revolutionary!
I remember seeing a analog device used to evaluate lenses. Light was beamed trough the lens and a uniformity pattern, or something similar, was projected on the other side of the device. I think it was on a "How it's made" episode. I guess that with your experience you could easily build such a device... if you could identify it from my vague description. :)
Since inner stress on manufacture the acrylic have huge birrefrigent or anisotropic.
I use a cook preesure pot with water to relax internal stress.
i would really recommend using electrical rotary tools (anything you have like handheld drill/lathe/dremel/straigh grinder) its so much more effektive than hand polishing
Do you think that a record player dust cover could be flame polished? I'm in the process of sanding a large area that was solvent damaged, and the sanding is taking a long time. Could flame polishing be considered a shortcut?
is that Novus stuff the same type of compound that comes with headlight polishing kits? I have seen those get really nice finishes on opaque acrylic
Would the polished acrylic make better lens if covered with release agent and used as a mold for some sort of clear resin? (obviously it would have to be a negative or used as a buck for a mold.)
Wonder if it would be possible to make concave lenses by spinning the mold as the resin cures?
Informative video, thanks
I am removing scratches from Acrylic Block window. I am successful in removing scratches, but not in the final step is to get it clear. I need your order of work: After you did the 2000 grit, you THEN used the Novus? Or was the 2000 IN PLACE of the Novus? I have cloudy results using auto compound, Turlte Wax's Premium Scratch Repair Kit which includes Clarifying Compound & up to 3600 grit. Chemicals or torches are beyond my comfort level.
Really nice video, thanks very much for sharing.
After drilling a hole long ways through acrylic. How can you polish the drilled hole? So it becomes clear.
hey ben could you make a video on refractor telescope? would be possible with your lenses...would be awesome!!
how did you mount the microscope objective to your camera? is it a special kit, or did you make it yourself? if the latter, can you make a video on that?
Ben, when I remove scratches from motorcycle helmet shields, I wet sand up to 3000 grit before polishing. Try that.
I wish to cut 10mm diameter colored acrylic rods into 50mm lengths in quantity and polish the cut edges. These will be used in jewelry production so a finished look is necessary. A slight bevel would be nice. These could be chucked on a drill press then spun against an abrasive or buffing surface. For instance a sanding/buffing block fixed at an appropriate angle. Some guidance would be greatly appreciated.
The vapour method might work better in a closed chamber.
The vapour stream seems pretty concentrated on one point, kind of like the flame.
I find Simichrome to be a suitable polish for machined acrylic, buffed with a cotton ball or swabs.
Have you tried cerium oxide paste too as a polish ?
What are these lenses going to be used for?
And would it be possible to build a lens that corrects for the chromatic aberration using two different blends of acrylics? Given that you would calculate the shapes of the lenses with for example OSLO...
Good stuff!
Measure the diffraction pattern from your lenses by passing a big, collimated HeNe beam through them. That's a quick and easy way to get an idea of your optical quality. You'll probably have to re-image the diffraction pattern onto a CCD with some magnification to get enough sampling.
astonishing: i knew tht Emanuel Swedenborg managed to build lenses. Today it is the first time I humbly know something more spcific about
Best REGARDS