Abstract:
An optical transceiver measures optical density of a toned image. The optical transceiver comprises:  
     a) a bifurcated optical bundle having first and second branches and a common end, each branch having a first plurality of fibers and a second plurality of fibers, respectively, wherein the branches are intermixed in the common end;  
     b) a radiation source for producing radiation, wherein the radiation source is aligned with the first branch;  
     c) a detector for detecting the radiation, wherein the detector is aligned with the second branch; and  
     d) a toner receptor having a surface, wherein the toned image is on the surface and the toner receptor is aligned with the common end.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a novel optical transceiver for use in measurement, evaluation and correction of optical density of a surface having toned images, more specifically, to the measurement of reflectance of radiation from a toned image surface and to the assignment of correct optical density to the measurement of toned images.  
           [0003]    2. Background of the Invention  
           [0004]    It is often useful to print large quantities of multi-colored prints to paper for the purpose of disseminating multiple copies of reports or brochure information. One objective of this kind of printing is to assure that all of the reports or brochures look the same, which means that the printing of each of the colors in multi-color pages and the color or shade of black in monochrome pages must maintain a consistent density as printing progresses. It is not desirable to allow the densities of primary colors to vary from page to page because the value and quality of the final product of the reports and/or brochures will be degraded if the colors are varying from document to document. Therefore, measuring density of images (i.e., plated toner) during the printing process is important.  
           [0005]    There have been many attempts made to achieve the above objective in the printing process or electrophotographic applications. For example, in U.S. Pat. No. 5,533,139 to Parker et al., a method has been disclosed for detecting coating density imperfections. A light is transmitted through a substrate that is to be inspected, and then significant image processing is applied to make decisions. A quadrifurcated fiber optic bundle has been used with annular illumination techniques so that incident radiation upon the sample is introduced at an angle to the collecting fiber optic axis, as disclosed in U.S. Pat. No. 4,947,348 to Van Arsdell. This disclosure implies that if the surface of the test piece exhibits a large component of specular reflection, then the illuminating radiation will reflect out of the collecting aperture. This kind of annular illumination is good for diffuse surfaces such as paper, but will not work for specular surfaces like polished metal or a photoconductor surface. Moreover, in the disclosed apparatus, all four of the quadrifurcated optical bundle channels are dedicated to detection of radiation and none are used to introduce radiation into the test piece. The annular illuminators are quite large in diameter compared to the fiber bundle and might therefore mechanically interfere with the system being observed.  
           [0006]    Another attempt to determine coating density imperfections is disclosed in U.S. Pat. No. 4,950,905 to Butler. This reference uses diffrimoscopic imaging that intentionally throws away directly reflected light and analyzes the diffusely reflected and/or scattered light to make decisions. Not only does this arrangement require a lot of space to set up, but also the signals from diffuse reflection and scattering are weak compared to the specularly reflected component. Consequently this system needs very high quality, high gain amplifiers to function properly.  
           [0007]    Bifurcated optical fibers are frequently used in sensors to detect a distance to a target. Typically, one sensor bundle comprises two sets of fibers, each set of fibers originating from a respective branch, the two sets of fibers being joined in a common end. Light is transmitted from a light source through one branch to the target and the corresponding light reflected from the target is conducted by the other branch to a light sensor. The intensity of the reflected light is indicative of the distance between the end of the common branch and the target.  
           [0008]    There is a present need for diminutive and inexpensive apparatus and a measurement method capable of indicating the optical density of the toned images on the surface of a substrate, yielding reliable, consistent and accurate results in different applications, including electrophotography.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with the invention, there is provided a novel optical transceiver and a method of using the optical transceiver for sensing toner density, particularly plated toner density on the surface of a toner receptor.  
           [0010]    In a first aspect, the invention features an optical transceiver that includes: (a) an at least bifurcated optical bundle having at least a first branch and a second branch and a common end, each of the first branch and second branch having a first plurality of fibers and a second plurality of fibers respectively, wherein the branches are combined or each set of plurality of fibers are intermixed in the common end; (b) a radiation source for producing or providing radiation, wherein the radiation source is aligned with the first branch to allow radiation to enter into and be carried by the first branch; (c) a detector for detecting the radiation, wherein the detector is aligned with the second branch to receive radiation transmitted by the second branch; and (d) a toner receptor having a surface, wherein a toned image is on the surface of the toner receptor and the toner receptor is optically aligned with the common end. The term optical alignment refers to the physical arrangement whereby light or radiation reflected from the surface of the toner receptor can enter into optical openings on the second branch. The orientation must be within a ±90° orientation to the surface, and preferably is within a ±45° orientation with respect to fibers being directly aimed at or perpendicular to the plane of the surface, more preferably within a ±30° orientation and within a ±20° orientation, or within a ±10° orientation towards the surface.  
           [0011]    In a second aspect, the invention features a method for measuring optical density of a toned image that includes: (a) providing radiation from a radiation source, wherein the radiation has stable intensity (e.g., by stable intensity it is meant that the energy at a wavelength or range of wavelengths, including white light, does not vary more than ±5% in energy/fluence during a measurement period, preferably not more than 3%, and more preferably less than or equal to 2% or less than or equal to 1%); (b) providing an at least bifurcated optical bundle having first and second branches and a common end, each branch having a first plurality of fibers and a second plurality of fibers respectively, wherein the branches are intermixed in the common end; (c) aligning the common end with a toner receptor having a surface (e.g., as with the alignment above, the range of orientations should be within the same ranges described above, most preferably within a ±20° orientation, or within a ±10° orientation towards the surface as measured from a line perpendicular to the plane of that surface), wherein the toned image is on the surface; (d) emitting the radiation from the radiation source through the first branch and the common end to the surface; (e) receiving radiation reflected from the surface through the common end to the second branch by a detector, wherein the detector generates an electrical signal depending upon the intensity of the reflected radiation; (f) providing means to move the toned surface with respect to the common end; (g) generating and storing a color specific look up table by evaluating density in a test patch of plated toner on the toner receptor; (h) adjusting imaging process parameters in a process used to generate a toned image in accordance to values in a look up table; and (i) preferably repeating the above procedures for at least one more color and preferably each color of a multi-color printing process.  
           [0012]    In a third aspect, the invention features an optical transceiver that includes: (a) an at least bifurcated optical bundle having at least first branch and second branch and a common end, each of the first branch and second branch having a first plurality of fibers and a second plurality of fibers respectively, wherein the plurality of the fibers in each branch or each of the branches are intermixed in the common end; (b) a radiation source for producing radiation, wherein the radiation source is aligned with the first branch to allow radiation to enter into and be carried by the first branch; (c) a detector for detecting the radiation, wherein the detector is aligned with the second branch to receive radiation transmitted by the second branch; (d) a toner receptor having a surface, wherein the toned image is on the surface and the toner receptor is aligned with the common end; and (e) a microprocessor for receiving and storing radiation data from the detector and for determining the position of the common end with respect to the toner receptor. It is to be noted that the intermixing of the fibers from each of the branches may be done so as to form a distinct and highly efficient distribution of both sets of fibers so as to maximize relative transmission from the first bundle (branch) to be reflected and carried by the second bundle (branch). For example, the fibers of the first branch may be distributed at the common end in concentric circles, in a circular distribution that defines the perimeter of all fibers in the common end or in a single core of fibers. In cooperation with that distribution, the fibers of the second branch may be distributed in concentric circles intermediate or adjacent to concentric circles of fibers from the first branch, fibers from the second branch may form a core within the perimeter defined by first branch fibers, or may themselves define a perimeter around a core of fibers from the first branch. It is also possible to have fibers from the first branch and fibers from the second branch evenly distributed within the common end, either in a defined pattern or a more approximately even distribution. These more designed mixtures of fibers will assist in maximizing return of light from the first branch back into fibers in the second branch after reflection from the surface.  
           [0013]    These and other aspects, advantages and features of the present invention will be more readily understood from the following detailed description of certain preferred embodiments thereof, when considered in conjunction with the appended claims and the drawings hereto. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]    [0014]FIG. 1 is a partial schematic diagram depicting the input portion of an optical transceiver pursuant to the present invention, consisting of a radiation source and a first branch of the bifurcated optical bundle;  
         [0015]    [0015]FIG. 2 is a partial schematic diagram of a toner receptor and the bifurcated optical bundle of an optical transceiver pursuant to the present invention;  
         [0016]    [0016]FIG. 3 is a partial schematic diagram showing an arrangement that constitutes the detection portion of an optical transceiver pursuant to the present invention, consisting of a second branch of the bifurcated optical bundle and a detector;  
         [0017]    [0017]FIG. 4 is a schematic block diagram of a process of generating a digital output value pursuant to the present invention 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    While the present invention will hereinafter be described in connection with one or more preferred embodiments thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
         [0019]    Referring now to FIG. 1, radiation source  11  may be any of a variety of source types and wavelengths such as light emitting diode (UV, infrared, red, or other visible radiation wavelengths), laser, continuously tunable broadband source, or even incoherent white light. Radiation  12  is emitted from radiation source  11 . While the output intensity of radiation  12  may be controlled by neutral density filters or by electronic intensity controllers controlling the power supply for radiation source  11 , preferably the radiation is a stable radiation such that the intensity and the wavelength of the radiation varying less than 1% over several hours of printing operations. This stability may also be required during the life of printer, measuring energy intensity such as might be achieved by using the output signal from a laser diode internal detector to servo the forward current to the junction. In one embodiment of the present invention, radiation source  11  may be a solid state laser diode such as Model LT080MD, commercially available from Sharp Corporation, Mahwah, N.J.  
         [0020]    Referring to FIG. 2, first branch  14  shown is a part of bifurcated optical bundle  23  at the distal side of the bundle. Bifurcated optical bundle  23  may be any commercially available optical fiber bundle, composed of small individual optical fibers  13 . Bifurcated optical bundle  23  comprises first branch  14  and second branch  15  which join together at common end  16 . In a preferred embodiment of the present invention, bifurcated optical bundle  23  is a Model BMP 753P, commercially available from Banner Engineering Corp., Minneapolis, Minn. Radiation source  11  is by definition properly or appropriately aligned with first branch  14  when a percentage of photons (e.g., greater than 50%) approaching the maximum number of photons leaving the source are fired successfully into the end of the fiber bundle first branch  14 . This can be realized most easily by detecting photons radiating from the common end with a detector and maximizing the signal from that detector by properly adjusting the relationship between the radiation source and the fiber bundle. Radiation  12  is then emitted from radiation source  11  and collected by fibers in first branch  14 . In order to achieve stable, reliable and repeatable results the alignment of first branch  14  with radiation source  11  must be strictly maintained during testing.  
         [0021]    [0021]FIG. 2 also depicts the geometric alignment of toner receptor  25  and common end  16  of bifurcated optical bundle  23  of the optical transceiver in accordance with an aspect of the invention. Toned image  26  is deposited on the surface of toner receptor  25 . Suitable materials for toner receptor  25  include but are not limited to paper, metal, metal coated substrates, composites, photoreceptors and the like. Toner receptor  25  can be in the form of sheet, roll, drum, disk, or belt. Furthermore, in electrophotographic applications, toner receptor  25  may be a developer roll, an intermediate transfer member, a photoreceptor, or a receiving medium such as paper.  
         [0022]    In a preferred embodiment, toner receptor  25  is a photoreceptor. A photoreceptor may be in the form of a plate, belt, or roll (i.e., drum). A photoreceptor has an electrically insulating photoconductive element on an electrically conductive substrate and is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or solid toner is then deposited in either the charged or uncharged areas to create a toner receptor on the surface of the photoconductive element. The resulting visible toned image can be fixed to the photoreceptor surface or transferred to the surface of the suitable receiving medium. The imaging process can be repeated many times on the reusable photoconductive element.  
         [0023]    As briefly described above, bifurcated optical bundle  23  (a bifurcated bundle is a bundle in which two separate sources of fibers are combined) includes first branch  14  and second branch  15  and common end  16 , in which only the common end has the first and second plurality respectively of intermixed fibers, in which the fibers are combined from both first branch  14  and second branch  15  without any spatial ordering. The fibers may be segregated by function when intermixed (e.g., distributed in a specific pattern after combination, such as alternating functions, groups of fibers in an alternating association of functions, or may be randomly distributed in the mixture.) The structures are within the control and design of the manufacturer, and preferred orientations as individual fibers or as groups of fibers are described above. Radiation  12  received by the input portion of the optical transceiver propagates through first branch  14  and then to common end  16  of the bifurcated optical bundle  23 . Common end  16  is placed directing radiation to and aligned properly with toner receptor  25 . Common end  16  is by definition aligned properly to toner receptor  25  when common end  16  is approximately (e.g., within plusor-minus ten degrees from a line perpendicular to the central point of impact of radiation  24  on the surface  25 ) of perpendicular to toner receptor  25  with a standoff distance adjusted to within ten and preferably within five millimeters (e.g., approximately three or four millimeters) so that an approximately maximum signal is obtained by the detector. Toner receptor  25  bearing toned image  26  on its surface is then relatively translated (along at least one dimension as with rotation of the toner receptor and preferably along two dimensions of length and width) with respect to common end  16  to produce measurement data. This translation may be in a raster fashion or any other pattern of movement that covers the entire imaged area of the surface. The two dimensions may be effected by moving the common end  16  in two dimensions or by moving the common end in one dimension perpendicular or at an angle away from the dimension of movement of the toner surface  25  at the point where it is impacted by radiation  24 .  
         [0024]    A preferred embodiment is where the toner receptor  25  is a photoreceptor roll illuminated with radiation  24 , which illuminating radiation is derived from radiation  12  that exits from common end  16 . The wavelength of radiation  12  and thus radiation  24  is selected such that radiation  24  is transmissible in the form of a toned image  26  but with at least some small amount of absorption by the toner. For most colored toners, a light emitting diode or solid state laser diode used in this preferred embodiment of the invention produces a satisfactory wavelength of light for this purpose in the near infrared range, although a specific toner may be non-absorbing to other wavelengths of light. Radiation  24  passed through toned image  26  is reflected from the surface of toner receptor  25 . As the surface of toner receptor  25  (e.g., a photoreceptor) rotates such as by means of a DC motor or a stepper motor, the intensity of the specularly reflected radiation  27  varies in proportion to the thickness of toned image  26 . One of the advantages of bifurcated optical bundle  23  in the present invention is the diminutive size of common end  16 . That is, bifurcated optical bundle  23  allows one to design a small-sized probe of the interface in common end  16  and to place it in any area to be measured, which lowers the cost and broadens the applications of the present invention. Because variation of standoff distance between the common end of the bifurcated bundle and the toner receptor may adversely affect the density measurement, it is advisable to place this measurement system over a back up roll (in the case of a flexible toner receptor), but the system is not disadvantaged by any curvature of the toner receptor surface at the point of measurement. Similar to the importance of mechanical stability of the alignment described above, the stability of common end  16  is needed for obtaining reliable and repeatable results during the measurement. As used herein, specular reflection refers to that portion of reflected radiation wherein the angle of reflection is equal to the angle of incidence. For example, collimated incident radiation may be reflected as an angular distribution which is sharply peaked about the specular reflection angle. This would be considered to be specularly reflected. Specularly reflected radiation  27  from the surface of toner receptor  25  passes through toned image  26  and reenters common end  16  of bifurcated optical bundle  23 . A non-limiting example of the usefulness of specular reflection technique in the present invention is its broad applications, i.e., the technique may be used for surfaces that even exhibit a large amount of specular reflection, such as a polished surface. Specularly reflected radiation  27  propagates to second branch  15  of bifurcated optical bundle  23  and comes out as radiation  22 .  
         [0025]    [0025]FIG. 3 shows an arrangement that constitutes a detection portion according to a practice of the present invention. Detector  33  receives radiation  22  propagated from second branch  15  of bifurcated optical bundle  23 . Detector  33  may be any sensor that can quantitatively and/or qualitatively detect and provide relative measurements of radiation, such as a photo transistor, a photomultiplier tube, or the like. One preferred detector for this invention is a photodiode, Model L54-034, commercially available from Edmund Industrial Optics, Barrington, N.J. The Second branch  15  of bifurcated optical bundle  23  is positioned such that substantially all the radiation exiting from the end of the bundle is collected by the detector  33  in order to obtain the maximum signal-to noise ratio. In this arrangement, the mechanical stability described above has to be considered as well.  
         [0026]    [0026]FIG. 4 is a block diagram showing an analog electrical signal from detector  33  being processed and compared to look up table  44  values containing calibration information to yield a digital output that is directly proportional to the toner film thickness of the test sample being measured. A calibration procedure to generate and store look up table  44 , with given test sample (e.g., test patch) is described in an elaborated way below. The analog electrical signal from detector  33  is amplified and filtered by amplifier module  42 , and then processed as required by signal processor  43  which uses lookup table  44 , generated from a calibration procedure. The result of the processing yields a digital output value which actually indicates the optical density of toned image  26 .  
         [0027]    When the optical transceiver has been assembled according to the above outlined procedure and properly aligned with respect to radiation source  11 , detector  33  and toner receptor  25  according to the procedure outlined above, it is of interest to generate a specific relationship between the output of detector  33  located near second branch  15 , i.e., the proximal end of bifurcated optical bundle  23  and the density of a test patch of plated toner on toner receptor  25 . The calibration procedure is described in the following way.  
         [0028]    The baseline of digital output value is recorded when there is no toner on toner receptor  25  to provide a base or ‘white’ background density for the receptor. This value may be assumed or actually measured, but it is preferred to actually record a measured value. For the purpose of obtaining an extremely accurate value in the absence of any toner, the surface of toner receptor may be rotated for several revolutions and the average value of the digital output value is calculated and stored for use as a baseline of zero density. If the standard deviation of the average value of the digital output value is greater than a specifically defined amount such as 2% of the average value, then toner receptor  25  is seen to have variable reflection as a function of angular position and a table (lookup table) may be constructed for the recording of the digital output value as a function of angular position. If the extent of the toner patch to be measured is several angular degrees of rotation of toner receptor  25 , then a look up table of values should have an entry for zero density for every several angular degrees and the position of the toner test patch to be measured must be known so that the correct value of zero density may be chosen to calculate the correct density of the toner patch. A microprocessor or computer associated with the system translating the toner receptor provides the information on the angular position on the test patch and an actual image during operation of the system. If the standard deviation of the average value of the digital output value is less than, for example, 2% of the average value, then one global value (the average value) for zero density may be stored and used for all density calculations, regardless of the angular position of the toner test patch on toner receptor. With the measurement system in place and the zero density value determined, a toner test patch of known density may be printed on the toner receptor and the digital output value recorded as the toner patch is made to pass before the distal end of the bifurcated optical fiber bundle  23 , i.e., common end  16 . A next step is to repeat the process several times for several density values of the toner patch. The range of density values that are recorded and measured in this way should include at least the target density for the particular color being printed. The data collected in the above steps may be assembled into a lookup table for use by the printer CPU during printing to measure toner test patches and adjust operation process parameters, e.g., voltage bias, force, development gap, etc. in electophotographic application (EP parameters), in a printing machine to maintain the desired density for a particular color.  
         [0029]    In the case of printing of multiple colors, each color would need a separate optical transceiver and a separate lookup table in order to adequately control the multicolor printing of constant densities.  
         [0030]    The above process can also be used in an optical transceiver of the present invention for measuring and thereby controlling the optical density of toned images. The digital output value may enter into any system or component that can control toner density  45  wherein operation parameters of an apparatus are modulated for consistent optical density of toned images, using control schemes, e.g., feed-back control. For example, in an electrophotographic application, electrophotography process parameters such as applying voltage or amount of toners supplied can be controlled for consistent optical density of toned images during the printing process.  
         [0031]    Although several embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the particular embodiments described herein, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.  
         [0032]    Other enabled embodiments are described within the following claims.