Tuesday, 26 June 2012

No more tears laser scanning?

Assuming cheap 405nm laser diodes can be used to cure resin for a 3D printer design we are left with the issue of the scanning system. To ensure that the laser is focussed at the the plane across the entire plane and that the beam moves at the same speed an f-theta lens is used. This makes the relationship between the scan angle and position at the focal plane proportional to f.theta. The problem is that the focal length of the lens depends on wavelength so a different lens is required for 405nm than for the red laser used in a laser printer.

Here is a possible solution. High end photoplotters expose their film on the inside of a drum using a rotating mirror with a single face (monogon). They don't need correcting optics because the distance between mirror and  internal drum surface is always constant across the scan. 

Can this idea be used for a flat plane, I think so:

By putting the rotating mirror at a more oblique angle the beam forms a cone, that cone focusses at the same plane, perpendicular to the axis of the rotating mirror. As is shown on the right the optics are simple, a collimator followed by a focussing lens then the rotating mirror. There is an obvious disadvantage with the proposed system, that the beam follows a curved path, that however is just a software issue, distorting the bitmaps to suit. Another disadvantage is the scan speed, a polygon scanner with 6 sides can scan 6 times per rotation, this one will only do one scan (two sort of but lets not go there). That said it was likely that the the polygon would need to do multiple scans to get the required exposure so that may not be an issue anyway.

A further issue is diffraction if you try to make a FTB (from the bottom) system the scanned beam will be shifted by the transparent window. This would also be a problem for any other scanning system and indeed imaging systems (DLP) however the problem can probably be corrected in software for this case as the refraction will just tend to reduce the scan diameter slightly and consistantly.

Here is another idea about how to implement it, using a brushless motor with a hollow shaft makes it easy to get clearance from the motor and makes for a neat assembly. You could even spin the optics but I am not so sure that is a good idea, spinning the laser would be even better but ~10Mhz pulses via slip rings seems a bad idea :)


  1. You could try using spherical mirrors if your issue is chromatic abberations between 2 laser sources