Abstract:
A toner patch sensor arrangement in an electrophotographic machine includes a substantially hollow chamber having a reflective interior surface, a first opening exposing a toner patch, a second opening and a third opening. A light emitting element emits light onto the toner patch through the first opening and the second opening. A light detecting element receives through the third opening light reflected off of the toner patch such that the reflected light is received only after the light has also reflected off the interior surface of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to monitoring toner density of an unfused image in an electrophotographic machine, and, more particularly, to a toner patch sensor arrangement for monitoring toner density of an unfused image in an electrophotographic machine. 
     2. Description of the Related Art 
     Toner patch sensors are used in printers and copiers to monitor the toner density of unfused images and provide a means of controlling the print darkness. In color printers and copiers, the toner patch sensors are used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. It is a known problem that conventional reflection-based toner patch sensors will lose their calibration if the toner bearing surface changes in how the light is absorbed and scattered due to wear or toner filming. 
     Conventional reflection-based toner sensors use a single light source to illuminate a test patch of toner. In most cases the density of the toner patches are sensed on the photoconductor. With the advent of color laser printers with intermediate transfer belts, it is known to sense toner patches on the intermediate transfer medium rather than on the photoconductor surfaces. Toner patch sensing on the four photoconductor drums can be an unattractive option since it requires four sensors, and there may be no room for four such sensors between the cartridge and the intermediate belt. 
     It is known to use reflection signal ratios as opposed to differences in the toner patch signals. In a ratio control system, the reflectivity of a toner-free surface is sensed and compared to the reflectivity of the toned patch. By taking the ratio of these two signals, signal variations due to the variations in the light source, the detector, and the relative positions of these elements cancel out. However, this method of image density control is not self-compensating for degradation of the toner bearing surface, such as the photoconductive drum or intermediate belt, due to wear or toner filming. 
     Similar methods of maintaining accurate density control include sensing special toner patches with “saturated” toner densities. Saturated patches on an intermediate surface can be sensed, and the resulting values can be used for density control and gradation correction. 
     Intermediate belts are prone to toner filming and mechanical wear. Since changes in the surface roughness of the intermediate belt will affect the amount of light that is scattered at the belt surface and the direction in which the light is scattered, the toner patch sensor needs to be made insensitive to the surface roughness of the intermediate belt surface. 
     What is needed in the art is a toner patch sensor arrangement that can accurately measure the toner thickness on a surface having various degrees of surface roughness. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of maintaining accurate density control independent of the intermediate belt surface roughness. 
     The invention comprises, in one form thereof, a toner patch sensor arrangement in an electrophotographic machine. A substantially hollow chamber has a reflective interior surface, a first opening exposing a toner patch, a second opening and a third opening. A light emitting element emits light onto the toner patch through the first opening and the second opening. A light detecting element receives through the third opening light reflected off of the toner patch such that at least a majority of the reflected light is received only after the light has also reflected off the interior surface of the chamber. 
     An advantage of the present invention is that toner thickness can be accurately measured on a surface having various degrees of surface roughness. 
     Another advantage is that only one photosensitive device is needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a side, sectional view of a first embodiment of a toner patch sensor arrangement of the present invention; 
     FIG. 2 is a side, sectional view of a second embodiment of a toner patch sensor arrangement of the present invention; 
     FIG. 3 is a side, sectional view of a third embodiment of a toner patch sensor arrangement of the present invention; and 
     FIG. 4 is a side, sectional view of a fourth embodiment of a toner patch sensor arrangement of the present invention. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and, more particularly to FIG. 1, there is shown one embodiment of a toner patch sensor arrangement  10  of the present invention, including a reflective chamber  12  and a toner patch sensor  14 . 
     A single toner patch sensor  14 , including an infrared light emitting diode  16  and a silicon photosensitive diode  18 , is used to measure a toner patch  20  that has been developed and transferred to an intermediate transfer belt  22 . Sensor arrangement  10  is positioned in close proximity to an intermediate belt drive roll (not shown), after the last color transfer station (not shown). Light emitting diode  16  can have a narrow output beam, such as that of diode SFH 480  produced by Infineon. 
     Chamber  12  includes an integrating optical cavity  24  which allows photosensitive diode  18  to detect light reflected off the surface of belt  22  at multiple angles of incidence and/or reflection, thereby providing toner patch sensor  14  with a high level of accuracy. Chamber  12  may have the shape of a box, cylinder, sphere or other hollow three-dimensional volume. Chamber  12  may be molded from a thermoplastic such as polystyrene which has been loaded with titanium dioxide to produce a high reflectivity, such as SC 24-244  from RTP Imagineering Plastics. Because light may undergo multiple reflections inside chamber  12  before it reaches photodiode  18 , the reflectivity of cavity  24  can be above 90%. 
     Alternatively, chamber  12  can be formed of a material having low surface reflectivity, and an inside surface  25  that defines cavity  24  of chamber  12  can be coated with barium sulfate to create a highly reflective but non-specular surface. 
     Chamber  12  includes openings  26 ,  28  and  30  associated with the illumination source  16 , the photodiode  18 , and the test patch  20 , respectively. Light source  16  is disposed in a collimating unit  27  molded from polycarbonate loaded with 2%-3% carbon black to produce a highly absorptive material at the wavelength of emitter  16 . Collimating unit  27  has three apertures  29 ,  31  and  33  which are disposed between emitter  16  and entry aperture  26 . Apertures  29 ,  31 ,  33  serve to define the extent and direction of the light beam so that it can pass through entry aperture  26  and sampling aperture  30  without reflecting off either of the surfaces surrounding apertures  26  and  30 . 
     In the embodiment of FIG. 1, openings  26 ,  28  and  30  are configured to allow direct illumination with indirect detection. More particularly, the illuminating light from light emitting diode  16  enters reflective cavity  24  through a small opening  26  and most or all of the light reflects off the surface of test patch  20  before undergoing further reflections off of interior surface  25  of chamber  12 . Entrance aperture  26  is positioned off-center relative to the location of test patch  20  so that any specularly reflected light is diffusely reflected by interior surface  25  of chamber  12  rather than passing immediately back out entrance aperture  26 . 
     Opening  30 , through which test patch  20  is illuminated, is in the form of a circular aperture located about 1.5 mm from the surface of intermediate belt  22 . The diameter of aperture  30  is approximately 8 mm, which is much larger than the 1.5 mm gap between aperture  30  and the surface of intermediate belt  22 . This arrangement ensures that most of the light that is reflected by belt  22  or toner patch  20  will re-enter optical cavity  24  where it can be detected by photodiode  18 . 
     The size and locations of the three apertures  26 ,  28  and  30  influence to what extent sensor  14  is affected by changes in the surface roughness. The geometry described above was selected based on computer simulation of light reflected off the surface of intermediate belt  22  and interior surface  25  of chamber  12 . Calculations were performed using OptiCAD ray tracing software to compare the amount of light detected from a highly specular surface and from a non-specular surface. The computer simulation indicated that differences in the detected light intensity were minimized for this combination of cavity geometry, hole sizes, and hole locations. 
     Photodiode  18  is placed behind aperture  28  to sample the light intensity in optical cavity  24 . Photodiode  18 , made by UDT Sensors, Inc. of Hawthorne, Calif., has a relatively large surface area (4 mm×4 mm) and a wide angular sensitivity (+/−40 degrees). The large surface area increases the fraction of the light that is detected by photodiode  18  before it is either absorbed by interior surface  25 , or exits cavity  24  through aperture  30  or aperture  26 . In the embodiment of Fig. 1, test patch  20 , whether bare or toned, is illuminated at a well defined angle of incidence and the reflectance is sensed over a wide range of reflection angles by photodetector  18 . Arrangement  10  is relatively insensitive to variations in the roughness of the belt surface because cavity  24  samples the light from many reflected light directions, not just one. 
     In a second embodiment, toner patch sensor arrangement  32  (FIG.  2 ), the illuminating light enters optical cavity  24  through a small aperture  26  and is diffusely reflected by interior cavity surface  25  before reaching test patch  20 . A small opaque baffle  34  protruding from a lower chamber wall  36 , intersecting an imaginary line between light emitting diode  16  and test patch  20 , serves to block direct exposure of photodiode detector  18  to the light from light emitting diode  16 . A circular opaque flange  37 , intersecting an imaginary line between light emitting diode  16  and test patch  20 , and also intersecting an imaginary line between test patch  20  and photodiode  18 , surrounds and defines opening  30 . This second embodiment produces a diffuse illumination of test patch  20 . The light reflected by test patch  20  also undergoes diffuse reflection in optical cavity  24 . In this arrangement  32 , a portion of the light reaching photodiode  18  has reflected around cavity  24  without reflecting off the surface of test patch  20 . The remaining portion of the light has reflected off test patch  20  one or more times. This configuration provides the greatest immunity to surface roughness reflectivity errors since it illuminates sample  20  with light from many different directions and detects the light reflected or scattered into many different directions. As with the first arrangement  10 , chamber  12  is made from, or interior surface  25  is coated with, a highly reflective non-specular material, and the diameter of opening  30  is much larger than the gap between bottom wall  36  and intermediate belt  22 . 
     In the third embodiment, toner patch sensor arrangement  38  (FIG.  3 ), test patch  20  is diffusely illuminated as in FIG. 2, but the field of view of photodiode  18  is largely or completely limited to test patch  20 . Infineon photodiode SFH 203 A has a limited field of view, an integral lens for focusing the collected light onto the light sensitive area, and is an example of an inexpensive photodetector that could be used as photodiode  18  in the arrangement  38  of FIG.  3 . 
     An optical cavity with specular reflecting surfaces could also be used to sample a variety of reflection directions. Specular reflection cavity surfaces can be ellipsoidal in shape. In a fourth embodiment, toner patch sensor arrangement  40  (FIG.  4 ), a reflector chamber  42  is ellipsoidal in shape and has test patch  20  at one focus point and photodiode detector  18  positioned at the other focus point. Chamber  40  can be molded out of acrylic and the top interior surface  44  can be aluminized to produce a high specular reflectivity. A bottom interior surface  46  is covered with black paint so as to be highly absorbing, and has an aperture  30  to allow exposure of test patch  20 . A side interior surface  48  is also painted black to absorb any light which does not land on photodiode  18 . Light emitting diode  16  has a narrow emission beam and photodiode  18  is a low cost Infineon photodiode BPW 34  with a +/−60 degree field of view in this embodiment. The position of light emitting diode  16  allows specularly reflected light from test patch  20  to reach photodiode  18 . Since the scattered light intensity tends to diminish as the scattering direction deviates from the specular direction, it is advantageous to arrange for a specularly reflected beam  50  to impinge near a center  52  of surface  44  of the elliptical mirror. 
     Thus, the openings for the illumination source, the test patch, and the photodiode may be configured to produce three different design scenarios: 1) direct illumination with indirect detection, 2) indirect illumination and detection, and 3) diffuse illumination with direct detection. 
     In the embodiments shown herein, the chambers have been shown as being hollow. However, it is to be understood that it is also possible for the cavity of the chamber to be filled with a transparent material, such as acrylic. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.