(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2012/0092461 A1
`(43) Pub. Date:
`Apr. 19, 2012
`Fisker et al.
`
`US 20120092461A1
`
`(54)
`
`(76)
`
`FOCUS SCANNINGAPPARATUS
`
`Inventors:
`
`Rune Fisker, Virum (DK); Henrik
`Ojelund, Lyngby (DK); Rasmus
`Kjaer, Kobenhavnk (DK); Mike
`van der Poel, Rodovre (DK); Arich
`A. Qazi, Toronto (CA); Karl-Josef
`Hollenbeck, Kobenhavn () (DK)
`
`(21)
`
`Appl. No.:
`
`13/376.427
`
`PCT Fled:
`
`Jun. 17, 2010
`
`PCT NO.:
`
`S371 (c)(1),
`(2), (4) Date:
`
`Dec. 6, 2011
`
`(22)
`
`(86)
`
`
`
`Related U.S. Application Data
`(60) Provisional application No. 61/187,744, filed on Jun.
`17, 2009, provisional application No. 61/231,118,
`filed on Aug. 4, 2009.
`Publication Classification
`
`(51) Int. Cl.
`(2006.01)
`H04N I3/02
`(52) U.S. Cl. ................................... 348/46; 348/E13.074
`(57)
`ABSTRACT
`Disclosed is a handheld Scanner for obtaining and/or measur
`ing the 3D geometry of at least a part of the Surface of an
`object using confocal pattern projection techniques. Specific
`embodiments are given for intraoral scanning and scanning of
`the interior part of a human ear.
`
`Align Ex. 1005
`U.S. Patent No. 9,962,244
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`FOCUS SCANNINGAPPARATUS
`
`0001) The present invention relates to an apparatus and a
`method for optical 3D scanning of surfaces. The principle of
`the apparatus and method according to the invention may be
`applied in various contexts. One specific embodiment of the
`invention is particularly Suited for intraoral scanning, i.e.
`direct scanning of teeth and surrounding soft-tissue in the oral
`cavity. Other dental related embodiments of the invention are
`Suited for scanning dental impressions, gypsum models, wax
`bites, dental prosthetics and abutments. Another embodiment
`of the invention is suited for scanning of the interior and
`exterior part of a human ear or ear channel impressions. The
`invention may find use within scanning of the 3D structure of
`skin in dermatological or cosmetic/cosmetological applica
`tions, scanning of jewelry or wax models of wholejewelry or
`part of jewelry, scanning of industrial parts and even time
`resolved 3D scanning, such as time resolved 3D scanning of
`moving industrial parts.
`
`BACKGROUND OF THE INVENTION
`0002 The invention relates to three dimensional (3D)
`Scanning of the surface geometry of objects. Scanning an
`object surface in 3 dimensions is a well known field of study
`and the methods for scanning can be divided into contact and
`non-contact methods. An example of contact measurements
`methods are Coordinate Measurement Machines (CMM),
`which measures by letting a tactile probe trace the surface.
`The advantages include great precision, but the process is
`slow and a CMM is large and expensive. Non-contact mea
`Surement methods include X-ray and optical probes.
`0003) Confocal microscopy is an optical imaging tech
`nique used to increase micrograph contrast and/or to recon
`Struct three-dimensional images by using a spatial pinhole to
`eliminate out-of-focus light or flare in specimens that are
`thicker than the focal plane.
`0004. A confocal microscope uses point illumination and
`a pinhole in an optically conjugate plane in front of the detec
`tor to eliminate out-of-focus information. Only the light
`within the focal plane can be detected. As only one point is
`illuminated at a time in confocal microscopy, 2D imaging
`requires raster scanning and 3D imaging requires raster scan
`ning in a range of focus planes.
`0005. In WO 00/08415 the principle of confocal micros
`copy is applied by illuminating the surface with a plurality of
`illuminated spots. By varying the focal plane in-focus spot
`specific positions of the surface can be determined. However,
`determination of the surface structure is limited to the parts of
`the surface that are illuminated by a spot.
`0006 WO 2003/060587 relates to optically sectioning of a
`specimen in microscopy wherein the specimen is illuminated
`with an illumination pattern. Focus positions of the image
`plane are determined by characterizing an oscillatory com
`ponent of the pattern. However, the focal plane can only be
`adjusted by moving the specimen and the optical system
`relative to each other, i.e. closer to or further away from each
`other. Thus, controlled variation of the focal plane requires a
`controlled spatial relation between the specimen and the opti
`cal system, which is fulfilled in a microscope. However, such
`a controlled spatial relation is not applicable to e.g. a hand
`held Scanner.
`0007 US2007/0109559 A1 describes a focus scanner
`where distances are found from the focus lens positions at
`
`which maximum reflective intensity of light beams incident
`on the object being scanned is observed. In contrast to the
`invention disclosed here, this prior art exploits no pre-deter
`mined measure of the illumination pattern and exploits no
`contrast detection, and therefore, the signal-to-noise ratio is
`Sub-optimal.
`0008. In WO 2008/125605, means for generating a time
`Variant pattern composed of alternating split images are
`described. This document describes a scanning method to
`obtain an optical section of a scan object by means of two
`different illumination profiles, e.g. two patterns of opposite
`phases. These two images are used to extract the optical
`Section, and the method is limited to acquisition of images
`from only two different illumination profiles. Furthermore,
`the method relies on a predetermined calibration that deter
`mines the phase offset between the two illumination profiles.
`SUMMARY OF THE INVENTION
`0009. Thus, an object of the invention is to provide a
`Scanner which may be integrated in a manageable housing,
`such as a handheld housing. Further objects of the invention
`are: discriminate out-of-focus information and provide a fast
`Scanning time.
`0010. This is achieved by a method and a scanner for
`obtaining and/or measuring the 3D geometry of at least a part
`of the surface of an object, said scanner comprising:
`0011 at least one camera accommodating an array of
`sensor elements,
`0012 means for generating a probe light incorporating
`a spatial pattern,
`0013) means for transmitting the probe light towards the
`object thereby illuminating at least a part of the object
`with said pattern in one or more configurations,
`0014) means for transmitting at least a part of the light
`returned from the object to the camera,
`10015) means for varying the position of the focus plane
`of the pattern on the object while maintaining a fixed
`spatial relation of the scanner and the object,
`0016 means for obtaining at least one image from said
`array of sensor elements,
`0017 means for evaluating a correlation measure at
`each focus plane position between at least one image
`pixel and a weight function, where the weight function is
`determined based on information of the configuration of
`the spatial pattern;
`0018 data processing means for:
`(0019 a) determining by analysis of the correlation
`measure the in-focus position(s) of:
`0020 each of a plurality of image pixels for a range
`of focus plane positions, or
`0021 each of a plurality of groups of image pixels
`for a range of focus plane positions, and
`0022 b) transforming in-focus data into 3D real
`world coordinates.
`0023 The method and apparatus described in this inven
`tion is for providing a 3D surface registration of objects using
`light as a non-contact probing agent. The light is provided in
`the form of an illumination pattern to provide a light oscilla
`tion on the object. The variation/oscillation in the pattern may
`be spatial, e.g. a static checkerboard pattern, and/or it may be
`time varying, for example by moving a pattern across the
`object being scanned. The invention provides for a variation
`of the focus plane of the pattern over a range of focus plane
`positions while maintaining a fixed spatial relation of the
`
`0020
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`scanner and the object. It does not mean that the scan must be
`provided with a fixed spatial relation of the scanner and the
`object, but merely that the focus plane can be varied (scanned)
`with a fixed spatial relation of the scanner and the object. This
`provides fora handheld Scanner Solution based on the present
`invention.
`0024. In some embodiments the signals from the array of
`sensor elements are light intensity.
`0025. One embodiment of the invention comprises a first
`optical system, Such as an arrangement of lenses, for trans
`mitting the probe light towards the object and a second optical
`system for imaging light returned from the object to the
`camera. In the preferred embodiment of the invention only
`one optical system images the pattern onto the object and
`images the object, or at least a part of the object, onto the
`camera, preferably along the same optical axis, however
`along opposite optical paths.
`0026. In the preferred embodiment of the invention an
`optical system provides an imaging of the pattern onto the
`object being probed and from the object being probed to the
`camera. Preferably, the focus plane is adjusted in Such a way
`that the image of the pattern on the probed object is shifted
`along the optical axis, preferably in equal steps from one end
`of the scanning region to the other. The probe light incorpo
`rating the pattern provides a pattern of light and darkness on
`the object. Specifically, when the pattern is varied in time for
`a fixed focus plane then the in-focus regions on the object will
`display an oscillating pattern of light and darkness. The out
`of-focus regions will display Smaller or no contrast in the light
`oscillations.
`0027 Generally we consider the case where the light inci
`dent on the object is reflected diffusively and/or specularly
`from the object's surface. But it is understood that the scan
`ning apparatus and method are not limited to this situation.
`They are also applicable to e.g. the situation where the inci
`dent light penetrates the Surface and is reflected and/or scat
`tered and/or gives rise to fluorescence and/or phosphores
`cence in the object. Inner Surfaces in a Sufficiently translucent
`object may also be illuminated by the illumination patternand
`be imaged onto the camera. In this case a Volumetric scanning
`is possible. Some planktic organisms are examples of Such
`objects.
`0028. When a time varying pattern is applied a single
`sub-scan can be obtained by collecting a number of 2D
`images at different positions of the focus plane and at differ
`ent instances of the pattern. As the focus plane coincides with
`the scan Surface at a single pixel position, the pattern will be
`projected onto the Surface point in-focus and with high con
`trast, thereby giving rise to a large variation, or amplitude, of
`the pixel value over time. For each pixel it is thus possible to
`identify individual settings of the focusing plane for which
`each pixel will be in focus. By using knowledge of the optical
`system used, it is possible to transform the contrast informa
`tion vs. position of the focus plane into 3D surface informa
`tion, on an individual pixel basis.
`0029. Thus, in one embodiment of the invention the focus
`position is calculated by determining the light oscillation
`amplitude for each of a plurality of sensor elements for a
`range of focus planes.
`0030. For a static pattern a single sub-scan can be obtained
`by collecting a number of 2D images at different positions of
`the focus plane. As the focus plane coincides with the scan
`surface, the pattern will be projected onto the surface point
`in-focus and with high contrast. The high contrast gives rise to
`
`a large spatial variation of the static pattern on the Surface of
`the object, thereby providing a large variation, or amplitude,
`of the pixel values over a group of adjacent pixels. For each
`group of pixels it is thus possible to identify individual set
`tings of the focusing plane for which each group of pixels will
`be in focus. By using knowledge of the optical system used, it
`is possible to transform the contrast information vs. position
`of the focus plane into 3D surface information, on an indi
`vidual pixel group basis.
`0031. Thus, in one embodiment of the invention the focus
`position is calculated by determining the light oscillation
`amplitude for each of a plurality of groups of the sensor
`elements for a range of focus planes.
`0032. The 2D to 3D conversion of the image data can be
`performed in a number of ways known in the art. I.e. the 3D
`surface structure of the probed object can be determined by
`finding the plane corresponding to the maximum light oscil
`lation amplitude for each sensor element, or for each group of
`sensor elements, in the camera's sensor array when recording
`the light amplitude for a range of different focus planes.
`Preferably, the focus plane is adjusted in equal steps from one
`end of the scanning region to the other. Preferably the focus
`plane can be moved in a range large enough to at least coin
`cide with the surface of the object being scanned.
`0033. The present invention distinguishes itself from WO
`2008/125605, because in the embodiments of the present
`invention that use a time-variant pattern, input images are not
`limited to two illumination profiles and can be obtained from
`any illumination profile of the pattern. This is because the
`orientation of the reference image does not rely entirely on a
`predetermined calibration, but rather on the specific time of
`the input image acquisition.
`0034. Thus WO 2008/125605 applies specifically exactly
`two patterns, which are realized physically by a chrome-on
`glass mask as illuminated from either side, the reverse side
`being reflective. WO 2008/125605 thus has the advantage of
`using no moving parts, but the disadvantage of a compara
`tively poorer signal-to-noise ratio. In the present invention
`there is the possibility of using any number of pattern con
`figurations, which makes computation of the light oscillation
`amplitude or the correlation measure more precise.
`
`DEFINITIONS
`
`0035 Pattern: A light signal comprising an embedded spa
`tial structure in the lateral plane. May also be termed “illumi
`nation pattern'.
`0036 Time varying pattern: A pattern that varies in time,
`i.e. the embedded spatial structure varies in time. May also be
`termed “time varying illumination pattern'. In the following
`also termed “fringes”.
`0037 Static pattern: A pattern that does not vary in time,
`e.g. a static checkerboard pattern or a static line pattern.
`0038 Pattern configuration: The state of the pattern.
`Knowledge of the pattern configuration at a certain time
`amounts to knowing the spatial structure of the illumination at
`that time. For a periodic pattern the pattern configuration will
`include information of the pattern phase. If a Surface element
`of the object being scanned is imaged onto the camera then
`knowledge of the pattern configuration amounts to knowl
`edge of what part of the pattern is illuminating the Surface
`element.
`
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`0039 Focus plane: A surface where light rays emitted
`from the pattern converge to form an image on the object
`being scanned. The focus plane does not need to be flat. It may
`be a curved surface.
`0040. Optical system: An arrangement of optical compo
`nents, e.g. lenses, that transmit, collimate and/or images light,
`e.g. transmitting probe light towards the object, imaging the
`pattern on and/or in the object, and imaging the object, or at
`least a part of the object, on the camera.
`0041. Optical axis: An axis defined by the propagation of
`a light beam. An optical axis is preferably a straight line. In
`the preferred embodiment of the invention the optical axis is
`defined by the configuration of a plurality of optical compo
`nents, e.g. the configuration of lenses in the optical system.
`There may be more than one optical axis, if for example one
`optical system transmits probe light to the object and another
`optical system images the object on the camera. But prefer
`ably the optical axis is defined by the propagation of the light
`in the optical system transmitting the pattern onto the object
`and imaging the object onto the camera. The optical axis will
`often coincide with the longitudinal axis of the Scanner.
`0042 Optical path: The path defined by the propagation of
`the light from the light source to the camera. Thus, a part of
`the optical path preferably coincides with the optical axis.
`Whereas the optical axis is preferably a straight line, the
`optical path may be a non-straight line, for example when the
`light is reflected, scattered, bent, divided and/or the like pro
`vided e.g. by means of beam splitters, mirrors, optical fibers
`and the like.
`0043 Telecentric system: An optical system that provides
`imaging in Such a way that the chief rays are parallel to the
`optical axis of said optical system. In a telecentric system
`out-of-focus points have Substantially same magnification as
`in-focus points. This may provide an advantage in the data
`processing. A perfectly telecentric optical system is difficult
`to achieve, however an optical system which is substantially
`telecentric or near telecentric may be provided by careful
`optical design. Thus, when referring to a telecentric optical
`system it is to be understood that it may be only near telecen
`tric.
`0044 Scan length: A lateral dimension of the field of view.
`If the probe tip (i.e. scan head) comprises folding optics to
`direct the probe light in a direction different such as perpen
`dicular to the optical axis then the scan length is the lateral
`dimension parallel to the optical axis.
`0045 Scan object: The object to be scanned and on which
`surface the scanner provides information. “The scan object”
`may just be termed “the object’.
`0046 Camera: Imaging sensor comprising a plurality of
`sensors that respond to light input onto the imaging sensor.
`The sensors are preferably ordered in a 2D array in rows and
`columns.
`0047 Input signal: Light input signal or sensor input sig
`nal from the sensors in the camera. This can be integrated
`intensity of light incident on the sensor during the exposure
`time or integration of the sensor. In general, it translates to a
`pixel value within an image. May also be termed “sensor
`signal'.
`0048 Reference signal: A signal derived from the pattern.
`A reference signal may also be denoted a weight function or
`weight vector or reference vector.
`0049 Correlation measure: A measure of the degree of
`correlation between a reference and input signal. Preferably
`the correlation measure is defined such that if the reference
`
`and input signal are linearly related to each other then the
`correlation measure obtains a larger magnitude than if they
`are not.
`0050. In some cases the correlation measure is a light
`oscillation amplitude.
`0051
`Image: An image can be viewed as a 2D array of
`values (when obtained with a digital camera) or in optics, an
`image indicates that there exists a relation between an imaged
`Surface and an image Surface where light rays emerging from
`one point on said imaged surface Substantially converge on
`one point on said image Surface.
`0.052
`Intensity: In optics, intensity is a measure of light
`power per unit area. In image recording with a camera com
`prising a plurality of individual sensing elements, intensity
`may be used to term the recorded light signal on the individual
`sensing elements. In this case intensity reflects a time inte
`gration of light power per unit area on the sensing element
`over the exposure time involved in the image recording.
`
`Mathematical Notation
`0053 A A correlation measure between the weight func
`tion and the recorded light signal. This can be a light
`oscillation amplitude.
`0054 I Light input signal or sensor input signal. This can
`be integrated intensity of light incident on the sensor during
`the exposure time or integration of the sensor. In general, it
`translates to a pixel value within an image.
`0055 fReference signal. May also be called weight value.
`0056 n. The number of measurements with a camerasen
`sor and/or several camera sensors that are used to compute
`a correlation measure.
`0057 HImage height in number of pixels
`0058 W Image width in number of pixels
`0059 Symbols are also explained as needed in the text.
`
`DETAILED DESCRIPTION OF THE INVENTION
`0060. The scanner preferably comprises at least one beam
`splitter located in the optical path. For example, an image of
`the object may be formed in the camera by means of a beam
`splitter. Exemplary uses of beam splitters are illustrated in the
`figures.
`0061. In a preferred embodiment of the invention light is
`transmitted in an optical system comprising a lens system.
`This lens system may transmit the pattern towards the object
`and images light reflected from the object to the camera.
`0062. In a telecentric optical system, out-of-focus points
`have the same magnification as in-focus points. Telecentric
`projection can therefore significantly ease the data mapping
`of acquired 2D images to 3D images. Thus, in a preferred
`embodiment of the invention the optical system is substan
`tially telecentric in the space of the probed object. The optical
`system may also be telecentric in the space of the pattern and
`CaCa.
`
`Varying Focus
`0063 A pivotal point of the invention is the variation, i.e.
`scanning, of the focal plane without moving the scanner in
`relation to the object being scanned. Preferably the focal
`plane may be varied, such as continuously varied in a periodic
`fashion, while the pattern generation means, the camera, the
`optical system and the object being scanned is fixed in rela
`tion to each other. Further, the 3D surface acquisition time
`should be small enough to reduce the impact of relative move
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`ment between probe and teeth, e.g. reduce effect of shaking.
`In the preferred embodiment of the invention the focus plane
`is varied by means of at least one focus element. Preferably
`the focus plane is periodically varied with a predefined fre
`quency. Said frequency may be at least 1 Hz, such as at least
`2 Hz, 3, 4, 5, 6, 7, 8, 9 or at least 10 Hz, such as at least 20, 40,
`60, 80 or at least 100 Hz.
`0064 Preferably the focus element is part of the optical
`system. I.e. the focus element may be a lens in a lens system.
`A preferred embodiment comprises means, such as a transla
`tion stage, for adjusting and controlling the position of the
`focus element. In that way the focus plane may be varied, for
`example by translating the focus element back and forth along
`the optical axis.
`0065. If a focus element is translated back and forth with a
`frequency of several HZ this may lead to instability of the
`scanner. A preferred embodiment of the invention thus com
`prises means for reducing and/or eliminating the vibration
`and/or shaking from the focus element adjustment system,
`thereby increasing the stability of the scanner. This may at
`least partly be provided by means for fixing and/or maintain
`ing the centre of mass of the focus element adjustment sys
`tem, such as a counter-weight to Substantially counter-bal
`ance movement of the focus element; for example, by
`translating a counter-weight opposite to the movement of the
`focus element. Ease of operation may be achieved if the
`counter-weight and the focus element are connected and
`driven by the same translation means. This may however,
`only substantially reduce the vibration to the first order. If a
`counter-weight balanced device is rotated around the counter
`weight balanced axis, there may be issues relating to the
`torque created by the counter-weights. A further embodiment
`of the invention thus comprises means for reducing and/or
`eliminating the first order, second order, third order and/or
`higher order vibration and/or shaking from the focus element
`adjustment system, thereby increasing the Stability of the
`SCa.
`0.066. In another embodiment of the invention more than
`one optical element is moved to shift the focal plane. In that
`embodiment it is desirable that these elements are moved
`together and that the elements are physically adjacent.
`0067. In the preferred embodiment of the invention the
`optical system is telecentric, or near telecentric, for all focus
`plane positions. Thus, even though one or more lenses in the
`optical system may be shifted back and forth to change the
`focus plane position, the telecentricity of the optical system is
`maintained.
`0068. The preferred embodiment of the invention com
`prises focus gearing. Focus gearing is the correlation between
`movement of the lens and movement of the focus plane posi
`tion. E.g. a focus gearing of 2 means that a translation of the
`focus element of 1 mm corresponds to a translation of the
`focus plane position of 2 mm. Focus gearing can be provided
`by a Suitable design of the optical system. The advantage of
`focus gearing is that a small movement of the focus element
`may correspond to a large variation of the focus plane posi
`tion. In specific embodiments of the invention the focus gear
`ing is between 0.1 and 100, such as between 0.1 and 1, such as
`between 1 and 10, such as between 2 and 8, such as between
`3 and 6, such as least 10, such as at least 20.
`0069. In another embodiment of the invention the focus
`element is a liquid lens. A liquid lens can control the focus
`plane without use of any moving parts.
`Camera
`0070 The camera may be a standard digital camera
`accommodating a standard CCD or CMOS chip with one A/D
`
`converter per line of sensor elements (pixels). However, to
`increase the frame rate the scanner according to the invention
`may comprise a high-speed camera accommodating multiple
`A/D converters per line of pixels, e.g. at least 2, 4, 8 or 16A/D
`converters per line of pixels.
`
`Pattern
`0071 Another central element of the invention is the probe
`light with an embedded pattern that is projected on to the
`object being scanned. The pattern may be static or time vary
`ing. The time varying pattern may provide a variation of light
`and darkness on and/or in the object. Specifically, when the
`pattern is varied in time for a fixed focus plane then the
`in-focus regions on the object will display an oscillating
`pattern of light and darkness. The out-of-focus regions will
`display Smaller or no contrast in the light oscillations. The
`static pattern may provide a spatial variation of light and
`darkness on and/or in the object. Specifically, the in-focus
`regions will display an oscillating pattern of light and dark
`ness in space. The out-of-focus regions will display Smaller or
`no contrast in the spatial light oscillations.
`0072 Light may be provided from an external light
`Source, however preferably the Scanner comprises at least one
`light source and pattern generation means to produce the
`pattern. It is advantageous in terms of signal-to-noise ratio to
`design a light source Such that the intensity in the non-masked
`parts of the pattern is as close to uniform in space as possible.
`In another embodiment the light source and the pattern gen
`eration means is integrated in a single component, such as a
`segmented LED. A segmented LED may provide a static
`pattern and/or it may provide a time varying pattern in itself
`by turning on and off the different segments in sequence. In
`one embodiment of the invention the time varying pattern is
`periodically varying in time. In another embodiment of the
`invention the static pattern is periodically varying in space.
`0073 Light from the light source (external or internal)
`may be transmitted through the pattern generation means
`thereby generating the pattern. For example the pattern gen
`eration means comprises at least one translucent and/or trans
`parent pattern element. For generating a time varying pattern
`a wheel, with an opaque mask can be used. E.g. the mask
`comprises a plurality of radial spokes, preferably arranged in
`a symmetrical order. The scanner may also comprise means
`for rotating and/or translating the pattern element. For gen
`erating a static pattern a glass plate with an opaque mask can
`be used. E.g. the mask comprises a line pattern or checker
`board pattern. In general said mask preferably possesses rota
`tional and/or translational periodicity. The pattern element is
`located in the optical path. Thus, light from the light source
`may be transmitted through the pattern element, e.g. trans
`mitted transversely through the pattern element. The time
`varying pattern can then be generated by rotating and/or
`translating the pattern element. A pattern element generating
`a static pattern does not need to be moved during a scan.
`
`Correlation
`0074. One object of the invention is to provide short scan
`time and real time processing, e.g. to providelive feedback to
`a scanner operator to make a fast scan of an entire tooth arch.
`However, real time high resolution 3D scanning creates an
`enormous amount of data. Therefore data processing should
`be provided in the scanner housing, i.e. close to the optical
`components, to reduce data transfer rate to e.g. a cart, work
`
`0023
`
`

`

`US 2012/0092461 A1
`
`Apr. 19, 2012
`
`station or display. In order to speed up data processing time
`and in order to extract in-focus information with an optimal
`signal-to-noise ratio various correlation techniques may be
`embedded/implemented. This may for example be imple
`mented in the camera electronics to discriminate out-of-focus
`information. The pattern is applied to provide illumination
`with an embedded spatial structure on the object being
`scanned. Determining in-focus information relates to calcu
`lating a correlation measure of this spatially structured light
`signal (which we term input signal) with the variation of the
`pattern itself (which we term reference signal). In general the
`magnitude of the correlation measure is high if t