TECHNICAL FIELD

The present invention is related to the area of spectrometers, and especially for increasing contrast and resolution for spectra recorded using the same.

BACKGROUND

Nearly all fields within natural science benefit from optical

spectroscopy as it reveals information that otherwise lies hidden within the spectral characteristics of the object of study. A spectrometer can be employed to disperse a light source into a spectrum comprising spectral lines with different colors. By analyzing the spectral lines e.g. the molecular composition of the light source can be determined. This type of analysis is commonly known as optical spectroscopy and is used in a wide variety of scientific fields. Applications of optical spectroscopy span from detection and treatment of tumors, classification of stars, species identification, validation of physical models, etc.

A spectrometer comprises of a number of optical components such as a dispersive material, often a reflection grating, mirrors and/or lenses. The light of interest is guided into the spectrometer through a thin adjustable slit, wherein the angle of the light dispersed by the grating depends on the wavelength. The light exiting the spectrometer then becomes spatially separated, according to the original color composition, into a spectrum.

One way of acquiring the spectrum for analysis is to mount a camera, typically digital camera, at the output of the spectrometer for recording an image of the spectrum. This type of spectrometer setup will hereinafter be referred to as a spectrograph.

An alternative approach is to only let light within a narrow wavelength region to exit the spectrometer by mounting a slit or an iris at the output. This type of spectrometer setup will hereinafter be referred to as a

monochromator. To record a full spectrum, using a monochromator, it is necessary to continuously alter the inclination angle of the incoming light onto the grating (a.k.a. scanning) and thereby changing the color of the light that exits the spectrometer. The scanning procedure is often not required for a spectrograph.

To maximize spectral resolution the optical components within the spectrometer are arranged to form an image of the entrance slit onto the output of the spectrometer. Unfortunately, unavoidable imperfections such as dirty optical components, striae, grating defects and reflection upon the inner walls of the housing etc., give rise to interference commonly referred to as "stray-light" being present at the output of the spectrometer. Hence, "stray light" is extraneous light not carrying any valuable information for the analysis regarding the color composition of the light being analyzed using the spectrometer. Examples of signal being regarded as stray light are light that is leaking into the spectrometer, imperfections in detectors (cross-talk blooming, etc.) and light that is scattered within the spectrometer, due to dirty optical components, striae, grating defects and reflection upon the inner walls of the housing, etc. Hence, the stray-light leads to reduced contrast and resolution, and in severe cases it may even completely obscure the actual signal photons.

A method for reducing the stray-light is presented in JP2141630 (A). To determine the stray-light level, the intensity of the light that is transported along the optical path of a monochromator is continuously altered. This is achieved by means of a fast revolving wheel, which either completely block or fully allow light to pass. A lock-in amplifier is then used to extract the signal photons and to discard the stray-light. However, this methodology is only suitable for monochromators and therefore the technique can only produce ensemble-averaged spectra. In addition, the use of a lock-in amplifier will make the acquisition time for a single scan longer since the device is based on detecting temporal variations in the detected intensity. Furthermore, a lock-in amplifier is an expensive type of equipment. Finally, the solution is only capable of reducing stray-light that significantly deviates from the optical path within the spectrometer. It can therefore only provide marginal improvements in the spectral resolution.

Therefore, finding a way to handle stray-light in both spectrographs and monochromators, and thereby increasing the contrast and resolution of the resulting spectrum, is highly sought after.

SUMMARY OF THE INVENTION

With the above description in mind, then, an object of the present invention is to provide a spectrometry system, which seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination.

According to a first aspect of the present invention a spectrometry system adapted for determining a stray-light induced intensity offset in a spectrum is provided. The spectrometry system comprises a spectrometer and a front pattern. The spectrometer having an entrance slit arranged to receive light from a light source emitting light carrying spectral information, a dispersive material arranged to disperse light entering the spectrometer, and an output wherein the dispersed light exits the spectrometer as a spectrum, the spectrum comprising signal photons being photons travelling along an intended optical path within the spectrometer and carrying spectral information pertaining to the light emitted from the light source and stray-light photons being photons not travelling along the intended optical path within the spectrometer and not contributing to the spectral information pertaining to the light emitted from the light source. The front pattern is mounted between said light source and said output of the spectrometer, wherein said front pattern is arranged to partially block and partially transmit light emitted from said light source giving said light from said light source a predefined imprint when passing through said front pattern such that the spectrum is spatially modulated comprising at least one brighter region containing both signal photons and stray-light photons and at least one darker region containing only stray-light photons. By making an imprint on the light entering the spectrometer, stray light and actual signal can be distinguished on a pixel basis, leading to a nearly 100% rejection of stray light. This will lead to the ability to generate spectra having higher contrast and better resolution as compared with a conventional spectrometer system. Possibly, the spectrometer system according to the present invention will have the ability to reveal otherwise hidden features in a spectrum due to the enhanced contrast and resolution. The spectrometry system according to the present invention is moreover time efficient since no time-consuming background measurements are needed. Furthermore, the solution to the problem of reducing stray light contribution presented by the spectrometry system according to the present invention is inexpensive since the only hardware part needed to be added to the spectrometry system in view of a conventional spectrometry system is the front pattern. Hence, apart from the grating, no other (hardware) modification is made to the

spectrometer system as compared to a conventional spectrometer system. Moreover, the spectrometry system according to the present invention provide for a simple, fast and automatic analysis procedure since the brighter regions containing both signal photons and stray-light photons and the darker regions containing only stray-light photons are compared in order to find the stray light contribution to the spectra being recorded by the spectrometer system. Hence, a stray light evaluation routine may be made automatic and fast and can be incorporated in the data acquisition software for performing real-time stray light suppression.

A spectrometer s to be understood as an instrument being employed to disperse light of interest into a spectrum comprising spectral lines with different colors. The spectrometer comprises of a number of optical components such as a dispersive material, mirrors and/or lenses. The light of interest is guided into the spectrometer through a thin, typically adjustable, entrance slit. The light entering the spectrometer ^' s dispersed by the dispersive material forming a spatially separated color spectrum at an output of the spectrometer. Moreover, the spectrometer ^' s arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer.

The dispersive material may be any material being arranged to disperse incident light into a color spectrum. Examples of dispersive materials are a reflection grating or a prism.

The spectrometry system may further comprise a detector mounted at said output of the spectrometer and adapted for detecting the spectrum; and a processing unit adapted to process said detected spectrum using said predefined imprint to distinguish between intensity contributions of said detected spectrum pertaining to signal photons and stray-light photons, respectively, for determining the stray-light induced intensity offset in the spectrum.

The front pattern may be in the form of a Ronchi grating.

The front pattern may be arranged directly in front of or directly behind the entrance slit of the spectrometer. By arranging the front pattern directly in front of or directly behind the entrance slit of the spectrometer the pattern induced by the front pattern will be sharp. This since the spectrometer is arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer. The spectrometer is arranged to have the entrance slit in focus.

The front pattern may comprise alternating opaque and transmissive stripes adapted to produce the darker and brighter regions in the spectral decomposition.

The stripes may be arranged perpendicular in relation to said entrance slit.

The spectrometer system may further comprise a shift device arranged to shift a pattern of said front pattern between at least a first and a second position. This will allow for an increased spatial resolution of the spectrum.

The spectrometer system may further comprise a rear pattern mounted between said front pattern said detector. Hence, the rear pattern is mounted downstream from said front pattern. Mounting the rear pattern downstream from said front pattern meaning that that the rear pattern is mounted downstream from said front pattern as seen in relation to an optical path of the light being emitted from the light source, entering the

spectrometer and ending at the output of the spectrometer. Hence, light emitted from the light source is first subjected to the front pattern and thereafter subjected for the rear pattern.

The spectrometer may further comprise a shift device arranged to shift a pattern of said rear pattern between at least a first and a second position.

The imprint induced by the front pattern may be any type of repetitive pattern such as a square wave, sinus wave, saw tooth wave, or triangle wave.

The detector may be a point detector, a one-dimensional detector array or two-dimensional detector array.

The spectrometer may further comprise a second slit arranged at the output of the spectrometer.

The spectrometer may be a monochromator or a spectrograph.

According to a second aspect of the present invention a method for determining a stray-light induced intensity offset in a recorded spectrum in a spectrometer. The method comprises the following steps: arranging a front pattern between an output of said spectrometer, having an entrance slit arranged to receive light from a light source emitting light carrying spectral information and a dispersive material arranged to disperse said light entering the spectrometer into a spectrum at the output of the spectrometer, and said light source, wherein said front pattern is arranged to partially block and partially transmit light emitted from said light source giving light from said light source a predefined imprint when passing through said front pattern such that a spectrum at said output of the spectrometer is spatially modulated comprising at least one brighter region containing both signal photons and stray-light photons and at least one darker region containing only stray-light photons; recording said spectrum at said output of said spectrometer using a detector; and processing, using said processing unit, said recorded spectrum by using said predefined imprint to distinguish between intensity contributions in said recorded spectrum pertaining to signal photons and stray-light photons, respectively, for determining the stray-light induced intensity offset in the recorded spectrum.

The step of processing may further comprise: calculating a Fourier transform of the recorded spectrum for determining a spatial frequency and a spatial phase of said recorded spectrum; determining at least one location of a brighter and a darker region in the recorded spectrum; calculating a difference in intensity between the at least one brighter region and darker region; and using said calculated difference for determining the stray-light induced intensity offset in the recorded spectrum, which can be done with a lock-in algorithm.

The recording and processing may comprise: positioning a pattern of said front pattern in a first position; recording a first spectrum using the detector and the processing unit; positioning the pattern of said front pattern in a second position; and recording a second spectrum using the detector and the processing unit; and comparing, using said processing unit, said first and second spectrum for determining the stray-light induced intensity offset in the recorded spectrum.

The recording and processing may comprise: positioning a pattern of said front pattern to spatially match pattern of a rear pattern arranged between said front pattern and said detector such that both signal photons and stray-light photons are transmitted through the rear pattern; recording a first spectrum; shifting either the pattern of the front pattern or the pattern of the rear pattern by means of a shift device so that its spatial phase changes ±180 degrees for positioning the pattern of the front pattern to spatially mismatch the pattern of the rear pattern such that signal photons are blocked by the rear pattern; and recording a second spectrum; and determining the stray-light induced intensity offset by calculating the difference between the first and the second spectrum.

The recording and processing may comprise: providing, between said front pattern and said detector, a rear pattern having a first region having a first pattern spatially matching a pattern of the front pattern such that both signal photons and stray-light photons are transmitted through the first region of the rear pattern and a second region having a second pattern spatially mismatching the pattern of the front pattern such that signal photons are blocked by the second region of the rear pattern; and determining the stray- light induced intensity offset by calculating the difference between a portion of the recorded spectrum comprising both signal photons and stray-light photons and a portion of the recorded spectrum comprising only stray light photons.

The above mentioned features, when applicable, apply to this second aspect as well. In order to avoid undue repetition, reference is made to the above.

According to a third aspect of the present invention a computer- readable recording medium is provided. The computer-readable recording medium having recorded thereon a program for implementing, when executed on a processing unit having processing capabilities, the following: recording data pertaining to a spatially modulated spectrum comprising at least one brighter region containing both signal photons and stray-light photons and at least one darker region containing only stray-light photons, determining a spatial frequency and a spatial phase of the spatially modulated spectrum, determining at least one location of a brighter and a darker region in the detected spectrum; calculating a difference in intensity between the at least one brighter region and the at least one darker region; and using said calculated difference for determining a stray-light induced intensity offset in a recorded spectrum, which can be done with a lock-in algorithm.

The step of determining a spatial frequency and a spatial phase of the spatially modulated spectrum may be performed using a Fourier transform calculation.

The above mentioned features, when applicable, apply to this third aspect as well. In order to avoid undue repetition, reference is made to the above. The features of the above-mentioned embodiments may be combined in any way.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention will appear from the following detailed description of the invention, wherein embodiments of the invention will be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 shows a block diagram of a spectrometer system according to an embodiment of the present invention;

FIG. 2 shows an example of a spectrum that is improved by means of the present invention;

FIG. 3 shows an example of a square wave pattern;

FIG. 4 illustrates a method to measure and discard the stray-light level using a system according to an embodiment of the present invention;

FIG. 5 shows a block diagram of a spectrometer system according to another embodiment of the present invention;

FIG. 6 shows a block diagram of a spectrometer system according to another embodiment of the present invention;

FIG. 7 illustrates an alternative method to measure and discard the stray-light level using a system according to an embodiment of the present invention;

FIG. 8 shows a block diagram of a spectrometer system according to an embodiment of the present invention;

FIG. 9 shows an example of two patterns, where half of the patterns match up and the other half is mismatched; and

FIG. 10 illustrates an alternative method to measure and discard the stray-light level using a system according to an embodiment of the present invention. DETAILED DESCRIPTION

Embodiments of the present invention relate, in general, to the field of spectroscopy. A preferred embodiment relates to a spectrometer such as a spectrograph or a monochromator. However, it should be appreciated that the invention is as such equally applicable to other similar devices such as streak cameras. However, for the sake of clarity and simplicity, the

embodiments outlined in this specification are related to spectrometers.

Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which

embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference signs refer to like elements throughout.

The term spectrometer will hereinafter be used for indicating both a spectrograph as well as a monochromator.

FIG. 1 shows a spectrometry system 100, according to an

embodiment of the present invention, comprising a spectrometer 1 10, a light source 150, a front pattern 120, a detector 130 and processing unit 140. The processing unit 140 is connected to said detector 130. The processing unit 140 comprises a processor and a memory. The processing unit 140 is adapted for processing data recorded by said detector 130.

The front pattern 120 is mounted between the output of the

spectrometer 1 10 and the light source 150. The front pattern 120 is arranged to give the light from the light source 150 entering the spectrometer 1 10 a predefined imprint which can be used for recognizing the light that has traveled along an intended optical path within the spectrometer 1 10. The imprint on the light entering the spectrometer 1 10 may be seen as a periodic shadowing. According to the embodiment illustrated in FIG. 1 the front pattern 120 is mounted between the entrance slit and the light source 150. However, the front pattern 120 may be mounted anywhere between the output of the spectrometer and the entrance slit. It is, however, preferred to mount the front pattern 120 as close to the entrance slit as possible. Hence, directly in front of or behind the entrance slit. By arranging the front pattern 120 directly in front of or directly behind the entrance slit of the spectrometer 1 10 the pattern induced by the front pattern 120 will be sharp. This since the spectrometer 1 10 is arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer 1 10. Hence, the spectrometer 1 10 is arranged to have the entrance slit in focus.

The front pattern 120 may be in the form of a Ronchi grating. None- limiting examples of embodiments of front patterns 120 according to the present invention are static pattern such as a mechanical gratings or an adjustable target, such as a computer controlled transmissive screen such as a LCD screen gratings.

The light from said light source 150, which carries the desired spectral information will hereinafter be referred to as signal photons, whereas the light from said light source 150 that has not travel along the intended optical path within the spectrometer 1 10 will hereinafter be referred to as stray-light photons. Thus the spectrum at the output of the spectrometer is comprised of signal photons carrying spectral information and stray-light photons which do not contribute anything to the spectrum being a spectral decomposition of light from the light source 150. An exemplifying illustration of a front pattern 120 according to the present invention is given in FIG. 3.

The main purpose of the invention is to suppress the intensity contribution from the undesired stray-light photons at the output of the spectrometer, as these merely contaminate the acquired spectrum by e.g. reducing the spectral resolution via blurring. The invention relies on the fact that signal photons in the spectrometer form an image of the entrance slit at the output of the spectrometer, and that the stray-light photons does not constructively participate in the image formation process, but rather lead to blurring and a loss of contrast and resolution. By knowing the transmission characteristics of the front pattern 120 it is possible, in the post-processing in the processing unit, to distinguish between signal photons and stray-light photons. Note that there is no particular restriction for the transmission characteristics of the front pattern 120, as long as it partially blocks and partially transmits light. However, patterns such as a square wave, sinus wave, saw tooth wave, triangle wave are generally easier to recognize in the image post-processing (e.g. by means of Fourier analysis), and they may also provide a better estimate of the stray-light photon intensity, should this contribution exhibit rapid spatial variations.

In more detail, assume that the front pattern 120 consists of a number (N) of alternating opaque and transmissive stripes (a.k.a. square wave as shown in FIG. 3), and that these stripes are arranged perpendicular in relation to the slit. Since the spectrometer 1 10 creates an image of the entrance slit at the output, each spectral line will thus contain N number of darker regions and N number of brighter regions at the output of the spectrometer 1 10. An example of this is given in FIG. 2 where a two- dimensional spectrum 201 is shown. In the example, each spectral line 206 contains 13 bright regions 202 (black dots), i.e. each spectral line is intensity modulated in space. By closer studying the cross-section 203 of such an intensity modulated spectral line, it can be observed that the intensity does not reach zero between the bright regions 202. This is because of the intensity arising from stray-light photons, which did not follow the intended optical path of the spectrometer and can therefore, in contrast to the signal photons, end up in the darker regions 204. The stray-light photons thus generate an intensity offset 205. Note that, although this offset is represented here with a constant value, it varies across the spectrum in real

measurement situations. By determining this offset the level of stray-light photons can be deduced, and may, through data post-processing in the processing unit, be suppressed. Below alternative approaches to determine the intensity offset due to the stray light contribution will be presented.

As mentioned above the front pattern 120 is mounted between the output of the spectrometer 1 10 and the light source 150, thereby spatially modulating the light that falls onto the spectrometer. The light from the light source 150 is guided through the spectrometer 1 10 by optical components and dispersed into a spectrum using e.g. a reflection grating. The light that falls on the output of the spectrometer 1 10 is detected using a one- dimensional (1 D) or two-dimensional (2D) detector array as a detector 130. Hence, the detector 130 is mounted at the output of the spectrometer 1 10. The detected light is composed of (1 ) signal photons, which both carry spectral information and form an image of the entrance slit and (2) stray-light photons, which generate the unwanted intensity offset. These two

components are distinguishable since the signal photons follow the predetermined transmission characteristic, determined by the front pattern 120. The detected spectrum is then processed by the processing unit where an algorithm determines the amount of light that has the transmission characteristic (signal photons). Depending on the pattern of the front pattern 120, different algorithms may be employed.

For example, if a square wave pattern as illustrated in FIG. 3, which contains brighter and darker regions with a certain spatial frequency and spatial phase, is used, the algorithm first calculates the Fourier transform of the acquired data. This allows for the spatial frequency of the pattern and its spatial phase to be determined. By knowing the spatial frequency of the pattern and its spatial phase it can be determined where within the acquired data the brighter and darker regions are located. The intensity in a brighter region contains both signal photons and stray-light photons, whereas the intensity in a darker region contains only stray-light photons. By calculating the difference in intensity between the brighter regions and the darker regions, the stray-light photon intensity can be significantly reduced. Note that, since the front pattern 120 is not moved, the spatial frequency and spatial phase will not change and can thus for instance be determined in a calibration process before the data acquisition. This would allow the Fourier transform calculations to be avoided, thus reducing the time needed to process the data.

The steps of the algorithm are illustrated in FIG. 4 for a square wave modulation. In FIG. 4(a) the unprocessed 2D spectrum 401 is shown, wherein intensity modulated spectral lines 402 can be observed. With access to the spatial frequency and spatial phase of the modulation, the algorithm knows where the brighter regions FIG. 4(b) and darker regions FIG. 4(c) are located. The stray-light corrected spectrum is then obtained by calculating the difference between the two intensity maps, FIG. 4(d).

Figure 5 shows a spectrometry system 500 according to an alternative embodiment of the present invention. The spectrometry system 500 comprises a light source 150, spectrometer 1 10, a front pattern 120, a shift device 160 adapted to shift a pattern of the front pattern 120, a detector 130 and processing unit 140.

The processing unit 140 is in connecting with said detector 130. The processing unit 140 comprises a processor and a memory. The processing unit 140 is adapted for carry out post-processing of data coming from said detector 130 and the shift device 160.

According to the embodiment illustrated in FIG. 5 the front pattern 120 is mounted between the entrance slit and the light source 150. However, the front pattern 120 may be mounted anywhere between the output of the spectrometer and the light source. It is, however, preferred to mount the front pattern 120 as close to the entrance slit of the spectrometer 1 10 as possible. Hence, directly in front of or behind the entrance slit. By arranging the front pattern 120 directly in front of or directly behind the entrance slit of the spectrometer 1 10 the pattern induced by the front pattern 120 will be sharp. This since the spectrometer 1 10 is arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer 1 10. Hence, the spectrometer 1 10 is arranged to have the entrance slit in focus.

As previously described, the purpose of the spectrometry system 500 is to determine the intensity offset induced by the stray-light photons. The benefit of the set-up according to the embodiment illustrated in Fig. 5, as compared to the previously described embodiment, is that by shifting the position of the pattern of the front pattern 120 it will be possible to measure the signal photons in the regions that previously were dark, thereby improving the spatial resolution of the spectrum. To take advantage of the improved spatial resolution in this arrangement, a 2D detector is thus used. Hence, in order to improve the spatial resolution, the pattern of the front pattern 120 is arranged to be shifted using the shift device 160. This allows the pattern of the front pattern 120 to be physically shifted in space. We will now explain the benefits with the set-up of the spectrometry system according to this embodiment: Assume once again that the pattern of the front pattern 120 consists of a square wave pattern. The pattern of the front pattern 120 is first positioned at a first position; the exact absolute position is not of importance. The signal photons that fall onto the detector 130 will thus have a number of bright and dark regions, repeated with a certain spatial frequency and spatial phase. The spectrum with the pattern of the front pattern 120 in the first position is recorded with the detector 130. The pattern of the front pattern 120 is then moved/shifted in space to a second position, whereupon the bright and dark regions move accordingly. The spectrum with the pattern of the front pattern 120 in the second position is recorded with the detector 130. If the pattern of the front pattern 120 is moved so that the spatial phase is changed ±180 degrees (or ±π radians), the brighter and darker regions change place, which, in effect, provides access to the information that was previously hidden in the darker regions when the pattern of the front pattern 120 was positioned in the first position. Combining the two recordings provide access to the signal photons in the darker regions, thereby improving the spatial resolution of the spectrum, compared to the previously described embodiment.

One way of shifting the pattern of the front pattern 120 is to mount the front pattern 120 onto a translational stage, acting as the shift device 160, with a sufficiently small step size so that the shift can be performed accurately. Moreover, in case the front pattern 120 is embodied as a computer controlled transmissive screen such as a LCD screen grating the shift device 160 may be embodied as a device being arranged to turn on and off pixels of the computer controlled transmissive screen.

Equation 1 can then be used to calculate the distance Ah the pattern of the front pattern 120 should be shifted for optimal performance. 1

ΔΙ = (1 )

M ^■ P wherein M is the number of spectra and P is the periodicity of the pattern of the front pattern 120, given in repetitions (grooves) per length scale. Note that Eq. 1 is only valid for repetitive (periodic) patterns, such as square wave, sinusoidal, saw tooth, to name a few. The number M of spectra needed depends on the shape of the pattern of the front pattern 120. For instance, nearly full spatial resolution may be obtained with only two measurements if a square wave is employed, whereas three measurements are needed to obtain the same result if the pattern of the front pattern 120 has a sinusoidal shape. If a square wave pattern is employed, the stray-light corrected spectrum is obtained using Eq. 2. wherein l (y) and / ₂ (y) are the two recorded spectra at a wavelength λ and / ^A (y) is the stray-light-suppressed spectrum. If three (or more) spectra are recorded, the stray-light corrected spectrum is instead acquired according to Eq. 3.

wherein / (y) and l (y) are spectra recorded at a wavelength λ but between which the pattern of the front pattern 120 is shifted in space. / ^A (y) is the stray-light-suppressed spectra and M is the number of recorded spectra.

Figure 6 shows a spectrometer system 600, according to an

embodiment of the present invention. The spectrometer system 600 comprises a light source 150, a spectrometer 1 10, a front pattern 120, a rear pattern 170, a detector 130, processing unit 140 and a shift device 160.

The processing unit 140 is connected to said detector 130. The processing unit 140 comprises a processor and a memory. The processing unit 140 is adapted for carrying out post-processing of data coming from said detector 130 and the shift device 160.

The shift device 160 is arranged to shift the pattern of the front pattern 120. The shift device 160 may, in a variant of the embodiment of the present invention, also be able to shift the pattern of the rear pattern 170 independent of shifting the pattern of the front pattern 120. In the following description the pattern of the rear pattern 170 may either be fixed or shiftable between a first and a second position (independent of the first and the second position of the pattern of the front pattern 120).

In FIG. 6 the front pattern 120 is mounted between the entrance slit and the light source 150. However, the front pattern 120 may be mounted anywhere between the output of the spectrometer and the light source 150. It is, however, preferred to mount the front pattern 120 as close to the entrance slit as possible. Hence, directly in front of or behind the entrance slit. By arranging the front pattern 120 directly in front of or directly behind the entrance slit of the spectrometer 1 10 the pattern induced by the front pattern 120 will be sharp. This since the spectrometer 1 10 is arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer 1 10. Hence, the spectrometer 1 10 is arranged to have the entrance slit in focus.

In FIG. 6 the rear pattern 170 is mounted between the detector 130 and the output of the spectrometer 1 10. However, the rear pattern 170 may be mounted anywhere between the detector 130 and the entrance slit. The rear pattern 170 is to be mounted between the front pattern 120 and the detector 130. Hence, the rear pattern 170 is mounted downstream the front pattern 120. Mounting the rear pattern 170 downstream from said front pattern 120 meaning that that the rear pattern 170 is mounted downstream from said front pattern 120 as seen in relation to an optical path of the light being emitted from the light source150, entering the spectrometer 1 10 and ending at the output of the spectrometer 1 10. Hence, light emitted from the light source 150 is first subjected to the front pattern 120 and thereafter subjected for the rear pattern 170.

This set-up of the spectrometry system is especially suitable when a point detector is employed. Assume that patterns of the front pattern 120 and the rear pattern 170 both have a square wave shape. Their individual spatial frequencies are chosen so that the spatial frequency of the image of the front pattern 120 matches up with the spatial frequency of the rear pattern 170. For example, if the spectrometer 1 10 performs a 1 :1 image of the entrance slit, the patterns of the front pattern 120 and the rear pattern 170 should be identical. With this arrangement it is possible to either (1 ) block all signal photons by the rear pattern 170 by completely mismatching the pattern of the front pattern 120 and the pattern of the rear pattern 170, or (2) to allow all signal photons to pass the rear pattern 170 by perfectly matching up the pattern of the front pattern 120 and pattern of the rear pattern 170.

Hence, a first step is to adjust the pattern of the front pattern 120 so that the spatial phase of its image at the output of the spectrometer 1 10 matches the spatial phase of the pattern of the rear pattern 170. Both the signal photons and the stray-light photons will therefore be transmitted through the rear pattern 170 and detected by the detector 130, which is situated after the rear pattern 170. This first acquisition will be referred to as I _0ri . The next step is to shift either the pattern of the first pattern 120 or the pattern of the rear pattern 170 by means of the shift device 160 so that its spatial phase changes ±180 degrees (or ±π radians). The signal photons will thus be completely blocked by the mismatched patterns and only the stray- light photons will pass the rear pattern 170. This second acquisition will be referred to as / ₀ ff. The intensity offset, the stray-light photons, can thereafter be suppressed by calculating the difference between the intensities, i.e. / _0n - / ₀ ff.

Figure 7 shows an example of the method to determine the intensity offset induced by the stray-light photons in the system 600. Figure 7(a) shows a 2D spectrum recorded with using square wave patterns. Note that the rear pattern 170 is not inserted in the spectrometer 1 10 in FIG. 7(a), this is instead a representation of the spectrum that falls onto the output of the spectrometer 1 10 without any rear pattern 170 present. Figure 7(b) and 7(c) shows the pattern of the rear pattern 170 and its two positions, where FIG. 7(b) shows the phase matched position and FIG. 7(c) shows the phase mismatched position. Figure 7(d) shows the light that is transmitted through the rear pattern 170 when the spatial phase is matched, i.e. containing both signal photons and stray-light photons. Figure 7(e) shows the light that is transmitted through the rear pattern 170 when its spatial phase differs ±180 degrees (or ±π radians) from the spatial phase of the front pattern 120, i.e. only stray-light photons are allowed to pass the rear pattern 170. The stray- light can be suppressed by calculating the difference between the two acquisitions (FIG. 7(f)).

Note that in order to be compatible with monochromators, the sum of the intensity in each of the two recordings is the measured quantity and not an image, as shown in FIG 7. Also, a monochromator may also have an output slit at its output and it is therefore not possible to record a 2D spectrum with a monochromator. The example in FIG. 7 should therefore only be viewed as an illustration of the individual steps of the method.

Figure 8 shows a spectrometry system 800, according to an embodiment of the present invention. The spectrometry system 800 comprises a light source 150, a spectrometer 1 10, a front pattern 120, a detector 130, a rear pattern 870and processing unit 140.

The processing unit 140 is connected to said detector 130. The processing unit 140 comprises a processor and a memory. The processing unit 140 is adapted for carry out post-processing of data coming from said detector 130.

In FIG. 8 the front pattern 120 is mounted between the entrance slit of the spectrometer 1 10 and the light source 150. However, the front pattern 120 may be mounted anywhere between the output of the spectrometer and the light source. It is, however, preferred to mount the front pattern 120 as close to the entrance slit as possible. Hence, directly in front of or behind the entrance slit. By arranging the front pattern 120 directly in front of or directly behind the entrance slit of the spectrometer 1 10 the pattern induced by the front pattern 120 will be sharp. This since the spectrometer 1 10 is arranged as an imaging system arranged to form an image of the entrance slit onto the output of the spectrometer 1 10. Hence, the spectrometer 1 10 is arranged to have the entrance slit in focus.

In FIG. 8 the rear pattern 870 is mounted between the detector 130 and the output of the spectrometer 1 10 but could be mounted between the output of the spectrometer 1 10 and the entrance slit as well .

The benefit of the set-up of the spectrometry system as illustrated in Fig. 8, compared to the previously described set-ups, is that it compatible for monochromators and, unlike the previous set-ups, requires only a single acquisition to determine the intensity offset induced by the stray-light photons. To explain, assume that the pattern of the front pattern 120 is a square wave pattern and that the spectrometer 1 10 forms an image of this at the output. The image of the square wave pattern will, as before, have a given spatial frequency and spatial phase. The pattern of the rear pattern 870 is also a square wave pattern, with the same spatial frequency as of the image of the pattern of the front pattern 120. To allow the intensity offset to be determined from a single acquisition, the rear pattern 870 is divided into two regions; region P _0n and region P _0ff . In the first region (region P _0ri ), the spatial phase of the square wave pattern equals that of the image of the pattern of the front pattern 120. In the second region (region P _0f f), the spatial phase is shifted so that it differs ±180 degrees (or ±π radians) from that of the image of the pattern of the front pattern 120. An example of the front pattern 120 and the rear pattern 870 is given in FIG. 9. With this arrangement it is thus possible to block the signal photons with the mismatched part (region P ₀ ff) and thereby make a reading of the intensity of the stray-light photons, while simultaneously measuring the sum of the signal photon intensity and the stray-light photon intensity with the matched part of the patterns (region P _0n ). The intensity offset due to the stray light is then suppressed by calculating the difference between the average intensity in P _0ri and P _0ff .

Figure 10(a) shows a 2D spectrum recorded with the set-up according to Fig. 8. Just as discussed above this image is a representation of the spectrum before it is transmitted through the rear pattern 870. Figure 10(b) shows the pattern of the rear pattern 870, with its two regions indicated. Figure 10(c) shows how the 2D spectrum in FIG. 10(a) is modified as it is transmitted through the rear pattern 870. The upper region ("Phase-matched region") of the rear pattern 870 allows signal photons and stray-light photons to pass, whereas the bottom region ("Phase-mismatched region") only transmits the stray-light photons. By calculating the difference between two intensities the intensity offset induced by the stray-light can be suppressed.

The embodiments of spectrometry systems in the present invention provides better rejection of stray-light photons compared to conventional systems, since photons that only deviate a small distance from the optical path of the spectrometer may be detected and removed. This, in turn, leads to better spectral resolution and higher contrast. In addition, the invention can be adapted for both spectrographs as well as for monochromators. None of the embodiments in the present invention require any calibration to work, but if a calibration is performed, the data post-processing becomes, at least for some of the embodiments, less complicated and more rapid. Single acquisition solutions are available, thus making the invention more time- efficient than solutions based on lock-in amplification. The embodiments of the present invention are also relatively inexpensive and their performance is not affected by the wavelength of the light source under investigation. In addition, the signal-to-noise ratio can be improved by using an evaluation algorithm based on lock-in detection/amplification which uses the frequency and phase to demodulate the signal .

In contrast to most other spectroscopic methods, Coherent anti-Stokes Raman spectroscopy (CARS) has the advantage that the stray light can be estimated by recording a special type of background spectrum. However using the spectroscopy system according to the present invention the extra background recording in CARS can be avoided, which enables single-shot CARS measurements.

According to one embodiment of the present invention a spectrometry system adapted for increasing resolution and contrast of a recorded spectrum is provided. The spectrometry system comprising a light source emitting a light carrying spectral information; a spectrometer having an entrance slit wherein said light enters the spectrometer and an output wherein a spectral decomposition of said light exits the spectrometer, and wherein said spectral decomposition is comprised of signal photons carrying spectral information and stray-light photons which not contribute anything to the spectral decomposition; a detector, mounted at said output of the spectrometer, adapted for recording said spectral decomposition of said light at the output of the spectrometer; a pattern, front pattern, is mounted between said output of the spectrometer and said light source giving said light from said light source a predefined imprint when passing through said pattern; and a processing unit adapted to process said recorded spectral decomposition using the transmission characteristics of said predefined imprint to distinguish between signal photons and stray-light photons in said recorded spectrum and thereby increasing resolution and contrast of said recorded spectrum.

The imprint on the pattern may consist of N alternating opaque and transmissive stripes adapted to produce N darker regions in the spectrum and N brighter regions in the spectrum at the output of the spectrometer.

The pattern may be mounted on a shift device capable of moving the pattern between at least a first and a second position.

The spectrometer further may be comprised of a rear pattern mounted between the entrance slit and the detector. The rear pattern may be mounted on a shift device capable of moving the pattern between at least a first and a second position.

Moreover a method for increasing resolution and contrast of a recorded spectrum in a spectrometer provided. The method comprising the steps of: applying a pattern between an output of said spectrometer and a light source giving a light from said light source a predefined imprint when passing through said pattern; recording a spectrum at an output of said spectrometer wherein said spectrum is a result of the light with said predefined imprint passing through the spectrometer and comprising signal photons and stray-light photons; and processing said recorded spectrum using the transmission characteristics of said predefined imprint to distinguish between signal photons and stray-light photons in said recorded spectrum and thereby increasing resolution and contrast of said recorded spectrum.

The step of processing may further comprise calculating a Fourier transform of the recorded spectrum to determine a spatial frequency and a spatial phase of said imprinted pattern in the spectrum; determining at least one location of a brighter and a darker region in the recorded spectrum; calculating the difference in intensity between the at least one brighter region and darker region and using said calculated difference to increase resolution and contrast of said recorded spectrum, which can be done with a lock-in algorithm.

The recording may further comprise the steps of positioning said pattern in a first position; recording a first spectrum with a one-dimensional detector or a two-dimensional detector; positioning said pattern in at least a second position; recording at least a second spectrum with said one- dimensional detector or two-dimensional detector; combining said at least two recordings to determine signal photons in said darker regions, and thereby improving the spatial resolution of the spectrum.

The method may further comprises the steps of setting up said pattern to match a rear pattern applied between the entrance slit and the detector such that signal photons or stray-light photons are blocked; recording a first spectrum (l_"On" ); shifting either the pattern or the rear pattern by means of the shift device so that its spatial phase changes ±180 degrees; recording a second spectrum (l_"0ff ); and determining an intensity offset induced by stray-light photons by calculating the difference between the first and the second spectrum (l_"On" - l_"Off' ). The method may further comprise the steps of dividing a rear pattern into two regions (region P_"On" and region P_"Off' ), wherein said regions are shifted ±180 degrees in relation to each other; record a spectrum, determining an intensity offset induced by stray-light photons by calculating the difference between the first and the second region (P_"On" - P_"Off' ).

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this

specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should be regarded as illustrative rather than restrictive, and not as being limited to the particular embodiments discussed above. The different features of the various embodiments of the invention can be combined in other combinations than those explicitly described. It should therefore be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.