prism laser scanner

bringing additive manufacturing to the next level

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An open hardware fast high resolution LASER suited for Printed Circuit Board (PCB) manufacturing or 3D printing. The laser head can be used to write to a substrate. I am working on reading from a substrate.

The goal of this project is to develop a laser head for 3D printing or PCB manufacturing which uses a rotating prism and is easy to assemble.
Cyanotype paper is currently used as it can be developed with water.


Specifications are determined by exposure onto a camera without lens and OpenCV. More technical details are available in the whitepaper or the business case pitch.

  • wavelength: 405 nm
  • rotation frequency:  up to 21000 RPM, current 2400 RPM
  • line speed: up to 34 meters per second @ 21000 RPM
  • spot size FWHM: circular, 25 micrometers diameter
  • cross scanner error: 40 micrometers  (error orthogonal to scan line)
  • stabilization accuracy scanning direction:  2.2 micrometers (disabling/enabling scan head)
  • jitter: 35 microns (error parallel to scan line)
  • duty cycle: 47%
  • laser driver frequency: 2.6 MHz
  • maximum scan line length: 24 mm
  • typical scan line length: 8 mm
  • optical power: 500 mW
  • facets: prism has 4 facets
  • prism dimensions: 30x30x2 mm


  • FPGA ICE40UP5K-SG48 with Icestorm toolchain
  • Firestarter cape  (laser driver, 3x TMC2130 stepper drivers, PWM spindle and fan control)
  • Raspberry 4


An image can be exported from Kicad to SVG and converted to instruction for the laserhead. An exposure result on a printed circuit board with close up is shown below.

Resolution below 100 microns can be achhieved. Stitching is between lanes is quite good on cyanotype paper but still a bit uneven on PCB. Exposure below is done without cylinder lenses.

An exposure goes as follows (for the result see above).

Special thanks go to Henner Zeller for his work on LDGraphy. The electronics and software in this project helped me a lot with constructing the laser scanner, see video.
The initial idea of using a prism for laser scanning originates from Dr. Jacobus Jamar. The first system used a plurality of laser bundles in a thicker prism under an angle.  TNO, a research institute of the Netherlands, still pursues this idea, in an entity known as Amsystems.
The current gateware relies on Amaranth HDL and some components from Luna. This is the work of white-quark and ktemkin.

Amaranth HDL gateware for Hexastorm
Optical design

PCB design

Hardware designs
CAD files

Literature Research
White paper @ Reprap

Other Links
Official website


Technical deck presented to selected audience. Deck does not cover LIFT, laser microscope and latest improvements on tech.

Adobe Portable Document Format - 9.11 MB - 12/10/2021 at 08:28



New pitch deck of 16-9-2021, also available as video on Youtube.

Adobe Portable Document Format - 2.03 MB - 09/16/2021 at 17:50


calibration data of current scanhead

Zip Archive - 278.28 kB - 01/25/2019 at 18:50



measurements and algorithm used to determine stabilization accuracy, see blog

RAR Archive - 594.17 kB - 01/25/2019 at 14:50


  • 1 × prism, 30x30x2 mm edges parallel to < 1 arc minute 40pcs @ 20 euro per piece @ ebetteroptics
  • 1 × Raspberry Pi 3B / 4 with 16GB sd card at least 35 euro's + 9 euro
  • 1 × Brass Laser Module Housing for 5.6mm TO-18 LD 5 euro
  • 1 × BDR-S06J 405nm / Nichia S06J 405nm 22 euro (eBay)
  • 1 × 10x10x1 first surface PMMA mirror 2.5 euro per 20 aliexpress (alternative edmund optics #45-517 18 euro per 10 including shipping)

View all 12 components

  • New design needed

    Hexastorm05/20/2022 at 16:39 0 comments

    I am able to spin the prism and the power is more than sufficient.
    Still, I decided to redesign the PCB.

    The prism now spins at 5 Volt and 0.7 amps and the board reaches a temperature of 60 degrees Celsius.
    In my ideal design, the magnets hover over the board with 100-200 microns. This is to ensure facet overlay.
    The idea is that the structure completely supports on the prism.  The prism is ideal, and rotation invariant.
    During the rotation some flexibility is needed in the axis. I want to 3D print the encasing and need some tolerance. As such, 100-200 microns seems reasonable.

    My current PCB thickness is 1.6 mm, so the distance to the center is 0.8 mm.
    If I increase it to 1 mm, I lose 36 percent of my force (due to square law).
    I can increase the voltage but the heat generated by the PCB is squarely dependent on the current (P=I^2*R).
    Furthermore, the resistivity of copper increases 0.39 percent for each degree. As a result, at a 200 microns offset the motor works barely.
    I looked online and could have ordered a PCB with a thickness of 0.8 mm instead of 1.6 mm. The price is the same and would be a lot better (factor 4, due to square law).
    The six coils are now placed at a distance of 8.25 mm from the center. However, the torque you can exert on the rotor increases linearly with the radius. (torque = radius  * force)
    So further away, is better as the magnets have a lever! This would also allow you to add more turns as there is more space between the coil centers.

    As result, I concluded;

      - i need to decrease the thickness of the PCB to 0.8 mm
      - increase the radius on which the coils are placed
      - add calculations to calculate the new resistance and maybe change the driver
      - I could do some experiments with a ferrite sheet on the current board, but don't have high hopes.

    Finally, there are other issues with the current design

      -  I should use a mirror and not measure the bundle after three reflections ( see optical flaw post)
      -  the laser driver and motor driver should not be so close (heat)
      -  the pinout is not really compatible with my current boards and requires an adapter in between. 

  • Prism motor new magnets

    Hexastorm05/18/2022 at 13:35 0 comments

     - Magnets increased from N52 8x2 to N52 10x5 and the pull force went from 1.1 kg to 3.6 kg.
     - Stronger magnets made me realize I had a mistake in my algo, improving the algo also increased torque.

     - The motor died due to ESD at my workplace. I am still waiting for material so I can properly ground myself
        I had to replace the FPGA and the motor chip

      - motor can reliable self start
      - motor requires lower current and voltage (5 volt suffices), heat has become less of an issue.

  • Prism spinning using feedback

    Hexastorm05/06/2022 at 12:05 0 comments

    In short;  - I  am able to increase the speed of the motor using the hall encoders
                   - prism speed seems sufficient for exposure 
                   - the motor driver dies over time, unknown why
                    - I will now focus on the mechanical design             

    In the figure below, you see the commutation states of a motor.

    I have three Hall sensors which detect the position of the magnets. There is a total of 8 states,
    but the state (1,1,1) and the state (0,0,0) are invalid.
    Furthermore the states are ordered, so (1, 1, 0) --> (0, 1,  0) is an allowed transition.
    But not (1,1, 0) -->(0, 1, 1) as it skips either (0,1,0) or (0,0,1).
    If a state is detected then some coils are turned on and off. The coils also only have six states.
    The starting point is random now.  That's why in the video, I have to kick it in the beginning.
    It's a very crude feedback algorithm but able to spin up the prism. I can also read out the frequency which is detected by the FPGA.
    A full cycle is 180 degrees, so the resolution by the Hall sensors is 30 degrees.
    The prism ends up in exactly the same position after a turn of 180 degrees. This is not dictated by the quartz but the four magnets below it, which have this symmetry.

    The main challenge is that the motor driver, the STSPIN230,  dies over time.  I start with three working bridges but end up with one working bridge.  I have no idea why. I limit the current to the chip below 1A and do not go outside the Voltage range.

    An algorithm can also be designed by simulation of the motor,  see article.  An old implementation in python is provided by open-blcd. This is quite dated. I looked at GYM electric motor.  I think it is able to do the same but could not find the time, right examples to set up something fast.

  • PCB motor rotating a prism with Hall encoder

    Hexastorm05/03/2022 at 18:43 0 comments

    A quick update:

  • Optical weakenss in new design

    Hexastorm04/03/2022 at 20:03 0 comments

    In the new design, I removed the mirror and track the position of the photo diode via a reflection in the prism.
    This technique has a weakness.

    The weakness is as follows, if the center of the rotor is not stable but moves microns during  the rotation.
    This will propagate to the photo-diode.

    To simplify if the rotor translates in the old design nothing happens;

    In the figure below the prism is moved upward by 10 mm..  Still the photodiode is hit at the same position.

    In the figure below the prism is moved 2 mm sidewards
    This is very different than the original
    This behavior already becomes a problem at say 20 microns. The error is propagated back twofold. The relation seems non linear and become more complex at larger displacement.

    The angular position is assumed to be stable due to angular momentum build up.

  • Levitron Revolution with EZ Float Technology

    Hexastorm03/23/2022 at 21:31 0 comments

    @gravis pointed me to a project which reversed engineered levitron revolution

    In the vertical position, the magnet holds like a pound of weight without consuming
    any additional power.  Video below shows the horizontal position. The device can be tilted 90 degrees!

    There are people who reached out to me for the current laser head.  The current laser head is hard to reproduce while still making a profit. Currently, I will focus on simply producing a cost effective device which is easy to produce.

    I admit that a magnetic levitated prism in combination with a laser scanner would be very cool.
    I think it should be totally possible to do this with pcb coils.

    Maybe one day...

    Gravis also had a link for customizable magnets;

  • Laser module with PCB motor

    Hexastorm03/11/2022 at 11:26 1 comment

    I made a video explaining the design of the new laser module with PCB motor

    AMSystems (the company sponsored and powered by the dutch state) released an interesting video. It shows a simulation of the plurality version of the laser prism scanner from 2013. The design was made when I worked there. I think it is made in Siemens NX. Hackaday might like this now they are a Siemens company.
    It is a nice render and would be even cooler if it actually worked.. I think they target a price of 60K euro's per module. I hope they also come up with a new working design.

  • Update

    Hexastorm03/04/2022 at 18:13 1 comment

    A couple of screenshots for those who can't wait;

    The mirror and cylinder lenses are removed.  Five percent of the light is reflected. This is used to trigger the photo-diode.

    For each ray drawn in freecad, the prism is in a different position. I draw them so that I know how to design the system.

    A simulation using pyoptools at 42 degrees is shown below.

    A ray goes through an ideal lens is refracted by a tilted prism at 42 degrees. Five percent of the light gets reflected at the second prism facet and is directed  to the diode as it hits the third facet at an angle greater than the critical angle (total reflection).

    The prism is not fixed to anything at the moment. I still have to think about this. I envision two solution.
    A metal base plane below the pcb using the magnetic force of the magnets or a mechanical push from above.

    If the prism is assumed "perfect" and somehow sticks to the teflon outer ring, which does not rotate!!, the system is rotational invariant. 
    A teflon cylinder is glued to the prism, at the bottom four magnets are placed which are 5 mm above the pcb.

    Designing the thing will take some more time. An issue, I currently have is that there is too much play between the teflon inner and outer ring. Still, this might be easy to resolve. I might post a video when I send it to the fab.
    I have also mostly replaced the raspberry pi with an ESP 32 board running micropython.
    Most libraries have been ported to the board. Also the libraries which were in C or C++ (fomuflash and TCMStepper). I made a custom board, which can be put in the raspberry pi connector of my main board.

  • More details on the PCB motor

    Hexastorm02/05/2022 at 17:21 2 comments

    There is a great write up on PCB motors by Kevin Lynagh with links to patents and videos.

    I did some more test with the pcb motor;

    I am now at a speed of 10 Hertz or 600 RPM.  The black plastic is acrylic cut with a laser cutter. The metal parts are pieces from a stanley blade. I use an acrylic dummy prism and the metal blade pieces ensure that the magnets are fixated.
    An acrylic piece is kept between the magnets so they do not move horizontally. Vertically they are fixed by the attraction of the metal part. Note that the prism does not has a shaft. There is a hole in the board but it is not used. The shaft is the acrylic holder of the magnets. This rotates in the square support.  It can be assumed that the prism is perfect, as it is manufactured with a high tolerance. The acrylic is not perfectly planar. Still if the prism rotates it will most likely end up in the same position.

    The PCB motor is pulsed with FPGA and the typical six step commuation loop.  I also switch the field off at the end of a move. This prevents it from being slowed down. I need to optimize this further.
    My current plan is to simple reduce the duty cycle if the prism speeds up.
    I will also replace the acrylic with Teflon. This will reduce the kinetic friction coefficient from 0.4 to 0.03.
    I aim to keep the prism in position by either pushing from above or by placing metal under the board.
    Metal under the board to create a downward force is possible, but the whole thing is quite "delicate"  and interferes with the other forces and risks that the prism doesn't rotate.

    The acrylic dummy prism already rotates at 5V and a current of 0.7 Amps. Heat is not a problem if a duty cycle of say 0.7 is used.  Temperature is around 50 degrees Celsius.

  • Spinning prism with PCB motor (bugeja style)

    Hexastorm01/31/2022 at 13:05 4 comments

    made a pcb motor bugeja style,

    Thanks @gravis, for the hint

    it took a lot of effort but I can spin prisms with a PCB motor.
    The idea is that this enables me to further reduce manufacturing costs, assembly costs
    and shrinks the size of my laser head. 
    The coefficient of static friction is higher than kinetic friction, the prism should be able to go quite fast

View all 111 project logs

Enjoy this project?



Hexastorm wrote 12/30/2019 at 15:53 point

No it wouldn't work.. you need edges polished, not the top and the bottom.
I am not shipping out prism at the moment as they r not balanced yet (working on this). I am also working on a FPGA toolchain, will post an update on my progress here soon.

  Are you sure? yes | no

Ben Wishoshavich wrote 10/27/2019 at 04:56 point

Can you recommend a specific polygon motor module on alibaba? I don't know if there are any differences and I'd like to order one. THanks!

  Are you sure? yes | no

Hexastorm wrote 10/27/2019 at 13:37 point

Yes, you should buy exactly this motor . It uses the NBC3111 chip. I had problems with other motors. Make sure you buy at least 2, although they are quite hard to break.

  Are you sure? yes | no

Ben Wishoshavich wrote 10/23/2019 at 21:59 point

Dude, this is some pretty awesome work. I'd love to help refine some of the manufacturing and board designs, let me know how I can best help

  Are you sure? yes | no

Hexastorm wrote 10/24/2019 at 17:55 point

Thanks! There are two other people who have shown interest. I need at least 10 people to do a run as I have to buy the prisms in bulk. Turn around time would be significant. Producing the prism takes at least a month.
I am fixing the low hanging fruits at the moment. It is hard to help with these as you don't have a laser head. I am building a new one but there is still only one in the world :-).
Things you could look in to;

- a better alignment system for the laser; this uses four screws and is over dimensioned --> see my free cad design files

- a better alignment system for the cylinder lenses; the screw system is not really comfortable --> see my freecad design files

-  how do i put my state machine on an fpga; I have been looking into as a replacer for the beaglebone.
This task seems rather complicated but I guess this knowledge could be really help full. The statemachine now runs on pru of the beaglebone and is limited at around 2 MHz.

- how do i balance the prism; i have done some preliminary experiments but still works needs to be done. You could also try to figure this out with a regular polygon motor. Add an imbalance and try to analyze this.

- the slicer or interpolater can only be run on a computer, as it overflows the ram of the beaglebone . Remove this problem, optimize the code.

In the mean time, I am cleaning up the code, building a second head and still have to do more experiments.
I am also waiting for news on the hackaday prize. That's also why I have been quiet on the blog.

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Ben Wishoshavich wrote 10/24/2019 at 22:57 point

I'm wondering if there isn't a BLDC motor that could be substituted for the polygon motor, hopefully something that is more available. And then I'd need an encoder. I'll take a look at this, although I suspect going greater than 20k rpm requries air bearings and a custom design.

The rectangular prism does seem to be a challenge. I'll let you worry about that. Out of curiosity, do you know how much it'd cost for a custom order?

An FPGA should be able to handle the state machine. I'm working on the FPGA that's in the Hackaday Superconference badge, and it's got an open toolchain. It might be overkill though. The ICE40 may be better(and cheaper). I might be in the minority in that I'm not sure this is a huge priority, as your electronics are pretty cheap. It's probably a pretty significant time sync to rewrite the assembly for the PRUs into a state machine. I'd probably focus on a command and control system to interact with GCODE or something else.

 I could jump into the MCAD, but I only know Solidworks. I suspect someone else might be better suited for that.
At work, we have specifications for balancing motors for EVs. The most relevant standard is ISO1940. Let me know if that's useful for you. Usually a balancing machine is used to detect vibrations while the part is rotated. They're pretty rare and fairly expensive though. They do make some simple ones for balancing quadcopter props you could look into. Usually you have to add or remove material in a specific spot to make it work.

I already have some of the parts you've used. Perhaps it won't be as hard to duplicate some version of this as I expect.(expect for that lens).

  Are you sure? yes | no

Hexastorm wrote 10/25/2019 at 17:13 point

-  i wouldn't substitute the motors; they have been used in this application for years by a large industry. Rotating polygon mirrors are produced in the tens of thousands. Motors can handle up to 21000 RPM.  This is more than what is needed at the moment, 2400 RPM, as the beaglebone can't go faster.
- custom order is around 500-600$ and MOQ is 10. for the prisms
- Henner zeller wrote a GCODE parser for the beaglebone called beagleg; my idea is that this used and you simply flip the machine between gcode mode and laser writing mode
- never heard of this standard good tip
- all parts are easy to obtain including lenses, motor etc; the only challenge is the prism and time. You will need time :-)

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rajhlinux wrote 01/01/2021 at 08:17 point

well you can count me in, i'm interested with this project

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Robert Mateja wrote 07/31/2019 at 11:48 point

Congratulations on winning Hackaday Prize 2019!  (at least in my opinion)

  Are you sure? yes | no

Hexastorm wrote 08/01/2019 at 09:30 point

Robert, thank you for supporting me! Winning the prize would be amazing.  My current target is to get other people to try out the technology, I am really trying to make it more accessible. I hope I can show an improved prototype of the scan head soon.

  Are you sure? yes | no

Conny G wrote 05/27/2019 at 12:36 point

What material is the prism made from? How is it manufactured?
Can i make it in my "maker lab"?

  Are you sure? yes | no

Hexastorm wrote 05/29/2019 at 11:39 point

Prism has the following properties; 2 mm thick, 30x30 mm square,  faces 60/40 < 5 arc min, chamfers 0.10 – 0.30mm, edges Polished 60/40, top bottom polished 60/40.   You will have to discuss details with manufacturers in China over Alibaba. I made the first in a maker styled lab, a description of the process is in this log

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Gravis wrote 02/03/2019 at 19:16 point

I'm also interested in the possibility of using a motor from a hard disk drive instead of a breaking down a polygon motor.  HDD motors are cheap to buy and (as I understand it,) contain an encoder and have screw holes which makes affixing things easier.

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Hexastorm wrote 02/04/2019 at 08:48 point

HDD seem too slow.  They typically spin at 5400 or 7200 RPM.  At the moment, I can go up to 24000 RPM with polygon motor. For some applications, I would like 50000 RPM or 70.000 RPM, like the roadrunner sold by precision laser scanner. Also the motors are not too expensive, they are like 20 euros. I understand 20 euro's can still be a lot but if you look at total costs; you can better pay attention to other components.

  Are you sure? yes | no

Gravis wrote 02/05/2019 at 06:00 point

Oh, I had no idea you were planning on high speed.  Where can you get the motors in the 20 euros range?

  Are you sure? yes | no

Hexastorm wrote 02/06/2019 at 13:40 point

You can find polygon motors at alibaba ( .  The system is a proof of concept; for desktop PCB prototyping 2400 RPM is fine. If you plan to compete with Kleo . You will need at least 50K RPM.  An option would be to encase the prism and remove the air. This will reduce the drag. You could also fill the encasing with Helium as it has low drag and a high thermal conductivity.  Like the roadrunner the encasing windows would be tilted out of plane to minimize back reflections, see

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Gravis wrote 02/03/2019 at 15:57 point

My suggestion for this project is to isolate the scanner from the 3 axis robot part so that the scanner could be made into a tool that can be changed out.  I would also ditch using BB's PRU and instead use a dedicated chip (and maybe a RAM buffer) and connect it via CAN bus.

  Are you sure? yes | no

Hexastorm wrote 02/04/2019 at 08:58 point

I intend to isolate the scanner, and design it for specific machines. I like the idea of having a dedicated chip. In the past I used a FPGA (Xula-LX25) with RAM as  bufffer. I can imagine there are even better options. The problem is that developing a dedicated board costs time, money and a lot of experience. Zeller made a very accessible code for the Beaglebone, so I went with that. You are looking at a proof of concept. It's a technology demonstrator. Anyway if you have recommendations; or some example code; feel free to share. 

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Gravis wrote 02/05/2019 at 07:48 point

Considering this is a project where accurate timing is vital, I think an XMOS processor (e.g. XS1-L4A) would be a good fit.  Each 100MIPS processing unit is 100% deterministic with fast GPIOs.  I don't know the rate of data throughput you need but it may be easier to just cache to workload on a local FLASH chip than stream it.

  Are you sure? yes | no

Hexastorm wrote 02/05/2019 at 10:53 point

XS1-L4A is nice... but I rather like something with lot of support and examples... probably first gonna optimize the current code and balance my prism.

  Are you sure? yes | no

Gravis wrote 02/05/2019 at 20:37 point

XMOS stuff actually does have lot of support (, examples ( and even an IDE but I somehow missed the part where you wrote that didn't want to build a custom board.  Sorry about that.  XMOS chips make it easy to glue things together since it's 99% software so it doesn't take too much skill to make a board with them.  Consider enlisting help to make a board as there is a good chance it will alleviate timing related issues.  Good luck! :)

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rajhlinux wrote 01/03/2021 at 13:24 point

This is a good idea, but honestly I think the best thing is to focus on getting the precision as perfect as possible. There's no consumer grade PCB maker device that can give traces/spaces resolution down to 0.01mm. It's crazy we are still using tonner transfer and UV exposure. I'm planning to build a CNC gantry made from granite, magnetic actuators and air bearings to get the most accurate, friction free, repeatable long term PCB manufacturing with >0.7 traces and spaces. But yes, would be nice down the road to have the scanner head removed for another tool... but for now precision is priority. Precision & Resolution is the reason why this project brings my interest and everyone here.

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Paul wrote 11/05/2017 at 03:49 point

You've certainly done your homework very thoroughly, and I see that in your application with very small optical cone angles (large focal ratio), the optical aberrations and field curvature appear to be tolerable.  That's great.

One question: You say that previous scan techniques require a large (and therefore expensive, you argue) f-theta lens, which must have one dimension at least as wide as the scan line.  In your approach, your polygons must be larger than the scan line length, but in *two* dimensions, making the volume of your optical element much larger.  Since either shape would use identical materials and fabrication processes in volume production (i.e. injection molding), one would naively expect the smaller element (the f-theta lens) to be cheaper.   How is this line of reasoning flawed?

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Hexastorm wrote 11/05/2017 at 10:51 point

A telecentric f-theta lens requires one dimension at least as wide as the scan line. A non-telecentric f-theta lens does not require that.  In my approach, the polygon must have one dimension longer than the scan line. The second dimension is 2 mm. An f-theta lens consists out of multiple lens elements, e.g. a 3 element f-theta lens. These elements have curved surfaces. The prism consists out of a single element with a flat surface. You can't make the f-theta lens out of plastic, so you would have to injection mold quartz. I am unfamiliar with the prizes for that. Besides most likely a higher price and the fact you have to use multiple elements, you also need to worry about patents. Envisiontec patented the usage of reflective polygons; US 9079355 B2 . Finally, you need a thick reflective polygon as thick polygons can deflect collimated bundles with a large diameter and these can be focused to smaller spots.  A transparent prism uses a focused bundle and therefore typically can be thinner, which keeps the price of the bearing lower.

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Paul wrote 11/05/2017 at 15:55 point

You have clearly thought about this a great deal, and have your arguments worked out well.

I wasn't considering that you might be requiring quartz.  I would have guessed even BK7 would be overkill for this application.

I mention injection molding because I look around my offices and see several laser printers.  Each of them contain a laser scanning unit with a rotating (reflective) polygon and a f-theta lens arrangement, with the final element being very large (>200 mm long).  All the optical elements of the ones I have inspected appear to be injection-molded PMMA or similar plastic.  The entire laser scanning unit must cost considerably less than $50, given the prices of the printers (all less than $200, two less than $70).  Granted, these are produced in huge volumes, but they serve as existence proof that these scanning systems with large optical elements are not intrinsically costly.

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Hexastorm wrote 11/05/2017 at 16:52 point

PMMA absorbs light at 405 nm. The laser printers at your office use 800 nm and low power lasers. I use 405 nm and high power lasers. So yes; there is concrete proof that PMMA injection molded systems are not intrinsically costly. There is no proof that quartz systems are not intrinsically costly.  It's unclear what the prices of these systems would be .

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Paul wrote 11/03/2017 at 13:21 point

A good variation on the usual way.  I build a few instruments based on this and similar methods between 1985-1989.  A couple of important notes:  

1. The scanned field is NOT flat: The optical distance to the target plane *increases* as the polygon rotates away from normal.  Not only is it geometrically longer, some of the increased  path length is in the high-index polygon too, increasing the path length even more.  The result is a curved focal or scan plane.  One of my instruments actually depended on this: a rotating polygon was used to tune the optical path length inside an optical resonator cavity, to adjust a tunable laser that was phase-locked to that cavity. 

2. You get some significant spherical aberration when you focus through a thick window like that, for similar reasons: the light rays at the periphery of the optical cone take a longer path to the target plane than rays going through the middle of the cone, with the result that they focus at a different depth.  For a laser at f/50 (or whatever it is), this probably isn't significant, but in an imaging system at f/2, it seriously degrades focus. 

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Hexastorm wrote 11/04/2017 at 22:09 point

Paul, thank you for your reply!
A transparent polygon scanner with a single laser bundle was first patented by Lindberg in 1962.  The scan field is not flat and you can get some significant optical aberrations. A full numerical model is available here . A description of this model is available in the technical presentation. The result shows that in practice the scan field is flat and the optical aberrations are not significant for the current spot size and line length.

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Hexastorm wrote 11/03/2017 at 11:58 point

Well this is the first comment! If Hackers are interested in transparent polygon scanning let me know :P ..

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