SCANNING OCT OFF-WALL PARTICLE SIZING

Abstract

A method for determining a characteristic of a flowing fluid having particles in a sample space by Fourier domain optical coherence tomography includes estimating a velocity of the fluid in the sample space; controlling an optical scanner to radiate a beam of light along an optical path to the fluid in the sample space and to sense a signal of interference of measurement light scattered back along the optical path mixed with reference reflected light, while moving the optical scanner, where the beam of light is moved with a scanner velocity being aligned with a velocity component of the velocity of the fluid perpendicular to the optical axis of the beam of light, processing the signal into a corresponding complex-valued optical path length resolved OCT signal, where the OCT signal represents the fluid in the sample space; determining the characteristic of the fluid based on the OCT signal.

Claims

1. A method for determining a characteristic of a flowing fluid in a sample space by Fourier domain optical coherence tomography (FD-OCT), the fluid comprising particles, wherein the method comprises: estimating a velocity of the fluid in the sample space; controlling an optical scanner to radiate a beam of light along an optical path to the fluid in the sample space and to sense a signal of interference of (i) measurement light-scattered back along the optical path mixed with (ii) reference reflected light, while moving the optical scanner, wherein the beam of light is moved with a scanner velocity being aligned with a velocity component of the velocity of the fluid perpendicular to the optical axis of the beam of light, wherein a ratio of a magnitude of the scanner velocity and a magnitude of the velocity of the fluid is in the range of 0.1-10; processing the signal into a corresponding complex-valued optical path length, z, resolved OCT signal, a(t, z), wherein the OCT signal a(t, z) represents the fluid in the sample space, and determining the characteristic of the fluid based on the OCT signal a(t, z), wherein the characteristic of the fluid comprises one or more of a size of the particles in the fluid, a shape of the particles in the fluid, a diffusion coefficient of the particles in the fluid, a particle size distribution (PSD) of particles in the fluid, a local velocity, .sub.l, of the fluid, a velocity profile of the fluid in the sample space, and an mean velocity, <>, of the fluid in the sample space.

2. The method according to claim 1, wherein processing the signal comprises deriving the complex-valued optical path length, z, resolved OCT signal, a(t, z), from a time-resolved OCT wavelength spectrum of interference, and, wherein determining the characteristic of the fluid comprises determining at least one of (i) an autocorrelation function of the OCT signal, a(t, z) in a time domain and (ii) a frequency power spectrum of the OCT signal a(t, z) in the spectral domain, wherein the autocorrelation function of the OCT signal a(t, z) in the time domain comprises a z-resolved temporal autocorrelation function, G(, z), of a(t, z), in which t represents a lag time, and wherein the frequency power spectrum of the OCT signal a(t, z) in the spectral domain comprises a z-resolved frequency power spectrum, (, z), of a(t, z), in which represents an angular frequency.

3. The method according to claim 2, wherein determining the characteristic of the fluid comprises one or more of repeatedly determining the autocorrelation function of the OCT signal a(t, z) in the time domain and/or the frequency power spectrum of the OCT signal a(t, z) in the spectral domain for voxels in the sample space that are irradiated with the measurement light, while moving the optical scanner.

4. The method according to claim 2, wherein determining the characteristic of the fluid based on the autocorrelation function of the OCT signal a(t, z) comprises one or more of: determining, from G(, z), z- and -dependent decorrelation factors, g.sub.F(, z), related to a flow of the fluid; determining, from G(, z), z- and -dependent autocorrelations, g.sub.B(, z), representative of Brownian motion of the particles; determining, from G(, z), a characteristic optical path length, Z.sub.ss, representative of a photon mean free path in the flowing fluid, for which g.sub.B(, z) for z<Z.sub.ss in the flowing fluid are independent of z within a measurement noise; and determining, based on g.sub.B(, z) for z<Z.sub.ss in the flowing fluid, an averaged autocorrelation function, <g.sub.B()>, representative of single scattered measurement light, and performing a photon correlation spectroscopy (PCS) analysis using <g.sub.B()> to extract information related to the characteristic of the fluid; and wherein determining the characteristic of the fluid based on the frequency power spectrum of the OCT signal a(t, z) comprises one or more of: determining, from (, z), z-resolved power spectra, .sub.F(, z) related to the flow of the fluid; determining, from (, z), z-resolved power spectra .sub.B(, z) representative of Brownian motion of the particles; determining, from (, z), a characteristic optical path length, Z.sub.ss, representative of the photon mean free path in the flowing fluid, for which .sub.B(, z) for z<Z.sub.ss in the fluid are independent of z within the measurement noise; and determining, based on .sub.B(, z), for z<Z.sub.ss in the flowing fluid, an averaged power spectrum, <.sub.B()>, representative of single scattered light, and deriving, from <.sub.B()>, information related to characteristic of the fluid.

5. The method according to claim 2, comprising determining a diffusion coefficient of the particles from the autocorrelation function of the OCT signal, a(t, z) and/or the frequency power spectrum of the OCT signal a(t, z) and calculating the size of the particle from the diffusion coefficient.

6. The method according to claim 5, wherein the method further comprises calculating the particle size distribution of a plurality of particles from sizes calculated for the respective particles.

7. The method according to claim 1, wherein the characteristic of the fluid is determined based on a first-order autocovariance of the OCT signal, a(t, z).

8. The method according to claim 1, wherein the scanner direction and an overall flow direction of the of the fluid define an oblique angle, , wherein the method further comprises numerically aligning comprising translating OCT signal a(t, z) values along their respective optical paths relative to each other, such that for a first range of optical path lengths z.sub.0 to z.sub.1 indicative for a presence of fluid at a first time t.sub.1 in the sample space, and a second range of optical path lengths z.sub.2 to z.sub.3 indicative for the presence of fluid at a second time t.sub.2 in the sample space, z.sub.2 and z.sub.0 are substantially equal and z.sub.0 and z.sub.3 are substantially equal, wherein numerically aligning is performed prior to determining the characteristic of the fluid.

9. The method according to claim 8, wherein the method comprises: processing the signal of interference into the corresponding complex-valued optical path length, z, resolved OCT signal, a(t, z) and successively spatially shifting the transformed signal of interference, or applying a phase multiplication of the signal of interference in a frequency domain and successively Fourier transforming the phase multiplicated signal of interference to provide the complex-valued optical path length, z, resolved OCT signal, a(t, z).

10. The method according to claim 9, further comprising normalizing the signal of interference before numerically aligning, wherein the OCT signal a(t, z) is adjusted for a confocal point spread function, h(z), or wherein the OCT signal a(t, z) is normalized based on an average optical path, z, resolved amplitude signal.

11. The method according to claim 1, wherein the fluid is flown through a channel comprising the sample space, wherein estimating the velocity .sub.est of the fluid in the sample space comprises: determining the mean velocity, <>, of the fluid based on a fluid volume flow rate through the channel and a flow-through area of the channel, and estimating the velocity, .sub.est, as a local fluid velocity at a predetermined location in the channel based on a laminar flow profile of the fluid in the channel.

12. The method according to claim 11, wherein a channel wall of the channel defines a channel width (d), and wherein the estimated velocity, .sub.est, is estimated at a position in the channel, wherein a ratio of a minimal distance (d.sub.min) between said position and the channel wall is in the range of 0.1-0.2.

13. The method according to claim 1, wherein a ratio of a magnitude of the scanner velocity, .sub.b, and a magnitude of the velocity, .sub.est, of the fluid is in the range of 0.1-1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which FIGS. 1A-1D, 2, and 3 schematically depict general aspects of the invention; and FIG. 4 depicts some further aspects of the numerically alignment of the method. The schematic drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0080] FIG. 1A schematically depicts a geometry for OCT flow measurements, depicted in 2D. The fluid 10 flows in a channel 25 comprising the sample space 20. The channel 25 is oriented at an angle with respect to an x-y plane 201 perpendicular to the illumination direction in z. In general, a flow of the fluid 10 being laminar with transverse (that is in x and y direction) .sub.t(z) and axial .sub.z(z) components as a function of depth may be assumed. Given a total flow .sub.0(z), the flow components may be expressed as .sub.t=.sub.0(z)cos() and .sub.z=.sub.0(z)sin(). The angles and may not be equal due to a beam refraction. The depicted OCT beam is a Gaussian beam characterized by the waist w.sub.0 in focus, defined as a distance from a center of the beam 129 where the field amplitude is 1/e of its maximum value. Yet, in further embodiments the beam may also have another shape (as is known by the skilled person). The OCT beam 129 can be moved with the scanner velocity .sub.b, i.e. in the x-y-plane 201 (perpendicular to the beam 129). Herein also the phrase moving the scanner with a scanner velocity and comparable phrases are used especially indicating that the beam 129 is moved with the scanner velocity (or beam velocity) .sub.b. Based on this movement effectively the fluid 10 in the sample space 20 is scanned with scan speed .sub.s. Because of this, it may be described herein that the projection of the scan speed .sub.s in the transverse plane 201 is .sub.b. Likewise it may be stated that the projection of the scanner speed .sub.b onto the fluid flow .sub.0 direction is the .sub.s.

[0081] In FIGS. 1B-D this is further depicted in 3D. In FIG. 1B the orientation of the different velocity vectors with respect to the transverse plane or x-y plane 201 and the axial plane 210 are depicted in 3D, whereas FIG. 1C schematically depicts their projection in the axial plane 210, and FIG. 1D depicts the projection in the transverse plane 201. Here .sub.t and .sub.z are the angles between the scanning and flow vectors in the transverse and axial planes, respectively. The projection of the scan speed (or scan velocity) .sub.s in the axial plane is given by .sub.s. The projection of the scan speed .sub.s in the transverse plane is given by .sub.b.

[0082] A commonly used method for measuring axial flow velocity is through phase-resolved Doppler OCT. Due to the Doppler effect, the frequency of light scattered from a particle undergoing axial motion is shifted. The Doppler shift in the scattered light leads to a phase change of the OCT signal, (z). From the phase change the axial depth-resolved velocity .sub.z(z) may be determined using .sub.z(z)=(z)/qt, where t is the sampling time, and q=2nk.sub.0 the scattering wavenumber for the backscattering probe configuration, wherein n is the refractive index, n=c/ with c being the speed of light in vacuum and the phase velocity of the light in the fluid. The total and axial flow velocities are related with the expression .sub.0(z)=.sub.z(z)/sin(). The maximum velocity that can be estimated using this expression is limited by the Nyquist sampling criterion. The maximum unambiguously measurable axial velocity is .sub.z,max=/qt for low transverse velocity. Especially at high transversal velocities, because of the changing intensity of the illuminating beam on the moving particles, the phase change does not increase linearly with the velocity and approaches a constant value making it impossible to extract the velocity information.

[0083] Basically, Doppler methods may only be used to determine axial flows. To determine both axial and transverse flows, correlation-based methods may be used. For a Gaussian illuminating beam and Gaussian-shape spectral envelope, the depth-dependent autocovariance of the OCT complex signal in a backscattering geometry may be given by the next formula:

[00004]$g_1\left(z,\right)=A_1\left(z\right)e^{{\mathrm{iqv}}_z\left(z\right)}{e}^{-{\mathrm{Dq}}^2}{e}^{-\frac{{v_z\left(z\right)}^2{}^2}{2w_z^2}}{e}^{-\frac{{v_t\left(z\right)}^2{}^2}{w_0^2}},$ [0084] wherein where D is the diffusion coefficient, w.sub.z is the coherence function waist (1/e radius) in the sample, and is the correlation time lag. For the Gaussian spectrum, w.sub.z=1/(.sub.kn{square root over (2)}), with .sub.k being the source wavenumber spectrum standard deviation and n the sample refractive index. The parameter A.sub.1(z) is the autocovariance amplitude containing the effect of a diminishing signal-to-noise in depth and takes values between 0 and 1. Note that the decorrelation only depends on the in-focus beam radius w.sub.0 than the field decorrelation and can be expressed using the second-order autocovariance:

[00005]$g_2\left(z,\right)=.Math.g_1\left(z,\right).Math.=A_2\left(z\right)e^{-2{\mathrm{Dq}}^2}{e}^{-\frac{{v_z\left(z\right)}^2{}^2}{w_z^2}}{e}^{-\frac{2{v_t\left(z\right)}^2{}^2}{w_0^2}},$ [0085] wherein where A.sub.2(z) is a depth-dependent amplitude factor. The above given formulae describe some examples that may be used in the method and are only given for further explaining the methods. Further examples are known to the skilled person. Below further reference is made to the second-order autocovariance g.sub.2(z, ).

[0086] When the autocovariance function is used for estimating the flow, the 1/e decay time of the autocorrelation must especially be equal or larger than the temporal sampling t. From this requirement, the maximum measurable transverse and axial flow speeds are .sub.t,max=w.sub.0/2t, and .sub.z,max=w.sub.z/t. These maximum flow speeds are derived under the assumption of ideal sampling. However, when the measurements are performed while integrating over a specific detector time, defined T=t/C (with C, the multiplicative constant, being larger or equal to 1), the axial motion of a sample during the integration time may cause a significant SNR degradation that limits the axial velocity to .sub.z,max=C/qt.

[0087] The maximum measurable flow speed .sub.t,max=w.sub.0/2t, and .sub.z,max=w.sub.z/t. limit the maximum measurable transverse and axial velocity components from a correlation perspective: when the effective particle displacements become comparable to the transverse and axial resolutions, the acquired signals become completely decorrelated within a single acquisition time. Formula .sub.z,max=C/qt limits the axial velocity due to the fringe washout at the detector. For a spectrometer-based OCT systems, the detector integration time may be comparable to the sampling time, i.e. C1. Such systems may especially operate in the visible and infrared wavelength ranges with q>>1/w.sub.z, therefore limiting the axial velocity by the fringe washout, i.e., through .sub.z,max=C/qt., rather than by the axial resolution, i.e., through .sub.z,maxw.sub.z/t.

[0088] To circumvent the limit imposed .sub.t,max=w.sub.0/2t, the method of the invention comprises implementation of flow quantification using B-scan correlation analysis. When moving the OCT beam in any direction with constant vs, while acquiring the signal, .sub.t(z) and .sub.z(z) in the autocorrelation function should be replaced by effective transverse and axial velocities, respectively .sub.r(z) and .sub.z(z) given by:

[00006]${v_t\left(z\right)}^2=\left[{\left(v_0\left(z\right)-v_s\cos \left({}_t\right)\right)}^2+{\left(v_s\sin {}_t\right)}^2\right]{\cos }^2=\left[{v_0\left(z\right)}^2--2v_0\left(z\right)v_s\cos \left({}_t\right)+v_s^2\right]{\cos }^2,\mathrm{and}$${v_z\left(z\right)}^2=\left[{\left(v_0\left(z\right)-v_s\cos \left({}_z\right)\right)}^2+{\left(v_s\sin {}_z\right)}^2\right]{\sin }^2=\left[{v_0\left(z\right)}^2-2v_0\left(z\right)v_s\cos \left({}_z\right)+v_s^2\right]{\sin }^2,$ [0089] wherein .sub.t and .sub.z are the angles defining the scan direction, shown in FIGS. 1B-D. When the beam motion is sufficiently aligned with the flow velocity, so that cos(.sub.t) and cos(.sub.z) are about one, these equations may be simplified to:

[00007]$v_t\left(z\right)=\left(v_o\left(z\right)-v_s\right)\cos =v_o\left(z\right)\cos ,\mathrm{and}$$v_z\left(z\right)=\left(v_o\left(z\right)-v_s\right)\sin =v_o\left(z\right)\sin .$

[0090] Hence, with an ideal scanning alignment, the ratio of the effective transverse and axial velocity components remains unchanged irrespective of the beam scanning speed. This simplification removes extra unknowns from the decorrelation rate and makes it possible to determine the velocity components in the identical way with or without scanning the beam. Therefore, a generalized autocovariance model of the OCT signal magnitude incorporating the beam motion can be written as

[00008]$g_2\left(z,\right)=A_2\left(z\right)e^{-2{\mathrm{Dq}}^2}{e}^{-\frac{{\left(v_0\left(z\right)-v_s\right)}^2{\sin }^2{}^2}{w_z^2}}{e}^{-\frac{2{\left(v_0\left(z\right)-v_s\right)}^2{\cos }^2{}^2}{w_0^2}}$

[0091] This modified formula especially holds for systems wherein the optical path length z is close to, especially the same, for all physical depths in the B-scan. For this modified formula the axial velocity limit is unchanged and limited by the fringe washout via .sub.z,max=C/qt. However, the transverse velocity limit is modified and is now limited by the relative velocity |.sub.t.sub.s cos()| rather than the absolute velocity .sub.t. This implies that for flows uniform along the length of the B-scan the maximum measurable flow is limited by the absolute difference between flow and scanning speeds. The application of B-scanning in correlation analysis can give a significant improvement because for a typical OCT flow geometry the transverse flow is much higher than the axial flow and the limitation caused by the transverse flow is more restrictive than that for axial flow. For a flow profile .sub.t(z) the most optimum scan speed may in embodiments be selected such that the decorrelation rate is at its maximum for the highest and lowest flows, i.e., the scan speed, is mid-range in the flow velocity with .sub.s cos()=(max(.sub.t(z))+min(.sub.t(z)))/2. This scanning speed may cause maximum and equal decorrelation rates for the maximum and minimum flow speeds.

[0092] Hence, by moving the scanner, the maximum transverse velocity may be limited at

[00009]${\max }_{B-\mathrm{scan}}\left(v_t\left(z\right)\right)=\frac{\sqrt{2}{w}_0}{t}+\frac{\min \left(v_t\left(z\right)\right)}{2},$

for flow profiles with min (.sub.t(z))=0. Hence, the B-scan flow limit is a factor of 2 larger than the M-scan correlation flow limit. This equation is based on the assumption that there is only a small angle between scanning and flow vectors. Experimentally, it appeared that with angles up to 10 the method using the small angle approximation provided good results. To obtain the actual flow speeds, the scan speeds need to be added to the effective velocities after acquisition. Therefore, for flows with a non-uniform transverse component, assuming that the beam can be moved at sufficiently high velocities, the maximum flow that can be determined with B-scanning is at least twice the flow that can be determined without. Same relations can be applied to the axial flow if a numerical alignment is applied.

[0093] In FIGS. 2-3 some further aspects of the method are depicted. The method is used for determining a characteristic of a flowing fluid 10 in a sample space 20 by optical coherence tomography (OCT). In the fluid 10 only a single particle 11 is depicted for clarity reasons. The method may especially be used to determining one or more characteristics of a diffusion coefficient D of particles 11 in the fluid 10, a size 12 of the particles 11 in the fluid 10, a shape of the particles 11 in the fluid 10, a particle size 12 distribution PSD of particles 11 in the fluid 10, a local velocity, .sub.l, of the fluid 10, a velocity profile of the fluid 10 in the sample space 20, and a mean velocity, <>, of the fluid 10 in the sample space 20.

[0094] The method may comprise different steps, such as estimating a velocity .sub.est of the fluid 10 in the sample space 20; controlling an optical scanner 110 to radiate a beam 129 of measurement light 120 along an optical path 121 to the fluid 10 in the sample space 20 and to sense a signal 122 (of interference) of (measurement) light 120 scattered back along the optical path 121 mixed with reference reflected measurement light 123. Because, sensing a signal 122 of interference of reference light with backscattered light, is more or less standard for OCT measurement, this part is only very schematically depicted in the figure by mixing (or combining) the reference light 123 with backscattered light 120, that successively propagate to give a signal of interference 122 on a sensor or detector of the scanner 110. The last being very schematically indicated by the arrow pointing to a location where the lights 123, 120 interfere on the sensor/detector. Further, the optical scanner 110 is moved with a scanner velocity, .sub.b. This scanner velocity .sub.b refers to a physical movement of the scanner 100 and may herein also be indicated as the beam velocity .sub.b or the velocity of the beam 129. This scanner velocity .sub.b, and there with the scan velocity vs, is especially (set based on) a function of the velocity, .sub.est, of the fluid 10. The method further comprises processing the signal 122 into a corresponding complex-valued optical path 121 length z resolved OCT signal a(t, z), herein also (simply) indicated as OCT signal. The OCT signal may especially represent the fluid 10 in the sample space 20. In embodiments processing the signal 122 comprises deriving the complex-valued optical path (121) length, z, resolved OCT signal, a(t, z), from a time-resolved OCT wavelength spectrum of interference.

[0095] In embodiments, the scanner 110 is moved in a scanner direction (a direction of the scanner velocity .sub.b) being aligned with a velocity component of the velocity, .sub.est, of the fluid 10 (in a plane) perpendicular to the optical axis 125 of the beam 129 of measurement light 120. In further embodiments, the scanner 110 is moved in alignment with an overall flow direction of the fluid 10 in the sample space 20. The directions of the scanner velocity .sub.b may thus differ between embodiments. In the FIG. 2, this is schematically indicated by two scanner velocities .sub.b1 and .sub.b2. The movement of the scanner 10 relative to the sample space 20 may be selected based on an estimated velocity .sub.est at a specific location in the sample space 20. The local velocity .sub.l may differ significantly depending on the location in the samples space 20. For instance, in the depicted embodiment in FIG. 2, a laminar flow profile 15 is depicted. Here, the velocity .sub.l,1 or speed at a first location is about 25% higher than the value of the velocity .sub.l,2 at the second location. The scanner velocity .sub.b may especially be selected to be the mean value of the lowest (estimated) local velocity .sub.l and the highest (estimated) local velocity .sub.l in the scanning space 20. It is noted that in FIG. 2 the local velocity (speed) is indicated with .sub.l, whereas in FIGS. 1 .sub.0 refers to the local velocity (speed).

[0096] The method may further comprise determining the characteristic of the fluid 10 based on the OCT signal a(t, z).

[0097] Above, many different ways are described to determine the characteristics. Determining the characteristic may especially (repeatedly) determining the autocorrelation function of the OCT signal a(t, z) in the time domain and/or determining the frequency power spectrum of the OCT signal a(t, z) in the spectral domain (for both ways), especially for voxels in the sample space 20 that are irradiated with the measurement light 120 (while moving the optical scanner 110).

[0098] In further embodiments, e.g., the characteristic of the fluid 10 is determined based on a first-order autocovariance of the complex-valued OCT signal.

[0099] The method may comprise Fourier domain low coherence tomography FD-LCT.

[0100] The method may further comprise calculating the size 12 of one or more particles 11, especially more particles and/or a particle size distribution (PSD) from their diffusion coefficients, wherein the diffusion coefficients are determined from the autocorrelation function of the OCT signal and/or from the frequency power spectrum of the OCT signal 122.

[0101] FIG. 2 further depicts an angle between the optical path 121 and (the direction of) the mean velocity <> of the fluid 10. Essential the angle relates to the angle , e.g., given in FIG. 1. The sum of these angle , may be equal to 180. Furthermore, some different optical paths 121 based on the movement of the scanner 110 are depicted with reference 121A and 121B. Especially a plurality of optical paths 121 may define the sample space 20.

[0102] FIG. 3, further schematically depicts some further aspects of the method. In the figure, the mean velocity <> is depicted, which can be defined for the depicted channel 25 as a ratio of the total fluid 10 flow (rate) through the channel 25 to the flow-through area 27 (herein also indicated as cross section 27, or cross-sectional area 27). This is especially the inner cross section 27 defined by the channel wall 26 and perpendicular to the channel axis 29. It is further noted that in FIG. 3 further the depth direction 29 and the z-direction are depicted; showing that they are not aligned.

[0103] FIG. 4, further schematically depicts the numerical aligning when the scanner direction and an overall flow direction as indicated by the arrow of <> of the of the fluid 10 define an oblique angle the method may comprise numerically aligning. In the figure this is very schematically depicted. Basically, measured (processed) signals at a respective z value may be translated to a new z value such that, such that for a first range of optical path lengths z.sub.0 to z.sub.1 indicative for a presence of fluid 10 at a first time t.sub.1 and a second range of optical path lengths z.sub.2 to z.sub.3 indicative for the presence of fluid 10 at a second time t.sub.2 in the sample space, z.sub.2 and z.sub.0 are substantially equal (have the same value) and z.sub.0 and z.sub.3 are substantially equal

[0104] Numerically aligning is especially performed prior to determining the autocorrelation function of the OCT signal and/or prior to determining the frequency power spectrum of the OCT signal.

[0105] The term plurality refers to two or more. Furthermore, the terms a plurality of and a number of may be used interchangeably.

[0106] The terms substantially or essentially herein, and similar terms, will be understood by the person skilled in the art. The terms substantially or essentially may also include embodiments with entirely, completely, all, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term substantially or the term essentially may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms about and approximately may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms substantially, essentially, about, and approximately may also relate to the range of 90%-110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

[0107] The term comprise also includes embodiments wherein the term comprises means consists of.

[0108] The term and/or especially relates to one or more of the items mentioned before and after and/or. For instance, a phrase item 1 and/or item 2 and similar phrases may relate to one or more of item 1 and item 2. The term comprising may in an embodiment refer to consisting of but may in another embodiment also refer to containing at least the defined species and optionally one or more other species.

[0109] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0110] The devices, apparatus, or systems may herein, amongst others, be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

[0111] The term further embodiment and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment but may also refer to an alternative embodiment.

[0112] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

[0113] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0114] Use of the verb to comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words comprise, comprising, include, including, contain, containing and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of including, but not limited to.

[0115] The article a or an preceding an element does not exclude the presence of a plurality of such elements.

[0116] The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to an advantage.

[0117] The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

[0118] The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

[0119] The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.