GB2589614A

GB2589614A - Apparatus, systems and methods for detecting light

Info

Publication number: GB2589614A
Application number: GB1917788.0A
Authority: GB
Inventors: Yuan Xin; Wilford Paul; Qiao Mu; Liu Xuan
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2019-12-05
Filing date: 2019-12-05
Publication date: 2021-06-09
Also published as: WO2021111237A1; GB201917788D0

Abstract

Apparatus for optical coherence tomography comprises a modulating unit 3 for spatially modulating an input beam 9 of light and emits a modulated beam 11 of light, a dispersing unit 5 which emits a dispersed beam 13 and a detecting unit 7 that detects a of light and converts the detected beam of light into an electrical output signal 15. The detector unit is arranged to be moveable relative to the dispersing unit so as to allow the simulation of a frequency shift and thus enable calibration of the modulating unit. A monochromatic light source may be provided so as to allow the simulation of differing wavelengths

Description

TITLE

Apparatus, Systems and Methods for Detecting Light

TECHNOLOGICAL FIELD

Examples of the disclosure relate to apparatus, systems and methods for detecting light. In particular some examples relate to apparatus, systems and methods for detecting light that has been subjected to compressive sensing.

BACKGROUND

According to the theory of compressive sensing, traditional sampling is replaced by measurements of inner products with random vectors.

Light modulated by reflection from or transmission through an object, when detected directly by a two-dimensional pixelated detector, is an oversampled field that has a sparser representation in some domain. As a consequence, detecting spectrally dispersed coded fields (sparse incoherent fields rather than the whole field) can capture sufficient information to characterise the object. For example, spectral images of the object can be determined from the detected spectrally dispersed coded fields.

BRIEF SUMMARY

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector.

In at least some examples, the means for causing relative movement of the dispersing means and the detector simulates a frequency shift of the beam of light to enable calibration of at least the modulating means.

In at least some examples, the means for causing relative movement of the dispersing means and the detector is configured to change a relative position of the detector compared to the dispersing means from a first relative position to a second relative position, wherein the second relative position is offset in a first direction from the first relative position, at the detector, by an offset value, and wherein the apparatus is configured such that when the dispersing means and the detector are at the first relative position, light of a first wavelength in the beam of light is dispersed in the first direction onto a first area of the detector and light of a second wavelength, different to the first wavelength, in the beam of light is dispersed in the first direction onto a second area of the detector overlapping the first area and offset from the first area by the offset value, and when the dispersing means and the detector are at the second relative position, light of the first wavelength in the beam of light is dispersed in the first direction onto the second area of the detector.

In at least some examples, the apparatus comprises a monochromatic light source for providing, during calibration, a monochromatic beam of light, wherein the beam of light during a calibration is the monochromatic beam of light.

In at least some examples, the means for causing relative movement of the dispersing means and the detector simulates, for the monochromatic beam of light, arrival at the detector of the monochromatic beam of light as if it were a different wavelength.

In at least some examples, the dispersing means provides dispersion in a dispersion direction, and wherein the means for causing relative movement of the dispersing means and the detector causes relative movement in the dispersion direction.

In at least some examples, the dispersion direction is aligned with rows or columns of pixels in the detector and/or the dispersion direction is aligned with rows or columns of modulating pixels in the modulating means.

In at least some examples, the dispersing means comprises one or more refractive elements or one or more diffractive elements.

In at least some examples, the means for causing relative movement of the dispersing means and the detector is configured to move the dispersing means.

In at least some examples, the means for causing relative movement of the dispersing means and the detector is configured to rotate the dispersing means.

In at least some examples, the dispersing means is a diffraction grating wherein rotation by an angle de simulates a wavelength shift dA, proportional to de, of the beam of light to enable calibration of at least the modulating means.

In at least some examples, the modulating means comprises a two-dimensional spatially coded aperture comprising at least a first plurality of portions, having a first transparency, and at least a second plurality of portions, having a second different transparency, wherein the first plurality of portions and the second plurality of portions are spatially distributed in two dimensions.

In at least some examples, the first portions and the second portions are arranged in an array of non-overlapping pixelated portions and are arranged in rows and columns.

In at least some examples, the apparatus comprises a double path interferometer comprising a sample path for an object and a reference path; means for superposing the sample path and reference path during measurement to create the beam of light for detection; means for blocking the sample path during calibration.

In at least some examples, the apparatus comprises a broadband light source for providing, during measurement, a broadband beam of light for illuminating an object, wherein the beam of light during the measurement is the broadband beam of light.

In at least some examples, a system comprises the apparatus.

In at least some examples, the system or apparatus comprises processing means for processing output of the detector for a position of the dispersing means, after calibration, to produce a three-dimensional image of the object.

In at least some examples, the system or apparatus comprises processing means for processing outputs of the detector for different relative positions of the dispersing means and the detector to calibrate the system.

In at least some examples, there is provided a calibrating method comprising causing relative movement of the dispersing means and the detector of the apparatus of any preceding claim.

In at least some examples, there is provided a computer program for calibrating an apparatus comprising causing relative movement of the dispersing means and the detector of the apparatus of any preceding claim.

According to various, but not necessarily all, embodiments of the invention there is provided a system comprising: n apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector wherein the system or apparatus comprises processing means for processing outputs of the detector for different relative positions of the dispersing means and the detector to calibrate the system.

According to various, but not necessarily all, embodiments of the invention there is provided a calibrating method for an apparatus, the apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector, wherein the method comprises causing relative movement of the dispersing means and the detector of the apparatus.

According to various, but not necessarily all, embodiments of the invention there is provided a computer program for calibrating an apparatus, the apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector.

wherein the computer program is configured to cause relative movement of the dispersing means and the detector of the apparatus.

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus comprising: a coded aperture configured to spatially modulating a beam of light to produce a modulated beam of light; one or more disperser elements configured to disperse the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and an actuator configured to cause relative movement of the disperser element and the detector.

According to various, but not necessarily all, embodiments of the invention there is provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which: Fig. 1 illustrates an example apparatus for measurement and for calibration; Fig. 2 shows the compressive sensing principle of examples of the disclosure; Fig. 3 illustrates an optical coherence tomography arrangement for measurement; and Fig. 4 illustrates an optical coherence tomography arrangement for calibration; and Fig. 5A and 5B illustrates how a modulated and dispersed beam of light, having respective wavelengths Ao and Al, is incident on a detector, when the dispersing means and the detector have a first relative position; and Fig. 5C illustrates how a modulated and dispersed beam of light, having a wavelength Ao, is incident on the detector when the dispersing means and the detector have a second relative position.

DETAILED DESCRIPTION

Examples of the disclosure relate to an apparatus 1 which performs compressive sensing (compressive sampling) using modulating means 3 for spatially modulating a beam of light 9, dispersing means 5 for dispersing the modulated beam of light 11 and detector 7 for detecting the spatially modulated and dispersed beam of light 13.

In particular the apparatus 1 is configured to allow relative movement of the dispersing means 5 and the detector 7 so that apparatus 1, including the modulating means 3, can be calibrated.

The calibration process can, for example occur at the processing means 10.

The beam of light 9 can, for example, arrive from any suitable source 17.

The beam of light 9 can, for example, comprise light that has been reflected from or that has passed through a scene or an object 21.

In some but not necessarily all examples, the beam of light 9 can be provided by an optical coherence tomography (OCT) arrangement, which acts as the source 17.

Fig. 1 schematically illustrates an example apparatus 1. The example apparatus 1 comprises modulating means 3 for spatially modulating a beam of light 9 to produce a modulated beam of light 11; dispersing means 5 for dispersing the modulated beam of light 11 to produce a spatially modulated and dispersed beam of light 13; a detector 7 configured to detect the spatially modulated and dispersed beam of light 13; and means for causing relative movement of the dispersing means 5 and the detector 7.

The modulating means 3 can, in some examples, comprise one or more spatial modulators.

An example of a spatial modulator 3 is a spatially coded aperture.

In the example of Fig. 1, a modulator 3 is arranged within the apparatus 1 so that when the apparatus 1 is coupled to source of the input beam of light 9, the input beam of light 9 is incident, at least in part, upon the spatial modulator 3.

The spatial modulator 3 may selectively remove information from the input beam of light 9 so that only portions of the input beam of light 9 are detected. In some examples the spatial modulator 3 may be arranged to convert a three-dimensional signal [x, y, A] into a two-dimensional signal [x, y].

The spatial modulator 3 may comprise any means which may be arranged to spatially modulate the input beam of light 9. The spatial modulation occurs over a transverse cross-sectional area of the input beam of light 9. The modulation comprises amplitude modulation that varies in dependence upon a location within the transverse cross-sectional area of the input beam of light 9.

In some examples the spatial modulator 3 comprises a spatially coded aperture. The spatially coded aperture provides for spatial modulation over a cross-sectional area of the input beam of light 9 that passes through the coded aperture. The coded aperture is coded to provide amplitude modulation that varies in dependence upon a location within the aperture.

The coded aperture defines a fixed two-dimensional pattern of spatially varying transparency. The spatially coded aperture physically modulates the beam of light to a spatially compressed/sparse format.

The spatially coded aperture may comprise a non-uniform optical mask or any other suitable type of aperture that provides amplitude modulation that varies in dependence upon a location within the aperture.

The spatially coded aperture may be a two-dimensional spatially coded aperture or any other suitable type of aperture. The two-dimensional spatially coded aperture defines a two-dimensional plane. The beam of light 9 may travel in a direction normal (orthogonal) to the two-dimensional plane.

In other examples the spatial modulator 3 could comprise a liquid crystal on silicon (LCOS) modulator, a digital micromirror device (DMD) array or any other suitable type of spatial modulator 3.

The spatial modulator 3 can comprise multiple different portions that have a particular transparency. In some examples the spatial modulator 3 may comprise at least a first portion having a first level of transparency to the input beam of light 9 and at least a second portion having a second, different level of transparency to the input beam of light 9. In some examples the spatial modulator 3 may comprise at least multiple spatially distributed non-overlapping first portions, that are distributed over an area in two dimensions and have a first level of transparency to the input beam of light 9 and at least multiple spatially distributed non-overlapping second portions that are distributed over the area in two dimensions and have a second, different level of transparency to the input beam of light 9. In at least some examples, the spatially distributed first portions and the spatially distributed second portions do not overlap. The spatially distributed first portions and the spatially distributed second portions can be contiguous and, in some examples, the spatially distributed first portions and the spatially distributed second portions completely fill the area. The different levels of transparency may allow different levels of light to pass through the spatial modulator 3. In some examples the spatial modulator 3 may be a binary modulator 3 so that only two different absorbencies are provided by the respective portions of the spatial modulator 3. In other examples the spatial modulator 3 may be a grey-scale modulator and may comprise more than two different levels of transparency in the different portions of the spatial modulator 3.

The different portions of the spatial modulator 3 may be arranged in any suitable pattern. In some examples the respective portions of the spatial modulator 3 having different transparencies are pixelated and arranged in a pixelated pattern. The pixelated arrangement may have the respective portions of the spatial modulator 3 arranged in an array of columns and rows of pixels. In some examples, the pixels are square or rectangular.

The coded aperture can comprise multiple different portions that are coded with a particular transparency, for example, the coded aperture can be pixelated and comprise multiple different portions (pixels) that are arranged as an array in rows and columns, where the pixels are coded with a particular transparency. The two-dimensional pattern of pixels (portions) that have a first transparency is different to the two-dimensional pattern of pixels (portions) that have a second 1 0 transparency, different to the first transparency.

The transparency at each pixel defines a fixed two-dimensional pattern of spatially varying transparency. In some examples, the transparency at each pixel in a row defines a fixed one-dimensional pattern of spatially varying transparency that does not repeat or does not repeat within a minimum number of columns. In some examples, the transparency at each pixel in a column defines a fixed one-dimensional pattern of spatially varying transparency that does not repeat or does not repeat within a minimum number of rows. In some examples, the transparency at each pixel defines a fixed two-dimensional pattern of spatially varying transparency that has a random or pseudorandom spatial distribution. In some examples, the pixels are coded as either opaque or transparent. In other examples, the pixels are coded using grey scale.

The size p of the pixels when projected onto a detector 7, can be directly proportional to a size d of pixels of the detector 7.

The number of transparent pixels, partially transparent pixels, and opaque pixels in a spatially coded aperture can vary in different implementations of the disclosure. In some examples approximately half of the pixels of the modulator could be absorbent so that half of the incident area of the modulator acts to block the input beam of light 9 while the other half allows the incident beam of light to pass, or partially pass through in a spatially-coded format.

In some examples the different portions (e.g. pixels) of the spatial modulator 3 may be arranged in a random pattern (which encompasses pseudo random patterns) that is random in two dimensions. The random pattern may be an irregular pattern. The random pattern might not be defined or arranged in relation to any specific object. In other examples the respective portions (e.g. pixels) of the spatial modulator 3 may be arranged in a predetermined pattern.

The predetermined pattern may be selected according to the source 17 of the broadband beam of light 9. It can for example be selected according to the object 21 or type of object that is to be imaged, for example, by an OCT arrangement.

In some examples the spatial modulator 3 may be fixed in position relative to the other components of the apparatus 1. In other examples the spatial modulator 3 may be arranged 1 0 to be moveable between imaging measurements relative to the other components of the apparatus 1. In particular the spatial modulator 3 may be moveable so that the spatial modulator 3 can be shifted relative to the dispersing means 5.

In some examples the transparency of the portions of the spatial modulator 3 may be wavelength dependent. In such examples the modulation of the input beam of light 9 by the respective portions of the spatial modulator 3 will be dependent upon the wavelengths within the input beam of light 9.

The spatial modulator 3 provides a spatially modulated beam of light 11 as an output.

The spatially coded aperture can be a fixed spatially coded aperture that remains fixed during integration time of detector 7.

The dispersing means 5 for dispersing the modulated beam of light 11 is arranged within the apparatus 1 so that the spatially modulated beam of light 11, or at least part of the spatially modulated beam of light 11, provided by the spatial modulator 3 is incident upon the dispersing means 5. Dispersion converts a spectral difference (a difference in wavelength of the light) into a spatial offset.

The dispersing means 5 can, in some examples, comprise one or more disperser elements.

The dispersing means 5 is configured to cause a wavelength dependent spatial shift of the same fixed spatially coded aperture, defined by the spatial modulator 3. In at least some examples the spatial shift is only in the plane of the aperture/beam (2D dispersion). In at least some examples, the spatial shift is only in one dimension (1D dispersion). That one dimension can be aligned with a row (or a column) of pixels in the spatially coded aperture and/or pixels of the detector 7.

The dispersing means 5 configured to disperse the modulated beam of light 11 can comprise one or more dispersing elements. The dispersing elements 5 may comprise any elements which cause different wavelengths of the modulated beam of light 11 to be dispersed by different amounts. The one or more dispersing elements 5 may comprise a refractive element, for example a prism, or a diffractive element, for example a grating, which can be a transmissive diffraction grating or a reflective diffraction grating or any other suitable elements.

The dispersing means 5 can be a prism or a combination of prisms. A prism is a polyhedron with two faces parallel, and with surface normals of the other faces lying in the same plane.

The or each prism can be a triangular prism. The triangular prism can have a constant triangular cross-section that has a shape of an isosceles triangle or an equilateral triangle.

The dispersing means 5 provides a spatially modulated and dispersed beam of light 13 as an output.

The detector 7 is configured to detect the dispersed beam of light 13. The detector 7 is arranged within the apparatus 1 so that the spatially modulated and dispersed beam of light 13, or at least part of the spatially modulated and dispersed beam of light 13, is incident on the detector 7 for detecting the modulated and dispersed beam of light 13.

The detector can be arranged to transduce an incident beam of light into an electrical output signal 15. In some examples the detector 7 may comprise a charge-coupled device, complementary metal-oxide semiconductor (CMOS) sensors or any other suitable type of sensors.

In some examples the detector 7 may comprise a two-dimensional array of sensors (pixels).

Measurement The beam of light 9 used during measurement has a broad spectrum (it is broadband). It comprises light that has a broad frequency spectrum. The broadband beam of light 9 can, for example, comprise light that has wavelengths that differ by over 20nm. The broadband beam of light 9 can, for example, comprise light that has wavelengths that differ by between 20nm and 50nm.

The bandwidth of the beam of light corresponds, through the dispersing means 5, to a maximal spatial shift between spatially coded aperture patterns for different wavelengths. In at least some examples, the pattern of the spatially coded aperture does not repeat in the direction of the spatial shift for at least a distance corresponding to the maximal spatial shift.

The spatially modulated beam of light 11 is a sparse three-dimensional data cube [x, y, A] with a two-dimensional slice [x,y] for each wavelength channel coded by the same fixed spatially coded aperture 3 that has variable transparency in the x-y plane. The spatially modulated and dispersed beam of light 13 represents a skewed version of the sparse three-dimensional data cube. The skew (offset), caused by the dispersing means 5, is within the x-y plane and is proportional to wavelength. In the example illustrated in Fig. 2 it is in the y-direction only. Each spatially coded two-dimensional slice [x,y] for each wavelength channel n is shifted (offset) yn. The detector 7 detects the superposition of the offset spatially coded two-dimensional slices [x,y] for each wavelength channel n. This reduces the sparse three-dimensional data cube to a two-dimensional projection in a single shot. It collapses overlapping differently masked spectrograms for different channels to a single spectrogram. The different masked spectrograms are incoherent.

In other examples the detector 7 may comprise a linear detector which may be scanned across a detecting plane.

Where of the broadband beam of light 9 has been reflected from or has passed through a scene or an object 21, then the output signal 15 provided by the detector 7 comprises information indicative of the scene or object 21. In some but not necessarily all examples, the processing means 10 uses the output signal 15 to provide a spectral image of the scene or object 21.

Where the source 17 of the broadband beam of light 9 is an optical coherence tomography (OCT) arrangement, the output signal 15 provided by the detector 7 comprises information indicative of the object 21 which can, for example, be a three-dimensional object imaged by the OCT arrangement 17.

In some but not necessarily all examples, the processing means 10 processes the output signal 15, for example using non-linear optimization, to produce a three-dimensional image of the object 21. This produced three-dimensional image can be rendered on a display or other suitable user output device.

The processing means 10 can be a part of the apparatus 1 or, as shown in Fig. 1, separate from the apparatus 1. In some examples, the processing means 10 is remote from the apparatus 1. The processing means 10 can comprise a processor or controller and memory.

The processing means 10 can comprise load and us a computer program stored in the memory to perform its functions.

Fig. 2 shows the compressive sensing principle of examples of the disclosure.

In the example of Fig. 2, the object 21 reflects broadband light which has been directed onto the object 21. Different wavelengths of the incident light are reflected differently depending upon the internal structure of the object 21. This provides a plurality of spatial images 23. Each of the spatial images 23 corresponds to a different wavelength of light A (Ai to AN). The different spatial images 23 therefore comprise information about the internal structure of the object 21.

The different spatial images 23 define a three-dimensional signal [x, y, The reflected beam of light 9 is a three-dimensional data cube [x, y, Ai] with a two-dimensional slice [x,y], a spatial image 23, for each wavelength channel A. In the example of Fig. 2 the spatial modulator 3 comprises a two-dimensional spatially coded aperture. Other types of modulator 3 may be used in other examples of the disclosure, for example as previously described.

The spatial images 23 in the input beam of light 9 are modulated by the spatially coded aperture to produce a spatially modulated beam of light 11.

The spatially modulated beam of light 11 is a sparse three-dimensional data cube [x, y, Ad with a two-dimensional slice [x,y] for each wavelength channel K, coded by the same fixed spatially coded aperture that has variable transparency in the x-y plane.

The spatially modulated beam of light 11 provided by the spatial modulator 3 is then spread by the dispersing element 5. In the example of Fig. 2 the dispersing element 5 comprises a triangular prism. Other types of dispersing element 5 could be used in other examples of the disclosure, as previously described. In this example, the dispersing element 5 refracts the modulated beam of light 11 to spatially spread the modulated beam of light in the y-direction only. Different wavelengths of the spatial images 23 are spread by a different amount in the y-direction as shown schematically in Fig. 2. The distance by which a spatial image 23 is spread by the dispersing element 5 is dependent upon (e.g. proportional to) the wavelength of the spatial image 23.

The spatially modulated and dispersed beam of light 13 represents a skewed version of sparse three-dimensional data cube. The skew (offset), caused by the dispersing means 5, is within the x-y plane and is proportional to wavelength. In the example illustrated in Fig. 2 it is in the y-direction only. Each spatially coded two-dimensional slice [x,y] for each wavelength channel i is shifted (offset) y,.

The spatially modulated and dispersed beam of light 13 is then incident upon the detector 7.

The detector 7 comprises a plurality of pixels 25. Only one pixel 25 is shown for clarity in Fig. 2. The plurality of pixels 25 may be arranged in any suitable array. In the example of Fig. 2 the plurality of pixels 25 may be arranged in a matrix array comprising rows and columns. Each pixel 25 detects the summation of the modulated and dispersed beam of light 13 for each of the different wavelengths Ai to AN for the area covered by the pixel 25.

As the different wavelengths Ai to AN in the dispersed beam of light 13 are shifted by different amounts the different wavelengths Ai to AN that are incident on a given pixel of the detector 7 have passed though different (incoherent) portions of the spatial modulator 3. This means that the different wavelengths A1 to AN that are incident on a given pixel 25 of the detector 7 may be modulated by different amounts.

The detector 7 detects the superposition of the offset spatially coded two-dimensional slices [x,y] for each wavelength channel A. This reduces the sparse three-dimensional data cube to a compressed two-dimensional projection in a single shot. It collapses overlapping differently masked spectrograms for different channels to a single spectrogram.

In the above examples the input beam of light 9 can be represented as NA wavelength channels. Each of the wavelength channels has a spatial size N, x N. The signal provided to the detector 7 may be represented as Sm(x, y) where: (x) = A So A)M(x)y).1)dA.

So(x, y, A) represents the input beam of light 9 that has been modulated by the object 23. M(x, y, A) represents the combined effect of the modulating means 3 and dispersing means 5.

Converting from the object space [x,y] to the detector space [i, j], the measurement z, of sin (x, y), obtained by the (i,j)th pixel where z E ilerxIVY is given by equation 1 z(i, j) = So(i na)M(i nA). (1) Where So(ijnA) is the (quantized) three-dimensional input signal and M (i nA) is a (quantized) function representing a combination of the spatial modulator 3 and the dispersing element 5. The value nA represents a spectral channel. The function M (i, j, nA) will be dependent on the transparencies of the portions on the spatial modulator 3, the spatial arrangement of the portions of the spatial modulator 3, the dispersing element 5 and any other suitable factors.

The function M(i,j, nA) can be modelled as a series of 2D masks for each wavelength, each 2D mask being generated by the same constant spatially coded aperture mask M*(0) with an appropriate wavelength dependent shift.

In this example, but not necessarily all examples, let us assume a one-to-one correspondence between the [i,i] space at the detector 7 where (i,j) c 10."Y and the [x,y] space at the coded aperture where (x, y) E llerXNY.

As an example, when the dispersing means 5 causes a spatial shift d(A) in the y-direction (where An is An -k, the spectral shift of the wavelength 4, from a central wavelength k) then: M(x, y + d(A" - = M*(x, y) The 2D mask for all wavelengths can be represented as a matrix tM01231N1 DIN,XNy This =.1

A

allows the measurement z obtained by each pixel 25 to be written in matrix form as z = Hs, (2) where z is a IV,Nyx1 vectorized version of the measurement obtained by each pixel 25, that is: [Z[1,1], Z[2.1]... Z[Ky]... Z[A/"Nyi] s is the NxNylVdd stacked vector of the three-dimensional input beam of light So (x, y, A), that is [5[1,11, 1, S[2,1], 1 * . S[x,y], 1* * .S[W,Ny], 1, 5[1,1], A, 5[2,1], A S[x,y], A S[iVx IVA A, S[1,11, NA, S[2,11, NA * S[Mi, NA S[NzNy], NA] and H cR(N.NY)x(N,NYNA) is the (A frAly) x NA) sensing matrix and can be represented by nia (Nol) x (NN) diagonal matrices: H = [Diag(M(1)), Diag(M(NA))] (3) In examples of the disclosure s is the spectral domain signal of the beam of light 9 from the object 21. This allows equation (2) to be rewritten as z = HFr (4) Where r E RNxNY Nk denote the three-dimensional image of the object and F is the Fourier transform F R(NxNYNA)x(NiNYNA). The image r can therefore be obtained by solving r = arg I lz HFrI12 + T R(r) (5) where R(r) denotes the regularizer imposed on the image r, and -t-balances the two terms in equations (5). The regularizer R can, for example, be based upon total variation of the three-dimensional image.

Any suitable compressive sensing inversion algorithms may be used by processing means 12 to solve equation (5) to obtain the desired image. For example, non-linear optimization can be used to produce a three-dimensional image of the object. The image has two spatial dimensions (x,y) and a third dimension in space (z) or wavelength (A) forming respectively a real image that has three spatial dimensions (x,y,z) or a hyperspectral (two-dimensional) image. Where an optical coherence tomography (OCT) arrangement 17 provides the beam of light 9 from an object, the non-linear optimization can be used to produce a real three-dimensional image of the object (a real image that has three spatial dimensions).

The sparsity of the transfer function H that represents the combination of the spatial modulating means 3 and the dispersing means 5 (e.g. the spatially coded aperture and prism) causes information compression.

Fig. 3 illustrates an example in which the source 17 of the broadband beam of light 9 provided to the apparatus 1 is an optical coherence tomography arrangement 17.

Optical coherence tomography (OCT) is also called low coherence light interferometry. A double path interferometer 17 comprising a sample path for an object 21 and a reference path. In this example, the double path interferometer arrangement 17 is used in a Michelson configuration to create interference between an optical beam reflected from an object 21 in the sample path and a reference optical beam in the reference path. The above described apparatus 1 can be used to detect the interference.

A double path interferometer, for example a Michelson interferometer, uses a beam splitter 33, to split light from a light source 31 into two beams of the same bandwidth that travel at 90 degrees to each other along different paths-a sample (object) path and a reference path. In the Michelson interferometer, each of those light beams is reflected back toward the beam splitter 33 which then combines their amplitudes using the superposition principle. The resulting interference pattern is directed to the detector 7 via spatial modulating means 3 and dispersing means 5.

In this example, a light beam is reflected back from the object 21 and the other reference light beam is reflected back from a mirror 35.

The arrangement 17 is a compressive spectral optical coherence tomography (CS-OCT) arrangement that uses a spatial modulating means 3 before dispersing means 5 on the interference signal before detection of a spectrogram of overlapping differently masked spectrograms for different wavelength channels.

The detector 7 can detect the different wavelengths simultaneously so that all the information is recorded in single image (one-shot operation). The apparatus 1 therefore enables NA channel signals to be recovered from a single measurement. The detector 7 produces, in a single shot, without spatial or spectral scanning, spectral domain information 15 that can be used to produce a three-dimensional image of the object 21.

In this example there is full field illumination of the object 21 by a light beam over an area. A reflected beam of light from the object 21 provides full field illumination of a spatially coded aperture 9 over an area. The spatially coded light provides full-field illumination of a disperser element 5 over an area. The dispersed, spatially coded light 13 provides full field illumination of the detector 7 over an area that corresponds to an array of N." by Ny pixels (sensels) in the detector 7.

The OCT arrangement 17 comprises a light source 31, a beam splitter 33, a static reference mirror 35, one or more microscope objectives 37, one or more compensators 39, and one or more focusing elements 41. The OCT arrangement 17 is a spectral domain arrangement. It uses a light source 31 of a fixed broad spectrum.

In examples of the disclosure the light source 31 used for measurement is a broadband light source which provides light having a range of wavelengths. The wavelength of the light that is used may depend on the type of object 21 that is to be imaged or any other suitable factor. In some examples the light used may be infrared light, in some examples the wavelength of the light used may have a range of wavelengths between 400nm and 1500nm. The "centre wavelength" can be for example at 830nm with a frequency range of 810nm to 850nm for a 40nm bandwidth or the "centre wavelength" can be 1300nm with a frequency range of 1280nm to 1320nm for a 40nm bandwidth or the "centre wavelength" can be 1500nm with a frequency range of 1480nm to 1520nm for a 40nm bandwidth. Other centre wavelengths and other bandwidths are possible.

The output light beam from the light source 31 is incident on the beam splitter 33. The beam splitter 33 may comprise a prism, a half silvered mirror or any other suitable component.

In the OCT arrangement 17 half of the split beam provides the reference beam and is provided to the static reference mirror 33. A microscope objective 37 and a compensator 39 are provided between the beam splitter 33 and the static reference mirror 35. The microscope objective 37 may comprise any means which may be arranged to focus the beam of light. In some examples the microscope objective 37 may comprise one or more lenses or any other suitable optical elements. The compensator 39 may comprise a compensator plate or any other suitable compensating element. In the example of Fig. 3 the compensator 39 comprises a decoupling compensator polarizer.

The other half of the split beam provides the object beam and is provided to the object 21. The object 21 may be arranged to be moved along the z axis, but not during imaging. This axis may enable the focussing of the three-dimensional images provided by the OCT arrangement 17. In the example of Fig. 3 the object 21 is provided on a motorised arrangement so as to enable movement along the z axis between one-shot measurements. In other examples a manual arrangement, or any other suitable type of arrangement, could be used.

A microscope objective 37 and a compensator 39 are provided between the beam splitter 33 and the object 21. The microscope objective 37 may comprise any means which may be arranged to focus the beam of light. In some examples the microscope objective 37 may comprise one or more lenses or any other suitable optical elements. The compensator 39 may comprise a compensator plate or any other suitable compensating element. In the example of Fig. 3 the compensator 39 comprises a decoupling compensator polarizer.

The different wavelengths of the light provide coherence of the object beam and the reference beam at different optical path lengths. Therefore, the different wavelengths of light provide information about different depths within the object 21. Different features within the object 21 reflect the incident light by different amounts. The interference between the reflected object beam and the reflected reference beam therefore provides information about the features within the object 21.

As the different frequencies of light provide information about different depths within the object 21 this enables three-dimensional imaging of the object 21. The three-dimensional imaging 21 may enable different features at different depths within the object to be identified and/or analysed. This ensures that the information obtained in the examples of the disclosure comprises information about the internal structure of an object 21 and not just information about the surface of the object 21 The OCT arrangement 17 also comprises one or more focusing elements 41. The focussing element 41 may comprise a lens or any other suitable means for focusing a beam of light. The focusing element 41 is arranged to focus the input beam of light 9 into the apparatus 1 for detecting.

In the example of Fig. 3 the apparatus 1 comprises a modulating means 3, a dispersing means 5 and a detector 7 as previously described. The input beam of light 9 provided to the apparatus 1 is from the OCT arrangement 17.

Calibration Referring back to Fig. 1 and 2, in order to process the compressively sampled output signals 15 from the detector 7 to recover information about the scene or object 21, the processing means 10 uses calibration information 110 that defines the effect of the modulating means 3 and the dispersing means 5. This calibration information 110 is used to define the sensing matrix H. The apparatus 1 comprises means 102 for causing relative movement of the dispersing means 5 and the detector 7. The means 102 causes a movement of the dispersing means 5 relative to the detector 7 or a movement of the detector 7 relative to the dispersing means 5.

The consequence is that there is relative movement between the dispersing means 5 and the detector 7.

The apparatus 1 therefore comprises: modulating means 3 for spatially modulating a beam of light 9 to produce a modulated beam of light 11; dispersing means 5 for dispersing the modulated beam of light 11 to produce a spatially modulated and dispersed beam of light 13; a detector 7 configured to detect the spatially modulated and dispersed beam of light 13; and means 102 for causing relative movement of the dispersing means 5 and the detector 7.

The detector 7 is configured to detect the spatially modulated and dispersed beam of light 13 and produce output signals 15 that are processed by the processing means 10. As previously described, the processing means 10 can be part of or separate to the apparatus 10.

The processing means 10 is configured to process outputs 15 of the detector 7, during calibration, for different relative positions of the dispersing means 5 and the detector 7.

The means 102 for causing relative movement of the dispersing means 5 and the detector 7 simulates a frequency shift of the modulated and dispersed beam of light 13 to enable calibration using a narrowband beam of light 9.

A light source 104 is a narrowband light source and provides a narrowband beam of light 9.

The bandwidth of the narrowband beam of light 9 used for calibration is significantly less than the bandwidth of the broadband beam of light 9 used for measurement. For example, the bandwidth of the narrowband beam of light 9 used for calibration can be one, two, three or more orders of magnitude less than the bandwidth of the broadband beam of light 9. In some but not necessarily all examples, the bandwidth of the narrowband beam of light 9 can be less than 0.02nm and the bandwidth of the broadband beam of light 9 can greater than 20nm. The bandwidth of the narrowband beam of light 9 used for calibration is consequently, in this example, three or more orders of magnitude less than the bandwidth of the broadband beam of light 9.

The narrowband light source 104 can, for example, be a monochromatic light source such as a laser or light-emitting diode. The monochromatic light source 104 is configured to provide, during calibration, a monochromatic beam of light 9. The term 'monochromatic' in this document should be interpreted to cover not only exactly monochromatic (a single wavelength) but also substantially monochromatic. In some but not necessarily all examples, the monochromatic light source has a bandwidth of less than 01m, in some examples significantly less than 01m.

It should be appreciated that the terms monochromatic and narrowband are interchangeable in this document, although defined differently.

In some examples, the monochromatic light source 104 is part of the apparatus 1 (as illustrated). In other examples, the monochromatic light source 104 is separate to the apparatus 1.

The modulating means 3 spatially modulates the monochromatic beam of light 9 to produce a modulated monochromatic beam of light 11, dispersing means 5 disperses the modulated monochromatic beam of light 11 to produce a spatially modulated and dispersed monochromatic beam of light 13; and the detector 7 detects the spatially modulated and dispersed monochromatic beam of light 13.

The means 102 for causing relative movement between the dispersing means 5 and the detector 7 simulates, for the modulated and dispersed monochromatic beam of light 13, arrival at the detector 7 of the modulated and dispersed monochromatic beam of light 13 as if it were a different wavelength.

Therefore, by causing relative movement of the dispersing means 5 and the detector, the modulating means 3 can be calibrated for use with the multiple wavelengths in the broadband beam of light 9 used during measurement, using the wavelength of the narrowband/ monochromatic beam of light 9 during calibration.

In some examples, the relative movement between the dispersing means 5 and the detector 7 is performed by an electrically controlled actuator 103. One example of an electrically controlled actuator is a motor, for example a servo motor.

In some examples, the actuator is controlled by the processing means 10 during calibration. The processing means 10 acts as a driver. In other examples, the actuator is pre-programmed to perform a defined series of relative movements between the dispersing means 5 and the detector 7.

The processing means 10 is configured to process outputs 15 of the detector 7, during calibration.

During calibration, the compressively sampled output signals 15 from the detector 7, for different relative positions of the dispersing means 5 and the detector 7, are processed by processing means 10 to produce calibration information 110 that calibrates the apparatus lby defining the effect of the modulating means 3 and the dispersing means 5 over then bandwidth of the light beam 9 used during measurement. This calibration information 110 is used to define the sensing matrix H. The function M(i,j,nao) which defines the 2D spatially coded aperture mask M*(i,j) for the monochromatic wavelength Ao is determined by the processing means 10.

Referring back to the example previously described for measurement, the dispersing means 5 is moved, during calibration, to cause sequential spatial shifts d" in the y-direction. Each spatial shift d" simulates a spatial shift d(L1.1.,,) where AA, is An -A0, the spectral shift of the 20 wavelength A." from the monochromatic wavelength Ao.

In the coded aperture space: M*(x, y) = M(x, y, /10) = m(x, y + dro An) In the detector space: M* (i,j) = M(i,j, Ao) = M(i, j + n, /1") The output of the detector 7 for (x, y + d") provides the matrix M012).

There then exists a common basis transformation that converts each matrix M("A) to diagonal matrix Diag(M(")) which can the be used, during measurement to solve equation (5) Let the dispersing means 7, have a dispersion equation F(0) = G(A), where 0 is the dispersion angle and A is the wavelength.

Then a change in the dispersion angle de is related to a change in the wavelength dA by dB = [ (dG/dA)/(dF/de)] dA.

Consequently, a change in the wavelength dA can be simulated by changing the dispersion angle de, for example, by moving the dispersing means 5 by de.

1 0 For example, where the dispersing means 5 is a transmission (or reflection) diffraction grating of grating spacing D, then for normally incident light, Dsin e = A and atB = Bo, dB= dA/Dcos 00.

Consequently, a change in the wavelength dA can be simulated by rotating the diffraction grating dB. In this example, rotation by an angle de simulates a wavelength shift dA, proportional to de, of the modulated and dispersed beam of light 13 to enable calibration. The rotation is about an axis parallel to the gratings. The rotation, for example, keeps the grating of the diffraction grating parallel to the rows/columns of the spatially coded aperture 3 and the rows/columns of the detector 7, so that the spatially coded aperture 3 is translated pixel by pixel across the detector 7 by different rotations dB.

The example illustrated in Fig. 4 adapts the source 17 of the broadband beam of light 9, which is provided during measurement, to also be the source 104 of the monochromatic beam of light 9. In this example, but not necessarily all examples, the source 17 of the broadband beam of light 9 is an OCT arrangement., for example, as described with reference to Fig. 3. The OCT arrangement 17 has been adapted or is adaptable for use during calibration. In the example of Fig. 4 the apparatus 1 comprises a modulating means 3, a dispersing means 5 and a detector 7 as previously described. The monochromatic input beam of light 9 provided to the apparatus 1 during calibration is provided from the OCT arrangement 17. During calibration, in the OCT arrangement 17, a monochromatic source of light 104 is used instead of the broadband source of light 31. The monochromatic light source 104 can, in some but not necessarily all examples, have a "centre wavelength" for example at 830nm with a bandwidth of 0.05nm. The sample path of the double path interferometer 17 is blocked by a shutter 120; only the reference path is used.

As previously described, the apparatus 1 comprises means for causing relative movement of the dispersing means 5 and the detector 7 so that the apparatus 1 including the modulating means 3 can be calibrated by the processing means 10 for use with the multiple wavelengths in the broadband beam of light 9 used during measurement, using the wavelength of the narrowband/monochromatic beam of light 9 during calibration.

In the example illustrated, the dispersing means 5 comprises a dispersion element that is rotated by de (which simulates a change in wavelength dA for the monochromatic light source 104).

In at least some examples, the rotation dB is about an axis that causes a spatial shift at the detector that is parallel to the rows/columns of the spatially coded aperture 3 and the rows/columns of the detector 7, so that the spatially coded aperture 3 is translated pixel by pixel across the detector 7 by different rotations de.

It will be appreciated that the double path interferometer of the OCT arrangement can have two configurations: a measurement configuration and a calibration configuration. In the calibration configuration, the sample path is blocked by the shutter 120 and the light source is the monochromatic light source 104. In the measurement configuration, the sample path is open, the light source is the broadband light source 31 for illuminating an object 21 using the open sample path, and the return sample path and the return reference path are superposed during measurement to create the beam of light for detection.

The effect of the relative movement of the dispersing means compared to the detector 7 can be appreciated from Figs 5A, 5B and 5C.

Fig. 5A illustrates how the modulated and dispersed beam of light 13, having a wavelength Ao the same as that of the monochromatic light source 104, is incident on the detector 7 when the dispersing means 5 and the detector 7 have a first relative position. This can, for example, be the position that is used (and fixed) during measurement. It is also a position used during calibration.

Fig. 5B illustrates how the modulated and dispersed beam of light 13, having a wavelength Ai different to the wavelength Ao of the monochromatic light source 104, is incident on the detector 7 when the dispersing means 5 and the detector 7 have the first relative position.

Fig. 5C illustrates how the modulated and dispersed beam of light 13, having a wavelength Ao the same as that of the monochromatic light source 104, is incident on the detector 7 when the dispersing means 5 and the detector 7 have a second relative position, different to the first relative position. The second relative position is not used during measurement but is used during calibration.

The dispersing means 5 provides dispersion in a dispersion direction 200. The dispersion direction 200 is the direction in which light is shifted spatially by dispersion means 5. The dispersion direction 200, when measured at the detector 7 is illustrated in Figs 5A, 5B, 50. The dispersing means 5 comprises one or more refractive elements or one or more diffractive elements.

The means 102 for causing relative movement of the dispersing means 5 and the detector 7 causes relative movement in the dispersion direction 200.

In at least some examples, the dispersion direction 200 (the direction in which light is shifted spatially by dispersion means 5), when measured at the detector 7, is aligned with rows or columns of pixels in the detector 7. Additionally or alternatively, in at least some examples, the dispersion direction 200 (the direction in which light is shifted spatially by dispersion means 5), when measured at the detector relative to a projection of the modulating means 3 onto the detector 7, is aligned with rows or columns of modulating pixels in the spatially coded aperture of the modulating means 3.

In at least some examples, the means 102 for causing relative movement of the dispersing means 5 and the detector 7 causes a series of relative movements. A different image of the modulating means 3 can be produced for each relative movement. In some examples, the series of relative movements are in the dispersion direction 200.

In some examples, the relative movement or the series of relative movements in the dispersion direction 200 are movements that correspond to whole numbers of pixels at the detector 7.

As previously described, the modulating means 3 can comprise a two-dimensional spatially coded aperture comprising at least a first plurality of portions, having a first transparency, and at least a second plurality of portions, having a second different transparency, wherein the first plurality of portions and the second plurality of portions are spatially distributed in two dimensions. The first portions and the second portions can be arranged in an array of non-overlapping pixelated portions and be arranged in rows and columns.

In at least some example, the means 104 for causing relative movement of the dispersing means 5 and the detector 7 is configured to change a relative position of the detector 7 compared to the dispersing means 5 from a first relative position to a second relative position, wherein the second relative position is offset in the dispersion direction from the first relative position, at the detector 7, by an offset value 202.

When the dispersing means 5 and the detector 7 are at the first relative position, light of a first wavelength Ao in the modulated and dispersed beam of light 13 is dispersed in the dispersion direction 200 onto a first area 211 of the detector 7 (Fig. 5A) and light of a second wavelength Ai, different to the first wavelength Ao, in the modulated and dispersed beam of light 13 is dispersed in the dispersion direction 200 onto a second area 212 of the detector 7 offset from the first area 211 by the offset value 202 (Fig. 5B). In at least some examples, the first and second areas overlap.

When the dispersing means 5 and the detector 7 are at the second relative position, light of the first wavelength A. in the modulated and dispersed beam of light 13 is dispersed in the dispersion direction 200 (substantially) onto the second area 212 of the detector 7 that is offset from the first area 211 by the offset value 202 (Fig. 5C).

Consequently changing the relative positioning between the dispersing means 5 and the detector 7 from the first relative position to the second relative position, simulates a change in the wavelength of the incident light of the first wavelength A. from wavelength A. to wavelength Al.The calibration process can therefore be performed for different (simulated) wavelengths A using a monochromatic light source by changing the relative positioning between the dispersing means 5 and the detector 7.

In some examples, during measurement, the spatial modulator 3 is arranged to be moveable relative to the other components of the apparatus 1 such as the disperser element 5 and the detector 7. The spatial modulator 3 is arranged to move in a direction which is different to the dispersion direction 200 in which the dispersing element 5 disperses the modulated beam of light 11. For example, as previously described, the disperser element 5 may disperse the light towards only a y direction and the spatial modulator 3 is then moved perpendicularly in the x direction. The effective spatial modulator 3 used for each sequential measurement is an x-shifted version of the same modulator 3. Having multiple measurements being obtained sequentially may enable more information to be obtained than from one single measurement.

During or after measurement, the electrical output 15 may be used for smart detection. The smart detection may comprise the use of algorithms, or any other suitable technique, to recognise features in the output signal 15. The smart detection can be performed by the processing means 10. In some examples, the output signal 15 may be used by the processing means 10 to reconstruct a three-dimensional image of the object 21, or at least part of the object 21.

The above described calibration uses relative movement of the dispersing means 5 and the detector 7 so that apparatus 1 can be calibrated. The calibration calibrates the combination of the modulating means 3, dispersing means 5 and the detector 7. For example, if the detector 7 has certain areas that respond differently than other areas, the calibration calibrates for this variation at the detector 7.

The term "comprise" is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use "comprise" with an exclusive meaning then it will be made clear in the context by referring to "comprising only one..." or by using "consisting".

In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term 'example' or "for example" or "may" in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus "example", "for example" or "may" refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

I/we claim:

Claims

CLAIMS1. An apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector.
2. An apparatus as claimed in claim 1, wherein the means for causing relative movement of the dispersing means and the detector simulates a frequency shift of the beam of light to enable calibration of at least the modulating means.
3. An apparatus as claimed in claim 1 or 2, wherein the means for causing relative movement of the dispersing means and the detector is configured to change a relative position of the detector compared to the dispersing means from a first relative position to a second relative position, wherein the second relative position is offset in a first direction from the first relative position, at the detector, by an offset value, and wherein the apparatus is configured such that: when the dispersing means and the detector are at the first relative position, light of a first wavelength in the beam of light is dispersed in the first direction onto a first area of the detector and light of a second wavelength, different to the first wavelength, in the beam of light is dispersed in the first direction onto a second area of the detector overlapping the first area and offset from the first area by the offset value, and when the dispersing means and the detector are at the second relative position, light of the first wavelength in the beam of light is dispersed in the first direction onto the second area of the detector.
4. An apparatus as claimed in any preceding claim comprising a monochromatic light source for providing, during calibration, a monochromatic beam of light, wherein the beam of light during a calibration is the monochromatic beam of light.
5. An apparatus as claimed in claim 4, wherein the means for causing relative movement of the dispersing means and the detector simulates, for the monochromatic beam of light, arrival at the detector of the monochromatic beam of light as if it were a different wavelength.
6. An apparatus as claimed in any preceding claim, wherein the dispersing means provides dispersion in a dispersion direction, and wherein the means for causing relative movement of the dispersing means and the detector causes relative movement in the dispersion direction.
7. An apparatus as claimed in claim 6, wherein the dispersion direction is aligned with rows or columns of pixels in the detector and/or the dispersion direction is aligned with rows or columns of modulating pixels in the modulating means.
8. An apparatus as claimed in any preceding claim, wherein the dispersing means comprises one or more refractive elements or one or more diffractive elements.
9. An apparatus as claimed in any preceding claim, wherein the means for causing relative movement of the dispersing means and the detector is configured to move the dispersing means.
10. An apparatus as claimed in any preceding claim, wherein the means for causing relative movement of the dispersing means and the detector is configured to rotate the dispersing means.
11. An apparatus as claimed in any preceding claim, wherein the dispersing means is a diffraction grating wherein rotation by a first angle simulates a wavelength shift, proportional to the first angle, of the beam of light to enable calibration of at least the modulating means.
12. An apparatus as claimed in any preceding claim, wherein the modulating means comprises a two-dimensional spatially coded aperture comprising at least a first plurality of portions, having a first transparency, and at least a second plurality of portions, having a second different transparency, wherein the first plurality of portions and the second plurality of portions are spatially distributed in two dimensions.
13. An apparatus as claimed in claim 12, wherein the first portions and the second portions are arranged in an array of non-overlapping pixelated portions and are arranged in rows and columns.
14. An apparatus as claimed in any preceding claim comprising a double path interferometer comprising a sample path for an object and a reference path; means for superposing the sample path and reference path during measurement to create the beam of light for detection; means for blocking the sample path during calibration.
15. An apparatus as claimed in claim 14 comprising a broadband light source for providing, during measurement, a broadband beam of light for illuminating an object, wherein the beam of light during the measurement is the broadband beam of light.
16. A system comprising the apparatus as claimed in any preceding claim, wherein the system or apparatus further comprises processing means for processing output of the detector for a position of the dispersing means, after calibration, to produce a three-dimensional image of the object.
17. A system comprising: n apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector, wherein the system or apparatus comprises processing means for processing outputs of the detector for different relative positions of the dispersing means and the detector to calibrate the system.
18. A calibrating method for an apparatus, the apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector, wherein the method comprises causing relative movement of the dispersing means and the detector of the apparatus.
19. A computer program for calibrating an apparatus, the apparatus comprising: modulating means for spatially modulating a beam of light to produce a modulated beam of light; dispersing means for dispersing the modulated beam of light to produce a spatially 1 0 modulated and dispersed beam of light; a detector configured to detect the spatially modulated and dispersed beam of light; and means for causing relative movement of the dispersing means and the detector. wherein the computer program is configured to cause relative movement of the dispersing means and the detector of the apparatus.