(57) An apparatus for monitoring toner concentration on a photoreceptor surface. The apparatus
includes a light emitting diode (90) a phototransistor (92), a beam splitter (94),
and a lens (96) disposed between the beam splitter (94) and the photoreceptor surface
(12) to collimate the light beam between the lens and the photoreceptor surface. A
portion of the light emitted from the LED is transmitted throught the beam splitter
and the lens to the photoreceptor surface. Collimated light is reflected from the
photoreceptor surface back through the lens and reflected from the beam splitter to
the phototransistor. The output signal from the phototransistor because of the incident
and reflected collimated light is independent of the distance of the lens (96) from
the photoreceptor surface (12). A second lens may be disposed between the beam splitter
and the phototransistor to enhance overall resolution.
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