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Thread: plastic lens vs glass lens for prototyping and low volume custom lenses

  1. #1

    Default plastic lens vs glass lens for prototyping and low volume custom lenses

    I don't wish to start a heated debate here over glass vs acrylic/pc and I'm sure people here and elsewhere have discussed all there is to pros and cons of each. But as I am not as skilled in optics as well as other related fields the information is hard for me to consume, so I hope to ask few questions here and have a friendly conversation and learn, nothing more.

    From what I have read, the transmittance of typical glass is 90% while acrylic is 92%. But this doesn't take into account the type of glass, type of plastic (pet, pcarb, acrylic), the optical finish quality, AR coatings.
    My question is, typically, given a typical average AR coated acrylic glass or lens and similar glass one, can we assume likely that they will have the same light transmittance? If not, which one will be better, how much and why?

    Refractive index is better for glass which allows thinner lenses for same results. Is this a good rule of thumb or is there more to it worth knowing?
    If this is the case, why are plastic lenses a thing? I understand plastic is less likely to shutter but it's used in places where there is not much need for that. Is plastic just easier to mold/machine with cheaper tools? If so, why is glass used for expensive lenses where few grams wouldn't change much?

    My goal is to find out the most affordable way to get prototype and low volume custom lenses made, with as little sacrifice to quality as possible.
    My current impression is acrylic is better for this purpose as its cheaper to mold and transmittance and lens properties are very close to glass and things get different with expensive high optical grade glass. But I may be totally wrong. As for polycarbonate vs acrylic for windows and lenses I managed to find even less info.

    PS: I am aware plastic can melt with high power laser beams.

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    Prototyping implies NOT making incredibly expensive molds to produce small quantities of lenses. Plastic lenses change their shape while cooling which implies controlling your process very carefully to achieve predictable results. There are a whole lot of different glasses - each with differering index of refraction and dispersion available. Dispersion (often referred to the Abbe number) and index refraction are how optical materials are specified. Back in the bad old days Abbe mined Calcium Fluorite to combine with normal glasses to make the first Apochromatic microscope objectives at Zeiss. He and Carl Zeiss invested in the startup of Schott Glass because having more glass options was important to getting where they wanted to go at Zeiss. The lasers we use for light shows today are essentially a single color it's possible to do a lot with a single lens - that's not the norm in optical design.
    "There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso

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    Can you 3d print a good lens yet? I doubt you can do normal ones but I bet you can do a fresnel using the liquid /laser type of printing. SLS. I think it is called. Can you yet print in glass and have it transmit light well?

    Sorry not you question. I’ve seen some plastic out perform glass and the other way too for laser. I think given the high power today the question is more about damage to the optic then the ability of the optic. Coatings are very important.

    If you below watt. Non of this much matters.

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    Quote Originally Posted by kecked View Post
    Can you 3d print a good lens yet? I doubt you can do normal ones but I bet you can do a fresnel using the liquid /laser type of printing. ...
    SLA printers *can* do it but the resins are not of optical quality and there would most likely be some post process grinding/shaping needed. Also, since these resins are photopolymers, they would be completely useless at 405nm and probably extremely lossy up to 450nm. So really, only red would be "usable" and even then, I doubt it.

  5. #5

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    Quote Originally Posted by laserist View Post
    Prototyping implies NOT making incredibly expensive molds to produce small quantities of lenses.
    Yes, but what options do I have for few prototype designs and then 10-100 custom lenses of the final design?
    I think precision milling 100 or even just 10 acrylic lenses will cost more than milling one aluminum block and injecting acrylic into it 10 times. Am I wrong?

    Plastic lenses change their shape while cooling which implies controlling your process very carefully to achieve predictable results.
    I was thinking of getting a mold and injection molding done by a 3rd party service rather than myself.
    There are some bench <1000 USD injection molding machines but I don't know how well they would work for lenses.
    https://www.youtube.com/watch?v=uD-kCf8amHk

    Plus I have access to a friend's professional CNC miller that can mill aluminum and steel. He can also let me use of his injection molding machine which his small company uses for making plastic toys.
    But whether these are precise enough I don't know. Are plastic optical molds made with specialized high precision CNCs and injection molding machines? Or should I check for some values in the spec sheets and give it a try if certain requirements are met?

    Plastic lenses change their shape while cooling which implies controlling your process very carefully to achieve predictable results.
    If the lens remains in the mold while cooling as well? If so, yes that might be a problem.

    I hope there are some online services that can do this as well. But I also hope they won't charge me my kidney.
    With general 3d printing and mold making Shapeways.com has been very affordable and good to me. But for metal they do manual polishing after printing the wax and casting the metal.


    Quote Originally Posted by kecked View Post
    Can you 3d print a good lens yet? I doubt you can do normal ones but I bet you can do a fresnel using the liquid /laser type of printing. SLS. I think it is called. Can you yet print in glass and have it transmit light well?
    In my experience even the best 3d printers and general purpose CNC machines produce a result with too much roughness that needs to be sanded then polished to be usable. Since the polishing is done by hand its not very precise. For fun optical illusion projects it seems more than enough ,but for prototyping optical design it is unreliable.
    Quote Originally Posted by kecked View Post
    Can you yet print in glass and have it transmit light well?
    Some researchers are working on decent quality glass 3d printers but nothing more than claims so far.

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    Thanks for the info. It would be truly a game changer to be able to make our own high quality lenses.

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    Quote Originally Posted by aku View Post

    From what I have read, the transmittance of typical glass is 90% while acrylic is 92%. But this doesn't take into account the type of glass, type of plastic (pet, pcarb, acrylic), the optical finish quality, AR coatings.
    My question is, typically, given a typical average AR coated acrylic glass or lens and similar glass one, can we assume likely that they will have the same light transmittance? If not, which one will be better, how much and why?
    Low iron glass such as Pilkington Optiwhite has a transmittance of 92%.

  8. #8

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    Also looking for molded polymer (plastic) lens manufacturers. Will need both low volume for initial prototyping and also bulk order.
    Preferrably cheap.
    Use case: pico projector.

    If I ask this in Physics Stack Exchange it will immediately get deleted as "off-topic".

  9. #9
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    I started down this road once for a corporate employer. Goal was a cheap, molded, spotlight reflector to replace a 500$ per unit electro-deposited, hand polished, first surface, ellliptical reflector. At the end of the process, we were still buying electrodeposited reflectors... Plastic could not hit the Navy specs for beam quality at 1000 feet at less then 10,000 pieces per year and be cost effective on the initial costs.
    ~
    The polish on the plastic mold has to be far better then most mold shops (95% of them can't do this) can do. When your prototype mold costs 10,000$ starting for one lens per mold shot, you'll quickly find having a "overseas" optics shop make your first batch of lenses from glass is far cheaper. Production optics molds for laser quality optics can easily hit 40-50,000$. They are usually produced by a interferometer controlled Diamond tooled CNC machine, with starting costs for the machine itself, being about 1 million Dollars, with another hundred thousand in molding machines and another hundred thousand or so in quality control tools. Getting ten slots or locations in the mold making matching optics requires decades of experience.
    ~
    A car or bike headlight finish, and +/- 9 microns is NOT good enough for a laser collimator or projector is produced by CNC as described in the link below.. You have to take the human hand out of the process to make a mold good enough for projection.
    Here is the process for something in low end optics, car headlight, plastic molded bike light reflector etc... This is not the process for lenses, as you see nothing in here about correcting the "figure" of the mold after the first run...
    ~
    Lenses are specified in many ways, and one of them is the deviation from perfect curve or shape measured in wavelengths of visible light. Lambda over four was good in the old days, now in the 2000s, a cheap projection lens should be Lambda over 20 or better, and the transmission is improved by AR coatings, not as much by choosing acrylic over glass. 540 nm green divided by four is +/- 135 nanometers tolerance, and that is a cheap lens with a modest finish. A better figure for a modern projection lens would be +/- 27 nanometers, Lambda over 20... Your single element plastic lens would not be achromatic, leading to color spreading issues as mentioned above, in other words, orange and blue halos around the edge of the image and fuzziness in the middle. Hence the need for achromatic lenses. Start with a book by Kingslake or Smith on lens design, You'll quickly find "off the shelf" is far cheaper.
    ~
    https://www.moldmakingtechnology.com...olish-aluminum <<<< Again, this is for a car headlight in plastic. If you think that kind of work is cheap, think again. One major slip of the hand from sneezing and they have to start all over again with the CNC and polishing.
    ~
    In reality, for lenses, a glass master would be made and the mold shaped to it, not by hand carving. Still, even with replication molding, your looking at tens of thousands of Dollars for a single mold, starting with diamond CNC controlled by an interferometer.
    ~
    In some cases, the old manual, glass, techniques are FAR, FAR cheaper for small production, as they have had centuries to get the costs down. Modified Draper polishing machines overseen by a skilled human are still far cheaper then Diamond CNC, and will be for the next few years.
    ~
    Steve
    Last edited by mixedgas; 01-09-2018 at 07:58.
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  10. #10

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    I agree and am aware that plastic injection mold and optical grade mold tolerances are quite different but I think comparing lens requirements for a beam with good profile at 1000 feet distance for navy vs a pico projector like the ones used in car HUDs is a bit of a stretch.

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