I am currently in the process of designing and building two new machines, one of them being a rotary plotter and the other being a Laser Direct Imager for PCBs, with the focus of this discussion being on the latter of the two.
I have a large portion of the design and mechanical operation in my head, and I have a large portion of the software already written. Information about the software can be found here: forums.parallax.com/discussion/166415/c-processing-windows-bitmaps-or-dibs-with-the-propeller/p1
Using a raster image file of a PCB and a 405nm laser, I intend to transfer the intended image onto a PCB, which will be covered with Laser Direct Imaging film. A variety of people have made attempts to accomplish this task, utilizing low dollar machines, with some of them, having much better success than others.
As I see it, my best option is to utilize 0.9 degree stepper motors for accuracy, and at which point I can shoot for 600, 1200, or 2400 dpi. My goal is to build a machine, which can easily expose boards having 3.2 in. X 4.0 in. dimensions, which basically matches the maximum dimension available in the freeware of EAGLE CAD. Now in EAGLE CAD, the highest resolution of an image which can be exported is 2400 dpi.
In most instances, whether it be a laser printer or an LDI machine, the laser beam will be spread across the surface of the target, by the use of a polygon mirror. Considering the complexity of this task, I have opted to expose pixel by pixel, however I foresee a potential for gaps within the exposure, because of the starting and stopping of a small round dot, instead of a constantly moving dot. My proposed solution to this problem is dot overlap. For example, let's say that I export an image at 600 dpi, but I actually build the machine to run at 2400 dpi which would give me 4 dots across the pixel and 4 dots down the pixel, so instead of having one dot per pixel, it would have 16, and the dots would overlap from one pixel to another. The ends of a line and the sides of a line might suffer a little because they will not have the same exposure as the center of the pixel, but perhaps with all the bleeding light, it might not make a bit of difference.
Anyhow, that is how I am starting this discussion, if anyone cares to participate.