As we print bigger and bigger objects we notice new artifacts in the prints. The newest artifact creates a sunspot effect on the part. The sunspot or banding seems to start where a model has a bulge. The thickness of the bands decrease as it moves further away from the centre of the bulge.
We did some research and it doesn't seem as if it has anything to do with standard speed or continuous speed printing methods. Companies like 3D Systems, Envisiontec, Thorgaard and Protec dental also seem to have it with their printers.
Example images from other companies also with banding. You might need to zoom in
Possible cause and possible solution
It seems we need more anti-alias in the PNG files. Looking at each file closely it is possible to see the stair stepping of the lines. These stairs are most likely what is creating the banding. By adding more anti-aliasing we will probably lose a bit of detail, but get smoother prints. We will be testing this in the coming weeks.
It also seems when Gizmetor generates the mp4 movie from the PNG files using ffmpeg the anti-aliasing gets removed. ffmpeg was used at the beginning because the thinking was that we needed to display 60 frames per second running on a Raspberry PI, but we don't. JavaFX can't fully run on a Raspberry Pi and we just display between 1 and 3 frames a second. The Java application is fast enough for that. It means displaying a frame every 333ms and that isn't a problem.
Update: Gizmetor has been updated to create the movie itself. It didn't solve the banding
One of our customers sent us an image to show what happens in Matlab when working with DICOM files. This also shows the banding that we see on 3D printed objects.
Other solutions: Go with an 8k projector that doesn't exist yet, but you will run into the issues again when you go even bigger. Another option is to produce an array of 4k projectors with a small projection size. Then your only limiting factor is your overdraft.