LED printers fascinate me. If it's a 600 dpi printer, you need over 5000 LEDs to image a typical row of A4. Are they using an OLED module or just discrete LEDs in some kind of custom package?
Neither. The LED assembly is, if I understand it correctly, assembled out of a few LED arrays, which are essentially “ICs” of LEDs, much the same way that we build billions of MOSFETs, diodes, etc. into a processor die (or pixels in an image sensor), we can make hundreds of LEDs within a single die.
Modern LED printers have 1200dpi printheads, so about 10,200 LEDs across an 8.5” maximum page width.
See
https://www.eevblog.com/forum/projects/led-_laser_-printer-illumination-bar-hacking/ where there is more discussion on the technology, including very informative source documents.
Is the module small and scaled to fit the page with lenses, or is it the size of the page?
It is the width of the page
and uses lenses. But the lenses are tiny and numerous. See the PDF attached to one of my replies in the thread linked above.
Early DLP projectors had a way of doubling resolution by 'shaking' the DLP chip up and down slightly, effectively creating an interlaced pattern. Maybe vibrating the drum could achieve some kind of micro-resolution improvement.
You really wouldn’t want to vibrate the imaging drum, as I would think that would blur and dull the image by physically shaking the toner that’s on it.
The now-discontinued Xerox wax printers (which were basically inkjet printers with page-wide printheads that sprayed molten wax instead of aqueous ink, and sprayed it onto a transfer drum rather than directly onto the paper) oscillated the entire head assembly horizontally to do exactly what you are thinking. (IIRC, it oscillated the width of 6 dots at the standard 600dpi, so that the printhead itself only needed to behave nozzles 1/100” apart.)
It’s unnecessary on LED printers since getting 1200dpi out of a semiconductor is trivial, by making the diodes small enough and really close together.