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`period is denoted n. The registration of the pattern position for each individual image
`
`combined with the independently known pattern variation over all sensing element (i.e.
`
`obtaining knowledge of the pattern configuration) and the recorded images allows for
`
`5
`
`an efficient extraction of the correlation measure in each individual sensing element in
`the camera. For a light sensing element with label j, the n recorded light signals of that
`element are denoted /11 , ... , ln 1 • The correlation measure of that element, AP may be
`expressed as
`
`i=l
`Here the reference signal or weight function f is obtained from the knowledge of the
`
`n
`
`Aj = I h,j Ii,j
`
`pattern configuration. /has two indices ij. The variation of /with the first index is
`
`1 O
`
`derived from the knowledge of the pattern position during each image recording. The
`
`variation of /with the second index is derived from the knowledge of the pattern
`
`geometry which may be determined prior to the 3 D scanning.
`
`Preferably, but not necessarily, the reference signal /averages to zero over time, i.e.
`
`15
`
`for all ywe have
`
`n ""'f =0
`
`L
`
`l,J
`
`i=l
`to suppress the DC part of the light variation or correlation measure. The focus position
`
`corresponding to the pattern being in focus on the object for a single sensor element in
`
`the camera will be given by an extremum value of the correlation measure of that
`
`sensor element when the focus position is varied over a range of values. The focus
`
`20
`
`position may be varied in equal steps from one end of the scanning region to the other.
`
`To obtain a sharp image of an object by means of a camera the object must be in focus
`
`and the optics of the camera and the object must be in a fixed spatial relationship
`
`during the exposure time of the image sensor of the camera. Applied to the present
`
`25
`
`invention this should imply that the pattern and the focus should be varied in discrete
`
`steps to be able to fix the pattern and the focus for each image sampled in the camera,
`
`i.e. fixed during the exposure time of the sensor array. However, to increase the
`
`sensitivity of the image data the exposure time of the sensor array should be as high as
`
`the sensor frame rate permits. Thus, in the preferred embodiment of the invention
`
`30
`
`images are recorded (sampled) in the camera while the pattern is continuously varying
`
`(e.g. by continuously rotating a pattern wheel) and the focus plane is continuously
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`moved. This implies that the individual images will be slightly blurred since they are the
`
`result of a time-integration of the image while the pattern is varying and the focus plane
`
`is moved. This is something that one could expect to lead to deterioration of the data
`
`quality, but in practice the advantage of concurrent variation of the pattern and the
`
`5
`
`focus plane is bigger than the drawback.
`
`In another embodiment of the invention images are recorded (sampled) in the camera
`
`while the pattern is fixed and the focus plane is continuously moved, i.e. no movement
`
`of the pattern. This could be the case when the light source is a segmented light
`
`10
`
`source, such as a segment LED that flashes in an appropriate fashion. In this
`
`embodiment the knowledge of the pattern is obtained by a combination of prior
`
`knowledge of the geometry of the individual segments on the segmented LED give rise
`
`to a variation across light sensing elements and the applied current to different
`
`segments of the LED at each recording.
`
`15
`
`In yet another embodiment of the invention images are recorded (sampled) in the
`
`camera while the pattern is continuously varying and the focus plane is fixed.
`
`In yet another embodiment of the invention images are recorded (sampled) in the
`
`20
`
`camera while the pattern and the focus plane are fixed.
`
`The temporal correlation principle may be applied in general within image analysis.
`
`Thus, a further embodiment of the invention relates to a method for calculating the
`
`amplitude of a light intensity oscillation in at least one (photoelectric) light sensitive
`
`25
`
`element, said light intensity oscillation generated by a periodically varying illumination
`
`pattern and said amplitude calculated in at least one pattern oscillation period, said
`
`method comprising the steps of:
`
`providing the following a predetermined number of sampling times during a pattern
`
`oscillation period:
`
`30
`
`o sampling the light sensitive element thereby providing the signal of said light
`
`sensitive element, and
`
`o providing an angular position and/or a phase of the periodically varying
`
`illumination pattern for said sampling, and
`
`calculating said amplitude(s) by integrating the products of a predetermined
`
`35
`
`periodic function and the signal of the corresponding light sensitive element over
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`said predetermined number of sampling times, wherein said periodic function is a
`
`function of the angular position and/or the phase of the periodically varying
`
`illumination pattern.
`
`5
`
`This may also be expressed as
`
`A= I:J(p, )I,
`
`where A is the calculated amplitude or correlation measure, /is the index for each
`
`sampling, tis the periodic function, p, is the angular position/ phase of the illumination
`
`pattern for sampling /and 1. is the signal of the light sensitive element for sampling /.
`
`10
`
`Preferably the periodic function averages to zero over a pattern oscillation period, i.e.
`
`To generalize the principle to a plurality of light sensitive elements, for example in a
`
`sensor array, the angular position / phase of the illumination pattern for a specific light
`
`15
`
`sensitive element may consist of an angular position / phase associated with the
`
`illumination pattern plus a constant offset associated with the specific light sensitive
`
`element. Thereby the correlation measure or amplitude of the light oscillation in light
`
`sensitive element /may be expressed as
`A1 = I:J(01 + P, )1,, 1 ,
`
`20
`
`where 0, is the constant offset for light sensitive element j.
`
`A periodically varying illumination pattern may be generated by a rotating wheel with an
`
`opaque mask comprising a plurality of radial spokes arranged in a symmetrical order.
`
`The angular position of the wheel will thereby correspond to the angular position of the
`
`25
`
`pattern and this angular position may obtained by an encoder mounted on the rim of
`
`the wheel. The pattern variation across different sensor elements for different position
`
`of the pattern may be determined prior to the 3 D scanning in a calibration routine. A
`
`combination of knowledge of this pattern variation and the pattern position constitutes
`
`knowledge of the pattern configuration. A period of this pattern may for example be the
`
`30
`
`time between two spokes and the amplitude of a single or a plurality of light sensitive
`
`elements of this period may be calculated by sampling e.g. four times in this period.
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`A periodically varying illumination pattern may generated by a Ranchi ruling moving
`
`orthogonal to the lines and the position is measured by an encoder. This position
`
`corresponds to the angular position of the generated pattern. Alternatively, a
`
`checkerboard pattern could be used.
`
`5
`
`A periodically varying illumination pattern may generated by a one-dimensional array of
`
`LEDs that can be controlled line wise.
`
`A varying illumination pattern may generated by a LCD or DLP based projector.
`
`1 O
`
`Optical correlation
`
`The abovementioned correlation principle (temporal correlation) requires some sort of
`
`registering of the time varying pattern, e.g. knowledge of the pattern configuration at
`
`each light level recording in the camera. However, a correlation principle without this
`
`registering may be provided in another embodiment of the invention. This principle is
`
`15
`
`termed "optical correlation".
`
`In this embodiment of the invention an image of the pattern itself and an image of at
`
`least a part of the object being scanned with the pattern projected onto it is combined
`
`on the camera. I.e. the image on the camera is a superposition of the pattern itself and
`
`20
`
`the object being probed with the pattern projected onto it. A different way of expressing
`
`this is that the image on the camera substantially is a multiplication of an image of the
`
`pattern projected onto the object with the pattern itself.
`
`This may be provided in the following way. In a further embodiment of the invention the
`
`25
`
`pattern generation means comprises a transparent pattern element with an opaque
`
`mask. The probe light is transmitted through the pattern element, preferably transmitted
`
`transversely through the pattern element. The light returned from the object being
`
`scanned is retransmitted the opposite way through said pattern element and imaged
`
`onto the camera. This is preferably done in a way where the image of the pattern
`
`30
`
`illuminating the object and the image of the pattern itself are coinciding when both are
`
`imaged onto the camera. One particular example of a pattern is a rotating wheel with
`
`an opaque mask comprising a plurality of radial spokes arranged in a symmetrical
`
`order such that the pattern possesses rotational periodicity. In this embodiment there is
`
`a well-defined pattern oscillation period if the pattern is substantially rotated at a
`
`35
`
`constant speed. We define the oscillation period as 2'fdro.
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`
`We note that in the described embodiment of the invention the illumination pattern is a
`
`pattern of light and darkness. A light sensing element in the camera with a signal
`
`proportional to the integrated light intensity during the camera integration time 6twith
`label j, 11 is given by
`
`5
`
`t+ot
`
`h=Kj 7i(t'); (t')df
`
`t
`Here K\s the proportionality constant of the sensor signal, tis the start of the camera
`integration time, T, is the time-varying transmission of the part of the rotating pattern
`light sensing element, and s, is the time-varying light
`element imaged onto the/th
`intensity of light returned from the scanned object and imaged onto the /th light sensing
`element. In the described embodiment T, is the the step function substantially defined
`by 7i{t) = 0 for sin(at+~) > 0 and 7i{t) = 1 elsewhere. <p5 is a phase dependent on the
`position of the/th
`imaging sensor.
`
`10
`
`The signal on the light sensing element is a correlation measure of the pattern and the
`
`15
`
`light returned from the object being scanned. The time-varying transmission takes the
`
`role of the reference signal and the time-varying light intensity of light returned from the
`
`scanned object takes the role of the input signal. The advantage of this embodiment of
`
`the invention is that a normal CCD or CMOS camera with intensity sensing elements
`
`may be used to record the correlation measure directly since this appears as an
`
`20
`
`intensity on the sensing elements. Another way of expressing this is that the
`
`computation of the correlation measure takes place in the analog, optical domain
`
`instead of in an electronic domain such as an FPGA or a PC.
`
`The focus position corresponding to the pattern being in focus on the object being
`
`25
`
`scanned for a single sensor element in the camera will then be given by the maximum
`
`value of the correlation measure recorded with that sensor element when the focus
`
`position is varied over a range of values. The focus position may be varied in equal
`
`steps from one end of the scanning region to the other. One embodiment of the
`
`invention comprises means for recording and/or integrating and/or monitoring and/or
`
`30
`
`storing each of a plurality of the sensor elements over a range of focus plane positions.
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`Preferably, the global maximum should be found. However, artifacts such as dirt on the
`
`optical system can result in false global maxima. Therefore, it can be advisable to look
`
`for local maxima in some cases.
`
`5
`
`Since the reference signal does not average to zero the correlation measure has a DC
`
`component. Since the DC part is not removed, there may exist a trend in DC signal
`
`over all focus element positions, and this trend can be dominating numerically. In this
`
`situation, the focus position may still be found by analysis of the correlation measure
`
`and/or one or more of its derivatives.
`
`10
`
`In a further embodiment of the invention the camera integration time is an integer
`number Mof the pattern oscillation period, i.e. ot= 2nMI OJ. One advantage of this
`embodiment is that the magnitude of the correlation measure can be measured with a
`
`better signal-to-noise ratio in the presence of noise than if the camera integration time
`
`15
`
`is not an integer number of the pattern oscillation period.
`
`In another further embodiment of the invention the camera integration time is much
`
`longer than pattern oscillation period, i.e. ot » 2r:cM I OJ. Many times the pattern
`oscillation time would here mean e.g. camera integration time at least 1 o times the
`oscillation time or more preferably such as at least 100 or 1000 times the oscillation
`
`time. One advantage of this embodiment is that there is no need for synchronization of
`
`camera integration time and pattern oscillation time since for very long camera
`
`integration times compared to the pattern oscillation time the recorded correlation
`
`measure is substantially independent of accurate synchronization.
`
`20
`
`25
`
`Equivalent to the temporal correlation principle the optical correlation principle may be
`
`applied in general within image analysis. Thus, a further embodiment of the invention
`
`relates to a method for calculating the amplitude of a light intensity oscillation in at least
`
`one (photoelectric) light sensitive element, said light intensity oscillation generated by a
`
`30
`
`superposition of a varying illumination pattern with itself, and said amplitude calculated
`
`by time integrating the signal from said at least one light sensitive element over a
`
`plurality of pattern oscillation periods.
`
`Spatial correlation
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`
`The above mentioned correlation principles (temporal correlation and optical
`
`correlation) require the pattern to be varying in time. If the optical system and camera
`
`provides a lateral resolution which is at least two times what is needed for the scan of
`
`the object then it is possible to scan with a static pattern, i.e. a pattern which is not
`
`5
`
`changing in time. This principle is termed "spatial correlation". The correlation measure
`
`is thus at least computed with sensor signals recorded at different sensor sites.
`
`The lateral resolution of an optical system is to be understood as the ability of optical
`
`elements in the optical system, e.g. a lens system, to image spatial frequencies on the
`
`10
`
`object being scanned up to a certain point. Modulation transfer curves of the optical
`
`system are typically used to describe imaging of spatial frequencies in an optical
`
`system. One could e.g. define the resolution of the optical system as the spatial
`
`frequency on the object being scanned where the modulation transfer curve has
`
`decreased to e.g. 50%. The resolution of the camera is a combined effect of the
`
`15
`
`spacing of the individual camera sensor elements and the resolution of the optical
`
`system.
`
`In the spatial correlation the correlation measure refers to a correlation between input
`
`signal and reference signal occurring in space rather than in time. Thus, in one
`
`20
`
`embodiment of the invention the resolution of the measured 3 D geometry is equal to
`
`the resolution of the camera. However, for the spatial correlation the resolution of the
`
`measured 3 D geometry is lower than the resolution of the camera, such as at least 2
`
`times lower, such as at least 3 times lower, such as at least 4 times lower, such as
`
`least 5 times lower, such as at least 10 times lower. The sensor element array is
`
`25
`
`preferably divided into groups of sensor elements, preferably rectangular groups, such
`
`as square groups of sensor elements, preferably adjacent sensor elements. The
`
`resolution of the scan, i.e. the measured 3D geometry, will then be determined by the
`
`size of these groups of sensor elements. The oscillation in the light signal is provided
`
`within these groups of sensor elements, and the amplitude of the light oscillation may
`
`30
`
`then be obtained by analyzing the groups of sensor elements. The division of the
`
`sensor element array into groups is preferably provided in the data processing stage,
`
`i.e. the division is not a physical division thereby possibly requiring a specially adapted
`
`sensor array. Thus, the division into groups is "virtual" even though the single pixel in a
`
`group is an actual physical pixel.
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`35
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`
`In one embodiment of the invention the pattern posseses translational periodicity along
`
`at least one spatial coordinate.
`
`In a further embodiment of the invention the spatially
`
`periodic pattern is aligned with the rows and/or the columns of the array of sensor
`
`elements. For example in the case of a static line pattern the rows or columns of the
`
`5
`
`pixels in the camera may be parallel with the lines of the pattern. Or in the case of a
`
`static checkerboard pattern the row and columns of the checkerboard may be aligned
`
`with the rows and columns, respectively, of the pixels in the camera. By aligning is
`
`meant that the image of the pattern onto the camera is aligned with the "pattern" of the
`
`sensor element in the sensor array of the camera. Thus, a certain physical location and
`
`1 O
`
`orientation of the pattern generation means and the camera requires a certain
`
`configuration of the optical components of the scanner for the pattern to be aligned with
`
`sensor array of the camera.
`
`In a further embodiment of the invention at least one spatial period of the pattern
`
`15
`
`corresponds to a group of sensor elements. In a further embodiment of the invention all
`
`groups of sensor elements contain the same number of elements and have the same
`
`shape. E.g. when the period of a checkerboard pattern corresponds to a square group
`
`of e.g. 2x2, 3x3, 4x4, 5x5, 6x6, 7x7, 8x8, 9x9, 10x1 O or more pixels on the camera.
`
`20
`
`In yet another embodiment one or more edges of the pattern is aligned with and/or
`
`coincide with one or more edges of the array of sensor elements. For example a
`
`checkerboard pattern may be aligned with the camera pixels in such a way that the
`
`edges of the image of the checkerboard pattern onto the camera coincide with the
`
`edges of the pixels.
`
`25
`
`In spatial correlation independent knowledge of the pattern configuration allows for
`
`calculating the correlation measure at each group of light sensing. For a spatially
`
`periodic illumination this correlation measure can be computed without having to
`
`estimate the cosine and sinusoidal part of the light intensity oscillation. The knowledge
`
`30
`
`of the pattern configuration may be obtained prior to the 3 D scanning.
`
`In a further embodiment of the invention the correlation measure, A 1 within a group of
`sensor elements with label /is determined by means of the following formula:
`n
`Aj = LAA,j
`i=l
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`
`Where n is the number of sensor elements in a group of sensors, t, = (f1 J, ... f ,i is the
`reference signal vector obtained from knowledge of the pattern configuration, and ~ =
`(?ii,, ... lnJ is input signal vector. For the case of sensors grouped in square regions with
`Nsensors as square length then n = f\l.
`
`5
`
`Preferably, but not necessarily, the elements of the reference signal vector averages to
`
`zero over space, i.e. for all /we have
`
`n 'f =0
`
`L
`
`i=l
`to suppress the DC part of the correlation measure. The focus position corresponding
`
`1,1
`
`to the pattern being in focus on the object for a single group of sensor elements in the
`
`1 0
`
`camera will be given by an extremum value of the correlation measure of that sensor
`
`element group when the focus position is varied over a range of values. The focus
`
`position may be varied in equal steps from one end of the scanning region to the other.
`
`In the case of a static checkerboard pattern with edges aligned with the camera pixels
`
`15
`
`and with the pixel groups having an even number of pixels such as 2x2, 4x4, 6x6, 8x8,
`
`1Ox10, a natural choice of the reference vector /would be for its elements to assume
`
`the value 1 for the pixels that image a bright square of the checkerboard and - 1 for the
`
`pixels that image a dark square of the checkerboard.
`
`20
`
`Equivalent to the other correlation principles the spatial correlation principle may be
`
`applied in general within image analysis. In particular in a situation where the resolution
`
`of the camera is higher than what is necessary in the final image. Thus, a further
`
`embodiment of the invention relates to a method for calculating the amplitude(s) of a
`
`light intensity oscillation in at least one group of light sensitive elements, said light
`
`25
`
`intensity oscillation generated by a spatially varying static illumination pattern, said
`
`method comprising the steps of:
`
`providing the signal from each light sensitive element in said group of light sensitive
`
`elements, and
`
`calculating said amplitude(s) by integrating the products of a predetermined
`
`30
`
`function and the signal from the corresponding light sensitive element over said
`
`group of light sensitive elements, wherein said predetermined function is a function
`
`reflecting the illumination pattern.
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`
`To generalize the principle to a plurality of light sensitive elements, for example in a
`
`sensor array, the correlation measure or amplitude of the light oscillation in group /may
`
`be expressed as
`
`n
`
`A1 = L f{i, j)l,. J,
`
`l=l
`
`5
`
`where n is the number of sensor elements in group j, l1
`J is the signal from the lth sensor
`element in group /and
`f(i,j) is a predetermined function reflecting the pattern.
`
`Compared to temporal correlation, spatial correlation has the advantage that no moving
`
`pattern is required. This implies that knowledge of the pattern configuration may be
`
`10
`
`obtained prior to the 3 D scanning. Conversely, the advantage of temporal correlation is
`
`its higher resolution, as no pixel grouping is required.
`
`All correlation principles, when embodied with an image sensor that allows very high
`
`frame rates, enable 3 D scanning of objects in motion with little motion blur. It also
`
`15
`
`becomes possible to trace moving objects over time ("4D scanning"), with useful
`
`applications for example in machine vision and dynamic deformation measurement.
`
`Very high frame rates in this context are at least 500, but preferably at least 2000
`
`frames per second.
`
`20
`
`Transforming correlation measure extrema to 30 world coordinates
`
`Relating identified focus position(s) for camera sensor or camera sensor groups to 3 D
`
`world coordinates may be done by ray tracing through the optical system. Before such
`
`ray tracing can be performed the parameters of the optical system need to be known.
`
`25
`
`One embodiment of the invention comprises a calibration step to obtain such
`
`knowledge. A further embodiment of the invention comprises a calibration step in which
`
`images of an object of known geometry are recorded for a plurality of focus positions.
`
`Such an object may be a planar checkerboard pattern. Then, the scanner can be
`
`calibrated by generating simulated ray traced images of the calibration object and then
`
`30
`
`adjusting optical system parameters as to minimize the difference between the
`
`simulated and recorded images.
`
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`
`In a further embodiment of the invention the calibration step requires recording of
`
`images for a plurality of focus positions for several different calibration objects and/or
`
`several different orientations and/or positions of one calibration object.
`
`5
`
`With knowledge of the parameters of the optical system, one can employ backward ray
`
`tracing technique to estimate the 2D -> 3D mapping. This requires that the scanner's
`
`optical system be known, preferably through calibration. The following steps can be
`
`performed:
`
`1. From each pixel of the image (at the image sensor), trace a certain number of rays,
`
`1 0
`
`starting from the image sensor and through the optical system (backward ray tracing).
`
`2. From the rays that emit, calculate the focus point, the point where all these rays
`
`substantially intersect. This point represents the 3 D coordinate of where a 2 D pixel will
`
`be in focus, i.e., in yield the global maximum of light oscillation amplitude.
`
`3. Generate a look up table for all the pixels with their corresponding 3 D coordinates.
`
`15
`
`The above steps are repeated for a number of different focus lens positions covering
`
`the scanner's operation range.
`
`Specular reflections
`
`High spatial contrast of the in-focus pattern image on the object is often necessary to
`
`20
`
`obtain a good signal to noise ratio of the correlation measure on the camera. This in
`
`turn may be necessary to obtain a good estimation of the focus position corresponding
`
`to an extremum in the correlation measure. This sufficient signal to noise ratio for
`
`successful scanning is often easily achieved in objects with a diffuse surface and
`
`negligible light penetration. For some objects, however, it is difficult to achieve high
`
`25
`
`spatial contrast.
`
`A difficult kind of object, for instance, is an object displaying multiple scattering of the
`
`incident light with a light diffusion length large compared to the smallest feature size of
`
`the spatial pattern imaged onto the object. A human tooth is an example of such an
`
`30
`
`object. The human ear and ear canal are other examples. In case of intra oral
`
`scanning, the scanning should preferably be provided without spraying and/or drying
`
`the teeth to reduce the specular reflections and light penetration. Improved spatial
`
`contrast can be achieved by preferential imaging of the specular surface reflection from
`
`the object on the camera. Thus, one embodiment of the invention comprises means
`
`35
`
`for preferential / selectively imaging of specular reflected light and/or diffusively
`
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`
`reflected light. This may be provided if the scanner further comprises means for
`
`polarizing the probe light, for example by means of at least one polarizing beam
`
`splitter. A polarizing beam splitter may for instance be provided for forming an image of
`
`the object in the camera. This may be utilized to extinguish specular reflections,
`
`5
`
`because if the incident light is linearly polarized a specular reflection from the object
`
`has the property that it preserves its polarization state
`
`The scanner according to the invention may further comprise means for changing the
`
`polarization state of the probe light and/or the light reflected from the object. This can
`
`10
`
`be provided by means of a retardation plate, preferably located in the optical path. In
`
`one embodiment of the invention the retardation plate is a quarter wave retardation
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`plate. A linearly polarized light wave is transformed into a circularly polarized light wave
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`upon passage of a quarter wave plate with an orientation of 45 degrees of its fast axis
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`to the linear polarization direction. This may be utilized to enhance specular reflections
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`15
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`because a specular reflection from the object has the property that it flips the helicity of
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`a circularly polarized light wave, whereas light that is reflected by one or more
`
`scattering events becomes depolarized.
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`The field of view (scanning length)
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`20
`
`In one embodiment of the invention the probe light is transmitted towards the object in
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`a direction substantially parallel with the optical axis. However, for the scan head to be
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`entered into a small space such as the oral cavity of a patient it is necessary that the tip
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`of the scan head is sufficiently small. At the same time the light out of the scan head
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`need to leave the scan head in a direction different from the optical axis. Thus, a further
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`25
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`embodiment of the invention comprises means for directing the probe light and/or
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`imaging an object in a direction different from the optical axis. This may be provided by
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`means of at least one folding element, preferably located along the optical axis, for
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`directing the probe light and/or imaging an object in a direction different from the optical
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`axis. The folding element could be a light reflecting element such as a mirror or a
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`30
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`prism. In one embodiment of the invention a 45 degree mirror is used as folding optics
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`to direct the light path onto the object. Thereby the probe light is guided in a direction
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`perpendicular to the optical axis. In this embodiment the height of the scan tip is at
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`least as large as the scan length and preferably of approximately equal size.
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`29
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`One embodiment of the invention comprises at least two light sources, such as light
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`sources with different wavelengths and/or different polarization. Preferably also control
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`means for controlling said at least two light sources. Preferably this embodiment
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`comprises means for combining and/or merging light from said at least two light
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`5
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`sources. Preferably also means for separating light from said at least two light sources.
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`If waveguide light sources are used they may be merged by waveguides. However,
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`one or more diffusers may also be provided to merge light sources.
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`10
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`Separation and/or merging may be provided by at least one optical device which is
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`partially light transmitting and partially light reflecting, said optical device preferably
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`located along the optical axis, an optical device such as a coated mirror or coated
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`plate. One embodiment comprises at least two of said optical devices, said optical
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`devices preferably displaced along the optical axis. Preferably at least one of said
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`15
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`optical devices transmits light at certain wavelengths and/or polarizations and reflects
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`light at other wavelengths and/or polarizations.
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`One exemplary embodiment of the invention comprises at least a first and a second
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`light source, said light sources having different wavelength and/or polarization, and
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`20
`
`wherein
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`a first optical device reflects light from said first light source in a direction different from
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`the optical axis and transmits light from said second light source, and
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`a second optical device reflects light from said second light source in a direction
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`different from the optical axis. Preferably said first and second optical devices reflect
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`25
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`the probe light in parallel directions, preferably in a direction perpendicular to the
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`optical axis, thereby imaging different parts of the object surface. Said different parts of
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`the object surface may be at least partially overlapping.
`
`Thus, for example light from a first and a second light source emitting light of different
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`30
`
`wavelengths (and/or polarizations) is merged together using a suitably coated plate
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`that transmits the light from the first light source and reflects the light from the second
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`light source. At the scan tip along the optical axis a first optical device (e.g. a suitably
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`coated plate, dichroic filter) reflects the light from the first light source onto the object
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`and transmits the light from the second light source to a second optical device (e.g. a
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`35
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`mirror) at the end of the scan tip, i.e. further down the optical axis. During scanning the
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`30
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`focus position is moved such that the light from the first light source is used to project
`
`an image of the pattern to a position below the first optical device while second light
`
`source is switched off. The 3 D surface of the object in the region below the first optical
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`device is recorded. Then the first light source is switched off and the second light
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`5
`
`source is switched on and the focus position is moved such that the light from the
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`second light source is used to project an image of the pattern to a position below the
`
`second optical device. The 3D surface of th