Patent Publication Number: US-7583418-B2

Title: Array based sensor to measure single separation or mixed color (or IOI) patches on the photoreceptor using MEMS based hyperspectral imaging technology

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 10/248,387, filed on 15 Jan. 2003, and entitled, “Systems and Methods for Obtaining a Spatial Color Profile and Calibrating a Marking System;” U.S. patent application Ser. No. 10/342,873, filed on 15 Jan. 2003, and entitled, “Iterative Printer Control and Color Balancing System and Method Using a High Quantization Resolution Halftone Array to Achieve Improved Image Quality with Reduced Processing Overhead;” U.S. patent application Ser. No. 09/566,291, filed on 5 May 2000, and entitled, “Online Calibration System for a Dynamically Varying Color Marking Device;” U.S. patent application Ser. No. 11/070,681, filed on 2 Mar. 2005, and entitled, “Gray Balance for a Printing System of Multiple Marking Engines;” U.S. patent application Ser. No. 11/097,727, filed on 31 Mar. 2005, and entitled, “Online Gray Balance Method with Dynamic Highlight and Shadow Controls;” U.S. patent application Ser. No. 11/428,489, filed 3 Jul. 2006 and entitled, “Pitch-to-Pitch Online Array Balance Calibration;” and U.S. Pat. No. 6,295,130, filed on 22 Dec. 1999, and entitled, “Structure and Method for a Microelectromechanically Tunable Fabry-Perot Cavity Spectrophotometer;” U.S. Pat. No. 6,975,949, filed on 13 Dec. 2005, and entitled, “Full Width Array Scanning Spectrophotometer;” U.S. Pat. No. 6,690,471, filed on 10 Feb. 2004, and entitled, “Color Imager Bar Based Spectrophotometer for Color Printer Color Control System;” U.S. patent application Ser. No. 11/535,385, filed 26 Sep. 2006 and entitled, “Color Sensor to Measure Single Separation, mixed Color or IOI patches;” U.S. patent application Ser. No. 11/535,382 filed 26 Sep. 2006 and entitled, “Mems Fabry-Perot Inline Color Scanner For Printing Applications Using Stationary Membranes;” and U.S. patent application Ser. No. 11/016,952, filed Dec. 20, 2004 and entitled, “Full Width Array Mechanically Tunable Spectrophotometer.” The disclosures of the related applications are incorporated by reference in their entirety. 
     BACKGROUND 
     This disclosure generally relates to light sensor devices for use in marking methods and systems. 
     This disclosure refers to “marking” as a process of producing a pattern, such as text and/or images, on substrates, such as paper or transparent plastic. A marking engine may perform the actual marking by depositing ink, toner, dye, or any other suitable marking material on the substrate. For brevity, the word “toner” will be used to represent the full range of marking materials, and is used interchangeably with the terms for other identifying materials in the full range of marking materials. 
     A popular marking engine is the xerographic marking engine used in many digital copiers and printers. In such a xerographic marking engine, a photoreceptor unit, such as, for example, a belt or roller, whose electrostatic charge varies in response to being exposed to light, is placed between a toner supply and the substrate. In systems including xerographic marking engines, the toner is typically an electrostatically chargeable or electrostatically attractable toner. A laser unit, bank of light emitting diodes, or other such light source, is used to expose the photoreceptor unit to light to form an image of a pattern to be printed on the photoreceptor unit. In simple, monochromatic xerographic marking engines, single color toner is electrostatically attracted to the image on the photoreceptor unit to create a toner image on the photoreceptor unit. The toner image is then transferred to the substrate from the photoreceptor unit. Different methodologies are then employed to heat-set or otherwise “fuse” the toner image onto the substrate. 
     In more complex systems, multiple colors of toner are applied. General categories of more complex color systems include those that are referred to as Image On Image (IOI) systems and/or tandem systems. In an IOI system, such as that shown schematically in exemplary manner in  FIG. 1 , the marking engine  10  includes a plurality of primary color applying units  11  that deposit toner on a photoreceptor belt  13 , which includes multiple image forming areas  14 , hereafter pitches  14 . A first pitch  14  of the photoreceptor belt  13  receives a first toner image in a first color. The first color remains on the photoreceptor belt  13  while second (and subsequent) toner images are created by applying second (and subsequent) colors atop the first image in the same pitch  14 . The first and second (and subsequent) toner images remain on the photoreceptor belt  13  and are subsequently built up on the photoreceptor belt  13 . Once all of the toner images are placed on the photoreceptor belt  13 , they are then transferred to a substrate, typically paper, and fused to the substrate. Furthermore, after the first pitch  14  has passed one of the color applying units  11 , the next pitch  14  comes into alignment with that color applying unit  11 , and the image forming process starts again in the next pitch  14 . 
     In an embodiment of a tandem system architecture, such as that shown in exemplary manner in  FIG. 2 , the marking engine  20  includes multiple primary color applying units  21  that first deposit their toner on respective photoreceptor drums  22  to form toner images. These toner images are deposited on an intermediate transfer belt (ITB)  23 , which includes multiple pitches  24 . Each toner image is transferred onto the ITB  23  before the next toner image is formed. Like in the  101  system, the toner images are transferred to a substrate once all toner images for a given pitch have been deposited on the ITB  23 . 
     In a variant of the tandem system shown in  FIG. 2 , an additional drum may be included between each photoreceptor drum  22  and the ITB  23 . The additional drum accepts the toner image from the photoreceptor drum  22  and deposits it on the ITB  23 . The inclusion of the additional drum aids in reducing a possibility of toner contamination by toner of one color getting into a toner source of another color due to electrostatic interaction between the toner image on the ITB  23  and the photoreceptor drum  22 . 
     SUMMARY 
     Marking engines using any of the printing techniques disclosed above seek to achieve consistency and reproducibility in generated output images. One approach by which consistency and reproducibility is effected is through the use of one or more image sensors to generate reflectance values from separate toned patches periodically output by the marking engine onto the photoreceptor unit and transferred to the substrate, based on stored test data. Measured reflectance values from a toned patch on an output substrate may be compared with stored target values and a difference value calculated. These difference values may be used to generate feedback control signals to the marking engine. In response to the feedback control signal, the marking engine may automatically adjust the amount of toner of one or more colors laid within one or more of the respective pitches that comprise an image to improve image quality, consistency and reproducibility. 
     Despite such feedback techniques, marking engines continue to suffer from color inconsistency or instability that may affect a final image. Such color instability may be attributed to such factors as temperature, humidity, age and/or amount of use of the photoreceptor unit, age and/or use of an individual toner color, or other like environmental and/or mechanical factors. 
     Further, media attributes (e.g., media weight) can also affect color stability. For example, changes in media weight may result in a need to adjust fuser temperature, decurler penetration force, and acceleration profiles to achieve micron level registration tolerances. 
     Mechanical control systems may also contribute to color instability in certain circumstances. For example, color to color registration errors can lead to color instability. By way of example only, in some marking systems, every pixel in all four color separations is registered on a image carrier to within, approximately, 85 microns. The placement of the separations is controlled by adjusting the speed of the photoreceptor belt, ROS position, and speed and location of the servo drive rolls. Color registration marks are placed on the photoreceptor and read with special sensors to produce a completely closed loop system that may achieve 40 micron accuracy of dot placement. However, such mechanical color to color registration processes are prone to error. 
     Control and sensor systems intended to correct color instability are not always effective in eliminating the color instability caused by such effects. For example, printers that use hierarchical control systems with Extended Toner Area Coverage Sensors (ETACS) are often unable to provide sufficient marking engine stability for multi-separation IOI images. This is because ETAC sensors are used to measure tone development on the photoreceptor before transfer and fuse stages for three different input tone conditions, referred to, for example, as low, mid and high area coverage, resulting in a photoreceptor developability control model with 3 states. However, although ETACS may be used in such a manner to measure color of single color control patches, ETACS do not measure color of multi-separation control patches accurately. 
     On-paper color measurements with image sensors, and specifically spectrophotometers, were believed to constitute a fix for this problem. On-paper spectrophotometer color measurements may be performed within a marking system as an integral part of the marking system image generation process, that is, “in-line”, or performed in a process separate from the marking system image generation process, that is, “off-line.” Both in-line and off-line on-paper spectrophotometric measurements may be used in various forms to construct 1D gray balance calibration tone reproduction curves (TRCs) and/or 2D, 3D or 4D correction Look-Up-Tables (LUTs). These TRCs and/or LUTs may be used by a marking engine to automatically adjust the amount of toner of one or more colors laid within one or more of the respective pitches that comprise an image to improve image quality, consistency and reproducibility, as discussed above. A drawback of relying solely upon on-paper spectrophotometer measurement techniques is the inability of the marking engine to correct colors at a sufficiently high frequency, for example, every photoreceptor belt revolution, to achieve color stabilization as is supported by the hierarchical control systems discussed above. 
     This disclosure describes embodiments of a Micro-Electro-Mechanical System (MEMS) based Fabry-Perot array sensor, or spectrophotometer, for non-invasively measuring spectral information from toned single or multi-color control patches. Embodiments of the disclosed sensor may be used to measure spectral information from toned patches and/or images placed upon a photoreceptor unit within a marking system. Such embodiments may allow spectral information from control patches and/or images to be measured as often as every photoreceptor belt revolution, or greater, as disclosed below. Other embodiments of the disclosed sensor may be used to measure spectral information from toned patches and/or images placed by a marking system upon a non-photosensitive output substrate, such as an intermediate belt or paper. 
     This disclosure will generally refer to the photoreceptor unit as a photoreceptor belt. The use of the term photoreceptor belt in this manner is for ease of understanding and clarity. It should not be regarded in any way as limiting or excluding other types of photoreceptor units, such as, for example, photoreceptor drums. The frequency sought to be achieved in toned patch and/or image monitoring of such a photoreceptor belt in operation is one or more measurements per photoreceptor belt cycle, with increased measurement accuracy. 
     Non-filtered spectrophotometers, which may use single or multiple photo-site light sensors, provide measured reflectance values that include combined contributions from all wavelengths received by the light sensor(s). Filtered spectrophotometers may use colored filters, for example, red, blue and/or green to filter out selected bandwidths of light entering select photo-sites. In this manner, a filtered spectrophotometer provides crude spectral information, that is, a measured reflectance value for each separately filtered photo-site or each separately filtered group of photo-sites. Spectrophotometers that use such crude filtering techniques may provide a color stabilization process with more spectral information than would be possible with an unfiltered spectrophotometer. The information provided is often limited by the number of photo-site/filter combinations used. Additionally, the spectral bands for which reflectance values are provided are not dynamically configurable. Thus, the usefulness of such spectral information to the color correction process is limited. 
     A Fabry-Perot cavity is an optical resonating chamber formed by two parallel, highly reflecting planes. Light entering the cavity will resonate between the two mirrors. Based upon the gap distance between the two parallel mirrors, the various frequencies of light entering the Fabry-Perot cavity will constructively and destructively interfere with one another, thereby allowing only a select band of light frequencies to emerge from the cell. Therefore, a Fabry-Perot cavity may be set to pass only selected wavelengths of light by setting the gap distance between the two parallel mirrors. In this manner, a Fabry-Perot cavity may be used as a light filter. 
     Such a Fabry-Perot cavity may be created between a moving plate and a CCD or CMOS photodetector array. The plate may be moved using two or more electrodes that, along with a grounded plate, form a parallel-plate capacitive actuator. Applying different voltages to the electrodes allows the plate to be tilted so that there is a gradient in the Fabry-Perot air gap. Such a tilt may also be achieved through mechanical or other means. A single cell Fabry-Perot plate may be replicated and arranged in an N×M matrix of any size array, where N and M are integers. 
     For example, in a Fabry-Perot array sensor embodiment used to measure spectral information from a photoreceptor belt, an N×M array may be configured to span an entire width of a photoreceptor belt for any length of the photoreceptor belt desired, for example, a width of a toned patch, or the width of an image. Alternatively, the N×M array may be configured to span only a portion of the width of the photoreceptor belt, for any length of the photoreceptor belt desired, and may be physically positioned within a marking system to allow measurement of a portion of interest of the photoreceptor belt. 
     Further, in a Fabry-Perot array sensor embodiment used to measure spectral information from a non-photosensitive output substrate, such as an intermediate belt or paper, an N×M array may be configured to span an entire width of the non-photosensitive output substrate for any length of the output substrate desired, for example, an entire length of a paper sheet. Alternatively, the N×M array may be configured to span only a portion of the width of a non-photosensitive output substrate, for any length of the output substrate desired, and may be physically positioned within a marking system to allow measurement of a portion of interest of the output substrate. 
     The gaps within the individual Fabry-Perot array cells may be set so that there is a resulting slope of the cell gaps across the Fabry-Perot array, tilted either toward or away from a scan direction. For example, the moving plate can be tilted either toward or away from the scan direction, so that a range of cell gaps is created. In this manner, different sections of the Fabry-Perot array sensor may measure different portions of the spectrum of light reflected from the photoreceptor belt or non-photosensitive output substrate. Once a portion of the photoreceptor belt or output substrate has completely passed a section of such a Fabry-Perot array configured in such a manner, spectral information of toned patches or other images on the paper/photoreceptor belt may be known. A width of the spectrum across which spectral information is recorded and a resolution with which the spectral information is recorded is dependent upon the size of the N×M Fabry-Perot array and the manner in which the gaps across the array are configured. 
     The cells within the Fabry-Perot array may be periodically reconfigured to measure different portions of the spectrum. This allows the Fabry-Perot array sensor to be dynamically reconfigured to provide spectral information needed by a color stabilization process to more accurately stabilize color within a marking system. For example, embodiments of the Fabry-Perot array sensor may provide measured reflectance values for any number of wavelengths throughout a dynamically selectable spectral range. 
     Once the Fabry-Perot plates in each of the array cells are set, no further mechanical array movement is required to support reflectance measurements. Therefore, a speed at which spectral information is collected may be limited only by a sensitivity of the associated CCD or CMOS photodetector array, that is, by a time needed for the CCD or CMOS photodetector array element(s) associated with each Fabry-Perot cell or group of cells to collect sufficient light to produce an accurate measurement. Such a Fabry-Perot array may measure spectral information, for example, at marking system inline processing rates in excess of 100 pages per minute. 
     In the disclosed Fabry-Perot array embodiments, the width of the spectrum being measured may be increased by increasing the gap in the respective Fabry-Perot cells in the array. For example, the gap in the respective Fabry-Perot cells in a direction across the Fabry-Perot array may be increased at a fixed rate in a direction spanning the Fabry-Perot array. The larger a rate of increase in the gap across the array, the larger a section of the overall spectrum the array is configured to measure. For example, a given Fabry-Perot array configured with a relatively small rate of increase in the gap, or tilt, will measure a relatively small section of the spectrum. The greater the tilt, the greater the section of the spectral range that will be measured by the Fabry-Perot array, but at a lesser resolution. However, the Fabry-Perot array is not limited to being set with a fixed rate of change in the gaps of the respective Fabry-Perot cells across the array. The cell gaps across the Fabry-Perot array may be set in any manner desired to collect any combination of spectral information to allow a color stabilization process to resolve a color instability issue. 
     Non-invasive measurement of toned patches and/or images at the photoreceptor belt rotation speed invariably requires some kind of illumination. Embodiments of the disclosed sensor illuminate toned patches and/or images using one or more illumination bands that are outside of the photo-generation response range of the photoreceptor belt upon which the toned patches and/or images are placed. 
     A common print quality problem in xerographic printing results from a build-up of residual potential and surface voltage on photoreceptors. Such a condition results in a vestigial image repeated at regular intervals down the length of a page and appearing as light or dark areas (in black and white printers) or often colored area in (color printers) relative to the surrounding field, referred to as ghosting. There are many sources of ghosting. Subsystems from charging, development, photoreceptor, to fusing can all produce ghosting. 
     Photo-generation of charge carriers in a photoreceptor belt takes place at the bottom of a charge generation layer when the photoreceptor belt is exposed with photons. The charge generation layer has photoconduction material that generates electron-hole pairs in response to the photons. These charges drift and migrate to the top surface, and neutralize the surface charges in the illuminated areas to form latent electrostatic images when the photoreceptor belt is exposed with images or toned patches. The strength of the photo generation response depends on a wavelength of the photons. 
       FIG. 3  presents a graphical plot  30  of the spectral sensitivity of an exemplary photoreceptor belt used in an exemplary marking engine. As shown in  FIG. 3 , photo generation of the photoreceptor belt has minimum electron-hole pair generation at ˜470 nm and above 900 nm (infrared). Threshold line  32  marks an exemplary threshold below which ghosting is not observed in subsequent toned patches and/or images. 
     Therefore, exemplary embodiments of the disclosed Fabry-Perot array sensor may illuminate patches on the photoreceptor belt using illumination bands centered at ˜470 nm and above 900 nm, without affecting the charge generation layer of the photoreceptor belt. In this manner, the patches on a photoreceptor belt may be illuminated and a corresponding reflected light response measured, without introducing ghost images. 
     Exemplary Fabry-Perot array sensor embodiments used to measure color reflectance from a non-photosensitive output substrate, such as an intermediate belt or paper, are not limited in the use of illumination bands. Therefore, in such exemplary Fabry-Perot array sensor embodiments, any number of illumination wavelengths may be used, even white light, that is, light that includes all wavelengths. 
     Fabry-Perot array sensor embodiments configured to measure spectral information from a photoreceptor substrate, as well as embodiments configured to measure spectral information from an output substrate, may illuminate the respective substrate using Low Cost Light Emitting Diode (LCLED) technology. The LEDs selected may be based upon the photosensitive characteristics of the particular substrate. 
     In Fabry-Perot array sensor embodiments used to measure spectral information from a photoreceptor substrate, LCLEDs that emit light centered at wavelengths outside the photo response range of the photoreceptor belt may be used, as discussed above. In Fabry-Perot array sensor embodiments used to measure spectral information from a non-photosensitive output substrate, LCLEDs that emit light at wavelengths throughout the spectrum may be used. 
     Exemplary embodiments of the disclosed Fabry-Perot array sensor may be used to measure spectral information from a non-photosensitive output substrate and may use, for example, 8 LED illuminators, centered at ˜437 nm, ˜468 nm, ˜507 nm, ˜523 nm, ˜573 nm, ˜596 nm, ˜626 nm, respectively. However, in the case of Fabry-Perot array sensor embodiments used to measure spectral information from patches on a photoreceptor belt, with a photo generation response discussed above, for example, with respect to  FIG. 3 , LEDS may be selected that illuminate at wavelengths centered at wavelengths for which photo generation of the photoreceptor belt has minimum electron-hole pair generation. 
     Exemplary embodiments of such Fabry-Perot array sensors may sequentially illuminate toned patches and/or images with LEDs at specific wavelengths, such as: (1) one or more LEDs that produce a narrow illumination band centered at a wavelength within a first low photosensitivity region of the photoreceptor belt, for example, below 525 nm, such as an LED that produces a narrow illumination band centered around 470 nm; and (2) one or more LEDs that produce a narrow illumination band centered at a wavelength within a second low photosensitivity region of the photoreceptor belt, for example, above 900 nm, such as an LED that produces a narrow illumination band centered around 940 nm and/or an LED that produces a narrow illumination band centered around 970 nm. 
     Disclosed Fabry-Perot array sensors may be configured to measure light reflectance from toned patches of colors throughout the color gamut, either on the photoreceptor or on an output substrate, and to generate measured spectral information that may be used to characterize the toned patch. The measured spectral information may be compared with a set of desired spectral information and used to produce and/or update a color correlating tone reproduction curve (TRC) and/or a look-up table (LUT). The TRCs and/or LUTs may then be used to alter a theoretical combination of toner to produce more accurate color with an actual combination. 
     For example, if one desires a process color of 128 cyan, 64 magenta, 64 yellow, and 0 black, but the marking engine used must employ 131 cyan, 67 magenta, and 69 yellow, and 0 black to achieve the desired result, TRCs may be employed to adjust the requested color amounts so that the marking engine deposits 131 cyan, 67 magenta, 69 yellow, and 0 black, yielding the desired process color. Preferably, a different TRC may be used for each toner that a marking engine uses. For example, a CMYK marking engine may include four TRCs. TRCs may include different ranges of saturation values, such as 0 to 1, 0 to 100, or 0-255. Regardless of input and output ranges, all TRCs may be used to adjust an amount of toner deposited by mapping an input value to an output value. 
     Exemplary embodiments of disclosed Fabry-Perot array sensors, using illumination bands outside the photosensitivity range of a photosensitive substrate, may be configured to support multi-axis control color control of a marking engine at a frequency, for example, every photoreceptor belt cycle or greater, that is higher than an update frequency possible with on-paper measurements. Embodiments of Fabry-Perot array sensors for measuring spectral reflectance information, or spectral information, from non-photosensitive substrates, using multiple simultaneous illumination bands, may also be configured to support multi-axis color control. In such on-paper systems, new measurements may only be taken each time a new output substrate is generated, which typically requires multiple photoreceptor belt cycles. 
     An exemplary color stabilization process may use measured spectral information received from one or more Fabry-Perot array sensor embodiments. For example, a color stabilization process may receive measured spectral information from one or more Fabry-Perot array sensor embodiments that measure spectral information from toned patches and/or images on a photoreceptor belt, as well as one or more Fabry-Perot array sensor embodiments that measure spectral information from toned patches and/or images on a non-photosensitive output substrate. In this manner, an exemplary color stabilization process may obtain and make use of spectral information available from photoreceptor-based toned patch and/or image reflections and/or paper-based toned patch and/or image reflections, thereby maximizing spectral information available to support the color stabilization process. 
     Exemplary Fabry-Perot array sensor embodiments may be used to measure single-color, mixed-color and/or IOI patches to enable multi-axis color control of a wide range of marking engines. Further, a relatively low cost of such an approach may allow color control features, previously reserved for only high-end printing systems, to be considered for use in less expensive printing systems. 
     These and other objects, advantages and salient features are disclosed in or apparent from the following description of exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will be disclosed with reference to the accompanying drawings, in which like numerals represent like parts, and wherein: 
         FIG. 1  schematically illustrates an Image On Image (IOI) marking engine showing multiple pitches on a photoreceptor belt; 
         FIG. 2  schematically illustrates a tandem marking engine showing multiple pitches on an intermediate transfer belt (ITB); 
         FIG. 3  is a graphical plot of the spectral sensitivity of an exemplary photoreceptor belt; 
         FIG. 4  is a side view of a first exemplary Fabry-Perot cavity structure embodiment; 
         FIG. 5  is a top view of the exemplary Fabry-Perot cavity structure embodiment shown in  FIG. 4 ; 
         FIG. 6  is a side view of a second exemplary Fabry-Perot cavity structure embodiment; 
         FIG. 7  is a top view of the second exemplary Fabry-Perot cavity structure embodiment shown in  FIG. 6 ; 
         FIG. 8  is a side view of a third exemplary Fabry-Perot cavity structure embodiment; 
         FIG. 9A  schematically illustrates a through-wafer wet etching technique used to create components shown in  FIG. 8 ; 
         FIG. 9B  is a scanning electron microscope photograph of an etch performed using the through-wafer wet etching technique of  FIG. 9A ; 
         FIG. 10  schematically illustrates an exemplary Fabry-Perot array in which exemplary Fabry-Perot cavity structures are placed side by side to form an N×M array; 
         FIG. 11  schematically illustrates an exemplary Fabry-Perot array in which exemplary Fabry-Perot cavity structures are overlapped to form a full-width array; 
         FIG. 12  schematically illustrates a first exemplary gap configuration in an exemplary Fabry-Perot array embodiment; 
         FIG. 13  schematically illustrates a second exemplary gap configuration in an exemplary Fabry-Perot array embodiment; 
         FIG. 14  schematically illustrates an exemplary marking engine with an exemplary inline Fabry-Perot array spectral information measuring system; 
         FIG. 15  is a block diagram illustrating the exemplary marking engine inline Fabry-Perot array reflectance measuring system shown in  FIG. 14 ; 
         FIG. 16  schematically illustrates a marking engine undergoing calibration according to a process that uses the first exemplary embodiment of a Fabray-Perot array sensor; 
         FIG. 17  schematically illustrates a marking engine undergoing calibration according to a process that uses the second exemplary embodiment of a Fabray-Perot array sensor; 
         FIG. 18  is a schematic representation of an exemplary pitch and an exemplary set of toned patches; and 
         FIG. 19  is a flow diagram representing exemplary methods of calibrating exemplary marking systems according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     To obtain a desired color on a target media, such as white paper, different amounts of base colors or marking materials, such as cyan, magenta and yellow, are marked on a photoreceptor unit or belt in preparation for transfer to the target media. A well-balanced marking engine should produce a pitch with color reflectance values which, when measured, match reflectance values that correspond to the desired color. However, a marking engine may not produce an exact desired color due to, among other factors, variations in color pigments of the primary colors used by the marking engine, and/or internal processes of the marking engine. To overcome such shortfalls, color balance TRCs may be developed by iterative methods, such as those described above, and as disclosed in U.S. patent applications Ser. Nos. 09/566,291, 11/070,681 and 11/097,727. These TRCs maybe employed to, for example, adjust amounts of cyan, magenta and yellow proportions for all color tone values, taking into account the state of the materials and the marking engine. This approach can be extended to produce color balanced and/or gray balanced TRCs for spatial uniformity corrections as disclosed, for example, in U.S. patent applications Ser. Nos. 10/248,387 and 10/342,873. 
     Iterative methods disclosed above to produce accurate TRCs may rely upon feedback in the form of measured reflectance values from toned patches output by the marking engine in response to a set of predetermined, often stored, toned patch pattern data. By comparing measured spectral information from a toned patch with a stored set of desired spectral information previously generated for the toned patch, TRCs may be created and/or updated. The new or updated TRCs may then be used by the marking engine to adjust and stabilize color output. However, measured reflectance values provided by the spectrophotometers used in the above processes includes limited spectral information, as discussed above. 
     Calibration and control methodologies disclosed above may be used to achieve high quality and consistent color balanced printing for marking engines with periodic pitch-to-pitch variations. To counter the effects of such factors as temperature, humidity, the age and/or amount of use of the photoreceptor belt, age and/or use of an individual toner color and other such related factors, TRCs are preferably continuously updated based upon measured spectral information that is measured one or more times during a single revolution of a marking system&#39;s photoreceptor belt. As discussed above, exemplary embodiments of disclosed Fabry-Perot array sensor devices can provide improved spectral information in support of such color stabilization processes. One exemplary embodiment may obtain improved spectral information from toned patches and/or images on a photoreceptor belt within a marking system, allowing spectral information to be collected each revolution, without introducing ghost images upon the photoreceptor belt. Another exemplary embodiment may obtain spectral information from toned patches and/or images on a non-photosensitive output substrate, such as an intermediate belt or paper. 
     The basic structure of a Fabry-Perot cavity spectrophotometer is disclosed in detail in U.S. Pat. No. 6,295,130, and co-pending application U.S. patent application Ser. No. 11/092,635, which are incorporated herein by reference in their entirety. The basic Fabry-Perot cavity includes two micro-mirrors separated by a gap. The gap may be an air gap, or may be filled with liquid or other material. The micro-mirrors may include multi-layer distributed Bragg reflector (DBR) stacks or highly reflective metallic layers, such as gold. A voltage applied between the two mirrors may be adjusted to change the distance between the two mirrors. The distance between the two mirrors may be referred to as the gap distance. Only light with certain wavelength may be able to pass the gap due to interference effect of incident light and reflective light. 
     For example,  FIG. 4  shows a side view of an embodiment of a micro-electro-mechanically tunable Fabry-Perot array sensor, or spectrophotometer, having a Fabry-Perot micro-electro-mechanically tunable cavity structure  100 .  FIG. 5  is a top view of the cavity structure  100 . As shown in  FIG. 4 , the cavity structure  100  may include a top mirror  120  and a bottom mirror  121 . In various exemplary embodiments, the bottom mirror  121  may be a bottom distributed Bragg reflector (DBR) mirror that includes three pairs of quarter wavelength Si/SiN x  stacks. The top mirror  120  may be a top distributed Bragg reflector (DBR) mirror that includes two pairs of quarter wavelength Si/SiN x  stacks. 
     As shown in  FIG. 4 , the cavity structure  100  may also include a top electrode  115  and a bottom electrode  116 . The top electrode  115  may be formed on the top mirror  115  via a transparent support element  145 . The bottom electrode  116  may be sandwiched between the bottom mirror  121  and a substrate  185 . 
     The substrate  185  may have a transparent substrate portion  186  that may be a hole or a transparent part. The transparent support element  145  may be a transparent substrate. The top electrode  115  and the bottom electrode  116  may be transparent electrodes. Indium tin oxide (ITO) may be used for the transparent bottom electrode  116  and the transparent top electrode  115 . 
     The top and bottom mirrors  120  and  121  may be separated by a gap cavity  125 . The gap cavity  125  may be maintained in a variety of ways. In various exemplary embodiments, the gap cavity  125  may be maintained using a plurality of springs  150 . As shown in  FIGS. 4 and 5 , each of the plurality of springs  150  corresponds to a respective one of a plurality of anchors  160 . The plurality of springs  150  are connected to the transparent support element  145  such that the top mirror  120  may be separated from the bottom mirror  121  by the gap cavity  125 . 
     The gap cavity  125  may be characterized by the gap distance  126  between the top and bottom mirrors  120  and  121 . The gap distance  126  represents a dimension of the gap cavity  125 , and may be referred to as a size or height of the gap cavity  125 . 
     The gap distance  126  may be changed or otherwise adjusted. For example, top mirror  120  may be deformed to a dimensional change in the gap cavity  125  by applying a voltage in an exemplary range of 5-100 volts across transparent bottom electrode  116  and transparent top electrode  115 , or a charge in an exemplary range of 10 −11  coulombs on transparent bottom electrode  116  and transparent top electrode  115 , to effect a change in the gap distance  126  of gap cavity  125  of about 300 to 500 nm. Hence, electrodes  115  and  116  may form a capacitor and the Fabry-Perot cavity structure  100  may have an associated capacitance. As the gap distance  126  of gap cavity  125  decreases, for example, the center frequency of the spectral band passed by the Fabry-Perot cell decreases to shorter wavelengths. 
     The gap distance  126  may be changed in a way in which the top mirror  115  stays stationary, while the bottom mirror  116  moves relative to the top mirror  115 . The gap distance  126  may be changed in a way in which the bottom mirror  116  stays stationary, while the top mirror  115  moves relative to the bottom mirror  116 . The gap distance  126  may be changed in a way in which both the top mirror  115  and the bottom mirror  116  are moving relative to each other. In various exemplary embodiments, the top mirror  115  and the bottom mirror  116  maintain a relationship substantially parallel with each other regardless of the relative movement therebetween. 
     Furthermore, the size of the gap cavity  125  may be changed by a mechanism other than application of a voltage. For example, the size of gap cavity  125  may be changed by a mechanical, thermal or magnetic mechanism. 
     In the cavity structure  100  shown in  FIG. 4 , light may be received at the top of the cavity structure  100  through the top electrode  115 . The received light may be transmitted through the gap cavity  125  and the transparent substrate portion  186  of the substrate  185  at a tuned wavelength. 
     Also, a photodetector may be formed on a separate chip (not shown) from the chip upon which the Fabry-Perot cavity structure  100  is formed, so that transmitted light may be detected, if necessary, by a photodetector formed on the separate chip. 
       FIG. 6  shows a side view of another embodiment of a micro-electro-mechanically tunable spectrophotometer having a Fabry-Perot micro-electro-mechanically tunable cavity structure  101 .  FIG. 7  is a top view of the cavity structure  101 . The exemplary embodiment shown in  FIG. 6  differs from the embodiment shown in  FIG. 4  in that the hole or transparent part  186  may be replaced with a light detector  175  supported by substrate  185 . Other, similar feature are numbered similarly to the features disclosed above with respect to  FIG. 4 , and will not again be introduced. As shown in  FIG. 6 , a photodetector  175  may be formed on the substrate  185 . Thus, light received at electrode  115  and transmitted via gap cavity  125  may be detected by the photodetector  175  supported by substrate  185 . 
       FIG. 8  shows a side view of another embodiment of a micro-electro-mechanically tunable Fabry-Perot array sensor having a Fabry-Perot micro-electro-mechanically tunable cavity structure  103 . The exemplary embodiment shown in  FIG. 8  differs from the embodiment shown in  FIG. 4  in that the transparent part  186  spans the entire base of the structure. Further, electrodes  115  and  116 , used to set the size of gap cavity  125 , have been repositioned opposite one another across gap cavity  125 . Further, a default spacing between transparent support element  145  and transparent substrate portion  186  is determined by a height of spacer  162  positioned between anchors  160  and transparent substrate portion  186 . The height of spacers  162  may be varied across Fabry-Perot cells in an array to change the dynamic range of the respective Fabry-Perot cells in the array, as described with respect to  FIGS. 12 and 13 , below. Other, similar feature are numbered similarly to the features disclosed above with respect to  FIG. 4 , and will not again be introduced. 
     To reduce the cost of manufacturing the Fabry-Perot array sensor embodiment shown in  FIG. 8 , a wet etching process may be used to separate form springs  150  that support top support  145 . In one exemplary embodiment, hydrofluoric acid (HF) may be used to etch through the silica wafer through holes in a protective etch mask of polysilicon. However, other wet etching solutions and techniques may be used. 
       FIG. 9A  schematically illustrates a through-wafer wet etching technique that may be used to separate springs  150  from movable, transparent support element  145  shown in  FIG. 8 . As shown in  FIG. 9A , polysilicon masking material,  902   a  and  902   b , may be placed on either side of a silica substrate  903  to be etched. Holes,  904   a  and  904   b , in polysilicon masking material,  902   a  and  902   b  are aligned on opposite sides of silica substrate  903 . Masked silica substrate  903  may then be immersed in a hydrofluoric acid (HF) solution. Lines  906 ,  908 ,  910  and  912 , shown in  FIG. 9A  represent how etched fronts grow over time from holes  904   a  and  904   b  from both sides of masked silica substrate  903  until the etched fronts meet, resulting in a wet-etch-created silica beam  905 . By aligning multiple holes on either side of silica substrate  903  in a configuration, for example, as shown by the outline of Fabry-Perot micro-electro-mechanically tunable cavity structure  100  in  FIG. 5 , moving silica plate  145  and springs  150  may be formed. For example, the disclosed wet etching process may be performed upon a typical wafer lot of 24 wafers, simultaneously and inexpensively, in only a few hours.  FIG. 9B  is a scanning electron microscope photograph  950  of an etched groove  952  performed using the through-wafer wet etching technique of  FIG. 9A . 
       FIG. 10  illustrates an exemplary addressable Fabry-Perot array  1000 . As shown in  FIG. 10 , the Fabry-Perot array  1000  may include a plurality of adjacently located Fabry-Perot cavities  100 . For example,  FIG. 10  shows a 5×5 array of cavities. However, in general, other arrays may also be used, such as an N×M array, where N and M are integers. Alternatively, the cavities may also be arranged in other geometrically shapes, such as a triangle, a diamond, a hexagon, a trapezoid, or a parallelogram. Each arrayed and/or shaped set of cavities form a block of cavities. A plurality of blocks may be used to form a larger Fabry-Perot array. 
     In various exemplary embodiments, the cavities each may include a silicon membrane attached directly to a silicon spring, so that the silicon membrane may move to change the size of the cavity. In various exemplary embodiments, the cavity may include membranes as parallel plates attached to a silicon frame. The cavities may be located close to each other without much wasted space in between, so that the amount of “dead space” between adjacent membranes may be reduced or minimized, and the space used for sensing may be increased or maximized. Alternatively, as shown in  FIG. 11 , a cross-section of two N×M fixed-gap cavity groups, shown in  FIG. 11  as  1102  and  1104 , respectively, demonstrates that two N×M fixed-gap cavity groups may be overlapped to achieve a larger total array size. Such overlapping may be used repeatedly to overlap multiple N×M fixed-gap cavity groups, in succession, to create a Fabry-Perot array that is big enough to address a whole page, i.e., a full-width array. 
       FIG. 12  depicts a cross-section of a fixed-gap cavity group arranged in an N×M array  1200 . As shown in  FIG. 12 , this array includes a number of N rows and a number of M columns of individual Fabry-Perot cavities. In  FIG. 12 , only one column is illustrated with N number of cavities. As shown in  FIG. 12 , the array  1200  includes cavities  1218 ,  1228 ,  1238  and  1248 . Each of the cavities may include a substrate portion, a top mirror, and a bottom mirror. For example, in cavity  1218 , a voltage  1211  may be applied between a contact point  1213  of a substrate portion  1212  and a contact point  1215  of a top mirror  1214 , so that the distance  1216  between the top mirror  1214  and a bottom mirror  1217  may be adjusted by adjusting the voltage  1211 . The gap distances  1216 ,  1226 ,  1236  and  1246  for gap cavities  1218 ,  1228 ,  1238  and  1248 , respectively, are different from each other, so the group may cover a spectrum range. The gap distances are fixed during an operation. The gap distances may be reconfigured before, after or between operations. 
     The reconfiguration may be accomplished electrically, mechanically, thermally or magnetically. The reconfiguration may also be achieved by recalibration to accommodate a new set of conditions. For example, this reconfiguration may change the spectral coverage of a fixed gap group from one spectral range to another spectral range. 
     As discussed above, a portion of a toned patch or test image may correspond to a gap cavity group. Thus, different spectral information may be obtained simultaneously from a portion of a toned patch or test image, with each gap cavity in the group of gap cavities obtaining a unique spectral signal corresponding to the fixed size of the respective gap cavity. Accordingly, motion of the Fabry-Perot membranes is not needed, thereby improving the reliability and lifetime of the gap cavities. 
     Compared to a system in which each gap cavity corresponds to a portion of a toned patch or test image, the arrangement shown in  FIG. 12  may be used to reduce the spectral resolution of a Fabry-Perot array by a factor of the size (number of gap cavities) of the gap cavity group. For example, when an N×M group is configured to pass a common wavelength, the spectral resolution may be reduced by a factor of N×M. On the other hand, because a number of N×M measurements may be simultaneously obtained, the speed of measurement may be increased by a factor of N×M. 
     The size of each gap cavity group may be the same, so that spectral information at each wavelength may be obtained by the same number of gap cavities whose gap cavity sizes correspond to the wavelength. However, the size of each gap cavity group may also be different, depending on different needs of applications. 
     The reconfigurability may be part of the calibration for each Fabry-Perot gap cavity. For example, there may be a thickness variation among gap cavities during the microfabrication process. Typical variation may be less than two percent. Nevertheless, two percent variation in the size of the gaps could cause significant optical quality degradation. However, in a system shown in  FIG. 12 , each Fabry-Perot gap cavity may be fine-tuned to the same gap size for a desired wavelength range with a set of offset initial voltages. The offset initial voltages may be stored in a memory area within the system and may be used to adjust the Fabry-Perot cell gaps across a Fabry-Perot array, and may be updated at any time. Another approach may be to calibrate the sensor with a known sample, and then, instead of fine-tuning the cells to the same gap size with a set of offset initial voltages, storing a table of the errors. Assuming that the errors don&#39;t change, the stored table may be used to correct readings that the sensor makes later. 
       FIG. 13  illustrates a second exemplary configuration  1300  of gap cavities  1318 ,  1328 ,  1338  and  1348 . As shown in  FIG. 13 , within gap cavity  1318 , a voltage  1311  maybe applied between a contact point  1313  of a top mirror  1312  and a contact point  1316  of a bottom mirror  1315 , such that the distance  1314  between the top mirror  1312  and the bottom mirror  1315  may be adjusted. For example, as shown in  FIG. 13 , the top mirror  1312  may be the part whose position is adjustable, while the bottom mirror  1316  may be stationary. 
     The arrangements shown in  FIGS. 12 and 13  may be used to detect spectral information from a portion of a toned patch or test image in various parts of the electromagnetic spectral range. Depending on the mechanical tuning range of the mirrors used to create the gap cavity and the number of such gap cavities, the spectral range resulting from the gap cavity system may range from ultraviolet to near, mid or high infrared wavelengths. Additionally, the wavelength resolutions may be fine-tuned to a narrow range, such as sub-nanometer range, based on fixed sizes of gap cavities. For example, the arrangements in  FIGS. 12 and 13  may also be reconfigured to be used as a conventional RGB filter, if desired, in which RGB spectral information may be collected for portions of a toned patch or test image. 
     The sizes of the cavity gaps in an N×M two-dimensional matrix may be arranged in an increasing, decreasing or other pre-determined fashion. Although each Fabry-Perot gap cavity may have a dynamically configurable fixed size, the size of the gap is not changed during marking engine operations. The fixed size of a gap cavity may be reconfigured, for example, before a marking engine operation, after a marking engine operation or between marking engine operations. 
     In a Fabry-Perot array, such as the Fabry-Perot arrays depicted in  FIGS. 11-13 , a plurality of gap cavity groups maybe provided within a single N×M. Each gap cavity group may have a set of fixed gaps and may be used to obtain spectral information from a portion of a toned patch or test image for a specific spectral band. 
     Each fixed gap only allows a narrow band of wavelength to transmit (or reflect) light. Within a gap cavity group, the sizes of fixed gaps may differ from one another so that the spectra of a portion of a toned patch or test image may be assembled from each wavelength band of each of the fixed gaps within the gap cavity group. In particular, each portion of a toned patch or test image may contain a group of spectral characteristics, each spectral characteristic being associated with a group of respective wavelengths. Thus, because a portion of a toned patch or test image corresponds to a group of gap cavities, each specific gap cavity in a gap cavity group may be designated to obtain a spectral characteristic associated with a specific wavelength that corresponds to the gap cavity size of the specific gap cavity. 
     Also, because the Fabry-Perot array contains a plurality of gap cavity groups and each gap cavity group may have one gap cavity having the specific gap cavity size, all the gap cavities having the specific gap cavity size form a sub-array of gap cavities. This sub-array may obtain a spectral image at the specific wavelength corresponding to the specific gap cavity size. 
     As discussed above with respect to  FIG. 10 , a gap cavity group may be arranged in a N×M array, where N and M are integers, so that the group occupies a square or rectangle. Alternatively, the group of gap cavities may also be arranged in other geometrically shapes, such as a triangle, a diamond, a hexagon, a trapezoid, or a parallelogram. 
     When the gap cavity group is arranged in a N×M array, where N and M are integers, N×M equals the number of wavelength bands available for detection by the array. For example, to obtain 12 points in the wavelength spectra between 400 mm and 700 mm, the gap cavity group may have a 4×3, 6×2 or 12×1 single Fabry-Perot design. The gaps may be configured and reconfigured to obtain spectral information of the portion of a toned patch or test image at a different range and/or resolution of optical spectrum. For example, one range of optical spectrum may be from 400 nm to 700 nm. Another range may be from 380 nm-730 nm. Yet another range may be 400-550 nm and 550-700 nm etc. 
       FIG. 14  illustrates an exemplary Fabry-Perot optical system  1400  for use within an exemplary marking system. As shown in  FIG. 14 , the Fabry-Perot optical system  1400  provides a Fabry-Perot tunable filter array  1402 . Each element of the filter array  1402  may be a gap cavity structure, for example, as shown in  FIGS. 4 ,  6  and  8 . Further, each Fabry-Perot cavity structure may be configured individually, or as a gap cavity group, as disclosed above with respect to  FIGS. 12 and 13 . 
     The filter array  1402  may be located between an optical lens  1406  and a light sensing array  1404 , such as a CCD or CMOS photodetector array. The lens may be selected from a variety of lenses, such as Selfoc® lens array with a fixed focal length selected according to need. The size of the gap cavity in each of the gap cavity structure may be adjusted by, for example, a switching circuit (not shown), to give a desired transmissive frequency. The switching circuit may be a controller that sets Fabry-Perot gap distances to achieve a desired filtering frequency within individual Fabry-Perot cells, or to achieve a desired set of frequencies across a Fabry-Perot gap cavity group or a Fabry-Perot array. The provision of a desired frequency or group of frequencies may be from, for example, a user interface that receives input from a user. The switching circuit may also be a sampling circuit that provides modulation data that contains modulation signals to select the desired gap cavity or gaps. One gap cavity structure, or a group of gap cavity structures, may correspond to a portion of a toned patch or test image. As a result, the incoming image may be filtered to produce a filtered spectral image produced by filtering the incoming image at a wavelength corresponding to the size of the respective Fabry-Perot gap cavities. The filtered image may be output through Fabry-Perot array  1402  to the light sensing array  1404 . 
     As discussed above, the gap distance of the gap cavity may be adjustable. Thus, the light passing through the Fabry-Perot cells in the Fabry-Perot array may be filtered at any wavelength covered by the spectral space within the adjustable range of the gap cavity. Thus, the filtered image will be generated in various wavelengths by adjusting the size of the gap cavities to transmit selectively very narrow wavelengths or collectively a group of wavelengths of a portion of a toned patch or test image. 
     The filter array may be a two-dimensional array of Fabry-Perot cells that are addressable as a group, or the Fabry-Perot cells may be addressable independently. If the Fabry-Perot cells are addressable as a group, all membranes in the group may be actuated by a single control signal, such as a voltage. If the Fabry-Perot cells are individually addressable, each Fabry-Perot cell may be actuated by a control signal, such as a voltage, specifically designated for each individual cell. Further, actuation of individual Fabry-Perot cells, or groups of Fabry-Perot cells, in a Fabry-Perot array may be performed after an appropriate offset voltage has been applied to compensate the cell(s) for variations in the manufacture. 
     Spectral resolution of the filter array  1402  may depend on the mean reflectivity of the mirrors forming the gap cavity. The spectral range of a gap cavity may depend on the initial size of the gap cavity and the quarter wavelength Si/SiNx stacks that may be used. For light in the infrared region, the size of gap cavity may be on the order of the infrared wavelength range. When the tuning range of the gap cavity is limited because of, for example, structural limitations, a system consisting of more than one membrane with different initial sizes of gap cavities and different quarter wavelength stacks may be used to cover a broader spectral range. The initial gap distance of Fabry-Perot cells may be determined during the manufacturing process by changing the height of spacer  162 , as shown in  FIG. 8 , in individual Fabry-Perot cells or groups of Fabry-Perot cells used in a Fabry-Perot array, for example. Such a system may be designed to cover a spectral range from ultra-violet (UV) to infrared (IR). A detailed description of such a system is provided in copending application Ser. No. 11/092,835 filed Mar. 30, 2005, by Wang et al., the entire disclosure of which is herein incorporated by reference. 
     In  FIG. 14 , the filter array  1402  may be made of an array of small-sized micro Fabry-Perot cells. Such a structure may ensure the simultaneous actuation of the Fabry-Perot cells. Such a structure may also improve the uniformity among the Fabry-Perot cells, because each gap cavity may be individually adjusted based on calibration data containing calibration signals. The calibration may be conducted by, for example, the switching circuit which may be connected to the filter array  1402 , as disclosed with respect to  FIGS. 12 and 13 . 
     As shown in  FIG. 14 , Fabry-Perot optical system  1400  may be configured within an exemplary marking engine so that the Fabry-Perot optical system  1400  is positioned above the path of a photoreceptor belt  13  or a non-photosensitive output substrate, such as an intermediate belt or paper, as the output substrate passes through the marking engine. Light emitted by one or more illumination sources  1408  may be reflected from a toned patch or test image on the photoreceptor belt  13  or output substrate and reflected through optical lens  1406 , into filter array  1402  where the light may be filtered, as disclosed above, prior to being measured by light sensing array  1404 . 
     Embodiments configured to measure spectral information from a photoreceptor, as well as, embodiments configured to measure spectral information from an output substrate, may illuminate the respective surfaces using LCLED technology. The LEDs selected may be based upon the photosensitive characteristics of the surface with which the Fabry-Perot array sensor will be used. 
     For example, one embodiment used to measure spectral information from non-photosensitive output substrate may use 8 LED illuminators, centered at ˜437 nm, ˜468 nm, ˜507 nm, ˜523 nm, ˜573 nm, ˜596 nm, ˜626 nm, respectively. However, in the case of Fabry-Perot array sensor embodiments used to measure spectral information from toned patches and/or images on a photoreceptor belt  13 , with a photoactive response discussed above with respect to  FIG. 3 , LEDS may be selected that illuminate at wavelengths centered at wavelengths for which photo generation of the photoreceptor belt  13  has minimum electron-hole pair generation. One exemplary embodiment of the disclosed Fabry-Perot array sensor illuminates the toned patches and/or images on a photoreceptor belt  13  with multiple LEDs at specific wavelengths: (1) a blue LED centered around 470 nm (near the low sensitive region of the photoreceptor belt  13 ); and (2) two or more LEDs in the infrared (over 900 nm) wavelength bands. 
       FIG. 15  is a block diagram of an exemplary Fabry-Perot optical system  1500 , similar to the Fabry-Perot optical system disclosed above with respect to  FIG. 14 . The spectral system  1500 , as shown in  FIG. 15 , may include a Fabry-Perot filter array  1502 , a light sensing array  1504 , a central controller  1506 , a memory  1508 , a Fabry-Perot array reconfiguration controller  1512 , an illumination controller  1514 , a spectral filtering processor  1518 , and an input/output controller  1516 , each connected by a connection or bus  15   10 . The Fabry-Perot array  1502  may be a Fabry-Perot array, for example, as disclosed above with respect to  FIGS. 10-14 . 
     The various elements shown in  FIG. 15  may perform their respective functions under control of central controller  1506 . For example, the Fabry-Perot reconfiguration controller  1512  may reconfigure the sizes of the gap cavities in the Fabry-Perot array  1502 , before, after or between marking operations, as disclosed above with respect to  FIGS. 12 and 13  based upon parameters stored in memory  1508  or received from controller  1506 . Illumination controller  1514  may control LEDs or other sources of illumination to illuminate a toned patch and/or images that has been placed by a marking engine upon a photoreceptor belt  13  or output substrate. Light may be reflected from the toned patch and/or image, passes through Fabry-Perot array  1502  and may be received by light sensing array  1504 . The spectral filtering processor  1518  may receive and process output generated by the light sensing array  1504  to generate spectral information and/or measured spectral information that may be output through input/output controller  1516 , for example, to an external processor supporting a color stabilization process, such as generating or updating TRCs, and/or may be stored in memory  1508  for future use. 
     Activities performed by the Fabry-Perot array reconfiguration controller  1512 , the illumination controller  1514 , the input/output controller  1516 , and the spectral filtering processor  1518 , may be performed under the coordinated control of central controller  1506 . However, each controller/processor may also perform tasks autonomously or semi-autonomously. For example, illumination controller  1514  may illuminate individual LEDs of specific illumination bandwidths and/or in a certain order based on instructions stored in memory  1508  and/or received from central controller  1506 . Further, upon receiving an instruction from central controller  1506 , Fabry-Perot array reconfiguration controller  1512  may coordinate arrangement of the Fabry-Perot cell gaps within Fabry-Perot array  1502  in accordance with a predetermined configuration stored in memory  1508  and/or received from central controller  1506 , and may be able to autonomously monitor and maintain, via small corrections, the precise gap distances assigned to specific Fabry-Perot cells. Further, spectral filtering processor  1518  may be programmed to autonomously receive raw output from light sensing array  1504  and may process the received values based upon knowledge of Fabry-Perot cell gap cavity group configurations received from Fabry-Perot array reconfiguration controller  1512 . In this manner, different Fabry-Perot array configurations set by Fabry-Perot array reconfiguration controller  1512  may be automatically and correctly interpreted and the derived information made available via input/output controller  1516  to other processes. 
       FIG. 16  schematically illustrates a marking engine undergoing calibration by producing a tone reproduction curve based upon feedback received from an exemplary Fabry-Perot array sensor, as disclosed above. This exemplary method is based on the method disclosed in U.S. patent application Ser. No. 11/097,727, incorporated by reference above. 
     As shown in  FIG. 16 , a storage device  1632  stores a toned patch pattern  1634  in the form of data. The toned patch pattern  1634  may include a number of toned patches and every toned patch has a desired a set of desired spectral information values  1636 . As such, the storage device  1632  may store a set of desired spectral information values  1636  for each toned patch pattern  1634 . A toned patch pattern  1634  may specify any color, including black and shades of gray. The marking engine  1646  accepts the toned patch pattern  1634  and produces a toned patch  1620 . The toned patch  1620  may include one or more toned patches. Every toned patch  1620  is associated with a toned patch pattern  1634  because each toned patch  1620  results from the printing of a toned patch pattern  1634 . 
     For example, as shown in  FIG. 16 , marking engine  1646  may include a photoreceptor belt  13  upon which both pitches  14  and exemplary toned patches  1620  have been applied. Marking engine  1646  may retrieve toned patch patterns  1634  from a storage device  1632  and use the toned patch pattern data to generate and place toned patches  1620  upon photoreceptor belt  13 , for example, in between pitches  14  upon the photoreceptor belt  13 . Ink may be applied to the respective toned patches by primary color applying units  11 . One or more Fabry-Perot array sensors  1602  may be positioned above photoreceptor belt  13  so that light emitted by illumination sources (not shown), and reflected from a toned patch or test image, passes through Fabry-Perot array sensors  1602 . 
     For example, in response to an enable command received from processor  1638 , the one or more Fabry-Perot array sensors  1602  may initiate a sequence that results in each of the one or more Fabry-Perot array sensors  1602  producing measured spectral information values  1640  that may be passed to processor  1638 . Processor  1638  may compare the measured spectral information  1640  with desired spectral information values  1636  retrieved from storage device  1632 . Processor  1638  may then generate and/or update TRC  1642  based on a difference between the measured spectral information and the desired spectral information value  1636  and store the new/updated generated TRC  1642  in storage device  1644  so that the TRC  1642  may be used by marking engine  1646  to control the output of future pitches  14  and toned patches  1620  upon the photoreceptor belt  13 . TRCs may be common for the whole page or may be common for a group of pixels or may be common or different for different pixels. 
     The one or more Fabry-Perot array sensors  1602  may be configured to detect toned patches and/or images on a photoreceptor belt  13 . As discussed above, such an embodiment may be configured with illumination sources, for example, LEDs that emit wavelengths of light that are outside the photosensitive response range of the photoreceptor belt  13  with which the one or more Fabry-Perot array sensors  1602  are used. By carefully selecting the illumination sources, as discussed above with respect to  FIG. 3 , the one or more Fabry-Perot array sensors  1602  may be configured for use with any photosensitive surface, for example, photoreceptor belt  13 , without ghosting. 
       FIG. 17  schematically illustrates a marking engine undergoing calibration by producing a tone reproduction curve based upon feedback received from an exemplary Fabry-Perot array sensor, similar to the process disclosed above with respect to  FIG. 16 . However, as shown in  FIG. 17 , the exemplary Fabry-Perot array sensor  1702  may be positioned to measure spectral information from toned patches and/or test images after the toned patches/test images have been transferred to a non-photosensitive output substrate  1704 , for example paper or an intermediate belt. Objects in  FIG. 17 , corresponding to similar objects disclosed above with respect to  FIG. 16 , are identical, and will not be reintroduced. However, as shown in  FIG. 17 , the Fabry-Perot array sensor  1702  is not positioned over the photoreceptor belt  13  of marking engine  1746 . Instead, Fabry-Perot array sensor  1702  may be positioned along the inline process to measure spectral information from toned patches and/or test images disposed on an output substrate as it emerges from marking engine  1746 . 
     As discussed above, because the output substrate upon which the toned patches and/or test images are disposed is not photosensitive, the illumination sources used by Fabry-Perot array sensor  1702  need not be preferably limited to emitting light at wavelengths outside the photosensitive range of a photoreceptor. Therefore, the Fabry-Perot array sensor  1702  may include illumination sources that emit any wavelength of light. In one exemplary embodiment, the Fabry-Perot array sensor  1702  may be configured so that it may illuminate subject matter with eight or more separate illumination band across the visible spectrum, for example, 437, 468, 507, 523, 573, 596, 626, and 662 nm. However, selected illumination bands should not be considered to be limited to such wavelengths. Any illumination wavelength may be used, including white light, that is, light include a broad range and/or all wavelengths. 
     Although only a single Fabry-Perot array sensor  1702  is shown in  FIG. 17 , a marking system could be configured with multiple Fabry-Perot array sensors. Each of the Fabry-Perot array sensors  1702  provides spectral information and/or reflectance measurement values in support of inline color stabilization processes. For example, depending upon the size and configuration of the respective Fabry-Perot arrays, multiple Fabry-Perot array sensors  1702  could be positioned at multiple locations along the internal photoreceptor belts of the marking engine and/or along the output path of the generated non-photosensitive output. Individual Fabry-Perot array sensors  1702  located at different locations within the marking engine may illuminate and may collect spectral information simultaneously, so long as each Fabry-Perot array sensor  1702  is shielded from all other light sources that may corrupt collected spectral information and/or measured spectral information. 
       FIG. 18  presents a detailed view of toned patches  110  and toned patches  120  placed upon a photoreceptor belt  13  relative to an image pitch  14 . As shown in  FIG. 18 , dashed lines  1802  and  1806  represent exemplary leading and trailing boundaries, respectively, of an exemplary output substrate to which the toned patches  120  and image pitch  14  may be transferred, for example, in a marking system in which spectral information may be collected from toned patches and images on the output substrate. The toned patches  120  may also be applied as a single patch stripe developed along the belt or photoreceptor to cover the entire region between boundaries  1804  and  1802 . Dashed lines  1804 ,  1806 ,  1807  and  1809  represent exemplary leading, trailing, left and right boundaries, respectively, of an exemplary output substrate to which the image pitch  14 , only, may be transferred, for example, in a marking system in which spectral information may be collected only from toned patches and/or images directly from a photoreceptor belt  13 , or intermediate transfer belt, and toned patches  110  and  120  are not output to an output substrate. 
     The toned patches and image represented in  FIG. 18  may be used to present colors throughout the color gamut or could be of single color. Spectral information collected from these toned patches may be processed as disclosed above with respect to  FIG. 16  or  FIG. 17  to support color stabilization processes. Details related to the nature and use of toned patches  110  and  120  are disclosed in copending U.S. patent application Ser. No. 11/428,489, entitled “Pitch-to-Pitch Online Array Balance Calibration.” Further, a test image based upon a stored image pattern may be periodically placed in one or more image pitches  14  and, may be processed in the same manner as a toned patch to support color stabilization processes. 
       FIG. 19  is a flow diagram representing an exemplary method for color calibrating exemplary marking systems according to this disclosure. The disclosed color calibration process may be initiated manually by a user, for example, in response to an observed marking system condition, or automatically, for example, as periodically scheduled maintenance or in response to a condition detected by a marking system controller or otherwise. For example, the process may be controlled and executed by a color calibration processor such as that described above with respect to  FIGS. 16 and 17  which may control a Fabry-Perot array sensor based upon communication with Fabry-Perot array sensor control modules such as those described above with respect to  FIG. 15 . 
     The exemplary method for color calibrating exemplary marking systems described below with respect to  FIG. 19  may use measured spectral information received from one or more Fabry-Perot array sensor embodiments. For example, a color stabilization process may receive measured spectral information from one or more Fabry-Perot array sensor embodiments that measure spectral information from toned patches on a photoreceptor belt, as well as one or more Fabry-Perot array sensor embodiments that measure spectral information from toned patches on a non-photosensitive output substrate. In this manner, an exemplary color stabilization process may obtain and make use of spectral information available from photoreceptor-based toned patch reflections and/or paper-based toned patch reflections, thereby maximizing spectral information available to support the color stabilization process. 
     As shown in  FIG. 19 , operation of the method begins at step S 1902  and proceeds to step S 1904 . 
     In step S 1904 , the color calibration processor may determine whether one or more Fabry-Perot array sensors should be reconfigured based, for example, upon the range and resolution of spectral information desired by a color calibration processor to support a color calibration process. 
     If, in step S 1904 , the color calibration processor determines that reconfiguration of the Fabry-Perot array sensor is required, operation of the method continues to step S 1906 . 
     In step  1906 , the Fabry-Perot array may be reconfigured by, for example, any one or all of the methods described above in paragraphs [0078-0091] and [0101-0103], above, with respect to  FIGS. 12 ,  13  and  15 . Once the Fabry-Perot array reconfiguration process is complete, operation of the method continues to step S 1908 . 
     If, in step  1904 , the color calibration processor determines that reconfiguration of the Fabry-Perot array sensor is not required, operation of the method continues directly to step S 1908 . 
     In step S 1908 , a desired image pitch maybe formed in a first area of the photoreceptor unit, which is an image area. Operation of the method continues to step S 1910 . 
     In step S 1910 , which may be substantially simultaneous with step S 1908 , a toned patch pattern containing data for generating one or more toned patches may be retrieved from a stored memory and provided to a marking engine. Operation of the method continues to step S 1912 . 
     In step S 1912 , the marking engine may produce a toned patch upon a photoreceptor belt  13  based upon the toned patch pattern retrieved from storage. Each toned patch pattern may include one or more toned patches, such as those discussed above in connection with  FIG. 11 . In some embodiments, and/or for some types of color calibration, it should be appreciated that the toned patch pattern may include only a single toned patch. For example, the toned patch could include a single mixture of color, and a measured reflectance value of the toned patch may be used to develop a calibration value that may be applied by the marking engine for that color. Calibrations for other colors could be performed separately with other toned patches on the same, or in subsequent, belt cycles. Operation of the method continues to step S 1914 . 
     In step S 1914 , generated patches upon the photoreceptor belt  13 , and/or upon an output substrate to which a generated has been transferred, are illuminated and a reflectance value for each toned patch is measured for each of the one or more illumination wavelengths, and made available to a calibration processor. Reflectance values measured by each of the respective Fabry-Perot array sensors may also be stored. Such measured reflectance values may be measured by one or more of the Fabry-Perot array sensor embodiments described above. Therefore, the measured reflectance values may include reflectance values measured from toned patches on a photoreceptor belt and/or reflectance values measured from toned patches on an output substrate. Operation of the method continues to step S 1916 . 
     In step S 1916 , desired reflectance values for each of the one or more patches disposed upon the photoreceptor belt  13  for each illumination wavelength may be retrieved from memory storage and made available to a calibration processor. Each toned patch pattern may include separate desired reflectance values for one or more of the Fabry-Perot array sensor embodiments described above. Therefore, desired reflectance values may include for each toned patch pattern, desired reflectance values for measurements taken from a photoreceptor belt  13  in response to illumination wavelengths outside the photosensitive response range of the photoreceptor belt  13 , as well as desired reflectance values for measurements taken from various output substrates, e.g., different types of plastic sheet, different types of paper sheet, in response to each of the illumination wavelengths and or bands of wavelengths used, including white light. Operation of the method continues to step S 1918 . 
     In step S 1918 , the calibration processor may determine a difference between retrieved desired reflectance values for each toned patch and the corresponding measured reflectance value measured for each toned patch for each illumination wavelength, and/or band of wavelengths, in step S 1910 . Operation of the method continues to step S 1920 . 
     In step S 1920 , the calibration processor may generate marking engine calibration data, e.g. a TRC or LUT, for each toner color that the marking engine uses. For example, a CMYK marking engine may have four TRCs or LUTs. Operation of the method continues to step S  1922 . 
     In step S 1922 , the calibration data generated in step S 1914  is applied to the marking engine for use in adjusting the amount of ink output by primary color applying units to a photoreceptor in response to a requested process color. Operation of the method continues to step S 1924 . 
     In step S 1924 , the generated marking engine calibration data may be stored in a memory store so that the calibration data may be later retrieved and used in subsequent marking operations, e.g. after a marking system restart, to stabilize color variations. Operation of the method continues to step S 1926 . 
     In step S 1926 , the differences between retrieved desired reflectance values for each toned patch and the corresponding measured reflectance value measured for each toned patch for each illumination wavelength, determined in step S 1918 , may be compared against a threshold value. Such a threshold represents an acceptable deviation from desired reflectance values, and may, for example, be one or more user configurable values that may be associated with, for example, one or more toned patch patterns and/or one or more desired reflectance values. If the difference is greater than a predetermined threshold, the method continues to step S 1904  to repeat the calibration process. However, if the difference is less than or equal to a predetermined threshold, the method continues to step S 1928  where operation of the method ceases. 
     In the above exemplary method, color balanced TRCs may be generated using spectral information measured from toned patches or test images on the photoreceptor belt  13  and/or on an output substrate, such as an intermediate belt or paper, using one or more of the disclosed exemplary Fabry-Perot array sensor embodiments. For example, color-balanced TRCs may be accurately generated according to embodiments using, for example, mixed CMY gray patches and K patches in similar fashion to that employed by some prior art methods, such as that disclosed in Mestha et al., “Gray Balance Control Loop for Digital Color Printing Systems,” Proceedings of 21 st  International Conference on Digital Printing Technologies, NIP21, pp. 499-505 (2005), which is incorporated by reference in its entirety. Exemplary embodiments of the disclosed systems and methods may use measured spectral information from relatively few gray and black patches and/or any number of color patch spectral information obtained directly from the photoreceptor belt  13  and/or output substrate in order to construct TRCs more frequently, thus reducing time-dependent drifts in performance. 
     From the foregoing description it will be appreciated that the exemplary embodiments of the disclosed systems and methods include a novel Fabry-Perot array sensor color stabilization process that allows spectral information values to be measured from toned patches on a marking system photoreceptor transfer device such as a photoreceptor belt  13 , and/or from toned patches upon an output substrate. Exemplary embodiments allow measured spectral information values to be collected at a higher frequency and improved accuracy for use in supporting color stabilization processes. The embodiments disclosed above and illustrated in the drawings represent only a few of the many ways of implementing the disclosed Fabry-Perot array sensor system and methodology for implementing color correction processes based upon an analysis of measured spectral information measured from toned patches upon a photoreceptor transfer device within a marking engine and/or from toned patches upon an output substrate. These exemplary embodiments are intended to be illustrative and in no way limiting regarding the manner by which such systems and methods may be implemented. 
     Spectral information values may be measured directly from a photoreceptor transfer device within the marking engine, such as a photoreceptor belt or drum. The sample rate may be limited only by the sensitivity of the light sensor and the time necessary to collect sufficient light for a reliable measurement. Therefore, spectral information values measured directly from a photoreceptor transfer device may be measured one or more times per rotation/revolution of the photoreceptor transfer device, if necessary, to support color stabilization. In paper based embodiments, spectral information values measured directly from an output substrate may be generated each time a new output substrate is produced. 
     The described color stabilization process may be implemented in any number of hardware/firmware/software modules and is not limited to the hardware/software architecture described or depicted above. It is to be understood that software modules supporting any selected hardware/firmware/software architecture process may be implemented in any desired computer language, and could be developed by one of ordinary skill in the computer and/or programming arts based on the functional description contained herein and the flow charts illustrated in the drawings. 
     Software modules generally can be composed of two parts. First, a software module may list the constants, data types, variable, routines and the like that that can be accessed by other modules or routines. Second, a software module can be configured as an implementation, which can be private (i.e., accessible perhaps only to the module), and that contains the source code that actually implements the routines or subroutines upon which the module is based. Such software modules can be utilized separately or together to form a program product that can be implemented through signal-bearing media, including transmission media and recordable media. 
     The disclosed color stabilization process may accommodate any quantity and any type TRCs, LUTs, and/or any quantity and any type of data set files and/or databases or other structures containing stored toned patch calibration data, measured reflectance values, and/or intermediate data sets, such as differences between measured reflectance values and stored toned patch calibration data. 
     Output from the disclosed color stabilization process may be presented to a user in any manner using numeric and/or visual presentation formats. However, output may be presented only in the form of output images with improved color stabilization. Input from a user may be input in any manner accessible to a user, for example, a marking system control interface and/or a network connection to the marking system, and may be stored in any manner accessible to the color stabilization process for controlling user configurable data and/or thresholds and/or control parameters used in the color stabilization process. 
     Further, any references herein of software performing various functions generally refer to computer systems or processors performing those functions under software control. The computer system may alternatively be implemented by hardware or other processing circuitry. The various functions of the disclosed color stabilization process may be distributed in any manner among any quantity (for example, one or more) of hardware and/or software modules or units, computer or processing systems or circuitry, where the computer or processing systems may be disposed locally or remotely of each other and communicate via any suitable communications medium (for example, LAN, WAN, Intranet, Internet, hardwire, modem connection, wireless, etc.). The processes disclosed above and illustrated in the flow charts and diagrams may be modified in any manner that accomplishes the functions disclosed herein. 
     Toned patches are not limited to any particular color, color combination or shade of black or gray. Exemplary Fabry-Perot array sensors may be used to measure accurate reflectance values from any toned patch, including single-color patches, mixed-color patches and multi-separation image-on-image colors. 
     Sensor capabilities may include single or multiple Fabry-Perot array sensor devices mounted within a marking system to allow measured reflectance values to be generated from one or several locations within the marking system. If measured reflectance values are collected simultaneously, by multiple Fabry-Perot array sensor devices, these devices may preferably be light isolated, so that a measured reflectance value is in response to light emitted from the same Fabry-Perot array sensor device used to generate the measured reflectance value. 
     In exemplary embodiments, the voltage source used to drive illumination sources, for example LCLEDs, may be pulsed at a level above what is sustainable in a continuous current mode, thereby producing higher flux detection signals and allowing a toned patch to be interrogated in a shorter time period. Further, by integrating output of the light sensor over one or more illumination periods, enhanced signal to noise ratios can be achieved. 
     While the LEDs in exemplary embodiments, disclosed above, are turned on one at time in sequence, it will be appreciated that the system is not limited thereto. There may be measurement modes in which it is desirable to turn on more than one LED or other illumination source, simultaneously, on the same toned patch. 
     Toned patches may be discretely applied to a photoreceptor transfer device at any location outside the respective pitch areas. Further, embodiments disclosed above use toned patches as the means by which reflectance values are measured. In such a manner, color correction processes may be supported without interfering with image process flow. Toned patches may alternatively be applied as, for example, test images within pitches. Reflectance values for such test images may be generated from one or more exemplary Fabry-Perot array sensors, such as a Fabry-Perot array sensor positioned over the pitch area of the photoreceptor transfer device. Such test images may be transferred to an output substrate or removed from the photoreceptor transfer device without being transferred to an output substrate. Further, such test images may be transferred to an output substrate and exemplary Fabry-Perot array sensor, such as a Fabry-Perot array sensor positioned over the image area of the output substrate, may generate spectral information values based upon the toned patches on the output substrate in addition to, or in place of, measured spectral information values of toned patches measured from the photoreceptor transfer device. 
     The use of color calibration processes using toned patches and/or test images and one or more Fabry-Perot array sensors for measuring spectral information from patches and/or test images on a photoreceptor unit and/or on an output substrate may be initiated at any time, either manually or automatically. Such color calibration processes may be executed simultaneously with image generation or as separately executed operations. Regardless of when such color calibration processes are performed, reconfiguration of Fabry-Perot array sensor Fabry-Perot cell gap distances are preferably set before, after or between spectral imaging operations supporting such color calibration processes. Further, in a fully automated system, reconfiguration of Fabry-Perot array sensor Fabry-Perot cell gap distances could be adjusted based on image content. 
     In the disclosed paper-based Fabry-Perot array sensor embodiments and related color stabilization processes, illumination wavelengths used to illuminate toned patches are not limited to any specific wavelengths. In the disclosed photoreceptor based Fabry-Perot array sensor embodiments and related color stabilization methods, the illumination wavelengths are not limited to any specific wavelengths, but may preferably include only wavelengths that are outside of the sensitivity range of the photoreceptor transfer device and, therefore do not result in ghosting. Wavelengths may be selected based upon the spectral response curve of the respective photoreceptor transfer device. The spectral response shown in  FIG. 3  is exemplary only. Similar spectral response curves for other photoreceptor transfer devices are commercially available and/or may be easily obtained and, therefore, wavelengths for which the photoreceptor transfer device is only nominally responsive may be easily determined. 
     The wet etching process disclosed above with respect to  FIGS. 9A and 9B  may be performed using any number of mask patterns, mask materials, and etching solutions. The technique is not limited to the exemplary Fabry-Perot cell designs disclosed. 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.