TECHN ICAL FIELD OF THE INVENTION

The invention relates to a method and device for determining gas concentration. Especially the invention relates to determining gas concentration of a volume in a closed space.

BACKG ROUND OF THE INVENTION

Determination of gases is very important task in many procedures in different kinds of industries. For example foodstuffs are packed in hermetically sealed foodstuff packets, which are additionally filled by protective gas, such as nitrogen, oxygen or carbon dioxide or mixture of those so that the food would not perish too early and that it would look good longer. The protection gases are widely used in a food industry, such as also in the beverage industry, where bottles, like soft drink, beer and wine bottles are filled by the protective gas. Determination of gases in the bottles, especially in wine bottles, are also very important for evaluating preservability, as well as also a development of flavour and a state of post- fermentation. Furthermore different kinds of gases are used also in a glass industry, such as especially in the insulating glass industry, where glass units are constructed typically with a configuration having two or more glass sheets with a closed spacing in between the sheets, and where the closed spacing is filled with gas with low thermal conductivity, such as monoatomic gases, like argon, and krypton, or mixture of those for minimizing the heat transfer through the glass unit. Typically oxygen is an unwanted gas within the insulating glass units. In addition the determination of gases is important also in other industry and procedures, such as also in biogas production processes. Different solutions for determination of gases are known from prior art, such as taking a sample from a closed volume by a needle and analysing the sample by a spectrometer or a gas chromatograph. There are however some disadvantages relating to the invasive methods for taking and determining the samples. At first the puncturing will break possible hermetic sealing of the closed volume after which the gases from the volume might diffuse easier outside the volume. In addition the measuring means (such as the needle or other analysing equipment) as well as also the sample and volume may be contaminated. In addition the invasive methods are very slow processes and thus essentially impossible test methods for testing all objects for example in a production line of a factory.

Infrared (I R) absorption spectroscopy represents a prior art method for determining gases, which is based on the fact that each gas compound has its own specific I R spectrum. However, several gases that occur to a significant extent are I R inactive, i.e. they cannot be detected using I R spectroscopy. Examples of such gases are all homonuclear diatomic gases, such as oxygen (0 ₂ ), nitrogen (N ₂ ), hydrogen (H ₂ ), as examples. Also non-invasive methods are known from the prior art for determining the gases inside the closed volume. One of the known prior methods is a method, where rapidly alternating electrical field is applied to the closed spacing in order to get characteristic emission peaks of the gases inside the spacing. However this method does not function properly when the spacing, such as e.g. spacing between the glass sheets, has a coating with metallic substances making the coating electrically conductive.

SUMMARY OF THE INVENTION

An object of the invention is to alleviate and eliminate the problems relating to the known prior art. Especially the object of the invention is to provide a method for determining gases of a closed space reliably, non-invasively and fast. In addition the object is to realise a compact and low-cost apparatus for determining the gases of a closed space reliably and non-invasively.

The object of the invention can be achieved by the features of independent claims.

The invention relates to a device according to claim 1 . In addition the invention relates to a method according to claim 1 2 and an arrangement according to claim 21 . According to an embodiment of the invention a gas concentration of a volume in a closed space is determined non-invasively. This has advantages that at first there is no need to open the volume for measuring purposes, which fastens the measurement. At second there is no contamination risk both for the volume to be measured but not also for the measuring device. The volume with a closed space may be e.g. a closed foodstuff package, a glass unit comprising at least two glass sheets forming said closed space between said glass sheets, a beverage bottle, a processing chamber, or a chamber comprising gases for medical purposes, as examples. According to an embodiment the determination is so fast that even all of the objects with closed spacing in a production line, such as foodstuff packets, can be determined, which is clear advantage compared to conventional puncture methods.

According to an embodiment the determination is performed by irradiating the volume by a laser beam in order to produce rotational Raman scattering of at least one gas component within the volume. The gas to be measured is a gas producing Raman signal, and especially rotational Raman signal, when excited by the laser beam. The gas may be e.g. 0 ₂ , CO, C0 ₂ , N ₂ , H ₂ or combination or mixture of those, as an example. The measuring of rotational Raman scattering is very advantageous when compared to e.g. vibrational Raman scattering, because the signal strength is much more intensive. Thus, the rotational Raman signal yields better sensitivity than e.g. vibrational Raman signal. In addition wavelengths of rotational Raman scattered beams of different gases are in the same wavelength area, whereupon the same configuration of the measuring components of the device can be used. Thus no special modifications for different gases are needed, which makes the device very simple with less components.

However, since the wavelengths of rotational Raman scattered beams of different gases are in the same wavelength area, distinguishing the different wavelengths from each other is very challenging task opposite to distinguishing wavelengths of vibrational Raman scattered beams. In order to determine all the wavelengths of scattered rotational Raman signals, the scattered beams are guided from the volume having the gases to be measured to a filtering means by a first guiding means, such as a lens. According to an advantageous embodiment the first guiding means is common both for the emitted laser beam and collected scattered beams. The first guiding means is configured to focus the spot of the emitted laser beam inside the closed space, such as inside the foodstuff box, window, or wine bottle. Using the common first guiding means ensures that the measured spot is exactly the same which is hit by the laser beam, which makes the determination very accurate. In other words the measuring geometry is backscattering geometry, whereupon only one optically transparent sheet is needed in the volume, because the scattered beams are collected in 1 80 ° angle in relation to the laser beam, which again offers advantage over known prior art where the measuring signal is collected in the back side of the volume.

The filtering means is configured to transmit essentially only bandwidth of the scattered beams and not the wavelength of the used laser. Advantageously the filtering means is an edge or notch filter as discussed in more details elsewhere in this document. The laser beam advantageously has very narrow bandwidth, such as e.g. 532 ± 0.1 5 or preferentially ± 0.01 5 nm.

The scattered beams are guided and spread onto a surface of a detector, such as e.g. CCD or line detector or spectrometer, by a second guiding means so that different wavelengths of scattered beams are guided onto different locations on the surface of the detector. Since each of the different wavelengths corresponds to a certain gas component of the volume detected and the intensities of the detected wavelengths are proportional to the concentrations of each gas components, the gas components with concentrations inside the volume of the closed space can be determined non-invasively.

According to an embodiment the filtering means is additionally used to guide the laser beam emitted from the laser source to the volume of said closed space in addition to function essentially non-transparently for the bandwidth of the laser beam emitted from the laser source. When the laser beam is guided, such as reflected, by the filtering means to the volume, the path of the laser beam guided by the filtering means and the path of the scattered beams guided by the first guiding means to the filtering means are essentially the same thus both making the structure very simple and also ensuring that the scattered beams are collected from the same point that where the laser beam is focused. According to an embodiment the first guiding means used for focusing said laser beam and collecting said scattered beams may be implemented by an implementation where the location of the focus spot of the guiding means inside the volume to be measured can be adjusted, advantageously continuously. According to an example the focus spot can be focused near the covering sheet of the volume so that the focus spot does not lie inside the medium contained by the volume (such as food) and thereby disturbing the determination. In addition according to an example the focus spot can be focused to different layers of the volume to be measured, such as at first to the first chamber and secondly to the second chamber of the volume, like different spaces of the insulating glass unit.

The volume between the filtering means and the first guiding means, where the emitted laser beam and scattered beams are transmitted, is according to an embodiment essentially a vacuum or filled with a medium producing no Raman scattering, such as Argon. However, according to another embodiment said volume between the filtering means and the first guiding means may be filled with medium producing Raman scattering (like H ₂ or CO), whereupon it can be used for calibration purposes.

The spreading or dispersion of the scattered beams into the different wavelengths and guiding them onto different locations on the surface of the detector may be implemented for example by a grating and a lens after the grating. The grating may be either transmission or reflection grating. According to an advantageous embodiment the transmission grating is used, because it is typically very compact and about three times more efficient than the reflection grating even though the diffraction line density is increased. The transmission grating has very high efficiency compared to the reflection grating with the same line density and especially very high line density can be applied with the transmission grating without losing the efficiency. In particular a Volume Phase Holographic (VPH) grating is one example of the grating type which can be used with the embodiments of the invention.

According to an embodiment an image is formed of the collected scattered beams, after which said formed image is collimated and guided for the dispersion into the different wavelengths. The forming of the image before guiding it onto the detector affects the resolution of the detecting device, namely the smaller the image is, the smaller the spot is on the detector surface and thus the higher the resolution of the detecting device.

In addition according to embodiment temperature of the gas concentration of the volume is determined based on the scattered rotational Raman signal. The temperature can be determined or calculated e.g. theoretically based on the intensity distribution of measured signals. Alternatively the temperature can be determined by comparing measured intensity distribution to known intensity distributions of gases in known temperatures. When determining the temperature based on the measured scattered rotational Raman signal, the temperature can be used e.g. for compensating deviations due to temperature in real time.

The present invention offers advantages over the known prior art, such as fastening the determination processes and make them also possible in a production line. The invention also removes the needs for puncturing the sample volume, as well as also minimizes possible contaminations risks. In addition the determination is very accurate and reliable when the measured spot is exactly the same which is hit by the laser beam. Furthermore the invention makes the determination of at least two different gases simultaneously possible by one measurement. In addition measurements can be performed very close to the Rayleigh wavelength (the wavelength of the light source). Thanks to this, a rotational Raman spectrum can be measured from samples, which permits a surprisingly high sensitivity for a gas measurement. The invention makes also possible to selectively detect and quantitatively analyse individual gases in a gas mixture. One example of an application is the analysis of compounds that cannot be detected by infrared spectroscopy. Some examples are oxygen, nitrogen, and hydrogen.

Furthermore, according to an embodiment, applying a steep-edge or notch filter in the measuring arrangement belonging to the apparatus permits a small and economical technical solution to the measurement of the Raman spectrum of gas mixtures, which can also be applied to commercial use. The quantitative composition of a gas mixture can be determined from the Raman spectrum, which can be measured through any transparent material whatever, for example, glass or a polymer film. Thus, the invention permits non-invasive measurement from a closed compartment. The measuring arrangement can be exploited in the manufacture of small and economical gas-mixture analysers.

Still, by using the geometry according to the embodiments of invention, better sample excitation and signal collection can be achieved, compared, for example, to techniques in which a beam splitter, for example, is used. The implementation without a beam splitter permits a signal that is as much as four times stronger. The other characteristic features of the apparatus and method according to the invention are stated in the accompanying claims while additional benefits achieved are itemized in the description portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 illustrates an exemplary Raman spectrum of air for rotational and vibrational Raman scattering,

Figure 2 illustrates a rotational Raman spectrum for different gases,

Figure 3a illustrates an exemplary measuring apparatus for determining gas concentrations of a closed volume according to an advantageous embodiment of the invention (transmission grating),

Figure 3b illustrates an exemplary measuring apparatus for determining gas concentrations of a closed volume according to an advantageous embodiment of the invention (reflection grating),

Figure 4 illustrates still another exemplary measuring apparatus for determining gas concentrations of a closed volume according to an advantageous embodiment of the invention,

Figures 5a-5d illustrate examples of adjusting a focus spot into different locations in a volume according to an advantageous embodiment of the invention, and Figure 6 illustrates a Raman spectra measured from air for different window materials.

DETAILED DESCRI PTION Figure 1 illustrates an exemplary Raman spectrum measured from air, from which the rotation and vibration signals can be detected. The complex structure at the Raman-shifts of 50 - 200 cm ^"1 is the rotational Raman spectrum 1 0 of air. The vibration spectra 1 1 , 1 2 of oxygen and nitrogen appear correspondingly at the Raman shifts of ~ 1 560 cm ^"1 and ~2330 cm ^"1 . In addition, the rotational spectra of gases, such as oxygen, nitrogen, and hydrogen, can be easily detected, so that their qualitative detection is easy. As it can be seen from Figure 1 the rotational Raman spectrum 1 0 is considerably more intensive than the vibrational spectra 1 1 , 1 2.

Figure 2 illustrates an exemplary Raman spectrum for different gases. The qualitative analysis of gas mixtures is based on the identification of the characteristic spectra of the various components and quantitative analysis is based on the linear dependence of the Raman signal on the concentration of the substance being measured:

I signal ~ ^ laser ^ in which I _si g _na i is the intensity of the signal being measured, I _laser is the intensity of the laser, and a is a constant typical of each substance, which depicts the strength of the Raman scattering, and c is the concentration of the gas.

Figure 2 shows the Raman spectra for pure air 20, nitrogen 22, and a mixture 21 of nitrogen, oxygen, and carbon dioxide, which is used, for example, in food packages. The spectra 21 was measured from a minced- meat package purchased from a local supermarket, in which a protective gas mixture was used. By comparing the spectrum 21 with the spectra 20, 22, it will be seen that the gas mixture contains nitrogen and oxygen. In addition, the carbon dioxide signal can be seen as closely spaced lines in the range 40 - 80 cm ^"1 . The quantitative analysis of a known ('known' means that the qualitative composition of the mixture is known) gas mixture can be made, for example, in two ways that are, as such, known. The first exemplary method is based on measuring the component gases of the mixture as pure substances in known contents and, with their aid, fitting the measured spectrum by varying the coefficients of the components in the mixture. Alternatively, it is possible to use a limited spectrum region in the fitting, and even a single line for each component. The second exemplary method is based on the theoretical simulation of the measured spectrum. The theoretical basis of the rotation spectrum of gaseous substances is known very precisely with the aid of quantum mechanics and spectroscopy. The measured spectrum can be modelled theoretically, if the device function and temperature are known. The device function can be determined by separate measurement while the temperature can either be measured separately, or can be determined from the measured spectrum, with the aid of simulation, for example. Thus, the method is also valid when determining temperature.

Figures 3a and 3b illustrates exemplary measuring devices 300, 400 for determining gas concentrations (e.g. 0 ₂ , CO, C0 ₂ , N ₂ , H ₂ ) of a closed volume 301 according to an advantageous embodiment of the invention. The device advantageously comprises a laser emitting means 302 for emitting laser beam 306 into said volume 301 . The laser emitting means may either be a laser source 303 as such, or it may be an arrangement configured to transmit laser beam from an external laser source (optional, dashed line 303 in figures 3a and 3b). In addition the device comprises a first guiding means 309, which advantageously according to an exemplary embodiment comprises a lens 304 for guiding scattered beams 307 from said volume for to a filtering means 305. The filtering means, such as an edge or notch filter, is chosen so that it advantageously transmit essentially only bandwidth of the scattered beams, but not the wavelength of the laser beam.

It is to be noted that the filtering means 305 may advantageously be configured to guide also the laser beam 303 to the volume of said closed space 301 . In addition the filtering means 305 and the lens 304 are arranged so that the path of the laser beam 306 guided by the filtering means 305 and the path of the scattered beams 307 guided by the lens 304 are essentially parallel. Thus using the same lens 304 both for focusing the laser beam 306 and collecting the scattered beams 307 ensures that the excitation of the gas and the signal collection take place in the same space or spot. As an example the first path 306 of the laser beam is arranged to form an angle (a) at the edge filter, the magnitude of which is 1 0 - 1 6 degrees. It is to be noted that according to an exemplary embodiment also a microscope objective may be used instead of the lens 304.

The device 300, 400 also comprises a second guiding means 308 before a detector 31 1 for guiding and spreading said scattered beams 307 onto the surface of said detector so that different wavelengths of scattered beams are guided onto different locations on the surface of said detector. The second guiding means advantageously comprises a grating, which may be either a transmission grating 31 0a (as depicted in figure 3a) configured to pass said scattered beams 307, or a reflection grating 31 0b (as depicted in figure 3b) configured to reflect said scattered beams. The detector 31 1 may be e.g. a CCD or line detector, or any other equivalent analyser, analysing means or spectrometer, which can be used for detecting intensities of the scattered beams 307 so that said intensities can be used for determination of the concentrations of the measured gases.

In an exemplary embodiment the spectral resolution of the detector is advantageously selected in such a way that it is capable of distinguishing, at least partly, individual rotation lines in the spectrum. According to an exemplary embodiment, the detector's resolution can be, for example, approximately 0.1 - 1 0 cm ^"1 , more particularly 0.5 - 5 cm ^"1 , and even more particularly 0.5 - 2 cm ^"1 . The width of the signal in pixels can then be 0.01 - 0.05 nm. One commercial example of the detector is the Apogee Alta U 1 1 07 spectrometer. However it is to be noted that the invention is not limited to this detector but naturally also other suitable detectors can be used. In advantageous example the line width of the laser source is configured to be smaller than the resolution of the detector. In addition the device may comprise a lens 31 2 before the grating for forming an image 31 3 (for example at the location of a slit element 31 3a) from the scattered beams, the size of the image 31 3 being advantageously less than 1 00 μιη (the smaller the image, the smaller the spot on the detector surface and thus the higher resolution achieved). Furthermore the second guiding means 308 may further comprise a lens 31 4 for collimating the image 31 3 of the collected scattered beams and guiding said collimated scattered beams to the grating 31 0a, 31 Ob, as well as a lens 31 5 for forming again an image of the collected scattered beams on the surface of said detector 31 1 . As already discussed the filter means 305 may either be an edge filter or notch filter. As an example, the edge filter may be of the long wave pass filter type. According to one embodiment, this is characterized by a high reflection coefficient at the wavelength of the laser source, but a high transmission coefficient for the Raman scattering coming from the sample. The filter's response to the reflection of the laser beam is configured to be sufficiently steep for the rotation Raman signal to pass through the filter, but for Rayleigh scattering to be reflected with a high efficiency. Of course, a short wave pass filter can also be applied as an edge filter, for example.

The following describes the features of an example edge filter in slightly greater detail in connection with an exemplary (but not limiting) embodiment of the invention. According to an exemplary embodiment, 50 % of the transmission, measured from the OD (optical density) = 6 value of the edge steepness of the edge filter, can be in a wavelength range, the extent of which is, for example, 0.5 %, or less of the wavelength of the laser light source, more particularly 0.2 % or less of the wavelength of the laser light source, i.e. < 1 .1 nm = 37.5 cm ^"1 , the wavelength of the laser light source being 532 nm. Optical density OD = -log(T). Thus OD = 6 means a transmission T value of 0.0001 %.

Again as an example, the transition width of the edge filter, which is measured from the filter's 50 % transmission wavelength to the wavelength of the laser, can be, for example, less than 0.5 % of the wavelength of the laser light source, i.e. < 2.7 nm = 90 cm ^"1 , the wavelength of the laser light source being 532 nm. The edge filter's transmission at a wavelength, the deviation of which from the wavelength of the laser light source is 0.5 %, can be at least 50 %. According to an exemplary embodiment, the transmission of the edge filter is high, at least at a distance of app. 50 cm ^"1 from the wavelength of the laser. One example of the filter is Semrock's filter, known by the commercial name RazorEdge (R) Long Wave Pass Filter (LP03-532RE-25). In addition the lens 304 is advantageously selected in such a way that the laser beam 306 is focussed on a relatively small point in the volume 301 , especially when the signal is measured from the gas. A point-like signal source can be achieved using a suitably selected lens 304. According to an exemplary (not limiting) embodiment, the lens 304 can be selected in such a way that it achieves a focus with a diameter of 1 - 20 μιη, more particularly 5 - 1 5 μιη, such as approximately 1 0 μιη. Correspondingly, the numerical aperture (NA) of the lens 31 2 can be selected in such a way that it corresponds to the numerical aperture of the focussing lens 31 4. According to an example the numerical aperture (NA) of the detector is configured to correspond to the numerical aperture of the lens 304 arranged in connection with the measuring head (illustrated in connection with Figures 5a-5d).

Again, it is to be noted that the signal paths in the devices 300, 400 are only exemplary and that also other transmission way can be used, such as a fibre (not depicted in figures) especially between the first 309 and the second 308 guiding means and especially if said first and second guiding means are not located in the same device, but are decentralised (not illustrated in figures, but it is understood that the invention also covers this embodiment). According to an example the Raman signal 307 can be collected into an optical fibre without significant losses occurring. According to an exemplary embodiment, the internal diameter of the optical fibre (leaving the first guiding means, for example) can be 5 - 1 00 μιη, more particularly 50 - 1 00 μιη, such as approximately 80 μιη. The other end of the fibre can be connected directly to the detector 31 1 , such as a spectrometer without any gap. Thus, the diameter of the fibre corresponds to the size of the image of the source arriving at the detector.

According to an exemplary embodiment a double magnification can be permitted in the optics of the detector. The signal beam can then be still focussed on the detector to roughly one to two pixels, assuming that the pixel size is approximately for example 1 6 μιη (a typical pixel size in CCD cells). A sufficient resolution will then be achieved. The numerical aperture of the input of the detector, such as a spectrometer is configured to be the same as the numerical aperture of the optical fibre (when the fibre is used). No signal will then be lost. For example, if the fibre's NA is 0.22, the input's NA will be 0.22, but an NA = 0.1 1 can be permitted from the spectrometer's array. By using these parameters and arranging the optics to be suitable, a sufficient resolution can be achieved without loss of signal. However, the minimizing of the size of the focus point in the sample plays a significant role in this. Further, in addition to the factors referred to above, the factor must also be taken into account that the line width of the laser is arranged to be smaller than the desired resolution, i.e. in the case according to the embodiment, approximately 1 0 cm ^"1 or less, in particular approximately 2 cm ^"1 or less. One commercial example is the CN I Laser MSL-111-532. It is to be noted that the embodiment described in this section is only as an example and that the invention is not limited to this particular embodiment. Still according to an exemplary embodiment the emitted laser beam 306 and scattered beams 307 are arranged to be transmitted especially between the filtering means 305 and the focus spot 501 essentially in a vacuum or in a medium producing no Raman scattering. However, according to another embodiment said volume between the filtering means 305 and the focus spot 501 may be filled with medium producing Raman scattering, whereupon it can be used for calibration purposes.

Furthermore, the device may also comprise means 31 6 (such as data processing means with suitable memory) configured to determine temperature of the gas concentration of the volume 301 based on the scattered Raman signal, especially based on the intensity distribution of the scattered Raman signal, where temperature can be determined or calculated theoretically from the intensity distribution or where temperature can be determined by comparing measured intensity distribution to known intensity distributions of gases in known temperatures. The device 300, 400 may also comprise an aperture 31 3b arranged advantageously in the connection with the slit element 31 3a (or between the first 309 and the second guiding means 308) in order to limit or control any diffused or ambient light, which otherwise might end up onto the surface of the detector and thereby causing anomaly signals. According to an exemplary embodiment the diameter 31 3c of the aperture 31 3b is in the range of 0.1 -1 0 mm, and most advantageously 1 -3 mm. However, as an example the diameter 31 3c of the aperture might be adjustable. In addition it is to be noted that the aperture may be arranged either side of the slit element 31 3a. Again, according to an exemplary embodiment the device 300, 400 may also be implemented without the slit element 31 3b, for example if the aperture 31 3b is selected with suitable properties, such as with the suitable sized diameter, which is according to an example in the range of 0.1 - 0.2 mm. However the alignment of the system is typically much easier if the both slit element 31 3a and the aperture 31 3b is used.

Figures 5a-5d illustrate examples (of a measuring head 500) of adjusting a focus spot 501 into different locations in a volume 301 according to an advantageous embodiment of the invention, where the first guiding means and especially a movable lens 304 of it is used for focusing the laser beam 306 into a different depths of the volume 301 (such as inside a foodstuff box, window, wine bottle, etc. as an example) and collecting the scattered beams 307 from the different depths of the volume 301 .

The measuring head 500 comprises advantageously a movable lens 304, which can be moved e.g. with the movable housing 502. Of course different kinds of variations can be applied, such as e.g. a lens fixed to the movable housing, or the like performing substantially the same function or same results. Again the housing may comprise light shielding means 503 (such as avoiding ambient light to enter into the device or lens 304). The "aperture" area 504 of the light shielding means 503 may be configured to be changing when the lens is moved towards to or away from the volume 301 , via which the possible ambient light is again minimized (illustrated only in figures 5 and 5b, but it is clear that they may also be applied in connection with figures 5c and 5d). Furthermore the measuring head 500 may also comprise a window 505 for covering the optics and inner volumes of the measuring head. The window 505 is advantageously chosen so that it is transparent for the laser beam used as well as also to the scattered beams measured or so that its effect to the intensities is known (according to an embodiment (depicted in connection figure 6) the window can also be used for calibration purposes).

As depicted in Figures 5a-5b, the focus spot can be focused into different depths of the volume 301 to be determined e.g. by changing the depth of focus spot in a volume 31 0 of the measuring head (such as moving window 505 in relation to the lens 304 in relation to the volume to be determined). In this way the gas can be measured either nearby the covering film of the package (e.g. if the gas volume as such is small) such as depicted in figure 5a, or the deeper part of the volume, as in figure 5b. Moreover separate (and separated) gas volumes in different depths can also be measured by focusing the spot at first into the first volume (figure 5c), and then to the second volume (figure 5d), etc. In addition it is to be noted that the volume inside the measuring head, such as a volume 506 between the lens 304 and the covering window 505 may be in a vacuum or filled with a gas, such as e.g. argon.

By the examples illustrated in figures 5a-5d the accuracy of the measurement is improved. In addition by using the measuring head 500 as depicted any contaminations can be avoided, thereby making the measuring head 500 suitable also for industrial use, for example. The measuring head 500 can be shaped as a truncated cone, which narrows towards the sample being examined. The angle of the narrowing of the cone can correspond to the angle of the beam being focussed. In addition, the measuring head's outer coating or window can be arranged at a suitable distance from the focus point.

By means of the arrangement depicted in figures 5a-5d, several different benefits can be achieved, which are depicted in great detail in the following. The first of the benefits achieved by the conical measuring head is the minimization of the distance between the focus point and the window. The small distance permits the signal to be measured from directly under the plastic film covering the package being measured. This is of great importance, if there is only a small gas space under the film. Such packages are used in the food industry, for example. A second benefit achieved is the effective elimination of external stray light. In that case, the cone can be made from an opaque material. A third benefit is the application of the measuring head to measurements from small areas. The measuring head with a cone permits the outermost point of the unit to be arranged to have a small diameter, which is advantageous if the area being measured is small. A fourth benefit is the elimination of sources of error. The measuring head can be pressed tightly against the package being examined. There will then be a tight and gap-less contact between the window and the package. This will ensure that air will not remain between the window and the package, which would introduce an error to the measurement. In addition, the tight contact also keeps the package covering or film flat, which reduces the scattering caused by the covering of the package, which affects both the laser light and the signal.

Figure 6 illustrates a Raman spectra measured from air for different window materials, such as windows 505 used in the measuring head of the device 300, 400. The devices 300, 400 (for example) shown in Figures 3, 4 on a rough schematic level contain also several other significant features in connection with exemplary embodiments. Because the aforementioned equation contains the intensity of the laser beam, the measurement should be normalized relative to it. Two alternative methods can be used for this purpose. In the first alternative, the measurement arrangement unit (such as volume 506 or other volume especially between the filter 305 and the volume 301 to be measured) is filled with a gas, whereupon the unit 506 (as example) is made gas-tight. The gas inside the unit 506 produces its own Raman signal, the intensity of which is proportional to the intensity of the laser beam. The actual measurement result can be calibrated by using the following formulae:

I signal ~

*-reference

. _ ^reference

Haser ~~ ~

^reference ^reference

. _ ^reference

* signal ~ ~ ~ & signal * signal

^reference ^reference in which 'signaf refers to the Raman signal of the measured component and 'reference' refers to the Raman signal of the reference gas. Thus, the measurement of one separate reference gas permits the internal calibration of the measurement arrangement. If air is used as the reference gas, the gas-tightness need not be as good and, in addition, the composition of air can be used in the internal calibration. However, if small amounts of the same components as those appearing in air appear, the strong reference- gas signal may obscure the weak signal of the measured component. In this case, it is better to use some other calibration gas. Some alternatives for this are hydrogen and deuterium gas, which have a narrow Raman line, which is far from typical measured signals. A second possibility is to use a suitable material in the window 505 of the measuring head 500. Figure 6 shows Raman spectra 30, 31 , 32 measured from air, using various window materials (sapphire, CaF ₂ , MgF ₂ ). Relative to the focus point suitably situated in a gas volume, the window 505 produces a suitable reference signal, which can be used for internal calibration in the same way as the signal of a reference gas.

One example of a suitable material is calcium fluoride, CaF ₂ , which has a single sharp Raman line at a Raman shift of 320 cm ^"1 , which is suitably separate from the typical measuring region. Other window materials too, which have suitable spectra, are suitable for the purpose. It is characteristic of the reference substance that its calibration signal appears in a region covered by the measuring region of the selected spectrometry solution. In this sense, the examples presented above are particularly suitable.

If a reference-window material is used, the measuring arrangement unit (e.g. volume 506 or similar volume as depicted in this document) can be filled with a gas that does not itself produce a Raman signal. Argon gas, for example, is suitable for this purpose. In several cases there is no need for a particular gas filling. This is particularly the case if high concentrations are measured. The signal produced by the air contained in the measurement- arrangement-unit can then be reliably deducted from the measurement result.

The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims. Especially it is to be noted that the components of the device, such as a laser light source and device can be integrated into a same device with the first and second guiding means, but alternatively they may locate also in separate units. In addition it is to be noted that even though it is said that the device comprise laser emitting means, it does not necessarily mean the said device would comprise also the laser source. Thus the laser emitting means may be interpreted as means which might in some embodiment be means configured to guide said emitted laser beams from the laser source itself into the gas concentration determination device in order to be used in said gas concentration determination device as depicted in this document elsewhere. However, the laser source to be used is advantageously continuous operation laser. In addition it is to be noted that the device depicted in this document and the figures may also comprise other components than presented here, such as power source.