HexastormThe polygon mirror (PM) is often used for fast scanning applications due to its superior scanning speed and large scanning angle. However, PM-based laser scanning systems are prone to cross-scan errors, restricting scanning precision. The facet tilt and scanhead dynamics are considered as two primary sources contributing to cross-scan errors. A treaty of cross scan error in polygon mirrors can be found here https://www.acin.tuwien.ac.at/file/publications/iat/pre_post_print/2024_cong_aim.pdf .
Luckily, cross scan error is zero in a laser prism scanner with two cylindrical lenses.
In a prism scanner, the laser diode is first collimated with an aspherical lens. The bundle is then focused by a cylindrical lens in a direction parallel to the prism. The bundle is deflected by refracting it through a tilted transparent plate. The bundle is finally focused by a second cylindrical lens orthogonal to the scan line.
This has two advantages; the bundle is circularized (cylinder lenses have different focal lengths) and the cross scan error is removed (explained hereafter). Scanhead dynamics introduce noise into the system BUT the refracted laser bundle remains parallel to the incoming laser bundle as this is always the case for a planar prism. As such, the second cylinder lens removes the cross scan caused by vibrations as all parallel rays are focused in the same point. This is a fundamental advantage of prism laser scanning, minimal cross scan error. In polygon mirror systems this is solved by active compensation via a galvo scanner, see https://repositum.tuwien.at/handle/20.500.12708/213517. In a polygon mirror system the outgoing rays are not parallel to the incoming rays so the cylindrical lens trick doesn't work as well.
Below we see a side view of the new laser head and a camera with neutral density filters. If you look carefully, you can see a two cylinder lenses. The one between the laser and the prism is the hardest to see.
Cross scan measurement is shown below, a line which is around 12 pixels wide implying around 36 microns error. Camera has a pixel size of 3 micron, see https://hackaday.io/project/21933-prism-laser-scanner/log/217210-cross-scan-error-measurements-new-bearing
two cylinder lenses, one channel, current 80 a.u. (scale 1-255), 4 facets, polygon speed 2000 rpm,
exposure time is set to minimize overexposure and have a stable image
An exposure without cylinder lenses is shown below. Here you can clearly see the individual lines produced by the facets which are removed by the cylinder lens.
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Isn't the polygon tilt a repeatable error? If so then wouldn't keeping track of the orientation of the polygon enable the negation of the wobble by either only using one side of the polygon every time or by having calibrated correction profiles for each side?
This of course presumes the error is repeatable but if it is then it seems like something that software could compensate for.
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Polygon tilt is a repeatable error and is caused by incorrect mounting or an imperfect prism.
The cylinder lens removes incorrect mounting, vibrations but not the imperfection of the prism.
You could fix the repeatable error with software but would lose energy doses uniformity. Many companies such as Manz (which bought Kleo LDI), use camera measurements to align lasers. Manz, however went bankrupt this year 2025, and was bought by Tesla. Commercial application is very hard and requires much more than POC.
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I am assuming by "lose energy doses uniformity" you mean the laser would not apply laser light in a uniform fashion. However, my thought was to effectively perform the task effectively as four different pieces, each piece with it's own unique output profile. This would mean that you are effectively doing 4 different jobs that are interleaved with one job for each side of the prism.
If you don't understand then consider this hypothetical situation: what if the laser out was always a line that was a 45 degree diagonal? Would you only use the very center that hits the location you want? Or instead would you change your software so that it would design the job to use a line with a 45 degree angle? Now consider for each face of the prism the laser line was at a radically different angle. Would you only use one face? Or perhaps instead would you design your software to "paint" the target using the "brushes" in a strange and overlapped fashion?
To me, it seems entirely possible to uniformly expose the target to the laser light if you design the job around the assumption that none of laser lines for each prism face will be perfectly aligned or even have the same angle.
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