The printer is a Xerox Phaser 6022 http://www.office.xerox.com/printers/color-printers/phaser-6022/spec-enus.html and claims 1200x2400 DPI. I doubt there are actually 1200 LEDs in each bar, and that the 1200 figure is interpolation by varying brightness of 2 adjacent emitters, and/or marketing wank of 4-color * 300dpi per color. I didn't count the distinguishable elements in the emitter strip, but it doesn't look like it is even 300dpi, though it's hard to tell what's the emitting element.
No, they really do have 1200 LEDs per inch, so 10,200 LEDs (per color!) for an 8.5” page width. The little things you’re seeing are the lenses, which focus the light from the LEDs on the photosensitive drum. The individual LEDs would require a microscope to be seen.
You absolutely don’t want to vary the brightness in a xerographic toner process, that causes irregularity and blurriness. (Remember trying to make grayscale copies on old analog photocopiers? It was awful. Digital copiers that scan in grayscale and then dither it to pure black and white produce far better grayscale copies.)
But yeah, when you remember how small the individual transistors in modern semiconductors can be, making an array of 1200 LEDs per inch is trivial. I mean, a 12MP smartphone camera sensor packs 4000 photosites into less than 6mm — over 16,000ppi.
This The attached white paper by Fuji-Xerox explains it in detail. (And indeed, they state that theirs is 10,240 LEDs, so I guess they gave themselves some wiggle room for borderless printing.)
The doc doesn’t say what color the LEDs are, but AFAIK, laser printers use either IR or red, so I’d expect the same from the LEDs.
Whether it’s fun to play with, or indeed how to drive them, I dunno!
Edit: replaced link with attachment, after the link went dead.