JP2008500667A - Optical position detection device with shaped illumination - Google Patents

Abstract

One embodiment relates to an optical displacement sensor for detecting relative movement between the data input device and the surface (304) by determining the displacement of the optical features in successive frames. The sensor includes an illuminator and a detector. The illuminator includes a light source and an illumination optical system (506) for illuminating a part of the surface (510) having a flat phase plane. The detector includes a plurality of photosensitive elements (502) and an imaging optical system (512). The illuminator and detector are configured so that the illuminated portion of the surface (510) is smaller than one that is 50% larger than the field of view of the photosensitive element (502) of the detector. Other embodiments are also described.

Description

Cross-reference to related applications This application is a "Optical position sensing device having" filed on May 21, 2004, filed on May 21, 2004, invented by Clinton B. Carlisle, Jahja I. Trisnadi, Charles B. Roxlo and David A. LeHoty. claim the benefit of US Provisional Application No. 60 / 573,394, entitled "shaped illumination,". The entire disclosure of this US provisional application is incorporated herein by reference.

  This application is also referred to as “Optical position sensing device having” filed on May 21, 2004, invented by David A. LeHoty, Douglas A. Webb, Charles B. Roxlo, Clinton B. Carlisle and Jahja I. Trisnadi. No. 60 / 573,075, entitled “a detector array using different combinations of shared interlaced photosensitibve elements,”. The entire disclosure of this US provisional application is incorporated herein by reference.

  The present invention relates generally to an optical positioning device (OPD) and a method for detecting motion using the device.

  A pointing device such as a computer mouse or a trackball is used to input data to a personal computer or a workstation or to interface with these devices. Such devices allow the cursor to be quickly repositioned on the monitor and are useful in many text, database and graphical programs. For example, the user moves the mouse on the surface and controls the cursor so that the cursor moves in a certain direction by a distance proportional to the amount of movement of the mouse. Alternatively, moving the hand over a stationary device can be used for the same purpose.

  Computer mice are available in optical and mechanical versions. A mechanical mouse typically uses a rotating ball to detect movement and a pair of shaft encoders in contact with the ball to generate digital signals that are used by the computer to move the cursor. One problem with mechanical mice is that they are prone to inaccuracies and failures, such as dust build-up due to continued use. Furthermore, the movement of the mechanical elements, in particular the shaft encoder and the resulting wear, inevitably shortens the useful life of the device.

  One solution to the mechanical mouse problem described above was the development of an optical mouse. Optical mice have become very popular because they are more robust and can provide more accurate positioning accuracy.

  The core prior art used in optical mice is a series of light emitting diodes (LEDs) that illuminate the surface with grazing incidence, a two-dimensional CMOS (complementary metal oxide semiconductor) detector that captures the resulting image Software that obtains the direction, distance, and speed of mouse movement by correlating images is used. This technique generally provides good accuracy, but has low optical efficiency and relatively high image processing requirements.

  Another approach uses a one-dimensional array of photosensors or detectors such as photodiodes. A continuous image of the surface is imaged by an imaging device, converted by a photodiode, and compared to detect mouse movement. In some cases, photodiodes are directly connected to form a group in order to facilitate detection of movement. This alleviates the photodiode requirements and allows for rapid analog processing. One example of such a mouse is disclosed in US Pat. No. 5,907,152 by Dandliker et al.

The mouse disclosed in the above patent by Dandliker et al. Also differs from standard techniques in that it uses a coherent light source such as a laser. Light from a coherent light source scattered from a rough surface produces a random intensity distribution of light known as speckle. The use of speckle-based patterns has several advantages, such as efficient laser-based light generation and high image contrast even when the illumination is incident at a right angle. This results in a more efficient system and saves current consumption, which is advantageous for extending battery life in wireless applications.
U.S. Pat.No. 5,907,152

  These speckle-based (ie, utilizing speckle patterns) devices offer significant improvements over traditional LED-based optical mice, but are completely satisfactory for several reasons It's not a success. Specifically, mice that use laser speckle are typically required for today's highest level mice-this accuracy is typically less than 0.5% or about 0.5% It is not desirable to have a smaller path error.

  The present disclosure describes and provides a solution to several problems with conventional optical mice and other similar optical pointing devices.

  One embodiment relates to an optical displacement sensor for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames. The sensor includes at least an illuminator and a detector. The illuminator has a light source and illumination optical system for illuminating a part of the surface. The detector has a plurality of photosensitive elements and an imaging optical system. The illuminator and detector are configured such that the illuminated portion of the surface is smaller than one that is 50% larger than the field of view of the photosensitive element of the detector.

  Another embodiment relates to a method for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames. Illumination is generated from the light source, and the illumination is mapped to a portion of the surface (positioned on a portion of the surface) by the illumination optics. Illumination is reflected from the illuminated portion of the surface, and the reflected illumination is mapped by the imaging optics to an array of photosensitive elements in the detector. The illuminated portion of the surface is smaller than one that is 50% larger than the field of view of the photosensitive element.

  Another embodiment relates to an optical displacement sensor for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames. The sensor includes at least a light source, an illumination optical system, an array of photosensitive elements, and an imaging optical system. The illumination optical system is configured to illuminate a portion of the surface with a first shape, and the array of photosensitive elements has a second shape similar to the first shape. The imaging optics is configured to map the reflected illumination such that the illumination reflected from the illuminated portion of the surface covers the array of photosensitive elements.

  Other embodiments are also disclosed.

  The above and various other features and advantages of the present invention will be more fully understood from the following detailed description and the accompanying drawings. However, the detailed description and accompanying drawings are not intended to limit the scope of the claims to the specific embodiments shown, but are presented for the sake of explanation and understanding only.

Problems with Illumination Misalignment and Inefficiency One problem with conventional speckle-based optical positioning devices is that reflected illumination (or illumination light) and detection to cover the entire detector photodiode array. There is a possibility that misalignment may occur in the alignment with the container. In order to reliably cover the entire detector array, conventional OPDs typically illuminate a portion of the image plane that is much larger than the detector field of view, which allows the photodiode array to address possible misalignment problems. Nevertheless, it is configured to be surely sufficiently covered by the reflected illumination (hereinafter also referred to as reflected illumination).

  However, when the illumination area is increased, the power intensity of the reflected illumination detected by the photodiode is reduced. Therefore, trying to eliminate or avoid the misalignment problem in conventional OPDs often results in a loss of reflected light available to the photodiode array, ie imposes higher requirements on illumination power It will be.

  As described in detail below, one aspect of the present invention discloses a solution to the above problem of inefficiency and inefficiency in illumination.

Embodiments of OPD Disclosed in This Specification This disclosure relates generally to sensors for optical positioning devices (OPDs, also referred to as optical positioning devices), and to randomness of light reflected from a surface, known as speckle. The present invention relates to a method for detecting relative movement between a sensor and a surface based on a displacement (or movement, hereinafter the same) of a strong intensity distribution pattern. OPD includes, but is not limited to, an optical mouse or trackball for inputting data into a personal computer.

  Although the term “one embodiment” or “one embodiment” is used herein, the use of these terms may imply that certain features, structures, or characteristics described in connection with the embodiments are It is meant to be included in at least one embodiment of the invention. The terms “in one embodiment” found in various places in the specification are not necessarily all referring to the same embodiment.

  In general, a sensor for OPD includes a light source for illuminating a part of a surface and an illuminator having an illumination optical system, a detector having a plurality of photosensitive elements and an imaging optical system, and each of the photosensitive elements. Signal processing electronics or mixed-signal electronics for combining the signals from and generating an output signal from the detector.

  In one embodiment, the detector and mixed signal electronics are fabricated using standard CMOS processes and equipment. Preferably, the sensor and method of the present invention are structured illumination that produces a uniform phase plane and telecentric speckle imaging, and simple signals using a combination of analog and digital electronics. By using a processing arrangement, an optically efficient detection architecture is provided. This architecture reduces the amount of power allocated for signal processing and displacement prediction within the sensor. Using speckle detection technology and properly configured in accordance with the present invention, all performance criteria generally expected for OPD—maximum displacement speed (maximum travel speed), accuracy, and path error rate (%) It has been found that-can be met or exceeded.

Introduction to Speckle-Based Displacement Sensors This section describes the operating principles of speckle-based displacement sensors as understood by the applicant. While these operating principles are useful for understanding, it is not intended that the disclosed embodiments be more limited than necessary by those principles.

  Referring to FIG. 1A, laser light of the indicated wavelength is shown as a first incident wave 102 and a second incident wave 104 on the surface. Each incident wave forms an incident angle θ with respect to the surface normal. A diffraction pattern 106 having a periodicity of λ / 2sinθ is generated.

  In contrast, with reference to FIG. 1B, any common surface having a concavo-convex shape with dimensions larger than the wavelength of light (greater than approximately 1 μm) will be completely in a Lambertian fashion. There is a tendency to scatter light 114 into the hemisphere. If a coherent light source such as a laser is used, the spatially coherent scattered light will cause a complex interference pattern 116 when detected by a square-law detector with a finite aperture. Will be generated. This complex interference pattern 116 in the bright and dark areas is called speckle. The exact nature and contrast of the speckle pattern 116 depends on the roughness of the surface, the wavelength of the light and its degree of spatial coherence, and the light collection or imaging optics. Often, although very complex, the speckle pattern 116 is a distinct feature of an area of any rough surface that is imaged by the optical system so that the position on the surface is a laser and optical system-detector. It can be used to identify its position when placed laterally (or transversely) to the assembly.

  Speckle is expected to appear in all magnitudes up to the spatial frequency set by the effective aperture of the optical system (which is conventionally defined using the numerical aperture NA = sin θ as shown in FIG. 1B). The "Statistical Properties of Laser Speckle" by JW Goodman in Goodman ("Laser Speckle and Related Phenomena" edited by JC Dainty (Topics in Applied Physics Vol. 9, Springer-Verlag (1984), especially see pages 39-40)) According to “Patterns”), the statistical distribution of size is represented using the autocorrelation of speckle intensity. The "average" speckle diameter,

Can be defined as λ is the wavelength of coherent light.

It is interesting to note that the spatial frequency spectral density of speckle intensity (which is due to the Wiener-Khintchine theorem) is simply the Fourier transform of the intensity autocorrelation . The finest speckles that can occur (represented by a _min = λ / 2NA) are unlikely to occur, ie, the main contribution is the extreme ray 118 of FIG. 1B (ie, the ray at ± θ). ) And the interference from the most “inner” ray is destructively interfering. The cut-off spatial frequency is therefore f _co = 1 / (λ / 2NA) or 2NA / λ).

  The numerical aperture is relative to the spatial frequency in the image along one dimension (or dimension, eg “x”) and the spatial frequency in the image along a dimension (or dimension, eg “y”) orthogonal to it. Note that may differ from that for. This can be caused, for example, by an optical aperture (eg, an ellipse rather than a circle) that is longer in one dimension than the other, or by an anamorphic lens. In these cases, the speckle pattern 116 is also anisotropic, and the average speckle size will be different between the two dimensions.

  One advantage of a laser speckle-based displacement sensor is that it can operate with illumination light that arrives at an angle of incidence close to the normal. A sensor that uses an imaging optical system and incoherent light (incoherent light) that reaches a rough surface at a grazing incidence angle can also be used to detect lateral displacement. However, this system is inherently optically inefficient because the grazing incidence angle of illumination is used to generate appropriately sized light-dark shadows of the surface area in the image. . This is because a significant proportion of the light is reflected from the detector as if it is reflected by a mirror and does not contribute to the image formed. In contrast, speckle-based displacement sensors can efficiently utilize a greater percentage of the illumination light from the laser source, which can lead to the development of optically efficient displacement sensors. It becomes possible.

Speckle Pace Displacement Sensor Configuration In the following detailed description, a CMOS photodiode with an analog signal coupling circuit, a moderate (medium) amount of digital signal processing circuitry, and, for example, a 850 nm surface emitting laser (Vertical The architecture of a laser speckle-based displacement sensor using a low power light source such as Cavity Surface Emitting Laser (VCSEL) will be described. Some implementation details are described in the following detailed description, but are intended to combine different light sources, detectors or photosensitive elements and / or signals without departing from the spirit and scope of the present invention. One skilled in the art will appreciate that different circuits can be used.

  A speckle-based mouse according to an embodiment of the present invention will be described below with reference to FIGS.

  FIG. 2 is a functional diagram of a speckle-based system 200 according to one embodiment of the invention. The system 200 includes a laser light source 202, an illumination optical system 204, an imaging optical system 208, at least two sets 210 of a plurality of CMOS photodiode arrays, a front-end electronic circuit 212, a signal processing circuit 214, and an interface circuit 216. Photodiode array 210 can be configured to provide displacement measurements along two orthogonal axes x and y. Groups of photodiodes in each array can be combined using passive electronic components in front-end electronics 212 to generate a group signal. The group signals can then be algebraically combined by the signal processing circuit 214 to generate a (x, y) signal that provides information regarding the magnitude and direction of the OPD displacement in the x and y directions. The (x, y) signal can be converted by the interface circuit 218 into x, y data 220 that can be output by the OPD. Sensors using this detector technology can have an array of interlaced groups of linear photodiodes, known as a “diffrential comb array”.

  FIG. 3 shows a schematic configuration (along one axis) of such a photodiode array 302. The surface 304 is illuminated by a coherent light source such as a surface emitting laser (VCSEL) 306 and illumination optics 308. The combination of interlaced groups in array 302 acts as a periodic filter for the spatial frequency of the light-dark signal generated by the speckle image.

  The speckle generated by the rough surface 304 is mapped to the detector surface (detection surface) by the imaging optical system 310. The imaging optical system 310 is preferably telecentric so as to obtain optimum performance.

  In one embodiment, comb array detection is performed on two independent orthogonal arrays to obtain a predicted value of displacement in x and y. A small version of one such array 302 is shown in FIG.

Each array in the detector consists of a set of multiple (N) photodiodes, each set comprising multiple (M) photodiodes (PD) arranged to form MN linear arrays. Have. In the embodiment shown in FIG. 3, each set consists of four photodiodes (four PDs) referred to as 1, 2, 3, 4. PD1 from all sets are electrically connected (wired sum) to form a group, and PD2, PD3, and PD4 are similarly drawn out of the four signal lines from the array. The corresponding currents or signals are I ₁ , I ₂ , I ₃ , I ₄ . These signals (I ₁ , I ₂ , I ₃ , I ₄ ) may be referred to as group signals. Background suppression (and signal enhancement) uses a differential analog circuit 312 to generate an in-phase differential current signal 314 (I ₁₃ ) = I ₁ −I ₃ and a differential analog circuit 316 to generate a quadrature ( (Or quadrature) achieved by generating a differential current signal 318 (I ₂₄ ) = I ₂ −I ₄ . These in-phase and quadrature signals may be referred to as line signals. By comparing the phases of I ₁₃ and I ₂₄ , the direction of movement can be detected.

  Preferably, the sensor of the present invention uses a plurality of comb arrays to reduce the introduction of phase errors that can be directly converted into displacement errors. Furthermore, although the embodiments described herein use a “4N” scheme for individual arrays, the principles of system configuration are applicable to other array configurations or schemes such as 3N, 5N, 6N, 7N, 8N (appropriate With minor changes). The term “4N” refers to a detector array in which all fourth detectors are wired together, and the resulting four photocurrent signals are as described in Dandliker et al. (US Pat. No. 5,907,152). Are subtracted from each other. However, many other groupings are possible for suitable schemes for combining signals.

An outline of an example of the imaging or light collection optical system 402 is shown in FIG. Light is scattered by a region _{A s} of the surface 404, it forms an image on the detector 406 of the region _{A det.} In the case of a Lambertian surface (also called a uniform diffusion surface), the collection efficiency of the detector is

Can be expressed as η _optics is the efficiency of the optical system components (absorption, Fresnel reflection, etc.), r is the effective surface reflectivity, Ω _s is the solid angle to the surface, and Ω _det is the solid angle to the detector.

Shaped illumination Applicants have noted that one means for maintaining good optical efficiency is the detector footprint in size and shape if the illumination beam has a flat or uniform phase plane. I think that it is custom-designing the lighting footprint so that it almost matches. More preferably, the lighting footprint has an extra size just enough to provide the desired margin for slight misalignment resulting from motion and structure (or manufacturing).

  One embodiment of such structured illumination is shown in FIG. One feature shown in FIG. 5 is that the illumination only strikes locations on the optical surface within the field of view (FOV) of the imaging optics. For the detector array 502 shown in FIG. 5, the geometry of the photodetector array (s) may be a normal shape, such as an “L” -shaped geometry. Or, if it has an asymmetric shape, the illumination optics 504 should ideally provide light only to that region on the rough surface so that optimal optical efficiency is obtained. Light hitting the rough surface outside this FOV region is wasted and reduces the net efficiency of the optical positioning system.

  In one embodiment, the asymmetric (ie, non-circular) shape of the illuminated portion will be formed by the configuration (or structure) of the illumination optics 504 rather than a direct image of the illumination source. There may even be cases where the shape of the illuminated area is not convex. The use of a standard refractive or reflective optical surface 506 combined with a diffractive structure 508, as shown in the example of FIG. Can be optimally matched to the FOV required by the imaging optics. Imaging optics 512 maps a particular illumination profile 510 onto detector array 502 so that the light detection elements are efficiently covered without covering an excessively large area outside the detection area. It is configured as follows. In other words, the optical system 500 of FIG. 5 is configured such that the reflected illumination substantially matches the shape of the detector array 502 so that light falling outside the detector field of view is minimized. Advantageously, this allows the power from the light source to be used more efficiently.

  Preferably, the optical system is configured such that the illuminated portion of the surface is smaller than one that is 50% larger than the field of view of the photosensitive element of the detector (eg, the size of the illuminated portion of the surface is sensitive). It does not exceed 150% of the field of view of the device). In other words, the reflected illumination preferably covers (covers) the area of the detector that is no more than 150% of the minimum area to cover (cover) all the photosensitive elements of the detector. More preferably, the optical system is configured such that substantially all (eg, 85% or more) of the illuminated portion of the surface falls within the field of view of the photosensitive element of the detector.

  The particular example shown in FIG. 5 shows that the illumination profile 510 can be mirrored with respect to the shape of the reflected illumination covering the photodetector array 502. This depends on the configuration of the imaging optical system 512.

  According to a particular embodiment of the present invention, the detector configuration is shown in FIG. 6 in which a plurality of interlaced “pixel” (detector element) arrays are arranged in parallel rows along two axes. It can be used in a state of being arranged so as to form. Specifically, FIG. 6 shows three interlaced arrays 602 arranged in parallel rows along the x-axis, and three arranged in parallel rows along the y-axis. An interlaced array 604 is shown.

As described above in connection with FIG. 3, each “4N” array produces N signals that are electrically connected (wired sum) producing _four signals S ₁ , S ₂ , S ₃ , S _4. It consists of four groups of vessel elements. Background suppression (and signal enhancement) is done by taking the difference signals S ₁₃ = S ₁ -S ₃ and S ₂₄ = S ₂ -S ₄ . The amount and direction of translation (movement) can be estimated from the orthogonal pairs S ₁₃ and S ₂₄ .

  In this particular embodiment, three “4N” arrays (602 and 604) are used for each axis (x and y) to reduce the accumulated phase error during displacement. For this particular embodiment, 24 lines (two axes, three arrays / axes, four signals / arrays) are sent to front-end electronics 606 that process the signals and provide inputs to digital signal processor 608. There is. The DSP 608 can have a universal serial bus interface (USB I / F) 610, for example.

  Other embodiments may use an interlaced detector array having a value of M different from 4 (ie, the number of element groupings). Other embodiments may use a different number of rows other than three per dimension. The number of rows need not be the same in the x and y dimensions.

  Other embodiments may be configured with an array of detector arrays that is not an “L” shaped array 702. Examples of other possible arrangements are shown in FIG. Examples include a T array 704, a square (or square) array 706, and a “+” or “X” array 708. Still other embodiments may be configured with an array that forms skew angles (slopes) where the axes (rows or columns) are not vertical, such as a “V” or “Δ” type array, for example.

  The foregoing descriptions of specific embodiments and examples of the present invention have been presented for purposes of illustration and description. While this invention has been illustrated and described with respect to several examples, they are not to be construed as limiting. The embodiments described herein are not intended to be exhaustive or to limit the invention to the precise form disclosed, and many variations are within the scope of the invention in light of the above teachings. Changes, improvements and variations are possible. The scope of the present invention is intended to encompass the entire disclosed field and be defined by the appended claims and equivalents thereof.

The diffraction pattern of the light reflected from the smooth surface is shown. Speckle forming an interference pattern of light reflected from a rough surface. 1 is a functional block diagram of a speckle-based mouse according to an embodiment of the present invention. FIG. 1 is a block diagram of a photodiode array according to an embodiment of the present invention. FIG. It is a functional block diagram of the light collection optical system by one Embodiment of this invention. FIG. 6 is an optical diagram illustrating structured illumination according to an embodiment of the present invention. FIG. 4 is a functional block diagram illustrating two axes of detector elements each having multiple rows, according to one embodiment of the invention. Fig. 5 shows various arrangements of detector elements according to an embodiment of the invention.

Claims (20)

An optical displacement sensor for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames,
An illuminator having a light source and illumination optics for illuminating a portion of the surface with an illumination beam having a substantially flat phase plane;
A detector having a plurality of photosensitive elements and an imaging optical system;
An optical displacement sensor, wherein the illuminator and detector are configured such that the illuminated portion of the surface is smaller than 50% larger than the field of view of the photosensitive element of the detector.   The optical displacement sensor of claim 1, wherein the illumination optical system comprises diffractive and reflective optical systems for minimizing light incident outside the field of view of the detector.   The plurality of photosensitive elements includes a first plurality of photosensitive elements arranged in a substantially straight line along a first axis, and a second axis that is not parallel to the first axis. The optical displacement sensor of claim 1, comprising a second plurality of photosensitive elements arranged in a substantially straight row.   The optical displacement sensor of claim 3, wherein the second axis forms an angle of about 90 degrees with respect to the first axis.   The first and second photosensitive elements may form an array of “L”, “T”, “+”, “X”, “V”, “Δ” or square shape. The optical displacement sensor of claim 3, arranged along a first axis and the second axis.   The illuminator is further configured such that the illuminated portion of the surface defines a corresponding “L”, “T”, “+”, “X”, “Δ” or square shape on the surface. 6. The optical displacement sensor of claim 5 configured.   The optical displacement sensor of claim 1, wherein the illuminator is further configured such that the illuminated portion of the surface corresponds (or matches) the shape and area of the field of view of the photosensitive element of the detector.   The illuminator is further configured such that the illuminated portion of the surface is sufficiently larger than the detector field of view to provide a predetermined margin for error in the operation and configuration of the components of the optical displacement sensor. The optical displacement sensor of claim 1, wherein the illuminated portion of the surface is less than 50% larger than the field of view of the detector.   The optical displacement sensor according to claim 1, wherein the photosensitive element is a photodiode, and the light source is a laser.   The optical displacement sensor is a speckle-based displacement sensor configured to identify a position on the surface based on a complex interference pattern generated by light reflected from the surface, and the laser The optical displacement sensor according to claim 1, wherein the optical displacement sensor is a surface emitting laser (VCSEL).   The optical displacement sensor of claim 1, wherein the illuminator and detector are configured such that substantially all of the illuminated portion of the surface falls within the field of view of the photosensitive element. A method for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames, comprising:
Generating illumination from a light source;
Mapping the illumination onto a portion of the surface by illumination optics such that the illumination beam has a substantially flat phase plane;
Reflecting illumination from an illuminated portion of the surface;
Mapping the reflected illumination to an array of photosensitive elements in the detector by imaging optics;
The method wherein the illuminated portion of the surface comprises less than 50% greater than the field of view of the photosensitive element.   The step of mapping illumination by the illumination optics comprises refracting and diffracting the illumination such that an illuminated portion of the surface matches the shape of the field of the photosensitive element in the detector. 12 methods.   The first plurality of photosensitive elements are arranged in a substantially straight line along the first axis, and the second plurality of photosensitive elements are arranged on a second axis that is not parallel to the first axis. 14. The method of claim 13, wherein the method is arranged in substantially straight rows along.   The photosensitive elements are arranged to form a shape from a group of shapes consisting of “L”, “T”, “+”, “X”, “V”, “Δ”, and a square shape, 15. The method of claim 14, wherein the illuminated portion of the surface comprises forming a corresponding (or matching) shape.   The step of mapping illumination comprises a portion of the surface to define a “L”, “T”, “+”, “X”, “V”, “Δ”, or square shape on the surface. 15. The method of claim 14, comprising illuminating, wherein the illuminated portion overlaps and conforms to the field of view of the detector. An optical displacement sensor for detecting relative movement between a data input device and a surface by determining the displacement of optical features in successive frames,
A light source;
Illumination optics configured to illuminate a portion of the surface with a first shape;
An array of photosensitive elements having a second shape similar to the first shape;
An optical displacement sensor comprising: an imaging optical system configured to map illumination reflected from an illuminated portion of the surface, and wherein the reflected illumination covers an array of the photosensitive elements.   The optical displacement sensor of claim 17, wherein the first shape is non-circular and is not a direct image of the light source.   The optical displacement sensor of claim 17, wherein the first shape is not convex.   18. The optical displacement sensor of claim 17, wherein the illuminated portion of the surface is smaller than one that is 50% larger than the field of view of the photosensitive element.

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