Birefringent interferometer for measuring photoluminescence properties of samples

Abstract

A measurement system of photoluminescence properties of a sample, comprises a radiation source module configured to generate a first radiation, an excitation optical path coupled to the radiation source module, a support structured to support a sample to be optically coupled to excitation optical path and adapted to provide a photoluminescence radiation, and collection path coupled to the sample and configured to propagate the photoluminescence radiation. The system also includes an analysis device configured to receive the photoluminescence radiation and provide data/information on photoluminescence properties of sample. At least one path between the excitation path and the collection path comprises a respective adjustable birefringent common-path interferometer module configured to produce first and second radiations adapted to interfere with each other.

Claims

1. A measurement system of photoluminescence properties of a sample, comprising: a radiation source module configured to generate a first radiation; an excitation optical path coupled to the radiation source module; a support structured to support the sample to be optically coupled to the excitation optical path and adapted to provide a photoluminescence radiation; a collection path coupled to the sample and configured to propagate said photoluminescence radiation; an analysis device configured to receive the photoluminescence radiation and provide data/information on photoluminescence properties of the sample; wherein at least one path between the excitation path and the collection path comprises a respective adjustable birefringent common-path interferometer module configured to produce first and second radiations reciprocally delayed to be adapted to interfere with each other and obtain a delay-dependent intensity modulation.

2. The system of claim 1, wherein said interferometer module comprises: an adjustable wedge pair, including an optical wedge and a movable optical wedge, configured to provide an adjustable time delay between radiation components having reciprocally orthogonal polarizations; the adjustable time delay is dependent on a variable position of the movable optical wedge; an actuator module (causing translation of the movable optical wedge; a birefringent optical element coupled with the adjustable wedge pair and configured to introduce a fixed time delay between radiations of orthogonal polarizations; a polarizer device coupled with the adjustable wedge pair and the birefringent component to provide said first and second radiations adapted to interfere with each other, having same linear polarization.

3. The system of claim 1, wherein: the system includes a single adjustable birefringent common-path interferometer module placed along the excitation path, said analysis device includes a spectrometer providing a first output signal depending on position values associated with a movable component of the single interferometer module and a detection wavelength; the system further comprises a computing module configured to perform a Fourier Transformation of the first output signal providing a representation of the photoluminescence of the sample.

4. The system of claim 3, wherein said source module includes a polychromatic optical source.

5. The system of claim 1, wherein: the system includes a first adjustable birefringent common-path interferometer module placed along the excitation path; the system includes a second adjustable birefringent common-path interferometer module placed along the collection path; said analysis device includes a detector configured to provide a second output signal depending on first position values associated with a first movable component of the first interferometer module and second position values associated with a second movable component of the second interferometer module; the system further comprises a computing module configured to perform a Fourier Transformation of the second output signal and providing corresponding representation of the photoluminescence properties of sample.

6. The system of claim 5, wherein: the optical source includes a pulsed light source; the analysis device is a time-resolved detector synchronized with the source module providing an output signal (depending on the radiative lifetime of the molecules in the sample.

7. The system of claim 6, wherein: the system includes a single adjustable birefringent common-path interferometer module placed along the collection path; said analysis device includes a respective detector configured to provide a third output signal, depending on position values associated with a movable component of the corresponding interferometer module placed along collection path; the system further comprises a respective computing module configured to perform a Fourier Transformation of the third output signal to provide a representation of the photoluminescence properties of sample.

8. The system of claim 6, further comprising: a beam splitter placed along the excitation path and configured to provide a transmitted radiation to be sent to the sample and a reflected radiation; a reference detector configured to receive the reflected radiation and provide a reference signal; an output detector configured to receive an output radiation corresponding to said transmitted radiation passed through sample; wherein the output detector is configured to provide an absorption signal dependent on the absorption of sample.

9. The system according to claim 6, configured such as that said data/information on photoluminescence properties of sample are at least one of the following: fluorescence Excitation-Emission-Matrix (EEM), fluorescence emission spectrum and fluorescence excitation spectrum, phosphorescence Excitation-Emission-Matrix (EEM), phosphorescence emission spectrum and phosphorescence excitation spectrum.

10. The system according to claim 6, wherein the radiation source module is configured to operate in one of the following spectral regions: the UV region, the visible region, the near-infrared region and the infrared region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further characteristics and advantages will be more apparent from the following description of a preferred embodiment and of its alternatives given as an example with reference to the enclosed drawings in which:

(2) FIG. 1 shows a particular embodiment of a measurement system of photoluminescence properties of a sample and shows an example of a common-path interferometer module;

(3) FIG. 2a shows an example of a two-dimensional fluorescence map obtainable with said measurement system;

(4) FIG. 2b shows an EEM map obtainable with said photoluminescence measurement system;

(5) FIG. 3 shows a second example of said photoluminescence measurement system adapted to record also absorption spectra of a sample, in addition to the possibility to measure fluorescence emission and fluorescence excitation spectra;

(6) FIG. 4 shows a third example of said photoluminescence measurement system having an interferometer module in both the excitation and the detection paths;

(7) FIG. 5 shows a fourth example of said photoluminescence measurement system having an interferometer module in the collection path.

DETAILED DESCRIPTION

(8) FIG. 1 shows an embodiment of measurement system 100 adapted to measure photoluminescence properties of samples. Particularly, measurement system 100 can be employed to measure fluorescence or phosphorescence properties of samples. The following description refers to the fluorescence measurements, but the same techniques also apply to phosphorescence properties of samples, as it will be clear to the skilled person.

(9) Sample 1 is suitably mounted on a support or holder 200 (schematically represented in the drawings).

(10) Measurement system 100 can be configured to measure at least one of the following quantities: the Excitation-Emission-Matrix (EEM), fluorescence emission spectra and fluorescence excitation spectra of samples. The above-indicated quantities can be measured over a broad bandwidth in any spectral region, including the UV region, the visible region, the near-infrared region and the infrared regions. The above-indicated quantities can be also measured using phosphorescence instead of fluorescence as a molecular signal. Moreover, measurement system 100 can be employed to measure the absorption spectrum of sample 1, as will be clarified later with reference to the first example.

(11) The sample 1 can be, as an example, a biological and/or pharmaceutical material. In biology and/or pharmaceutics, fluorescence and EEM measurements are performed regularly in order to retrieve crucial information about the sample's chemical composition, such as, for example: rapid identification and quality evaluation of cell culture media components. Such measurements are, in general, useful for sample tracking and quality assessment in biopharmaceutical industries. The EEM measurements can be also used to detect and quantitatively assess dissolved organic matter in water and contaminants of jet fuels with aero-turbine lubricating oil. Measurement system 100 can be employed, as an example, in general industrial quality test. Another important field is food analysis and food quality testing.

(12) Measurement system 100 comprises an electromagnetic radiation source module 2 coupled to an excitation path 3 ending on the sample 1. The electromagnetic radiation source module 2 (called, herein after, source module) is configured to generate a radiation (light) propagating along the excitation path 3.

(13) Measurement system 100 also comprises a collection (or detection) path 4 coupled to sample 1. Excitation path 3 defines a first propagation direction z1 and the collection path 4 defines a second propagation direction z2 which is, preferably, traversal to the first propagation direction z1. According to the example considered, the first propagation direction z1 is perpendicular to the second propagation direction z2.

(14) Collection path 4 is configured to allow propagation of radiation, i.e. fluorescence light emitted from sample 1 as a consequence of excitation with light coming from the excitation path 3.

(15) Source module 2 can be a coherent source (e.g. a laser) or an incoherent source (e.g. a lamp). Source module 2 may generate polychromatic radiation or monochromatic radiation of either linear or other types of polarizations, as will be clarified in subsequent examples.

(16) Measurement system 100 also includes an analysis device 5 coupled to collection path 4 in order to receive radiation propagated along collection path 4 and provide data/information on the fluorescence emitted from sample 1. According to the particular implementation, analysis device 5 may include a spectrometer or a detector.

(17) Measurement system 100 further comprises an adjustable birefringent common-path interferometer module 6 placed along excitation path 3 and/or detection path 4. The adjustable birefringent common-path interferometer module 6 is configured to produce first and second radiations, which are collinear and adapted to interfere with each other.

(18) The adjustable birefringent common-path interferometer module 6 includes at least one movable birefringent optical element 10. The movable birefringent optical element 10 allows varying a time delay between the first and second radiations. A possible implementation of the adjustable birefringent common-path interferometer module 6 is described with reference to the subsequent first example.

(19) Moreover, measurement system 100 may comprise further optical components, such as lenses or objectives, in order to suitably focus or collimate the radiation on sample 1 and on analysis device 5.

(20) Measurement system 100 can also be equipped with a computing and control module 13 connected to the analysis device 5 to perform additional processing of its output signal.

(21) It is noticed that measurement system 100 is an interferometric measurement system and is based on the interference of two replicas (the above-mentioned first and second radiations) of the incoming radiation when a delay T is imposed between the two. The light is modulated, by the birefringent common-path interferometer module 6, differently for each fixed delay and the radiation spectral intensity going out the interferometer module 6 is given by:

(22) $I\left(\tau \right)={.Math.E\left(\omega \right)+E\left(\omega \right)e^{i\mathrm{\omega \tau }}.Math.}^2={.Math.E\left(\omega \right).Math.}^2+{.Math.E\left(\omega \right).Math.}^2+2\mathrm{Re}\left(E^{*}\left(\omega \right)E\left(\omega \right)e^{i\mathrm{\omega \tau }}\right)==2I\left(\omega \right)+2I\left(\omega \right)\cos \mathrm{\omega \tau }$ a. where E(ω) and I(ω) are the electric field and intensity in the angular frequency domain.

(23) This delay-dependent intensity modulation I(τ) of the light can be employed in the same way either to excite the sample 1 or to extract information about the light emitted from the sample 1. In the first case, knowing the modulation imposed to the excitation light, it is possible to retrieve the fluorescence properties of the sample 1 by analyzing its delay dependent spectral fluorescence intensity as a response to the incoming radiation. In the second case, by looking at the delay dependent light intensity modulation of the light emitted by sample 1, it is possible to extract spectral information about the sample as will be explained afterwards.

(24) Particularly, in operation, source module 2 generates an excitation radiation RE which propagates along the excitation path 3 and reaches the sample 1. Sample 1 may generate fluorescence radiation RF that propagates along collection path 4 and reaches the analysis device 5, which produces an output signal S.

(25) The adjustable birefringent common-path interferometer module 6 (placed along the excitation path 3 and/or along the collection path 4) is adjusted during the measurement procedure by varying position Δx.sub.1 of the movable birefringent optical element 10, in accordance with a pre-established calibration procedure.

(26) Fluorescence radiation RF that reaches analysis device 5 depends on both the molecular properties of sample 1 (particularly, the emission spectrum) and the interfering radiation components produced by the adjustable birefringent common-path interferometer module 6.

(27) Analysis device 5 produces an output signal S depending on the known positions Δx.sub.1 of the movable birefringent optical element 10 and on the fluorescence light emitted from sample 1. Further processing of the output signal S (e.g. a Fourier Transform) allows providing additional measured data on the fluorescence behavior of sample 1.

First Example: Interferometer Along the Excitation Path

(28) In a first example, the adjustable birefringent common-path interferometer module 6 (hereinafter called, interferometer module) is provided in excitation path 3 and no interferometer module is provided into collection path 4. According to this example, the source module 2 produces a polychromatic radiation.

(29) Particularly, the interferometer module 6 (FIG. 1) is provided with an adjustable wedge pair 7 and an optical element 8.

(30) The adjustable wedge pair 7 is configured to provide an adjustable time delay between radiation components passing through it and having reciprocally orthogonal polarizations.

(31) The adjustable wedge pair 7 comprises, as an example, a first optical wedge 9 and a second optical wedge 10 (e.g. the above-mentioned movable optical element). Both first 9 and second 10 optical wedges are made of a birefringent material and, as an example, show optical axes OX1 parallel to each other. Particularly, the first optical wedge 9 and the second optical wedge 10 are optical prisms, having, preferably, the same apex angle. The first optical wedge 9 coupled to the second optical wedge 10 is equivalent to an optical plate having variable thickness.

(32) At least one of the two optical wedges 9 and 10 is movable along a direction transversal to the first direction z1 by means of an actuator 12, schematically represented. Particularly, the first optical wedge 9 is fixed and the second optical wedge 10 is movable.

(33) The adjustable time delay introduced by the wedge pair 7 is dependent on the variable position of the second optical wedge 10. Moreover, as an example, the actuator 12 may include a computer-controlled precision translation stage. As an example, the computing and control module 13 controls the actuator 12. Alternatively, the computing and control module 13 reads and suitably stores the position values assumed by the second optical wedge 10 shifted by the actuator 12.

(34) Optical element 8 is a birefringent plate having a respective optical axis OX2 perpendicular to the optical axis of wedge pair 7 and the first direction z1. Optical element 8 is coupled with the adjustable wedge pair 7 and configured to introduce a fixed time delay between the radiations having reciprocally orthogonal polarizations.

(35) Moreover, interferometer module 6 can be equipped with an input polarizer 11 to provide an output radiation of linear polarization transversal to the optical axes OX1 and OX2 and, preferably, having tilt of 45° with respect to such axes. Input polarizer 11 can be avoided in case the source module 2 already produces a suitably polarized radiation.

(36) Interferometer module 6 also includes an output polarizer 14, as an example, interposed between the adjustable wedge pair 7 and sample 1. As the skilled person can recognize, the order of the elements of the interferometer module 6 can be different from the one shown in the drawings.

(37) According to this first example, analysis device 5 is a spectrometer, i.e. a device measuring the intensity of fluorescence radiation RF as a function of its wavelength.

(38) In operation, the excitation radiation RE emitted by source module 2 reaches interferometer module 6, provided along excitation path 3. Input polarizer 11 provides a first radiation R1 of linear polarization, as an example at 45° with respect to the optical axes OX1 and OX2.

(39) The birefringent plate 8 introduces a fixed first delay between the two orthogonally polarized components of the first radiation R1 that propagate along the fast and slow axis of the material of the birefringent plate 8. The adjustable wedge pair 7 introduces a second delay between such orthogonally polarized components. The second delay is of opposite sign with respect to the first delay, allowing one to change from positive to negative values (and vice versa) the resulting relative delay.

(40) The delay introduced by the adjustable wedge pair 7 is varied by changing the position (variation Δx.sub.1) of the second optical wedge 10. Particularly, actuator 12 varies the position of the second optical wedge 10, preferably, in a controlled and continuous manner, within a position range, defined by a minimum and a maximum value. The position values are suitably stored in a memory in connection with the corresponding delay time.

(41) The second polarizer 14 projects the two delayed components exiting the adjustable wedge pair 7 to a common polarization state (as an example, at 45°), allowing the two radiation components to interfere.

(42) Output radiation R2 exits interferometer module 6 and reaches sample 1. If the spectrum of output radiation R2 has at least a partial overlap with the absorption spectrum of sample 1, then sample 1 absorbs light, thus reaching an excited state. If showing a fluorescence behavior, sample 1 produces fluorescence radiation RF. Fluorescence radiation RF is collimated and focused on spectrometer 5, which measures a fluorescence signal S(Δx.sub.1, λ.sub.2).

(43) Fluorescence signal S(Δx.sub.1, λ.sub.2) is a function of position Δx.sub.1 of the adjustable wedge pair 7 and detection wavelength λ.sub.2 of spectrometer 5.

(44) FIG. 2a shows an example of a two-dimensional fluorescence map corresponding to the fluorescence signal S(Δx.sub.1, λ.sub.2).

(45) The fluorescence spectrum signal S(Δx.sub.1, λ.sub.2) is transformed in the wavelength domain (S(λ.sub.1, λ.sub.2) through a Fourier transformation (FT) procedure, performed (as an example) by the computing and control module 13, leading to:

(46) $\begin{array}{cc}S\left({\lambda }_1,{\lambda }_2\right)=\mathrm{FT}\left\{S\left(\Delta x_1,{\lambda }_2\right)\right\}\left({\lambda }_1,{\lambda }_2\right)& \left(1\right)\\ S\left({\lambda }_1,{\lambda }_2\right)=\int S\left(\Delta x_1,{\lambda }_2\right)e^{\frac{i2\mathrm{\pi \Delta }{x}_1}{{\lambda }_1}}d\left(\Delta x_1\right)& \left(2\right)\end{array}$

(47) The signal after Fourier transform S(λ.sub.1, λ.sub.2) is the so-called EEM map containing information about the fluorescence properties with respect to the excitation wavelength λ.sub.1 and detection wavelength λ.sub.2, wherein:

(48) λ.sub.1 is the excitation wavelength, i.e. the wavelength of the radiation generated by source module 2 causing the fluorescence phenomena;

(49) λ.sub.2 is the emission wavelength, i.e. the wavelength of the fluorescence radiation generated by sample 1 and detected by spectrometer 5.

(50) An example of the EEM map S(λ.sub.1, λ.sub.2) is shown in FIG. 2(b).

Second Example: Measurement of the Absorption Spectrum

(51) FIG. 3 refers to a second example 100-A of the measurement system 100, described with reference to FIG. 1. Measurement system 100-A is employed to measure, in addition to the Excitation/Emission Matrix map, also the absorption spectrum of sample 1, by using additional components. Particularly, measurement system 100-A is further provided with a beam splitter 18 placed along excitation path 3, between source module 2 and sample 1. As an example, beam splitter 18 is placed between interferometer module 6 (if provided along the excitation path 3) and sample 1.

(52) Beam splitter 18 is configured to split (accordingly to a known split ratio) an incoming radiation (e.g. the output radiation R2) into a reflected radiation RR propagating along a reflection path 15 and a transmitted radiation RT propagating along the transmission path 16 (collinear with excitation path 3) up to the sample 1.

(53) Reflection path 15 (preferably, perpendicular to excitation path 3) comprises a first detector PD1 (e.g. a photodiode or a photomultiplier) which is configured to convert the incoming reflected radiation RR into a first electrical signal S.sub.1.

(54) Holder 200 supporting sample 1 is provided with an input port coupled to transmission path 16 (which partially corresponds to excitation path 3) and an output port for a pass-through radiation RP propagating along pass-through path 17. Pass-through path 17 includes a second detector PD2 (e.g. a photodiode or a photomultiplier) which is configured to convert the incoming reflected radiation RR into a second electrical signal S.sub.2.

(55) With reference to the measurement of fluorescence properties of the sample 1, the measurement system 100-A of FIG. 3 operates in a manner analogous to the one above described with reference to FIG. 1.

(56) With regard to the measurement of absorption spectra, it is noted that the electrical signals S.sub.1 and S.sub.2 provided by the first detector PD1 and the second detector PD2, respectively, are represented by interferograms:
S.sub.1=I.sub.0(Δx.sub.1)  (2)
S.sub.2=I(Δx.sub.1)  (3)

(57) The computing and control module 13 provides the Fourier transformation (FT) of the interferograms I.sub.0(Δx.sub.1) and I(Δx.sub.1). In particular, the first signal S.sub.1 is used as a reference, while the second signal S.sub.2 measures the intensity of the pass-through radiation RP, transmitted by sample 1.

(58) The absorption spectrum is calculated (by the computing and control module 13) through the following formula:

(59) $\begin{array}{cc}A\left({\lambda }_1\right)=\log \frac{\mathrm{FT}\left\{I_0\left(\Delta x_1\right)\right\}\left({\lambda }_1\right)}{\mathrm{FT}\left\{I\left(\Delta x_1\right)\right\}\left({\lambda }_1\right)}=\log \frac{I_0\left({\lambda }_1\right)}{I\left({\lambda }_1\right)}& \left(4\right)\end{array}$

(60) where the quantity I.sub.0(λ.sub.1) is the reference light intensity measured with the first detector PD1 and the quantity I(λ.sub.1) is the transmitted light intensity (measured with the second detector PD2).

(61) The example of FIG. 3 also shows collimating and focusing lenses (or objectives) L1-L6 employable in the measurement system 100-A.

Third Example: An Interferometer in the Excitation Path and Another One in the Collection Path

(62) FIG. 4 refers to a third example 100-B of measurement system 100 wherein interferometer module 6 is provided along excitation path 3 and further interferometer module 6 (similar or identical to the previous one) is provided along collection path 4. According to this example, source module 2 produces a polychromatic radiation.

(63) The additional interferometer module 6 of collection path 4 is provided with a corresponding second optical wedge 10, which is movable, i.e. its position Δx.sub.2 can be varied.

(64) The measurement system 100-B of FIG. 4 comprises as analysis device 5 a detector, which is single-detector-based. Particularly, the detector 5 of FIG. 4 is a single-element device having a single continuous detection surface that is illuminated by the polychromatic radiation exiting the additional interferometer module 6.

(65) Detector 5 is configured to convert the fluorescent radiation RF, which is passed through the additional interferometer module 6, into an electric signal S.sub.3.

(66) In operation, both second optical wedges 10 of both interferometer modules 6 are moved to vary the positions Δx.sub.1 and Δx.sub.2. The electrical signal S.sub.3 provided by detector 5 is a function of the positions Δx.sub.1 and Δx.sub.2.

(67) The computing and control module 13 transforms the electrical signal S.sub.3 into the wavelength domain by means of a double Fourier transformation as follows:

(68) $\begin{array}{cc}{S}_3\left({\lambda }_1,{\lambda }_2\right)=\mathrm{FT}\left\{\mathrm{FT}\left[S_3\left(\Delta x_1,\Delta x_2\right)\right]\left({\lambda }_1,\Delta x_2\right)\right\}\left({\lambda }_1,{\lambda }_2\right)& \left(5\right)\\ S_3\left({\lambda }_1,{\lambda }_2\right)=\int \left[\int S_3\left(\Delta x_1,\Delta x_2\right)e^{\frac{i2\mathrm{\pi \Delta }{x}_1}{{\lambda }_1}}d\left(\Delta x_1\right)\right]e^{\frac{i2\mathrm{\pi \Delta }{x}_2}{{\lambda }_2}}d\left(\Delta x_2\right),& \left(6\right)\end{array}$ where: a. λ.sub.1 is the excitation wavelength, i.e. the wavelength of the radiation generated by source module 2 causing the fluorescence phenomena. b. λ.sub.2 is the emission wavelength, i.e. the wavelength of the fluorescence radiation generated by sample 1.

(69) The quantity S(λ.sub.1, λ.sub.2) is the fluorescence Excitation-Emission-Matrix (EEM).

(70) The measurement system 100-B of FIG. 4 also allows measuring fluorescence emission and fluorescence excitation spectra. The former (fluorescence emission) is obtained by integrating the EEM along the excitation wavelength axis, while the latter (fluorescence excitation) is obtained by integrating the EEM along the emission wavelength axis.

(71) Providing the measurement system 100-B of FIG. 4 with the additional components described with reference to FIG. 3, it is also possible to measure the absorption spectrum of sample 1.

(72) It is noticed that the additional interferometer module 6, included into the collection path 4, allows increasing the sensitivity of the measurement system 100-B, taking advantage of the so-called Jacquinot's and Fellgett's advantages, thanks to the absence of an entrance slit followed by a multichannel detector as necessary for spectrometer 5 of the example of FIG. 1. Moreover, the interferometer module 6 can be calibrated in frequency with high precision using well-known spectral features, as an example a Helium-Neon laser (Connes' advantage).

(73) Moreover, the configuration of FIG. 4 can be of great advantage in the infrared spectral region where cheap multichannel detectors are not available.

(74) It is also noticed that the measurement system 100-B of FIG. 4 can be of great interest in areas where sensitivity is the key factor such us in single-molecule spectroscopy or in water contaminant detection.

(75) According to a particular embodiment, the measurement system 100-B of the third example can be used to measure not only EEM and absorption, but also radiative lifetimes of sample 1. According to this embodiment, detector 5 is a time correlated single photon counting (TCSPC) device (or similar time-resolved detector) synchronized with source module 2, by a synchronization signal SCK. Moreover, source module 2 can be a pulsed light source.

(76) This information relating to the radiative lifetime of sample 1 can be helpful to disentangle the different contributions to the fluorescence signals of chemical species that can have the same EEM and absorption features but different lifetimes.

(77) In accordance with this particular embodiment, detector 5 provides a measured signal S.sub.4(Δx.sub.1, Δx.sub.2, t), where t is the time. Thus, for each position, Δx.sub.1 and Δx.sub.2, of the two second optical wedges 10, fluorescence dynamics are recorded as a function of t. A double Fourier transform with respect to Δx.sub.1 and Δx.sub.2 (performed by the computing and control module 13) provides a series of EEM maps for different times t:

(78) $\begin{array}{cc}{S}_4\left({\lambda }_1,{\lambda }_2,t\right)=\mathrm{FT}\left\{\mathrm{FT}\left[S_4\left(\Delta x_1,\Delta x_2,t\right)\right]\left({\lambda }_1,\Delta x_2\right)\right\}\left({\lambda }_1,{\lambda }_2\right)& \left(7\right)\\ S_4\left({\lambda }_1,{\lambda }_2,t\right)=\int \left[\int S_4\left(\Delta x_1,\Delta x_2,t\right)e^{\frac{i2\mathrm{\pi \Delta }{x}_1}{{\lambda }_1}}d\left(\Delta x_1\right)\right]e^{\frac{i2\mathrm{\pi \Delta }{x}_2}{{\lambda }_2}}d\left(\Delta x_2\right)& \left(8\right)\end{array}$

(79) The wavelengths λ.sub.1 and λ.sub.2 have been defined above.

Fourth Example: Interferometer Along the Collection Path

(80) FIG. 5 shows a fourth example 100-C of the measurement system 100, wherein interferometer module 6 is provided along collection path 4 and no interferometer module is provided along excitation path 3.

(81) In accordance with the fourth example, source module 2 can be a monochromatic source (or a series of different monochromatic sources) or a polychromatic source, followed by a tunable monochromator, that sequentially selects different narrowband excitation wavelengths provided by the broadband polychromatic source.

(82) Depending on the different wavelengths provided by source module 2 it is possible to access one or more horizontal lines (corresponding to different excitation wavelengths) of the EEM map. For each excitation wavelength λ.sub.1, the second moving wedge 10 of interferometer module 6 (FIG. 5) is scanned so as to obtain the signal S.sub.5(Δx.sub.2) as an output of the detector 5. The computing and control module 13 computes the Fourier transformation of signal S.sub.5(Δx.sub.2) as a function of Δx.sub.2 and provides signal S.sub.5(λ.sub.2) at a fixed excitation wavelength λ.sub.1, corresponding to a horizontal line of the EEM map:

(83) $\begin{array}{cc}{S}_5\left({\lambda }_2\right)=\mathrm{FT}\left\{S_5\left(\Delta x_2\right)\right\}\left({\lambda }_2\right)=\int S_5\left(\Delta x_2\right)e^{\frac{i2\mathrm{\pi \Delta }{x}_2}{{\lambda }_2}}d\left(\Delta x_2\right)& \left(9\right)\end{array}$

(84) If the source module 2 can provide different excitation wavelengths, by repeating the procedure for different wavelengths λ.sub.1 it is possible to stack the different lines and retrieve the entire (or a part of the) EEM map. This configuration presents the same advantages as the third example considering the detection stage.

(85) Measurement system 100-C of FIG. 5 can be adapted to determine the radiative lifetime of molecules in sample 1 in a manner analogous to the one described with reference to FIG. 4 (third example). Moreover, the measurement system 100-C of FIG. 5 allows measuring absorption spectra as depicted with reference to FIG. 3.

(86) The described measurement system 100 and the corresponding examples allow overcoming the stability problems shown by the techniques of the prior art. This is also due to the common-path geometry of interferometer module 6, that ensures high stability (also in the ultraviolet spectral region) and accuracy (on the order of attoseconds) on the relative delay between the generated replicas with no need of active control. Moreover, the design of the described measurement system 100 is compact and robust, thus allowing a remarkable reduction of the footprint of the entire instrument.