High contrast ratio paper sensor

#This invention relates to paper sensors and in particular to a high contrast ratio paper sensor for use in a reproduction machine.

#Typical prior art sensors are shown in U.S. Pat. Nos. 3,435,240 and 4,092,068. In U.S. Pat. No. 3,435,240, the surface characteristics of a material are determined by the quotient of output signals from two photomultipliers. The magnitude of these signals from the photomultipliers is determined by the simultaneous transmittance of energy from a single light source through small and large areas of a material being examined. In U.S. Pat. No. 4,092,068, a single light source is directed onto a material surface and the surface characteristics are examined by comparing the amount of light reflected at two different angles from the surface onto a pair of detectors.

#It is also known in the prior art to provide sensors to be able to detect translucent papers. One technique comprises a light emitter on one side of the paper path and a transmittance detector on the other side of the path. As paper enters the path between the emitter and detector, the light may be attenuated sufficiently through the paper in order that the signal sensed by the detector indicates paper in the path. The difficulty is that the sensor circuitry is generally tuned only to detect paper having a particular transmittance characteristic. For papers or documents having different transmittance characteristics, the sensor circuitry is often insensitive to be able to detect this type of paper without adjustments having to be made to the sensor circuitry.

#For a better understanding of the present invention, reference may be had to the accompanying drawings wherein the same reference numerals have been applied to like parts and wherein:

#FIG. 1 is a prior art reflectance detector.

#FIG. 2 is a prior art transmittance detector.

#With reference to FIG. 1, there is shown a prior art reflectance detector sensor. Without paper in the paper path, a sufficient amount of light emitted from the light source 12 reflects from the mirror 14 to the detector 16 to indicate no paper. With paper 18 in the paper path, it is assumed that the light will be attenuated enough by the paper so that the amount of light reflected from mirror 14 to the reflectance detector 16 has diminished sufficiently to indicate the presence of paper in the path. Unfortunately, with highly translucent paper, not enough light will be attenuated by the paper such that the detector will not recognize the presence of the paper. Some of the light is diffused but some is passed through the paper reflected off the mirror and goes back up through the paper again.

#FIG. 2 shows another prior art sensor embodiment. In this case, the reflectance detector 16 has been replaced by transmittance detector 20.

#In accordance with the present invention as seen in FIG. 3, an additional detector, a diffuse detector 22 is inserted into the sensor of FIG. 1. This detector senses light that is diffused by the paper, that is, is not transmitted through the paper. Also, some of the light that is transmitted through the paper and reflected from the mirror 14 is diffused and scattered. Some of this light also is sensed by the diffuse detector 22. It should be noted that the diffuse detector 22 is positioned to minimize detection by detector 22 of light reflected from mirror 14 with no paper present. Thus, if there is a narrow light beam from source 12 so that no light is reflected from mirror 14 to detector 22, then the position of detector 22, as shown, is suitable. This configuration will give a high contrast ratio and a great deal of latitude in selecting components.