High contrast ratio paper sensor

摘要

A high contrast ratio sensor is provided by introducing a pair of photodetectors into the sensor circuitry. One of the detectors is a diffuse detector and the other is either a reflective or transmittance detector. The two photodetectors are connected in a manner to cause the diffuse detector output to subtract from the reflective (or transmissive) detector output. This subtraction of signals provides a high contrast ratio. In a first embodiment, a diffuse detector and a reflectance detector are located on the same side of the paper as the emitter light and in a second embodiment a diffuse detector and a transmittance detector are located on opposite sides of the paper to be sensed.