Besides the optical sensing elements that interact with the prism at the
bottom of the CLI-8 and PGI-5 cartridges, closer inspection of my ip4200
printer reveiled another optical sensor, located left, behind the front.
Its vertical position matches the top of the tapered lightguide or the
CLI-8 and PGI-5 cartridges.
This top ends in an polished surface under 45 degrees, acting as a
mirror so that the light is reflected through the dark-grey window in
the direction of the sensor; the window is apparently a mask, shielding
Hence the light of the LED is guided via the mirror to the sensor when
it is properly aligned horizontally and then a measurement of the light
intensity could take place. The light-guide is rather leaky, so if some
of the leaking light is reflected back, e.g. dependent on the ink level,
the sensor will receive more light.
At least this means that the light guide plays a role in the ink supply
management system. What role is not clear, but it seems that removing
the light guide prevents the printer to detect a refilled cartridge.
Anyway, after printing ca 20 A4 pages, my printer still indicates "low
ink level" for the cartridge with the removed light guide, it works
normally and I can easily see the ink level in the cartridge. Of course
the inklevel indication of the printer-driver is not functioning
anymore, but at least the printer still seems to "think" that an
original cartridge is present that was never refilled.
See also "Check Canon Cli-8 ink level without removing cartridge" and
Have fun, Bert
10. Canon CLI-8 Ink