A METHOD AND SYSTEM FOR DETECTION OF ELECTROMAGNETIC RADIATION

Abstract

The invention relates to a device and method for imaging electromagnetic radiation from an object. The device includes entrance optics for allowing the electromagnetic radiation to enter the device, including an image plane onto which an image of the object is to be imaged. The device includes an interferometer having a measurement arm, wherein the image plane is in the measurement arm. The device includes a transformation layer, at the image plane, for transforming the electromagnetic radiation into a spatiotemporal variation of the refractive index of the transformation layer for causing spatiotemporal optical phase differences in the measurement arm of the interferometer that are processed to result in a representative image of the object.

Claims

1. A device for imaging electromagnetic radiation from an object comprising: entrance optics for allowing the electromagnetic radiation to enter the device, comprising an image plane onto which an image of the object is to be imaged; an interferometer with a measurement arm, wherein the image plane is in the measurement arm; a transformation layer, at the image plane, for transforming the electromagnetic radiation into a spatiotemporal variation of the refractive index of the transformation layer for causing spatiotemporal optical phase differences in the measurement arm of the interferometer representative of the image of the object.

2. The device of claim 1, wherein the transformation layer comprises a portion shielded from the electromagnetic radiation for generating a compensation signal from the portion.

3. The device of claim 1, wherein the shielded portion is located at the image plane.

4. The device of claim 1, wherein the interferometer is a phase-shifting interferometer.

5. The device of claim 1, wherein the radiation source of the interferometer is a current modulated laser diode.

6. The device of claim 1, comprising a digital camera arranged for capturing an interference image generated by the interferometer.

7. The device of claim 5, wherein the current modulated laser diode is synchronized with the digital camera.

8. The device of claim 6, comprising a processing unit arranged for processing the camera signal of the digital camera, such as for display on a display device.

9. The device of claim 1, wherein the transformation layer comprises a solid, a liquid, a gas, or any combination thereof.

10. The device of claim 1, wherein the transformation layer comprises a polymer.

11. The device of claim 10, wherein the polymer is a doped polymer.

12. The device of claim 1, wherein the transformation layer includes comprises a liquid mixture, such as a water and glycol.

13. The device of claim 1, wherein the transformation layer comprises a colloidal suspension.

14. The device of claim 1 wherein the transformation layer is housed between two windows.

15. A thermal camera comprising a device according to claim 1.

16. A night vision camera comprising a device according to claim 1.

17. An add-on device for a mobile device, such as a smartphone or tablet, comprising a device according to claim 1, arranged for cooperating with the mobile device, such that a digital camera of the mobile device captures an interference image generated by the interferometer.

18. A method for imaging electromagnetic radiation from an object the method comprising: imaging an electromagnetic radiation image of the object on an image plane, wherein the image plane is in the detection arm of an interferometer; transforming, by a transformation layer at the image plane, the electromagnetic radiation into a spatiotemporal variation of the refractive index of the transformation layer for causing spatiotemporal optical phase differences in the measurement arm of the interferometer representative of the image of the object; and generating a visible image representative of the image of the object based on the optical phase differences.

19. The method of claim 18, comprising: generating, in the interferometer, a compensation signal by shielding a portion of the transformation layer from the electromagnetic radiation.

20. The method of claim 18, comprising: generating a plurality of visible images wherein a phase of the illumination light of the interferometer differs between individual images of the plurality of visible images; and generating a single visible image based on the plurality of visible images.

21. The method of claim 18, wherein the transformation layer comprises a solid, a liquid, a gas, or any combination thereof.

22. The method of claim 18, wherein the transformation layer comprises a polymer, such as a doped polymer.

23. The method of claim 18, wherein the transformation layer comprises a colloidal suspension.

24. A computer program product comprising software code portions arranged for, when executed on a programmable apparatus, generating a visible image, based on an interference image generated by the interferometer of the device according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0068] The invention will further be elucidated based on exemplary embodiments which are represented in a drawing. The exemplary embodiments are given by way of non-limitative illustration. It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example.

[0069] In the drawing:

[0070] FIG. 1 shows a schematic example of a device;

[0071] FIG. 2 shows a schematic example of a device using a Mach-Zehnder interferometric setup;

[0072] FIG. 3 shows a schematic example of a device using a lateral shearing interferometric setup;

[0073] FIG. 4 shows a schematic example of a device using a Fizeau interferometric setup;

[0074] FIG. 5 shows a schematic example of a device using a Twyman-Green interferometric setup;

[0075] FIG. 6 shows a schematic example of a device using a polarizing Twyman-Green interferometric setup;

[0076] FIG. 7 shows a schematic representation of a method; and

[0077] FIG. 8 shows a schematic representation of an add-on device for a mobile device.

DETAILED DESCRIPTION

[0078] FIG. 1 shows a schematic representation of an example of a device 100. The device is arranged for imaging electromagnetic radiation from an object 101. The electromagnetic radiation is preferably invisible radiation, such as near-infrared radiation (NIR), short-wave infrared radiation (SWIR), medium-wave infrared radiation (MWIR), long-wave infrared radiation (LWIR), very long-wave infrared radiation (VLWIR), millimeter wavelength radiation, and/or microwave wavelength radiation.

[0079] The device 100 includes entrance optics. In this example, the entrance optics includes an objective 102. In this example, the entrance optics includes an entrance window 104. The entrance window 104 allows the electromagnetic radiation to enter the device 100. The objective 102 can include classical and/or diffractive optical elements. The entrance optics includes an image plane, or focal plane, 107 onto which the object is to be imaged. The objective 102 can include refractive, reflective, cata-dioptric and/or diffractive optical elements for focusing the object 101 at the image plane 107. The device 100 includes an interferometer 106. Here, the interferometer 106 has a beam-splitter 159 and a reference mirror 158. The interferometer 106 has a measurement arm 108. The interferometer 106 in this example also has a reference arm 110. The interferometer 106 includes a light source 112. The light source 112 generates a coherent beam 114 of light. The light source can e.g. be a laser diode. The light source can include a collimator/beam expander 115 for expanding a cross-sectional dimension of the beam. In this example the beam of light 114 is split into a first beam 116, travelling along the measurement arm 108, and a second beam 118, travelling along the reference arm 110. It will be appreciated that the image plane 107 is in the measurement arm 108

[0080] The device 100 further includes a transformation layer 120 at the image plane 107. The transformation layer includes a material having a high

[00013]$\frac{\mathrm{dn}}{\mathrm{dT}}.$

Here, the image plane 107 is positioned at a back plane of the window 104, i.e. at a front plane of the transformation layer 120, or the image plane is positioned adjacent the window 104 in the transformation layer 120.

[0081] When the image of the object 101 is focused onto the transformation layer 120 at the image plane 107, the transformation layer will experience local heating as a function of the intensity of the radiation in the image. The high

[00014]$\frac{\mathrm{dn}}{\mathrm{dT}}$

of the transformation layer 120 causes transformation of the local intensity of the electromagnetic radiation in the image into a local variation of the refractive index of the transformation layer 120. The first beam 116 will be affected by the local changes in refractive index. The second beam 118 is shielded from the electromagnetic radiation, and thus remains unaffected. When recombined, the first beam 116 and the second beam 118 cause an interference pattern at detection plane 122 of the interferometer. It will be appreciated that the interference pattern will include local interferences across the cross-sectional area of the combined beam. The local interferences are representative of the image of the electromagnetic radiation at the image plane 107. A cold finger 196 is placed at the interface between the transformation layer 120 and the window 105. The cold finger 196 can create a temperature difference between the window 104 and the window 105. The cold finger 196 is connected to an external thermo-electric cooler 198. In this example, the device 100 includes a detector 124, such as a digital video camera, e.g. a CMOS camera, for capturing an image of the interference pattern at the detection plane 122. As a result, the device 100 can convert an image in invisible electromagnetic radiation at the image plane 107 into a visible image at the detection plane 122, or at a display device 126 coupled to the detector 124.

[0082] In this example, the transformation layer 120 includes a first area 128 where the object is imaged, and a second area 130, which is shielded from the electromagnetic radiation, e.g. using a cover 131. The cover 131 could be fixed or moveable. When it is moveable it can cover the first area 128 and the second area 130 such that the external electromagnetic radiation is blocked. The cover 131 could be used as a mechanical chopper allowing phase sensitive detection. The first beam 116 includes two portions. A probe beam portion 116A samples the first area. A compensation beam portion 116B samples the second area 130. This generates two portions in the image at the detection plane 122. The first area 122A corresponds to the probe beam 116A, the second area 122B corresponds to the compensation beam 116B. It will be appreciated that since the compensation beam samples the second area, shielded from the electromagnetic radiation, the compensation beam is not affected by the electromagnetic radiation. Nevertheless, the compensation beam can be affected by environmental factors, such as temperature, humidity, and the like. Therefore, the compensation beam 116B can be used for generating a compensation signal to be used for compensating the probe signal for such environmental factors. In this example, the device 100 includes a processor 132 arranged for automatically compensating the image of the first area 122A of the image on the detection plane 122 recorded by the detector using a signal, such as an average intensity, of the interferometer signal in the second area 122B on the detection plane 122.

[0083] In this example, the light source 112 includes a modulator 112A for modulating the optical phase of the light source. This allows to take consecutive readings, such as images, with the detector at mutually different optical phases of the light source 112. This is also known as phase shift interferometry. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198.

[0084] The transformation layer can be a solid, a liquid, a gas, or a combination thereof, e.g. a polymer or a colloidal suspension. The transformation layer can be housed between two windows. The transformation layer can e.g. be housed in a narrow space, e.g. 1 μm-100 μm wide between two windows. The two windows can be connected at a perimeter to form a cavity between the windows. The cavity can easily house the transformation layer in case it is a liquid, gas, or mixture thereof. An outer one of the two windows can e.g. form an entrance window of the device.

[0085] A balanced setup is used where one volume in the transformation layer is illuminated with radiation from the object and another volume is not illuminated by the radiation from the object. Both volumes are simultaneously illuminated by the first beam. Interferometric data from both volumes are used to detect the difference in refractive index and by doing so infer the temperature distribution of the object based on the equation:

[00015]$dn\left(x,y,z\text{;}t\right)=\frac{\partial n\left(x,y,z\text{;}t\right)}{\partial T}dT\left(x,y,z\text{;}t\right)$

[0086] The reconstruction of the temperature distribution in the image of the object allows visualization of the temperature distribution of the object. The interferometric data can be collected for example using a phase detection in a Michelson interferometer, in a Fizeau interferometer, in a Twyman-Green interferometer, in a Mach-Zehnder interferometer, in a Fabry-Perot interferometer, in a lateral shearing interferometer, in a Jamin interferometer, phase detection in a Doppler-based sensing or a resonator-based sensing. In all these examples of interferometric setups, that are a non-exhaustive list, the change in the interferometric data is being induced by local temperature changes in the detection volume. In these interferograms the interference order Δm(x, y) relates to the wavelength of the probe beam λ, the path length of the light in the sample L and the change in refractive index Δn(x, y) according to the equation:

[00016]$\begin{array}{cc}\Delta m\left(x,y\right)=\frac{L}{\lambda }\Delta n\left(x,y\right)& \left(5\right)\end{array}$

[0087] When the transformation layer is homogeneous, the optical path of the measurement arm through the transformation layer is equal for all positions (x, y). In such a case the fringes in the interferogram are straight and the number and their angle with respect to an arbitrary axis are e.g. determined by the tilt in a tilt mirror in the other, reference, interferometer arm. Thus, the fringes are equidistant and for a given y the interference order k(x, y.sub.0) is a periodic function. Solving the equation for the refractive index n(x, y, z) gives the equation:

[00017]$\begin{array}{cc}{n}^2\left(x,y,z\right)=\frac{2K\rho \left(x,y,z\right)+1}{1-K\rho \left(x,y,z\right)}& \left(6\right)\end{array}$

[0088] The optical path length OPL is based on Fermat's principle and is defined by the line integral along the path from A to B:

OPL=∫.sub.A.sup.Bn(s)ds  (7)

where s is the parameter along the light path.
One can obtain the equation:

[00018]$\Psi \left(x,y\text{;}t\right)=\frac{4\pi }{\lambda }{\int }_{\mathrm{path}}\Delta n\left(x,y,z\text{;}t\right)\mathrm{ds},$

where Ψ(x, y; t) is the variable phase in the interferogram at time t and z is the coordinate along the optical axis of the interferometer while (x,y) are the coordinates in the transverse direction to the optical axis of the interferometer. An interferogram has typically an irradiance I(x,y; t) given by the equation:

I(x,y;t)=I.sub.1+I.sub.2+2√{square root over (I.sub.1I.sub.2)} cos[(φ.sub.A−φ.sub.B)−Ψ(x,y;t)],

where I.sub.1 is the irradiance in the reference arm and I.sub.2 is the irradiance in the measurement arm. In an example, phase shifting interferometry (PSI) is used to improve the accuracy of the phase measurement. For this purpose, many algorithms have been described (see Optical Shop Testing, Daniel Malacara, 3rd Edition). A classical algorithm is the four-step PSI. In this algorithm four interferograms are captured I.sub.1(x, y), I.sub.2(x, y), I.sub.3(x, y) and I.sub.4(x, y) with respective phase shifts of 0, π/2, π and 3π/2, where (x, y) is measured in the transverse plane to the propagation of the probe beam. The spatial distribution of the wrapped phase Ψ(x, y) is retrieved through the equation:

[00019]$\begin{array}{cc}\psi \left(x,y\right)=\arctan \left(\frac{I_4\left(x,y\right)-I_2\left(x,y\right)}{I_1\left(x,y\right)-I_3\left(x,y\right)}\right)& \left(8\right)\end{array}$

[0089] In the case of the Hariharan algorithm there are 5 interferograms resulting for the wrapped phase ψ(x,y) in the equation:

[00020]$\psi \left(x,y\right)=\arctan \left(\frac{2\left(I_2\left(x,y\right)-I_4\left(x,y\right)\right)}{\left(2I_3\left(x,y\right)-I_5\left(x,y\right)-I_1\left(x,y\right)\right)}\right)$

[0090] It is known that the Hariharan algorithm is more robust than the four-step algorithm.

[0091] The processor 132 can be arranged for performing the calculations required for the phase shifting interferometry. The light source being a current modulated laser diode 112 can be synchronized with the detector being a digital camera 124 for phase shifting interferometry.

[0092] The phase-shifter can be of different kinds, for example the current of a laser diode of the probe beam can be modulated. Other phase-shifters based on the polarization of the radiation of the probe beam can be used, for example, using linear polarized light where the polarization is rotated by wave plates such that the obtained phase shift is 90 degrees. The relation of the effective refractive index n.sub.eff(x,y) and the measured spatial distribution of the phase φ(x, y) is given by the equation:

φ(x,y)=n.sub.eff(x,y)kL,  (9)

where

[00021]$k=\frac{2\pi }{{\lambda }_0}$

is the magnitude of the wave vector of the probe beam, λ.sub.0 is the vacuum wavelength of the probe beam and L is the thickness in the z-direction of the transformation layer 120. Because of the periodic characteristics of the interfering waves, the wrapped phase at a certain point φ(x, y) is uniquely defined only in the principal value range of (−π, π]. Therefore phase-unwrapping algorithms can be used to generate a continuous phase φ(x, y) where the relation with the wrapped phase is:

[00022]$\begin{array}{cc}\phi \left(x,y\right)=\psi \left(x,y\right)-2\pi \left[\frac{\psi \left(x,y\right)}{2\pi }\right]& \left(10\right)\end{array}$

where the symbol [x] means the nearest integer function of x.

[0093] As shown in FIG. 7, the device as described with reference to FIG. 1 can be used in a method 200 for imaging, e.g. invisible, electromagnetic radiation from an object as follows. In a step 201 an electromagnetic radiation image of the object is imaged on an image plane, wherein the image plane is in the detection arm of an interferometer. In a step 202 the electromagnetic radiation is transformed into a spatiotemporal variation of the refractive index of a transformation layer at the image plane for causing spatiotemporal optical phase differences in the measurement arm of the interferometer representative of the image of the object. In a step 203 a visible image is generated representative of the image of the object based on the optical phase differences.

[0094] Optionally in a step 204 in the interferometer measurement arm, at the location of the image plane, a compensation signal is generated by shielding a portion of the measurement arm from the electromagnetic radiation. Optionally in a step 205 a plurality of visible images is generated wherein a phase of the illumination light of the interferometer differs between individual images of the plurality of visible images, and in an optional step 206 a single visible image is generated based on the plurality of visible images.

[0095] The arrangement in FIG. 2 shows an exemplary arrangement of a device. The probe beam is passing only once through the transformation layer. FIG. 2 shows an object 101 emitting electro-magnetic radiation that is collected by an objective 102 that creates an image in the image plane 107. In doing so the electro-magnetic radiation is reflected specularly at an interface between a beam splitter rhombohedron 190 and an entrance prism window 104. The image plane 107 is embedded in the transformation layer 120. A cold finger 196 is placed at the interface between the transformation layer 120 and the output prism window 105. The image is formed in a measurement volume of the transformation layer 120. The transformation layer 120 is placed between the entrance prism window 104 and an output prism window 105. The radiation of the light source, here a laser diode, 112 operates as probe beam where a collimator 115 collects the radiation of the laser diode 112 such that it illuminates the transformation layer with, at least quasi, parallel radiation. The collimated radiation of the laser diode 112 is split by a first beam splitter rhombohedron 170 and a prism beam splitter 103 in two beams. One beam propagates through a second beam splitter rhombohedron 180 towards the entrance prism 104 and continues to the transformation layer 120A and the other beam propagates through the compensation arm of the Mach-Zehnder interferometer 106 along the second beam splitter rhombohedron towards the compensation volume 120B and continues such that it reflects specularly on the surfaces of a third beam splitter rhombohedron 175. The beam of the measurement arm and the reference arm are combined at the interface between the output prism window 105 and the third beam splitter rhombohedron 175. A light absorbing material 199 can be glued to the output prism 105 to suppress parasitic light. The combined beam passes through an objective 140 that creates an image of the interface between the entrance prism window 104 and the transformation layer 120 on a detector, here a, for example CMOS, imager 124. The signal of the imager 124 is processed by a processing unit 132 and subsequently visualized on a display 126. The processing unit 132 and a laser diode driver 112A are synchronized by an internal clock. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198. The signals of the thermo-electric cooler 198 are controlled by the processing unit 132.

[0096] The arrangement in FIG. 3 shows an exemplary arrangement of a device. The probe beam is passing only once through the transformation layer 120. FIG. 3 shows an object 101 emitting electro-magnetic radiation that is collected by an objective 102 that creates an image in the image plane 107. In doing so the electromagnetic radiation is reflected specularly at the interface between a beam splitter rhombohedron 190 and an entrance prism window 104. The image plane 107 is embedded in the transformation layer 120. A cold finger 196 is placed at the interface between the transformation layer 120 and the output window 105. The image is formed in a measurement volume of the transformation layer 120. The transformation layer 120 is placed between an entrance prism window 104 and an output window 105. The radiation of a light source, here laser diode, 112 operates as probe beam where a collimator 115 collects the radiation of the laser diode 112 such that it illuminates the transformation layer 120 with, at least quasi, parallel radiation. The transformation layer 120 in this example contains two volumes: one measurement volume 120A and one compensation volume 120B. Both volumes are initially at the same temperature. The collimated radiation of the laser diode 112 is reflected specularly at a parallel plate 192 in two beams. One beam is formed by the reflection at the front surface of the parallel plate 192 and another beam is formed at the back surface of the parallel plate 192. The specularly reflected beams are sheared laterally with respect to each other and result in a lateral shearing interferometer 106. Both beams pass through the objective 140 that creates two sheared images of the interface between the entrance prism window 104 and the transformation layer 120 on the, for example CMOS, imager 124. In the overlap region of the two beams an interference pattern is observed. This interference pattern is sensed at the imager 124 and is subsequently processed by the processing unit 132. The processing unit 132 transfers the data to be visualized to a display 126. The processing unit 132 and laser diode driver 112A are synchronized by an internal clock. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198. The signals of the thermo-electric cooler 198 are controlled by the processing unit 132.

[0097] The arrangement in FIG. 4 shows an exemplary arrangement of a device. The probe beam is passing twice through the transformation layer 120. FIG. 4 shows an object 101 emitting electro-magnetic radiation that is collected by an objective 102 that creates an image in the image plane 107. The image plane 107 is embedded in the transformation layer 120. A cold finger 196 is placed at the interface between the transformation layer 120 and the window 105. The image is formed in a measurement volume of the transformation layer 120. The transformation layer 120 is placed between an entrance window 104 and an output window 105. A cover 131 is placed in front of the entrance window 104 to prevent electromagnetic radiation to reach the compensation volume of the transformation layer 120. The cover 131 could be fixed or moveable. When it is moveable it will cover the first area 128 and the second area 130 such that the external electromagnetic radiation is blocked. The cover 131 could be used as a mechanical chopper allowing phase sensitive detection. The radiation of a laser diode 112 operates as probe beam where a collimator 115 collects the radiation of the laser diode 112 such that it illuminates the transformation layer 120 with, at least quasi, parallel radiation. The collimated radiation of the laser diode 112 is reflected specularly at the interface between the transformation layer 120 and the entrance window 104. The specularly reflected radiation is diverted by a beam splitter 155 towards an objective 140 that creates an image of the interface between the entrance window 104 and the transformation layer 120 on the, for example CMOS, imager 124. The setup is recognized as Fizeau interferometer 106. The signal of the imager 124 is processed by the processing unit 132 and subsequently visualized on a display 126. The processing unit 132 and laser diode driver 112A are synchronized by an internal clock. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198. The signals of the thermo-electric cooler 198 are controlled by the processing unit 132.

[0098] The arrangement in FIG. 5 shows an exemplary arrangement of a device. The probe beam is passing twice through the transformation layer. FIG. 5 shows an object 101 emitting electro-magnetic radiation that is collected by an objective 102 that creates an image in the image plane 107. The image plane 107 is embedded in the transformation layer 120. A cold finger 196 is placed at the interface between the transformation layer 120 and the output window 105. The image is formed in a measurement volume of the transformation layer 120. The transformation layer 120 is placed between an entrance window 104 and an output window 105. A cover 131 is placed in front of the entrance window 104 to prevent electromagnetic radiation to reach the compensation volume of the transformation layer 120. The cover 131 could be fixed or moveable. When it is moveable it will cover the first area 128 and the second area 130 such that the external electromagnetic radiation is blocked. The cover 131 could be used as a mechanical chopper allowing phase sensitive detection. The radiation of a light source, here laser diode, 112 operates as probe beam where a collimator 115 collects the radiation of the laser diode 112 such that it illuminates the transformation layer 120 with, at least quasi, parallel radiation. The collimated radiation of the laser diode 112 is split by the beam splitter cube 160 in two beams. One propagates to the transformation layer 120 and one propagates to the mirrored face 150 of the beam splitter cube 160. In the reference arm of the non-polarizing interferometer we find the optical equivalent of the output window 152, the optical equivalent of the transformation layer 151. Those optical equivalents can create an equal path interferometer instead of the nominal unequal path interferometer where the optical equivalents 152 and 151 are not present and where the mirror 150 is evaporated on one of the faces of the beam splitter cube 160. The specularly reflected beam at the interface between the transformation layer 120 and the entrance window 104 is again divided by the beam splitter cube 160 where one beam is diverted towards an objective 140 that creates an image of the interface between the entrance window 104 and the transformation layer 120 on the, for example CMOS, imager 124. The signal of the imager 124 is processed by the processing unit 132 and subsequently visualized on a display 126. The processing unit 132 and laser diode driver 122A are synchronized by an internal clock. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198. The signals of the thermo-electric cooler 198 are controlled by the processing unit 132.

[0099] The arrangement in FIG. 6 shows exemplary arrangement of a device. The probe beam is passing twice through the transformation layer 120. FIG. 6 shows an object 101 emitting electro-magnetic radiation that is collected by an objective 102 that creates an image in the image plane 107. The image plane 107 is embedded in the transformation layer 120. A cold finger 196 is placed at the interface between the transformation layer 120 and the output window 105. The image is formed in a measurement volume of the transformation layer 120. The transformation layer 120 is placed between an entrance window 104 and an output window 105. A cover 131 is placed in front of the entrance window 104 to prevent electromagnetic radiation to reach the compensation volume of the transformation layer 120. The cover 131 could be fixed or moveable. When it is moveable it will cover the first area 128 and the second area 130 such that the external electromagnetic radiation is blocked. The cover 131 could be used as a mechanical chopper allowing phase sensitive detection. The radiation of a light source, here laser diode, 112 operates as probe beam where a collimator 115 collects the radiation of the laser diode 112 such that it illuminates the transformation layer 120 with, at least quasi, parallel radiation. The collimated radiation of the laser diode 112 is split by the polarizing beam splitter cube 161 in two beams. One propagates through a quarter wave plate 127 towards the transformation layer 120 and one propagates to the mirrored face 150 of the polarizing beam splitter cube 161. In the reference arm of the polarizing interferometer we find the optical equivalent of the quarter wave plate 153, the optical equivalent of the output window 152, the optical equivalent of the transformation layer 151. Those optical equivalents can create an equal path interferometer instead of the nominal unequal path interferometer where the optical equivalents 153, 152 and 151 are not present and where the mirror 150 is evaporated on one of the faces of the polarizing beam splitter cube 161. The specularly reflected beam at the interface between the transformation layer 120 and the entrance window 104 passes for a second time through the quarter wave plate 127 and is again divided by the polarizing beam splitter cube 161 where one beam is diverted towards a CMOS objective 140 that creates an image of the interface between the entrance prism window 104 and the transformation layer 120 on the, for example CMOS, imager 124. The signal of the imager 124 is processed by the processing unit 132 and subsequently visualized on a display 126. The processing unit 132 and laser diode driver 122A are synchronized by an internal clock. The setup is recognized as a Twyman-Green interferometer 106. The critical parts of the setup can be placed in a vacuum chamber 195. The vacuum chamber 195 can be controlled in temperature through the thermo-electric cooler 198. The signals of the thermo-electric cooler 198 are controlled by the processing unit 132.

[0100] FIG. 8 shows an example of a device 100 arranged as an add-on device for a mobile device 200, in this example a smartphone. In this example the add-on device is a clip-on device. It is also possible that the add-on device is otherwise mounted to the mobile device, e.g. through an adhesive connection. It will be appreciated that it is also possible that the add-on device is part of, or included by, a cover for the mobile device 200. In this example, the imager is formed by a digital camera 224 of the mobile device 200. Thus, the digital camera 224 can image the interference image created by the interferometer. The digital camera 224 is processed by the processing unit 232 of the mobile device and subsequently visualized on a display 226 of the mobile device. If desired, synchronization of the light source 112 of the device 100 with the camera 224 of the mobile device 200 is possible, e.g. via a, wired or wireless, communications link.

[0101] Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein, without departing from the essence of the invention.

[0102] In the examples, the compensation volume or second area of the transformation layer is placed in the measurement arm of the interferometer. It is also possible that the compensation volume or second area of the transformation layer is placed in the reference arm of the interferometer. Preferably, the compensation volume or second area of the transformation layer is then still in thermal contact with the measurement volume or first area of the transformation layer.

[0103] For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged and understood to fall within the framework of the invention as outlined by the claims. The specifications, figures and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense. The invention is intended to embrace all alternatives, modifications and variations which fall within the spirit and scope of the appended claims. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location.

[0104] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.