dirty encoder

[kamocat]

I was reinstalling encoders today (after having made new mounts for the gearboxes), and saw some specks of dirt on the encoder wheel.
These are the 360 CPR, 1/4" bore E4P encoders from US Digital.

I was wondering how much dirt you can have on the encoder before it impacts the reading. How does the phototransistor look at the wheel? Does it look at the whole of the line, or just at a single radius?

If one of the encoder lines were to be blacked out, would the quadrature decoder miss TWO whole counts? (Is there enough space in between the two phototransistors that it would miss the count when the blacked line went under channel A, and again under channel B?)


An interesting thing to note about the wheel is that it's fairly scratch resistant. When I was taking a wheel off, that plastic ring (with those lines) came unstuck from the aluminum. It's a thin piece of plastic, but the black lines are on the sticky side up against the aluminum, so to not be scratched off by the wheel being too close to the LED/phototransistors. It wouldn't surprise me if a little speck of dirt or grease got trapped under the plastic ring when I stuck it back on. Is this something I can clean, or will that harm the adhesive or the lines?