The electrophotography (EP) process used in some imaging devices, such as laser printers and copiers, is susceptible to variations due to environmental changes and component life. This variability may have a greater impact on color EP printers since it may cause changes in the toner density of developed images, which in turn causes objectionable color shifts. It is general practice in the industry to incorporate sensors that measure the toner density of test images and provide feedback to the control system for making adjustments to various bias voltages and/or laser power. Ideally, these adjustments increase or decrease the amount of toner developed out to the latent image to achieve a desired density. Some conventional sensors currently used in the industry are reflective sensors that range from a simple emitter-detector arrangement to more complex arrangements. For instance, some sensors incorporate light-integrating cavities and collimated light sources. A limiting factor of the known art is the ability to tune the sensor to the toner that is being measured. As an example, the color toners cyan, magenta, and yellow are transparent to infrared light and reflect light in a diffuse manner. Conversely, black toner, which often includes carbon black pigment, absorbs infrared light. This absorption results in a reduction of specular light reflected off the substrate. Accordingly, conventional sensors may not be optimally suited for use in color EP printers.
Various embodiments disclosed herein are directed to EP image forming devices and an improved toner patch sensor that uses multiple light paths that are selectably activated depending on the color of a toner patch being measured. The toner patch sensor may include a detector and a source adapted to transmit light that is reflected off a toner patch and towards the detector. The source may be oriented so that the reflected light is specular and/or diffuse. A controller may selectively change the amount of one or both of the specular and diffuse light received by the detector. The source may include separate emitters for the specular and diffuse light, with the controller selectably turning off one of the emitters or selectably adjusting a ratio of illumination power between the emitters. Alternatively, the source may include a single emitter and an optical element to split light between paths that reflect specular light and diffuse light towards the detector. Diffuse light may be blocked when sensing black toner patches. Specular light and diffuse light may be transmitted to the detector when sensing toner patches with a color other than black.
Embodiments disclosed herein are directed to a toner patch sensor that may be used to measure toner density and provide feedback that is used in adjusting operating parameters to consistently develop an appropriate amount of toner during the image formation process. This type of optimization can be performed in a device such as the image forming apparatus as generally illustrated in
Within the image forming device housing 102, the image forming device 100 includes one or more removable developer cartridges 116, photoconductive units 12, developer rollers 18 and corresponding transfer rollers 20. The image forming device 100 also includes an intermediate transfer member (ITM) belt 114, a fuser 118, and exit rollers 120, as well as various additional rollers, actuators, sensors, optics, and electronics (not shown) as are conventionally known in the image forming device arts, and which are not further explicated herein. Additionally, the image forming device 100 includes one or more system boards 80 comprising controllers (including controller 40 described below), microprocessors, DSPs, or other stored-program processors (not specifically shown in
Each developer cartridge 116 may include a reservoir containing toner 32 and a developer roller 18, in addition to various rollers, paddles and other elements (not shown). Each developer roller 18 is adjacent to a corresponding photoconductive unit 12, with the developer roller 18 developing a latent image on the surface of the photoconductive unit 12 by supplying toner 32. In various alternative embodiments, the photoconductive unit 12 may be integrated into the developer cartridge 116, may be fixed in the image forming device housing 102, or may be disposed in a removable photoconductor cartridge (not shown). In a typical color image forming device, four colors of toner—cyan, magenta, yellow, and black—are applied successively (and not necessarily in that order) to a print media sheet 106 to create a color image. Correspondingly,
The operation of the image forming device 100 is conventionally known and is not explicitly described herein. For a thorough description of a conventional image forming device, reference is made to commonly assigned, co-pending U.S. patent application Ser. No. 11/240,217 filed Sep. 30, 2005, the contents of which are hereby incorporated by reference. The representative image forming device 100 shown in
The above description relates to an exemplary image forming unit 10. In any given application, the precise arrangement of components, voltages, and the like may vary as desired or required. As is known in the art, an electrophotographic image forming device may include a single image forming unit 10 (generally developing images with black toner), or may include a plurality of image forming units 10, each developing a different color plane separation of a composite image with a different color of toner (generally cyan, magenta, yellow, and black).
The density of toner 32 that is supplied by the developer roller 18 to develop the latent image areas 28 is measured using one or more toner patch sensors 126. The density of the toner 32 is checked because the effectiveness of toner development varies due to environmental conditions, differing toner formulations, component variation, difference in age or past usage levels of various components, and the like. Controller 40, via sensor 126, monitors toner 32 formation on media sheet 106 or belt 114 and may adjust the surface potential of the surface of photoconductive unit 12 (via charging unit 14) or the surface potential of developer roller 18 or imaging device 16 power levels.
In an exemplary embodiment, controller 40 at least partially manages the formation of a predetermined pattern of toner 32 on a substrate, which may comprise a media sheet 106 or belt 114 (e.g., a transfer or ITM belt). A toner patch sensor 126 detects a reflectance of the transferred pattern and controller 40 adjusts the bias voltage of the charging unit 14 and/or developer roller 18, and/or imaging device 16 power levels as needed to optimize image formation at least partly based on information provided by the toner patch sensor 126. The toner patch sensor 126 may be configured to sense the developed patterns 32 and a substrate 106, 114. Additionally, or alternatively, the toner patch sensor 126 may be configured to sense the developed patterns 32 on the surface of the photoconductive unit 12. Generally, the toner patch sensor 126 may be disposed adjacent any toner carrying surface to sense the reflectance of toner 32, the underlying toner carrying surface, or both. Also, in certain instances, it may be desirable to print toner on toner images (e.g., black on yellow or other combinations) to achieve greater contrast between the developed image and the toner carrying surface. Thus, the toner carrying surface may comprise a solid toner patch of a different color disposed on the substrate 106, 114 or the photoconductive unit 12. Controller 40 establishes an operating point that will optimize toner density. Further, the controller 40 may adjust operating points based not only upon toner patch sensor 126 readings for solid toner patches, but also various halftone patterns in an effort to optimize halftone linearization. Accordingly, a brief description of the optimization process is provided below.
Initially, one or more solid toner patches 32 are developed and transferred to the substrate 106, 114 to determine appropriate bias levels for developer roller 18 and charging unit 14 as well as an appropriate power level for the imaging device 16. The solid toner patches 32 are transported towards toner patch sensor 126, which measures a reflectance of the solid toner patch 32. A series of toner patches are produced over a range of developer bias 18 values and/or imaging devices 16 power levels and the reflectance of each patch is measured by the toner patch sensor 120. Data from empirical testing is used to correlate the toner patch reflectance values to the target mass of the solid area on the page. The controller 40 then adjusts the developer bias 18 values and/or imaging devices 16 power levels to achieve the target mass of the solid area.
After selecting an appropriate combination of charge bias, discharge exposure energy, and developer roll bias, controller 40 manages the implementation of a halftone linearization where desired color halftone screen corrections are obtained to achieve a linear halftone response. Color imaging devices sometimes use halftone screens to combine a finite number of colors (usually four) to produce many shades of colors. In order to print different colors, they are separated into several monochrome layers for different colorants, each of which is then halftoned. The halftone process converts different tones of an image into spatial dot patterns that fill some percentage of a given screen. Smaller halftone percentages are produced by smaller dots in a halftone screen. Conversely, larger halftone percentages are produced by larger dots in a halftone screen.
Ideally, the image forming device 10 will produce halftones screens that comprise theoretically desired amounts of toner 32 relative to the underlying substrate 106, 114. For example, a 50% halftone pattern should theoretically comprises about half toner 32 and half substrate 106, 114. The halftone linearization process measures reflectivity values for various halftone percentages and calculates halftone screen corrections that are necessary to adjust the actual halftone screens towards ideal values.
In light of the foregoing optimization procedures, a toner patch sensor 126 as shown in
The emitters 50, 52 may be identified as specular or diffuse by nature of their orientation relative to the detector 54. The term “specular” is generally understood to mean mirror-like or capable of reflecting light like a mirror. Accordingly, the specular emitter 50 is oriented at an incident angle Φ relative to a direction normal to the measurement surface (e.g., toner patch 32 or substrate 106, 114) and that is substantially the same as a reflectance angle Φ at which the detector 54 is oriented. Notably, the incident angle Φ and reflectance angle Φ are equal but opposite relative to the direction normal to the measurement surface. Accordingly, a substantial amount of energy emitted by the specular emitter 50 may be measurably detected by the detector 54. For the sake of size, the incident and reflectance angle Φ may be within a range between about 10 degrees and about 45 degrees relative to a direction that is normal to the measurement surface (32, 106, 114). Angles outside this range are certainly permissible.
By comparison, the diffuse emitter 52 is oriented so that the incident and reflectance angles are not the same. In one embodiment, the diffuse emitter 52 is oriented to project light along a direction substantially normal to the toner patch 32 (or substrate 106, 114). Accordingly, while a majority of the light emitted from the diffuse emitter 52 may not reach the detector 54, some measurable scattered energy (due in part to the scattering of light by the measured toner 32) will reach the detector 54.
In the present embodiment shown in
In addition, or instead, each operating point may reflect a change in imaging device 16 power. For instance, each operating point may have an associated power level that is some fraction (e.g., a PWM duty cycle) of full power for an imaging device 16 capable of producing an exposure level of about 1.1 micro-Joules per square centimeter at 100% power. Thus, for example, each operating point may represent some intermediate value falling between about 30% and 90% of full power. Other values and ranges are certainly permissible and expected for different forming devices 10.
Notably, the precise values for the operating points used in
The vertical axis shown in
In contrast to the results in
As discussed above, toner patch sensing may be used for halftone linearization as well as toner density optimization. Accordingly, it follows that the detector output should produce a measurable variation over all or a substantial majority of all halftone patterns.
In the embodiment shown in
As disclosed above, the diffuse emitter 52 may be turned off when black toner patch sensing is performed. Accordingly, the present embodiment of the toner patch sensor 226 may be implemented with a screen 170 that selectably blocks light traveling along the diffuse path 152. The screen 170 may be selectably switched between the solid line position shown in
In embodiments described above, the diffuse emitter 52 and the diffuse light paths 152, 252 were oriented to project light along a direction substantially normal to the toner patch 32 (or substrate 106, 114). This is not specifically required.
When powered, the physical temperature of emitters 50, 52, 150 may increase to elevated operating temperatures. Detector 54 signal samples taken during emitter 50, 52, 150 temperature transients may provide inaccurate results due to variation in light intensity. It may be advantageous to obtain detector 54 samples when the temperature of the emitters 50, 52, 150 has stabilized. However, one embodiment contemplates turning on a diffuse emitter 52 during non-black toner patch sensing and turning off that same diffuse emitter 52 during black toner patch sensing. Consequently, temperature variations may result from turning on and off the diffuse emitter 52 at unequal intervals. To ensure that the temperature of the diffuse emitter 52 does not drift while samples are taken from the detector 54, the diffuse emitter 52 may be modulated to cycle on and off during toner patch sensing.
In the exemplary timing diagram, waveform 140 reveals that the specular emitter is turned on and remains on for the duration of the toner patch sensing. This includes both non-black (which may include one or more non-black colors, including cyan, magenta, or yellow) and black toner patch sensing. By comparison, waveform 142 is modulated so that the diffuse emitter 52 cycles on and off during toner patch sensing. This modulation may be the same for black and non-black toner patch sensing so the diffuse emitter 52 reaches a consistent operating temperature. In order to achieve the desired operation as described herein, the sample timing given by waveform 144 may be adjusted so that the detector 54 is sampled (at point 130) while both emitters 50, 52 are on for non-black toner patch sensing. Further, the detector 54 is sampled (at point 132) while the diffuse emitter 52 is off (and only the specular emitter 50 is on) for black toner patch sensing. Alternatively, the sampling times may be held constant for black and non-black toner patch sensing with the modulation timing (and not necessarily the duty cycle) of the diffuse emitter 52 adjusted so that the samples 130, 132 are taken at the appropriate times.
The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For example, a single detector 54 is shown in the various embodiments, which may provide a simple advantageous solution. However, the teachings provided herein may be applied to systems where a diffuse emitter is used with a diffuse detector and a specular emitter is used with a specular detector and the outputs from the multiple detectors combined. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
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