Densitometer for measuring toner area coverage in a xerographic printer

Abstract: An apparatus measures the reflectivity of a selected region of a surface. A first light beam (122) is reflected from the selected region and substantially focused on a photodetector (106). A second light beam (124) is reflected from the selected region and is substantially unfocused on the photodetector (106). A signal is derived representative of the direct reflectance of light reflected from the surface onto the photodetector (106) as a function of the intensities of the focused light beam (122) and the unfocused light beam (124) detected by the photodetector (106).

Description: The present invention relates to a densitometer for measuring toner area coverage on a photoreceptor surface in an electrophotographic printer. More specifically, the present invention relates to the use of multiple light sources and a single photodetector in such a densitometer. In an electrophotographic printing machine, the photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing marking or toner particles into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration. Various types of development systems have hereinbefore been employed. These systems utilize two component developer mixes or single component developer materials. Typical two component developer mixes employed are well known in the art, and generally comprise dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image. Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. It is evident that in printing machines of this type, toner particles are depleted from the developer mixture. As the concentration of toner particles decreases, the density of the resultant copy degrades. In order to maintain the copies being reproduced at a specified minimum density, it is necessary to regulate the concentration of toner particles in the developer mixture. This is achieved by a closed loop servo system which regulates developability. Developability, as it pertains to an electrophotographic printing machine is the ability of the developer mixture to develop the latent image with at least a minimum specified density. However, densitometers used for black toner particles are generally unsuitable for use with colored toner particles because they are generally sensitive to the large component of diffusely reflected flux in the infrared light from colored toner particles, which gives false density measurements. In measuring the density of toner on a photoreceptor surface, even in the case of black toner, a crucial consideration is the ability to measure the specularly reflected light from the surface of the partially covered photoreceptor, while excluding light reflected as a result of the reflectivity of the toner itself. Generally, no matter what the color of a particular type of toner, the more densely the toner is applied to the photoreceptor, the "darker" (more absorptive, less reflective) the toner will appear on the photoreceptor up to some maximum saturation value. When the purpose of a densitometer is to measure the density of toner on a surface, the most important measured value is the light directly reflected from the surface, which is the difference between the light transmitted to the surface and the light blocked by the toner. However, because toner is not only absorptive but partially reflects light itself, a quantity of "scattered" or "diffuse" light will be returned to the detector from the toner as well. The light scattered by the toner itself will interfere with a reading of that blocked by the toner. Thus, it is desired that a system be devised to measure only the fraction of light blocked by toner on the photoreceptor surface, without interference from the scattered light reflecting from the toner itself, which can be considered a type of noise. US-A-5,083,161 describes an infrared densitometer which measures the reflectivity of a selected region on a surface by reflecting light rays from a single source off the selected region onto an array of photodiodes. The present invention provides an apparatus for measuring the reflectivity of a selected region of a surface, according to claim 1 of the appended claims. Also according to the present invention, there is provided a method for measuring the reflectivity of a selected region of a surface as set forth in claim 9 of the appended claims. Preferably, the light beams are of an infrared frequency. The method further comprises the step of reflecting each of the pair of light beams in an alternating manner. The method may further comprise the step of reflecting each of the pair of light beams in an alternating manner at a frequency of about 10 cycles per second to about 1000 cycles per second. The deriving step includes deriving a signal representative of direct reflectance of a light beam reflected from the surface a the photodetector as a function of the difference between maxima and minima of a signal output by the photodetector. The method may further comprise the step of moving the surface relative to the photodetector during the activating step. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional, elevational view of a densitometer according to the present invention; Figure 2 is an exploded view of the densitometer of Figure 1; Figure 3 is a plot showing a typical relationship between directly reflected light and scattered light as detected by a photodiode; Figures 4A-4E are a series of plan views showing possible arrangements of light sources in a densitometer according to the present invention; Figure 5 is a signal diagram showing a typical output of a photodiode in a densitometer according to the present invention; Figure 6 is a schematic diagram of circuitry suitable for an embodiment of the present invention; and Figure 7 is an elevational view of an electrophotographic printer. In the drawings, like reference numerals designate like elements, whether in physical views or schematic diagrams. Details of an electrophotographic printing machine incorporating the infrared densitometer of the present invention are described in USSN 884,977 (Fig. 7), a copy of which was filed with the present application. It will become evident from the following discussion that the densitometer of the present invention is equally well suited for use in a wide variety of electrostatographic printing machines, and is not necessarily limited in its application to the particular electrophotographic printing machine mentioned herein. Since the art of electrophotographic printing is well known, the description of the various processing stations employed in the Figure 7 printing machine is omitted from the present disclosure. Referring now to Figures 1 and 2, there is shown infrared densitometer 51 in greater detail. Densitometer 51 includes a generally rectangularly shaped molded housing 96 made preferably from an acrylic material or any other suitable optically transparent material. Housing 96 defines a chamber 97. A cover 98 encloses the bottom of housing 96. A printed circuit wiring board 100 is mounted between cover 98 and housing 96 in chamber 97. Printed circuit board 100 supports a primary source, here in the form of light emitting diode (LED) 102 for providing light rays to illuminate the marking particles adhering to the photoreceptor surface of belt 10. Positioned adjacent primary source 102 is at least one secondary source 104, which operates independently of primary source 102. In the embodiment shown, there are shown two secondary sources, each marked 104, although numerous arrangements of secondary sources 104 relative to the primary source 102 are possible, as will be explained in detail below. A photodiode light sensing element 106 is also mounted on printed circuit board 100. Connector 108 is also mounted on printed circuit board 100. An integrated circuit (not shown) is electrically connected to primary source 102, secondary sources 104 and photodiode 106 to provide drive current to sources 102 and 104 and photodiode 106. The top surface 110 of housing 96 defines a V-shaped recess, generally indicated by the reference numeral 112. One surface of the V-shaped recess 112 supports a condenser lens 116 which is an integral collimating lens. The other surface of the V-shaped recess 112 supports another condenser lens 114 which is an integral collector lens. Further details of the structure of the densitometer, exclusive of secondary sources 104 and photodiode 106, may be found in US-A 4,553,033, the relevant portions thereof being hereby incorporated into the present application. Primary source 102 generates near infrared light rays which are transmitted through an aperture 118 in housing 120 onto condenser lens 116. Condenser lens 116 collimates the light rays and focuses the light rays onto the marking or toner particles deposited on the test area recorded on the surface of photoreceptor belt 10. Secondary sources 104 also generate near infrared light rays, but because the secondary sources 104 are displaced a small distance from primary source 102, the light rays from secondary sources 104 are not focused directly through condenser lenses 114 and 116 onto photodiode 106, as are the light rays from primary source 102. However, the secondary sources 104 are sufficiently close to the primary source 102 such that light rays emitted by secondary sources 104 strike the surface of belt 10 in essentially the same general area as the rays from primary source 102, but only the scattered component of the returned light reaches photodiode 106. The relevent diffuse or scattered components of the light flux, shown by arrows 124, flood the area surrounding and including photodiode 106. Figure 3 is a plot showing the general relationship between reflected light, indicated as R, and scattered light, indicated as S, detected by a photodiode such as 106 as a function of toner coverage on a surface. The line indicated as R + S represents a sum of the values of R and S for different levels of toner coverage. It will be noted that, the greater the coverage of toner on the surface, the lower the intensity of reflected light R from the toner. On the other hand, as the toner coverage increases the reflectivity of the toner causing scattered reflected light, S, increases. Note also that the diffuse light scattered from a bare substrate, having no toner thereon, is not zero. The present invention seeks to isolate the value of R from the value of R + S to obtain an accurate reading of toner coverage on a surface. As mentioned above, the value of R, corresponding to directly or specularly reflected light, is inversely related to the light absorption of toner on the surface of belt 10, which is a direct indication of toner density. Returning to Figure 1, it can be seen that light emitted from primary LED 102 is focused through condenser lenses 114 and 116 in such a manner that the focal point of light emitted from primary LED 102 and reflected from surface 10 is more or less exactly on photodiode 106. Light rays so reflected and focused represent a value of R + S, illustrated in Figure 3, associated with a given coverage of toner on the surface 10. The densitometer of the present invention obtains a value of R by sensing the difference between a measured value of R + S and a substantially simultaneous but separate measurement of the value of S, yielding an accurate measure of R. In order to obtain the substantially simultaneous value of S, the secondary LEDs 104 are employed. Because the secondary LEDs 104 are not aligned with the condenser lenses 114 and 116 to focus light on photodiode 106, substantially none of the light emitted from secondary LEDs 104 and reflected on photoreceptor surface 10 will be focused on photodiode 106; that is, any light emitted from secondary LEDs 104 and subsequently detected by photodiode 106, because it is not directly reflected, will therefore be entirely scattered light, and thus entirely representative of S in the sum R + S. The value of R alone may be derived by taking the difference between this measured value of scattered light S and the value caused by direct reflection of light from primary LED 102 to photodiode 106, which is equal to R + S. Figures 4A-4E are a series of plan views, showing a variety of possible spatial relationships among a primary source 102 and a set of secondary sources 104. As can be seen in Figures 4A-4E, the primary source 102, which may include one or more separate light emitting diodes (LEDs), is generally situated at the center of an arrangement of LEDs forming secondary source 104. Figure 4E shows how a "ring source,"designed to provide symmetric averaging of secondary illumination paths, can be made from a molded plastic annular light pipe surrounding the single LED of source 102, and illuminated from one or more external LEDs. Figure 5 illustrates the operation of the densitometer of the present invention, when primary source 102 and secondary sources 104 are operated in alternating manner to convey light rays to photodiode 106. In the preferred embodiment of the present invention, primary source 102 and secondary sources 104 are alternately switched on and off at a reasonably high rate, preferably on the order of 1000 cycles per second. That is, for the first part of a cycle only source 102 will be activated, and then for the second part of the cycle, only secondary sources 104 will be activated. Figure 5 is a signal diagram showing an output of signals from photodiode 106 in response to this alternating operation of primary source 102 and secondary sources 104. In the portions of the cycle when primary source 102 is activated, the direct focusing of the light from source 102 (as shown by lines 122 in Figure 1) will cause a relatively high voltage output from photodiode 106, shown as V1 in Figure 5, and representative of the intensity of R + S (directly reflected light plus scattered light). In the second portion of each cycle, when secondary source 104 is activated, essentially no light rays will be directly focused onto photodiode 106, and any light detected by photodiode 106 will therefore be related entirely to scattered light (as shown by lines 124 in Figure 1) and which represents S. The output from photodiode 106 in response to light rays from secondary sources 104, which is the same as the S scattered component, is given as voltage V2 in Figure 5. Since the value of V1 is representative of R + S and the output V2 is representative of S alone, it follows that V1 - V2 represents a value of the R component alone, which is the most precise measurable value for determining toner area coverage. The alternating operation of primary source 102 and secondary sources 104 will result in the square-wave output shown in Figure 5. The frequency of this output is necessarily the same as the frequency of the alternating activation of the light sources, and is thus preferably on the order of 1000 cycles per second. The absolute values of R + S and S may be designed into the system by selection of specific types of LEDs or other devices. Preferably, the relative intensities of sources 102 and 104 should be arranged such that the signals S and R + S detected at photodiode 106 are on exactly the same scale, making it easy to carry out the subtraction of S from R + S electronically. Figure 6 is a schematic diagram of a simplified circuit for carrying out the alternating activation of sources 102 and 104, and also for detecting the light at photodiode 106 and separating the S component from the R + S signal. It will be understood that many alternate embodiments of an appropriate circuit may be apparent to one skilled in the art. Towards the top of Figure 6 is the circuit for alternating activation of primary source 102, shown as a single LED 102, and secondary source 104, shown as an array of parallel of LEDs. Flip flop 200 is connected with its inverted output driving the D input so that it toggles state continuously with each clock transition CK from an external frequency source. Transistor switches 202 and 204 act as simple power drivers for the operation of sources 102 and 104, respectively, which may as necessary be directly connected to an external power source. Light emitted from the LEDs associated with sources 102 and 104 strikes the sample area on photoreceptor belt 10 being measured and is then detected by photodiode 106 as indicated by dotted lines in Figure 6. When the sources 102 and 104 are operated in an alternating manner the output of photodiode 106 will be generally in the form of a square wave. Photodiode 106 is shown connected to a two stage fixed gain AC amplifier circuit providing a stable analog output signal. Several different techniques can be employed to adjust the relative intensities of the signal associated with R + S and S, such as a simple potentiometer control that can be calibrated during manufacture. With the proper ratio of diode intensities applied, the amplitude of the square wave output V2 - V1 itself is proportional to the difference (R + S) -S, and hence a direct measure of the desired value of R. The square wave signal can alternatively be processed through computer sampling of the sequential values of V1 and V2 by means of conventional analog-to-digital measurement techniques that will be evident to those skilled in the art, for effecting the numerical subtraction of V2 from V1 in a computer data acquisition system, leaving a difference value which may be interpreted as the R component of the reflected light into photodiode 106, which in turn can be used to derive an accurate measurement of the toner area coverage. The advantage of using a relatively high frequency for operating the sources 102 and 104 is that, in the actual operation of an electrophotographic apparatus, wherein the photoreceptor belt 10 is moving at a relatively high speed, the high frequency of operation ensures that the densitometer can perform measurements of substantially the same small area (such as a test patch) on the photoreceptor belt as the photoreceptor belt 10 passes over the densitometer. Preferably, a number of distinct measurements of S and R + S, or a continuous burst of sequential values accumulated with every pass of the test patch may be taken, thus enabling data averaging which ensures more accurate and reliable result. The alternating operation of sources 102 and 104, and the resultant square-wave generation of the output signal from phtodiode 106, are conducive to an AC-based design of the amplification system. The advantages of working with an effectively AC system, instead of a DC system, are well known. The differential nature of the sequential measurements effectively cancels any common mode response from unwanted inputs such as background light from other sources. It can also be appreciated that the gain of an AC signal can be made very high without most of the drift and stability problems usually associated with high gain DC amplification circuitry. Also, because the same amplifying system is used for both signals, circuit components need not be precisely matched and maintain long term stability as is the case when the S and R + S signals are individually amplified. Because the gain of an AC signal can be very high, the LED excitation power can be low and still yield a strong, unambiguous signal. Synchronous detection and integration systems can be employed to extract minute signals from a noisy background. On the whole, compared with the DC approach, fewer and cheaper components are needed to produce comparably accurate results.

Claims: An apparatus for measuring the reflectivity of a selected region of a surface, comprising: a first light source (102) and a second light source (104) for creating a pair of light beams (122,124) reflected from the selected region, the second light source (104) being sufficiently close to the first light source (102) so that the pair of light beams (122, 124) strike the same selected region of the surface; a photodetector (106), having one (122) of the pair of beams focused thereon so that the photodetector (106) senses focused reflected light, with the other (124) of the pair of light beams reflected from the selected region being unfocused thereon so that the photodetector (106) senses unfocused reflected light; processor means, operatively associated with the photodetector (106), for deriving a signal representative of the direct reflectance of a light beam reflected from the selected region of the surface into the photodetector as a function of the focused reflected light (122) and the unfocused reflected light (124) detected by the photodetector, wherein the processor means is adapted to derive a signal representative of direct reflectance of a light beam from the selected region into the photodetector based on the difference between maxima and minima of a signal output by the photodetector (106) as a result of the alternating activation of the first light source (102) and the second light source (104); and means for activating the first light source (102) and the second light source (104) in an alternating manner. An apparatus as in claim 1, wherein the first light source (102) is adapted for creating a light beam (122) substantially focused on the photodetector (106); and the second light source (104) is adapted for creating the light beam (124) substantially not focused on the photodetector (106). An apparatus as in claim 1or claim 2, wherein the photodetector (106) is adapted to measure the intensity of infrared light. An apparatus as in claim 2 or claim 3, further comprising a lens (114) associated with the photodetector (106), and/or a lens (116) associated with the first light source (102). An apparatus as in any of claims 2 to 4, wherein the first light source (102) is disposed so that the light beam (122) reflected from the selected region of the surface and substantially focused on the photodetector is aligned with the primary axis of the lens (116) associated with the first light source (102). An apparatus as in any of claims 2 to 5, wherein means for activating the first light source and the second light source is adapted to activate at a frequency from about 10 cycles per second to about 1000 cycles per second. An apparatus as in any of claims 2 to 6, wherein the second light source (104) includes a plurality of light emitters arranged adjacent the first light source (102), and preferably arranged substantially symmetrically relative to the first light source (102). An apparatus as in claim 7, wherein the second light source (104) includes a substantially circular light pipe disposed around the first light source (102). A method for measuring the reflectivity of a selected region of a surface, comprising the steps of: activating a first light source (102) and a second light source (104) in an alternating manner, thereby reflecting a pair of light beams (122,124) from the selected region of the surface, the second light source (104) being sufficiently close to the first light source (102) so that the pair of light beams (122, 124) strike the same selected region of the surface; using a photosensor (106), detecting focused light from one (122) of the pairs of light beams and unfocused light from the other (124) of the pair of light beams; and deriving a signal representative of the direct reflectance of a light beam reflected from the surface into the photodetector as a function of the focused and unfocused light detected in the detecting step, wherein said derived signal is based on the difference, wherein the processor means is adapted to derive a signal representative of direct reflectance of a light beam from the selected region into the photodetector based on the difference between maxima and minima of a signal output by the photodetector (106) as a result of the alternating activation of the first light source (102) and the second light source (104). The method of claim 9, wherein said light sources (102, 104) are activated at a frequency from about 10 cycles per second to about 1000 cycles per second.

Inventors: GENOVESE FRANK C, GENOVESE, FRANK C.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G01N21
IPC codes: G01N21, G03G15