# Patent

## Title
Calibration system and method for light modulation device

## Abstract

A calibration method for a grating light modulator includes calibrating light reflective ribbons on the modulator on a pixel-by-pixel basis. The method further includes performing a dark-state calibration and a bright-state calibration for each pixel. Once completed, the results of the dark-state calibration and the bright-state calibration may be combined to ensure a smooth transition between a dark state and a bright state for each pixel.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/113,977, filed, Nov. 12, 2008, entitled “Calibration System and Method for Light Modulation Device,” which is hereby incorporated by reference herein in its entirety, including but not limited to those portions that specifically appear hereinafter, the incorporation by reference being made with the following exception: In the event that any portion of the above-referenced application is inconsistent with this application, this application supercedes said above-referenced application.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND
1. The Field of the Invention.
The present disclosure relates generally to light modulation devices, and more particularly, but not necessarily entirely, to methods of calibrating light modulation devices.
2. Description of Background Art
A wide variety of devices exist for modulating a beam of incident light. Light modulating devices may be suitable for use in displaying images. One type of light modulating device, known as a grating light modulator, includes a plurality of reflective and deformable ribbons suspended over a substrate. The ribbons are parallel to one another and are arranged in rows and may be deflected, i.e., pulled down, by applying a bias voltage between the ribbons and the substrate. A first group of ribbons may comprise alternate rows of the ribbons. The ribbons of the first group may be collectively driven by a single digital-to-analog controller (“DAC”) such that a common bias voltage may be applied to each of them at the same time. For this reason, the ribbons of the first group are sometimes referred to herein as “bias ribbons.” A second group of ribbons may comprise those alternate rows of ribbons that are not part of the first group. Each of the ribbons of the second group may be individually controllable by its own dedicated DAC such that a variable bias voltage may be independently applied to each of them. For this reason, the ribbons of the second group are sometimes referred to herein as “active ribbons.”
The bias and active ribbons may be sub-divided into separately controllable picture elements referred to herein as “pixels.” Each pixel contains, at a minimum, a bias ribbon and an adjacent active ribbon. When the reflective surfaces of the bias and active ribbons of a pixel are co-planar, essentially all of the incident light directed onto the pixel is reflected. By blocking the reflected light from a pixel, a dark spot is produced on the display. When the reflective surfaces of the bias and active ribbons of a pixel are not in the same plane, incident light is diffracted off of the ribbons. Unblocked, this diffracted light produces a bright spot on the display. The intensity of the light produced on a display by a pixel may be controlled by varying the separation between the reflective surfaces of its active and bias ribbons. Typically, this is accomplished by varying the voltage applied to the active ribbon while holding the bias ribbon at a common bias voltage.
The contrast ratio of a pixel is the ratio of the luminosity of the brightest output of the pixel and the darkest output of the pixel. It has been previously determined that the maximum light intensity output for a pixel will occur in a diffraction based system when the distance between the reflective surfaces its active and bias ribbons is λ/4, where λ is the wavelength of the light incident on the pixel. The minimum light intensity output for a pixel will occur when the reflective surfaces of its active and bias ribbons are co-planar. Intermediate light intensities may be output from the pixel by varying the separation between the reflected surfaces of the active and bias ribbons between co-planar and λ/4. Additional information regarding the operation of grating light modulators is disclosed in U.S. Pat. Nos. 5,661,592, 5,982,553, and 5,841,579, which are all hereby incorporated by reference herein in their entireties.
As previously mentioned, all of the bias ribbons are commonly controlled by a single DAC and each of the active ribbons is individually controlled by its own dedicated DAC. Each DAC applies an output voltage to its controlled ribbon or ribbons in response to an input signal. Ideally, each DAC would apply the same output voltage in response to the same input signal. However, in practice, it is very difficult to perfectly match the gain and offset of all the DACs to the degree of accuracy that is required for optimum operation of a light modulator due to the differences in the individual operating characteristics of each DAC. Thus, disadvantageously, the same input values may not always result in the same output for different DACs. This discrepancy means that two active ribbons whose DACs receive the same input signal may be undesirably deflected in different amounts thereby making it difficult to display an image with the proper light intensities.
In view of the foregoing, it is understood that prior to use the combination of DACs and ribbons on a light modulating device must be calibrated to ensure that the desired light intensities are correctly reproduced in a displayed image. As mentioned, calibration is required due to the fact that the offset voltage and gain of each DAC may be different. Thus, given the same DAC input values for the active ribbons of two pixels, the displayed light intensities generated by the two pixels will likely be different because the active ribbons will be deflected in different amounts. Calibration is intended to ensure that the different operational characteristics of the DACs and ribbons are taken into account during operation of the light modulation device.
The calibration process may be divided into two separate calibration processes, namely, a dark-state calibration and a bright-state calibration. Generally speaking, the dark-state calibration is an attempt to determine the DAC input values at which the pixels produce the minimum amount of light possible and the bright-state calibration is an attempt to ensure that each pixel produces the same light intensity for the same source input values.
Prior to the present disclosure, known calibration techniques for light modulation devices did not always produce the best possible results. In particular, previously known dark-state calibration methods involved calibrating all of the pixels on a light modulating device at the same time using a group-calibration process. For example, using one previously available dark-state calibration process, all of the DACs for the active ribbons of a light modulation device were first set with an input value of 0. (However, due to the offset of each of the active ribbons' DAC, a small voltage of about 0.5 volts was actually applied to the active ribbons thereby pulling them slightly down.) Then, the input value to the single DAC controlling all of the bias ribbons was experimentally varied until the best overall dark state for all of the pixels was determined by visual inspection from a human. As a result of the above described group-calibration process for the dark state, the constituent ribbons of some of the pixels were not necessarily co-planar as is required for the minimum light intensity output. Thus, some of the pixels still produced some light output even when they were set to a dark state.
The previously available bright-state calibration processes used a brute force method to determine the correct input value for a DAC based upon a desired intensity level. In particular, the previous bright-state calibration methods used an 8-entry look-up-table (“LUT”) to store the DAC input value to use for each individual pixel (DAC values were interpolated for intensities in between). The desired DAC value for each of the 8 LUT intensities was found by performing a binary search on DAC values until the desired intensity was reached. This search was performed on each pixel for each of the 8 LUT entries. One drawback to this method is that it took over 8 hours to calibrate a light modulation device with just 1000 pixels.
In view of the foregoing, it would therefore be an improvement over the previously available calibration methods to provide a dark-state calibration that minimizes the light output of each pixel individually instead of on a collective basis. It would further be an improvement over the previously available dark-state calibration methods to provide an alternative to using visual inspection by a human to determine a minimum light intensity output. It would further be an improvement over the previously available bright-state calibration methods to provide a bright-state calibration method that is quicker and easier to implement for a light modulating device with a high number of pixels.
The features and advantages of the disclosure will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the disclosure without undue experimentation. The features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.



BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the disclosure will become apparent from a consideration of the subsequent detailed description presented in connection with the accompanying drawings in which:
FIG. 1 depicts a light modulation device having a plurality of deflectable ribbons;
FIG. 2 is a perspective view of a light detection device with a photodetector;
FIG. 3 depicts a cross-sectional view of the ribbons on the light modulation device shown in FIG. 1 in an uncalibrated and unbiased state;
FIG. 4 depicts a cross-sectional view of the ribbons on the light modulation device shown in FIG. 1 with the bias ribbons pulled down;
FIG. 5 is a graph of a dark-state curve for a pixel on the light modulation device shown in FIG. 1;
FIG. 6 depicts a cross-sectional view of the ribbons on the light modulation device shown in FIG. 1 in a dark state configuration;
FIG. 7 is a graph of a bright-state curve for a pixel on the light modulation device shown in FIG. 1;
FIG. 8 is a graph depicting a combined normalized dark-state curve with a bright-state curve;
FIG. 9 is a diagram of an exemplary system for calibrating a light modulation device; and
FIG. 10 is a flow chart depicting an exemplary calibration process for a light modulation device.



DETAILED DESCRIPTION
For the purposes of promoting an understanding of the principles in accordance with the disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications of the inventive features illustrated herein, and any additional applications of the principles of the disclosure as illustrated herein, which would normally occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the disclosure claimed.
Referring now to FIG. 1, there is depicted a light modulation device 10 having a plurality of ribbons 12-26 arranged in a one-dimensional array on a substrate 30. The ribbons 12-26 may be formed from a layer of silicon nitride using an etching process such that the ribbons 12-26 are suspended above the substrate 30. In particular, a gap may separate the ribbons 12-26 from the substrate 30.
Each of the ribbons 12-26 may include a reflective coating, such as an aluminum coating, on the top surface visible in FIG. 1. The substrate 30 may include a conductive material beneath all of the ribbons 12-26 such that a voltage difference may be applied between the ribbons 12-26 and the substrate 30. Further, the reflective coating on the ribbons 12-26 may be conductive such that a voltage difference may be applied between the ribbons 12-26 and the corresponding locations on the substrate 30.
A first group of ribbons may begin with ribbon 12 and include every second or alternate ribbon below it, namely ribbons 16, 20 and 24. For purposes of convenience, the ribbons of the first group will be referred to herein as “bias ribbons.” A second group of ribbons may begin with ribbon 14 and include every second or alternate ribbon below it, namely ribbons 18, 22 and 26. For purposes of convenience, the ribbons of the second group will be referred to herein as “active ribbons.”
The bias ribbons may be electrically connected to, and commonly controlled by, a DAC 32. The active ribbons may each be electrically connected to, and controlled by, a dedicated DAC. In particular, ribbons 14, 18, 22 and 26 are individually controlled by DACs 34, 36, 38 and 40, respectively. The DACs 32-40 may accept input values corresponding to a 16-bit architecture, such that the input values may have a range between 0 and 65535. In response to an input value, each of the DACs 32-40 may produce an output voltage which is applied to the ribbon or ribbons controlled by it. It will be further appreciated that the DACs 32-40 may be considered control devices as they control the amount of deflection of each of the ribbon or ribbons to which they are connected.
The ribbons 12-26 may be subdivided into separately controllable picture elements, or pixels. As used herein, the term “pixel” may refer to a combination of micro-electro-mechanical (“MEMS”) elements on a light modulation device that are able to modulate incident light to form a corresponding display pixel on a viewing surface. (The term “display pixel” referring to a spot of light on a viewing surface that forms part of a perceived image.) Each of the pixels on a light modulation device may determine, for example, the light intensity of one or more parts of an image projected onto a display. In a display system using a scan-based architecture, a pixel on a light modulation device may be responsible for forming an entire linear element of an image across a display, such as a row.
Each of the pixels on the light modulation device 10 may comprise, at a minimum, one bias ribbon and an adjacent active ribbon. In FIG. 1, then, the ribbons 12 and 14 form Pixel A, the ribbons 16 and 18 form Pixel B, the ribbons 20 and 22 form Pixel C, and the ribbons 24 and 26 form Pixel D. It will be appreciated that the number of pixels of the light modulation device 10 is exemplary only, and that, in an actual application, the number of pixels on the light modulation device 10 may exceed several hundred, or even several thousand, to obtain the desired resolution of the displayed image. In addition, it will be appreciated that a pixel may comprise more than one bias ribbon and more than one active ribbon.
During operation, a common bias voltage is applied, and maintained, between the bias ribbons and the substrate 30 by the DAC 32. The appropriate active ribbon of each of the pixels may then be individually controlled to thereby determine a light intensity output. As previously discussed, incident light will be reflected from a pixel when the reflective surfaces of its constituent bias and active ribbons are both co-planar. In a display system that blocks reflected light, a pixel's light intensity output will be at a minimum value, sometimes referred to herein as a “dark state,” when the reflective surfaces of its constituent bias and active ribbons are co-planar.
A pixel's light intensity output may be increased from its dark state by deforming the pixel's active ribbon from its co-planar relationship with the bias ribbon. It has been previously determined that the maximum light intensity output for a pixel will occur in a diffraction based system when the distance between the reflective surfaces of the bias ribbon and the active ribbon is λ/4, where λ is the wavelength of the light incident on the pixel. Intermediate light intensity outputs may be achieved by varying the distance between the reflective surfaces of the bias ribbon and the active ribbon in a range from 0, i.e., co-planar, to λ/4.
Calibration of the pixels of the light modulation device 10 according to the present disclosure may be broken down into a dark-state calibration and a bright-state calibration. One purpose of the dark-state calibration is to determine each active ribbon's DAC input value that will result in the minimum light intensity output for each pixel. One purpose of the bright-state calibration is to be able to accurately predict a light intensity output for each pixel for any given DAC input value.
Referring now to FIG. 2, there is depicted a detection device 50 for use in calibrating the light modulation device 10 (FIG. 1). The detection device 50 may include a support structure 52 and a mounting base 53. Mounted to the support structure 52 may be a stepper motor 54 having an output shaft 56. A moveable stage 58 may be mounted to the output shaft 56 of the stepper motor 54. The stage 58 may move up and down along the shaft 56 of the stepper motor 54. Mounted to the stage 58 is a reflective surface 60 for directing incoming light onto a photodetector 62. A slit (not visible) in front of the photodetector 62 may only allow light from a predetermined number of pixels to hit the photodetector 62 at any given time.
In one embodiment of the present disclosure, the slit is approximately 200 μm and may allow light from approximately 30 to 80 pixels to hit the photodetector 62 at a given time. The detection device 50 is placed in the path of diffracted light from the light modulation device 10 such that the stage 58 may accurately center light from any given pixel onto the photodetector 62. The stepper motor 54 may move the stage 58 along the shaft 56 as needed to calibrate any pixel of the light modulation device 10. In particular, the stepper motor 54 positions the photodetector 62 in an optical output path of a desired pixel.
An output signal from the photodetector 62 is received by a lock-in amplifier circuit (not explicitly labeled). The lock-in amplifier circuit may work at a frequency of approximately 10 KHz to filter out any unwanted noise, as is known to one of ordinary skill in the art. In particular, a pixel being calibrated may have its active ribbon toggled between the desired DAC input value and a reference DAC value of 0 (or a DAC input value that makes the pixel's output as dark as possible) at a frequency of 10 KHz. The lock-in amplifier is operable to measure the amplitude of this 10 KHz signal, which happens to be the light intensity corresponding to the input DAC value. When the DAC toggles the active ribbon of a pixel from the reference value of 0 to the desired DAC value, the photodetector 62 measures the intensity of the pixel at the desired DAC value along with the dark state intensity of the other pixels whose light is not filtered by the slit. However, since the lock-in amplifier only measures changes having a frequency of 10 KHz, the resulting signal is the difference in intensity between the desired DAC value and the reference value. It will be appreciated that the intensity from the other pixels whose light is allowed to pass through the slit is filtered out along with any other noise that is not related to the toggling of the pixel being measured since none of the ribbons of the other pixels are being toggled. It will be further appreciated that the use of a lock-in amplifier allows the intensity of a desired pixel to be measured without having to mechanically single out the desired pixel from the other pixels whose light is allowed to pass through the slit in front of the photodetector 62.
Still referring to FIG. 2, the first step to calibrate the light modulation device 10 (as represented in FIG. 1) is to place the detection device 50 into the diffracted light path from the light modulation device 10. This may be at a point to capture an intermediate image. The next step is to relate the position of each of the pixels of the light modulation device 10 with the position of the stepper motor 54 by briefly toggling the pixels one by one while moving the stage 58 through the beam of diffracted light. This step allows the photodetector 62 to be accurately centered up with each pixel on the light modulation device 10.
In an embodiment of the present disclosure, the position of each of the pixels of the light modulation device 10 in relation to the position of the stepper motor 54 may be determined by toggling less than all of the pixels and then determining the position of the other pixels by liner interpolation. Once the above recited steps are complete, each of the Pixels A-D (as represented in FIG. 1) may be calibrated for a dark state and a bright state as will be described below. In an embodiment of the present disclosure, not all of the Pixels A-D are calibrated and their dark state may be found through mathematical calculation (linear interpolation).
Dark-State Calibration
Referring now to FIG. 3, there is shown the ribbons 12-26 (which are also represented in FIG. 1) in an uncalibrated and undeflected state above the substrate 30. The ribbons 12-26 are held in this uncalibrated and undeflected state due to the natural tensile strength of the ribbons 12-26 and due to differences in DAC offset voltages. It will be noted that the bias ribbons 12 and 20 are positioned above their adjacent active ribbons 14 and 22, respectively, while the bias ribbons 16 and 24 are positioned below their adjacent active ribbons 18 and 26, respectively.
The first step of the dark-state calibration method according to the present disclosure is to apply a common bias voltage to all of the bias ribbons 12, 16, 20 and 24 such that each of them is deflected to a common biased position as shown in FIG. 4. The common biased position is characterized by the fact that it is below the reflective surfaces of all of the active ribbons 14, 18, 22 and 24. It will be noted that the bias ribbons 12, 14, 20, and 24 are maintained at the common biased position during calibration and operation of the light modulation device 10. Once the bias ribbons 12, 16, 20 and 24 have been deflected to the common biased position, a dark state for each pixel can then be determined. In an embodiment of the present disclosure, the position of each of the bias ribbons 12, 14, 20, and 24 when deflected to the common biased position may be slightly different.
The dark-state calibration of Pixel A, comprising the bias ribbon 12 and the active ribbon 14, will now be described. Again, the purpose of the dark-state calibration is to determine the input value for DAC 34 (FIG. 1) at which the active ribbon 14 is deflected in an amount such that the reflective surfaces of the bias ribbon 12, at the common bias position, and the active ribbon 14 are substantially co-planar. To find the input value for DAC 34 that produces the minimum intensity or dark state of Pixel A, the intensity output of the Pixel A is measured at several predetermined input values for the DAC 34 using the detection device 50 (FIG. 2).
As the input values for the DAC 34 are successively increased, the light output intensity of the Pixel A will decrease up until the point that the reflective surface of the active ribbon 14 is co-planar with the reflective surface of the bias ribbon 12. As the input values for the DAC 34 are increased past the input value at which the active ribbon 14 and the bias ribbon 12 are co-planar, the intensity of the Pixel A will begin increasing again since the active ribbon 14 will be deflected past the bias ribbon 12.
The predetermined input values for the DAC 34 and the corresponding light intensity outputs of the Pixel A may form a set of data points that may be graphed as shown in FIG. 5, where the input values for the DAC 34 are plotted along the x-axis and their corresponding intensity output levels are plotted along the y-axis. Using the data points in the graph shown in FIG. 5, any suitable curve fitting technique may be employed to find a curve that has the best fit to the data points.
In an embodiment of the present disclosure, a 4th order polynomial curve fit may be performed using the data points to create a curve that describes the intensity response of Pixel A with respect to the input values. This 4th order polynomial may take the form of Equation 1,

ID(V)=AV4+BV3+CV2+DV+E 

where ID(V) is equal to the light output intensity of Pixel A determined experimentally and V is equal to the voltage applied to the active ribbon 14 by DAC 34. (In order to use Equation 1, it is assumed that DAC 34 has a linear response so that one can easily convert the DAC input value to voltage or from voltage to the DAC input value.) The unknowns of Equation 1, namely variables A, B, C, D, and E, may be found using any suitable technique. In an embodiment of the present disclosure, the unknown variables A, B, C, D, and E may be determined by using the method of least squares. The resulting equation determined from the data points on the graph shown in FIG. 5 is sometimes referred to herein as the “dark-state equation” of Pixel A.
Once determined, the dark-state equation for Pixel A may then be used to determine the input value for the DAC 34 that produces the minimum intensity or dark state for the Pixel A. This point is where the intensity of the Pixel A is at a minimum as seen on the graph in FIG. 5. Thus, to reproduce the dark state of the Pixel A during operation of the light modulation device 10, one simply sets the input value to DAC 34 that corresponds to the minimum intensity as determined by the dark-state curve and the dark-state equation. The above described dark-state calibration process is then repeated individually for each of the remaining Pixels B, C and D of the light modulation device 10. Thus, each pixel on the light modulation device 10 will have its own unique dark-state curve and corresponding dark-state equation.
The dark-state calibration process pursuant to the present disclosure may start with the topmost pixel on the light modulation device 10, i.e., Pixel A, and continue in a sequential order until the bottommost pixel on the light modulation device 10, i.e., Pixel D, is calibrated. After a pixel's dark state has been determined through the above described process, the pixel should be left in this dark state while the other pixels on the light modulation device 10 are being calibrated. In this manner, all of the neighboring pixels above the pixel actually being calibrated are at their best available dark state.
For those pixels below the pixel being calibrated on the light modulation device 10, they may be set to their best known dark-states if such data is available. If no such data is available, then an estimated dark-state value may be used. The estimated dark-state value may be determined by performing a dark-state calibration on a group of neighboring and uncalibrated pixels below the pixel actually being calibrated. This group dark-state calibration involves moving all of the active ribbons of the group of neighboring and uncalibrated pixels at the same time and determining an estimated DAC input value that will result in a minimum intensity of the group as a whole. Once determined, each of the DACs of the active ribbons in the group of uncalibrated pixels is set to this estimated DAC input value.
The group of neighboring and uncalibrated pixels may comprise about 80 pixels beneath the pixel actually being calibrated. This group calibration may be repeated about every 20 pixels so that there are always at least 60 pixels below the pixel actually being calibrated that are set to the estimated DAC input value that produces a minimum intensity for the group as a whole. It will be appreciated that the use of the group dark-state estimation of the neighboring and uncalibrated pixels as explained above allows for a better solution than if the active ribbons of the neighboring and uncalibrated pixels were left at arbitrary positions.
Further, due to the fact that a pixel's own dark-state calibration may be affected by the subsequent dark-state calibration of adjacent pixels, the above described calibration process may need to be repeated at least twice for the Pixels A-D on the light modulation device 10 using an iterative calibration process. The end result of the dark-state calibration process should allow the active ribbon and bias ribbon of each pixel to be positioned such that they are substantially co-planar as shown in FIG. 6 using the appropriate input value as determined by the pixel's dark-state curve and dark-state equation. It will therefore be appreciated that a dark-state curve fitting process is undertaken for the light modulation device 10 on a pixel-by-pixel basis.
In addition to predicting a DAC input value that produces a minimum light intensity output for each pixel, each pixel's dark-state equation may also be used to predict a light intensity output of the pixel for any DAC input value that falls near the DAC input value that produces the minimum light intensity output for that pixel. Typically, the dark-state equation is used to predict a pixel's intensity output for input values falling in the lower end of the full range of acceptable DAC input values. For example, the dark-state equation may be used for DAC input values falling in a range between 0 and X, where X is a predetermined upper limit for using the dark-state equation.
The exact DAC input value chosen for X is dictated by the dark-state curve. The DAC input value chosen for X must be past the DAC input value that produces the minimum light intensity output or dark state. Also, the DAC input value of X must produce an intensity output that is bright enough that an accurate measurement can be obtained when measuring the bright state with low gains as will be described hereinafter. In a system using a 16-bit architecture, an acceptable value for X has experimentally been determined to be about 20,000. For DAC input values above X, a bright-state equation may be used instead of a dark-state equation as explained below.
Bright-State Calibration
The bright-state calibration according to the present disclosure may be based upon the electro-optic response for a ribbon, which can be modeled by the following Equation 2,




  
    
      I
      B
    
    ⁡
    
      (
      V
      )
    
  
  =
  
    C
    (
    
      
        
          sin
          2
        
        (
        
          
            -
            
              
                2
                ⁢
                π
              
              λ
            
          
          *
          0.4
          ⁢
          
            
              y
              0
            
            [
            
              
                
                  [
                  
                    1
                    -
                    
                      
                        (
                        
                          
                            
                              (
                              
                                V
                                *
                                
                                  V
                                  gain
                                
                              
                              )
                            
                            -
                            
                              V
                              offset
                            
                            -
                            
                              V
                              BC
                            
                          
                          
                            V
                            2
                          
                        
                        )
                      
                      2
                    
                  
                  ]
                
                0.44
              
              -
              1
            
            ]
          
        
        )
      
      +
      
        I
        Offset
      
    
    )
  




where IB(V) is the intensity of a pixel whose active ribbon is at voltage V; V is the voltage applied to the active ribbon of the pixel; λ is the wavelength of light incident on the pixel, VBC is the voltage difference between the bias ribbons and the substrate (common); Vgain is used to account for the fact that the precise value of V is unknown; Voffset is the offset voltage of the active ribbon; Ioffset is simply a variable to shift the curve created by Equation 1 up or down; V2 is the snap-down voltage of the ribbons; and C is a maximum intensity of the pixel. The other variable, y0, is a fitting parameter.

The variables IB(V), V, λ, and VBC are the known variables of Equation 2. In particular, IB(V) can be determined experimentally using the detection device 50. Although V is not known precisely, it can be estimated based upon the DAC input value (0-65535 for a 16-bit system) and based upon the assumption that the output voltage, V, is a linear ramp corresponding to the input values. λ is the wavelength of the source light and VBC is programmed via the DAC 32 for the bias ribbons. Equation 2, therefore, has six unknowns, namely, C, y0, Vgain, Voffset, V2, and Ioffset.
To determine the unknown variables of Equation 2 for a given pixel, say Pixel A, a bright-state curve, such as the one shown in FIG. 7, is built by measuring the intensity output, IB(V), for a set of predetermined DAC input values. The predetermined DAC input values may range from approximately X, the upper limit of the range for the dark-state equation, to the maximum DAC input value for the Pixel A, e.g., 65535 in a 16-bit system. Once these data points have been measured, any suitable mathematical technique may be utilized to solve for the unknowns in Equation 2 to determine a unique bright-state equation for the Pixel A.
In an embodiment of the present disclosure, a Levenberg-Marquardt type algorithm, or any other iterative algorithm, may be utilized to solve for the unknowns in Equation 2. Suitable starting values of the unknown variables of Equation 2 have been found to be as follows: C=Maximum intensity of the measured data points; y0=600; Vgain=1.0; Voffset=0.5; V2=15; and Ioffset=0. Once the unknowns of Equation 2 have been determined for Pixel A, Equation 2 may be utilized to predict the intensity output for any given DAC input value from X to the maximum DAC input value. It will be appreciated that a unique bright-state equation, and bright-state curve, is determined for each of the Pixels A-D on the light modulation device 10.
Combined Dark and Bright State Response
Once a bright-state equation and a dark-state equation have been determined for each of the Pixels A-D, the two equations, or curves, for each pixel can be combined such that the intensity output of the pixel can be predicted for any DAC input value. The process of combining the two equations first involves normalizing the dark-state equation for each pixel.
To normalize the dark-state equation for a given pixel, the minimum intensity of the pixel is set to a value of zero, and the intensity output at the DAC input value of X is normalized to a value of 1.0. This may be accomplished by first subtracting the minimum value of the dark state curve from the variable E to determine a new value, E′, (this will shift the minimum of the dark state curve to zero) and then dividing each of the values determined for variables A, B, C, D, and E′ of Equation 1 by ID(X) such that the resulting curve has a minimum intensity output of 0 and a maximum intensity of 1.0 at the DAC input value of X. To combine the dark-state and bright-state equations, the normalized values for variables A, B, C, D, and E′ are multiplied by the intensity of the bright-state curve at X as determined by IB(X).
As a result of the above described process for combining the dark-state and bright-state equations, there is a smooth transition between using the dark-state equation and the bright-state equation as shown in FIG. 8. In particular, when looking for an intensity for a DAC input value less than X, the dark-state equation is used and when looking for an intensity for a DAC input value greater than or equal to X, the bright-state equation is used. Thus, it will be appreciated that DAC input values less than X are for a first operating range of a pixel, while DAC input values greater that X are for a second operation range of the pixel.
Referring now to FIG. 9, there is depicted an exemplary system 100 for calibrating a light modulation device 102. A light modulation device 102 may include a plurality of ribbons, both bias ribbons and active ribbons, which are used to form a plurality of pixels. The system 100 may further include a computing device 104. The computing device 104 may include a computer memory device 105 configured to store computer readable instructions in the form of an operating system 107 and calibration software 106. In an embodiment of the present disclosure, the operating system 107 may be Windows XP®. The processor 109 may be configured to execute the computer readable instructions in the memory device 105, including the operating system 107 and the calibration software 106. The execution of the calibration software 106 by the processor may calibrate the light modulation device 102 using any process described above and that will be more fully described in relation to FIG. 10.
Referring now primarily to FIG. 10, the computing device 104 may be in communication with projector control electronics 108. The projector control electronics 108 may include a pair of field programmable gate arrays 110 and 112. The projector control electronics 108 may further include a lock-in amplifier 114 and a programmable gain circuitry 116. The projector control electronics 108 may further control a light source 126, such as a laser. The light source 126 may provide incident light onto the light modulation device 102. A detection device 118 may include a control board 120, a photodetector 122, and a stepper motor 124. The control board 120 may receive instructions from gate array 110. The control board 120 may send data collected by the photodetector 122 to the programmable gain circuitry 116.
The light modulation device 102 may include a plurality of ribbons having a first group of ribbons, i.e., bias ribbons, and a second group of ribbons, i.e., active ribbons. The first group of ribbons may be commonly controlled by a single DAC. The second group of ribbons may each be individually addressable and controlled by a single DAC. At least one ribbon from the first group and at least one ribbon from the second group may form a pixel on the light modulation device 102. It will be appreciated that the computing device 104 and the projector control electronics 108 may constitute a control device for positioning the first elongated elements of each of the pixels on the light modulation device 102 to a common biased position and for toggling the second elongated elements of each of the pixels one-by-one at a predetermined frequency such that a light intensity response for each of the pixels may be determined. It will be appreciated that as used herein, the term “light intensity response” may mean any information, mapping or data that allows a display system to determine one or more input values or settings for a pixel from the image source data. The image source data may include, for example, data encoding in a predetermined format for a picture, graphic, or video. The term “light intensity response” may further mean any set of data that includes the intensity output of a pixel based upon one or more predetermined input values or settings for the pixel. In this case, the intensity output may be determined experimentally. The processor 109 may determine the light intensity response for each of the pixels, including a bright state response and a dark state response. The processor 109 may also determine an input value for the active ribbon of each of the plurality of pixels at which the bias ribbon and the active ribbon are substantially planar.
Referring now to FIGS. 9 and 10, a flow diagram 150 is shown for calibrating the pixels of the light modulation device 102 using the system 100. The flow diagram 150 may be implemented by the calibration software 106 in the memory device 105. At step 152, the lock-in amplifier 114 is initialized by shifting the phase of its 10 KHz reference wave to match the phase of the 10 KHz toggling signal coming from the photodetector 122. At step 154, the position of the stepper motor 124 is calibrated to locate any given pixel on the light modulation device 102. At step 156, the programmable gains for the programmable gain circuitry 116 are determined by using a single pixel located in the middle of the light modulation device 102. The programmable gains may include dark state gains and bright state gains. Typically, the dark state gains will be high so as to be able to detect low levels of light, while the bright state gains are low so as not to saturate the lock-in amplifier 114.
At step 158, the programmable gain circuitry 116 is set to the dark state gains. At step 160, the dark state curve or equation for each of the pixels is determined on a pixel-by-pixel basis as described above. At step 162, the dark state curve or equation for each pixel is normalized and stored in computer memory. At step 164, the programmable gain circuitry 116 is set to the bright state gains. At step 166, the bright state curve or equation for each of the pixels is determined on a pixel-by-pixel basis. At step 168, the bright state curve or equation for each pixel is stored in a computer memory. At step 170, a look-up table for each pixel is constructed using the pixel's normalized dark state curve or equation and its corresponding bright state curve or equation. This may take the form of the table disclosed in U.S. Patent Publication No. 2008/0055618 (application Ser. No. 11/514,569), which is now hereby incorporated by reference in its entirety. The processor 109 may be operable to generate the look-up table for each of the pixels from their respective bright state curve or equation and dark state curve or equation.
From time to time, it may be necessary to re-normalize the bright state curve or equation determined at step 166 as shown at step 172. This may be required due to degradations or other changes in the amount of illumination produced by the projection lasers of the projection system. At step 174, the programmable gain circuitry 116 is set to the bright state gains. At step 176, a curve multiplier is determined for each pixel and the bright state curve or equation of each pixel found at step 166 is multiplied by this curve modifier. This may be accomplished by measuring a single intensity and then re-normalizing the previous bright state curve to this new intensity. It will be appreciated that this allows a system to be quickly re-calibrated to account for illumination changes. At step 178 the re-normalized bright state curve or equation is saved for each pixel in a computer memory. At step 180, a new look-up table for each pixel is constructed.
In the foregoing Detailed Description, various features of the present disclosure are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed disclosure requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description by this reference, with each claim standing on its own as a separate embodiment of the present disclosure.
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present disclosure and the appended claims are intended to cover such modifications and arrangements. Thus, while the present disclosure has been shown in the drawings and described above with particularity and detail, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, variations in size, materials, shape, form, function and manner of operation, assembly and use may be made without departing from the principles and concepts set forth herein.

## Claims

1. A method of calibrating a plurality of pixels of a light modulation device, each of said pixels comprising a first elongated element and a second elongated element, comprising:
applying a voltage to the first elongated elements of each of the plurality of pixels such that they are deflected to a common biased position; and
determining a light intensity response for each of the plurality of pixels, pixel-by-pixel, using a photodetector while said first elongated elements are held at the common biased position using a processing device.

2. The method of claim 1, wherein said common biased position resides below the second elongated elements in an undeflected state.

3. The method of claim 1, wherein determining a light intensity response for each of the plurality of pixels, pixel-by-pixel, further comprises:
selecting one of the plurality of pixels for calibration;
toggling the second elongated element of the pixel selected for calibration;
capturing light reflected off of the plurality of pixels using the photodetector;
generating a signal using the photodetector based upon the captured light, the signal comprising a first portion corresponding to the pixel selected for calibration and a second portion corresponding to the pixels not selected for calibration;
filtering the signal to thereby remove the second portion of the signal; and
using the first portion of the signal to determine a light intensity response for the pixel selected for calibration.

4. The method of claim 3, further comprising toggling the second elongated element of the pixel selected for calibration at a predetermined frequency.

5. The method of claim 1, further comprising determining an input value for the second elongated element of each of the plurality of pixels at which the first elongated element and the second elongated element of that pixel are substantially planar.

6. The method of claim 1, further comprising toggling the second elongated element of each of the plurality of pixels between one of a plurality of discrete positions and a reference position, and measuring a light intensity output for the pixel at each of the plurality of discrete positions.

7. The method of claim 6, further comprising determining from the measured light intensity output, an input value for the second elongated element of each of the plurality of pixels at which the light intensity output is at a minimum.

8. The method of claim 6, further comprising using the measured light intensity output in a polynomial curve fit to thereby determine an input value for the second elongated element of each of the plurality of pixels at which the light intensity output is at a minimum.

9. The method of claim 1, further comprising determining a light intensity response for each of the plurality of pixels in a sequential order.

10. The method of claim 1, further comprising positioning the second elongated elements of a group of uncalibrated pixels to an estimated minimum intensity position while determining the light intensity response of a pixel.

11. The method of claim 1, further comprising toggling the second elongated element of a pixel at a predetermined frequency.

12. The method of claim 1, further comprising determining the light intensity response for each pixel using a lock-in amplifier.

13. The method of claim 1, wherein determining a light intensity response for each of the plurality of pixels, pixel-by-pixel, further comprises determining a first light intensity response for a first operating range of each pixel and a second light intensity response for a second operating range of each pixel.

14. The method of claim 13, further comprising generating a look-up table from the first light intensity response and the second light intensity response for each pixel.

15. The method of claim 13, wherein said second light intensity response is for a state brighter than said first light intensity response.

16. The method of claim 1, further comprising generating a look-up table for each of the plurality of pixels.

17. A system for calibrating a plurality of pixels of a light modulation device, each of said pixels comprising a first elongated element and a second elongated element, said system comprising:
at least one light source;
a photodetector for measuring a light intensity output of each of the plurality of pixels;
a control device for positioning said first elongated elements to a common biased position;
said control device further operable to toggle the second elongated elements of the plurality of pixels, one-by-one, while the first elongated elements are positioned at the common biased position; and
a processing device for determining a light intensity response for each the plurality of pixels on a pixel-by-pixel basis.

18. The system of claim 17, wherein said control device is further operable for positioning said photodetector in an optical output path of each of the plurality of pixels.

19. The system of claim 17, further comprising a lock-in amplifier for isolating a light intensity output of a single pixel on the light modulation device.

20. The system of claim 17, wherein said common biased position resides below the second elongated elements of the plurality of pixels in an undeflected state.

21. The system of claim 17, wherein said control device is further operable to toggle each of the second elongated elements at a predetermined frequency.

22. The system of claim 17, wherein said processing device is further operable to determine an input value for the second elongated element of each of the plurality of pixels at which the first elongated element and the second elongated element are substantially planar.

23. The system of claim 17, wherein said light intensity response for each pixel comprises a first light intensity response for a first operating range of the pixel and a second light intensity response for a second operating range of the pixel.

24. The system of claim 23, wherein said second light intensity response is for a state brighter than said first light intensity response.

25. The system of claim 17, wherein said processing device is further operable to generate a look-up table for each of the plurality of pixels.

26. A system for calibrating a plurality of pixels of a light modulation device, each of said pixels comprising a first elongated element and a second elongated element, said system comprising:
means for deflecting a first group of elongated elements to a common biased position; and
means for determining a light intensity response for the plurality of pixels on a pixel-by-pixel basis while said first elongated elements are held at the common biased position using a processing device.

27. The system of claim 26, further comprising means for toggling the second elongated elements at a predetermined frequency.

28. The system of claim 26, further comprising means for measuring a light intensity output of each of the plurality of pixels.

29. The system of claim 26, further comprising means for isolating a light intensity output of a single pixel on the light modulation device.

30. The system of claim 26, wherein said light intensity response for each of the plurality of pixels comprises a first light intensity response for a first operating range of the pixel and a second light intensity response for a second operating range of the pixel.

31. The system of claim 30, wherein said second light intensity response is for a state brighter than said first light intensity response.

32. The system of claim 26, further comprising means for generating a look-up table for each of the pixels.

33. The system of claim 26, further comprising means for generating incident light onto the light modulation device.

34. A non-transitory computer readable medium for storing computer instructions that, when executed on a computer, enable a processor-based system to:
deflect a first group of elongated elements on a light modulation device to a common biased position; and
determine a light intensity response for the plurality of pixels on a pixel-by-pixel basis while the first elongated elements are held at the common biased position.