Optical system of spectrometer

Abstract

A spectrometer device (100) comprises an incident aperture (2), a diffraction grating (3), and two detectors (5) for measuring the spectrum of incoming light (L), the detectors being located on the same side of a deflection plane. Two vertical focusing mirrors (4) focus the light onto the detectors, which are arranged along two polygonal figures (6a, 6b) into a front row of mirrors (4b) and a rear row of mirrors (4a), the front row of mirrors and the rear row of mirrors being offset relative to each other and offset relative to the focus curve. The deflection angle of the front row of mirrors is less than 90°, which can minimize the offset (d1) of the front row of mirrors (4b) relative to the focus curve. The distance (d1) between the front row of mirrors and the front row of detectors (5b) is minimized while still avoiding collision between the front row of detectors (5b) and their brackets and the rear row of detectors (5a) and their brackets. The front row of mirror elements overlaps with adjacent rear row of mirror elements.

Description

Technical Field

[0001] The present invention relates to an optical system of a spectrometer and a spectrometer device comprising such an optical system. Background Art

[0002] The optical system of a spectrometer is a component used to measure the properties of light across a specific portion of the electromagnetic spectrum. The optical system is the part of a spectrometer instrument used to generate spectral lines through methods such as arc or spark excitation, LIBS, ICP, or glow discharge excitation. The wavelength and intensity of these spectral lines are measured to identify materials or analyze their chemical composition. Such spectrometer instruments can operate over a wide wavelength range, from the deep UV to the far infrared.

[0003] Spectrometer equipment requires an excitation generator for spectral analysis of a sample, which provides the excitation energy required for evaporation and ionization of a portion of the sample to generate a plasma; an optical system for separating the wavelengths of the spectral lines emitted by the plasma; a detector or detector arrangement capable of measuring the intensities of these spectral lines; a readout and control system for recording the measured values ​​and controlling the functions of the spectrometer equipment; and a computer with software for conveniently converting the obtained measured values ​​into the concentrations of the individual components of the sample. Modern spectrometers almost exclusively use diffraction gratings for spectral dispersion. If the grating surface is non-planar, these gratings have imaging capabilities. The shape and spacing of the grating grooves influence these imaging properties. When the optical system is arranged in a so-called Paschen-Runge setup, the grating is a Rowland grating whose substrate is a concave spherical surface with a radius of curvature R. The shape and spacing of the grooves are such that the resulting focus curve (the curve in the grating's deflection plane, where a sharp image of the entrance aperture occurs) is a circle with diameter R, known as a Rowland circle. All components of the optical system, such as the grating, entrance aperture, and detector, lie on a Rowland circle. The radius from the center of curvature of the grating to the center of symmetry of the groove pattern on the grating surface (which ideally coincides with the geometric center of the grating) is called the grating normal. This is the reference line used in the grating equation, and all angles are measured relative to it. In a Paschen-Runge setup, the focus curve (circle) is symmetric about this normal.

[0004] The grating substrate can be non-spherical, and the spacing and shape of the grooves can differ from the Rowland type, resulting in a non-circular focus curve that is asymmetric about the normal. This can be the case with so-called flat-field gratings, which have a focus curve optimized for use with spatially resolved linear (array) detectors at certain diffraction angles. The present invention can also be applied to some of these systems and is not limited to Paschen-Runge installations alone.

[0005] Optical systems using line detectors with a pixel height of <1 mm such as CCD sensors or CMOS sensors will benefit from a vertical focusing element, e.g. a rod lens, a cylindrical lens, a cylindrical mirror or an aspherical mirror or lens, for compressing the spectral line to be detected from a height of a few millimeters perpendicular to the deflection plane to ideally use the pixel height of the detector. Otherwise, signals from weak lines may not be detected due to poor (light) intensity levels. This is especially true for optical systems with large focal lengths. Such pure vertical focusing has to take place at the detector and cannot be done together, for example, at the entrance slit due to the imaging properties of the grating. It is often advantageous to combine vertical light focusing and focusing at a deflection plane offset to reduce the overall size of the optical system and to create the necessary installation space for the detector.

[0006] Several solutions to the problem of vertical focusing and out-of-plane deflection at the detector have been described over the years, and the prior art is covered by several patents:

[0007] In DE19853754B4 and US6,614,528B1, vertical focusing and deviation from the deflection plane are achieved by cylindrical mirrors. The longitudinal axes of these mirrors are mounted in the deflection plane along a polygonal figure, each part of which is parallel to and offset from a dedicated local tangent line of the actual focal curve of the optical system. Here, the radius of curvature ρ Cyl The offset or distance d between the local tangent line and the corresponding part of the polygonal figure is constant over the entire wavelength range or focal curve and is determined by the universally valid equation

[0008] (1)

[0009] in, is the angle from the deflection plane. Gaps between adjacent mirrors are necessary to prevent damage to the mirror edges during assembly of the detector units (each of which comprises a vertically focused cylindrical mirror element, a designated detector, and a mounting plate or base plate). To create the necessary space within the optical system for the detectors themselves, whose housings extend further than their photosensitive or receiving areas, cylindrical mirrors are selected. The normalized angle of deviation from the deflection plane is such that the detectors are located on two planes parallel to the deflection plane and equidistant (distance = d). Thus, one of these mounting planes is arranged above the dispersion plane and the other is arranged below the dispersion plane. The length of the cylindrical reflector is adapted to the length of the receiving area of ​​the specified detector used. Typically, the length of the reflector will be slightly less than the length of the receiving area of ​​the specified detector to ensure placement tolerances. Due to the increase in viewing angle, the reflector shape must become increasingly trapezoidal as the diffraction angle increases to ensure that the incident spectral lines fall completely on the reflector surface (or in the gap between adjacent reflectors). Moreover, as the diffraction angle increases, the mirror center point and the detector center point become more offset relative to each other along the portion of the polygonal diagram. Since the spectral lines are not yet focused at a position far from the focal curve, the spectral lines incident on the cylindrical reflector surface will extend horizontally over an area that is much wider than their expected FWHM on the illumination area of ​​the detector. The width of this area on the mirror surface will depend on the distance d between the longitudinal axis of the cylindrical reflector and the corresponding local tangent of the focal curve calculated according to formula (1). At the edges of a mirror, the light of a spectral line will only partially strike the mirror, and thus only a small fraction will reach the designated detector. Consequently, its intensity will vary if spatial drift occurs. Consequently, such lines must be considered unreliable and should not be used for spectral evaluation of samples. Consequently, in this system, spectral lines that partially or completely fall into the gaps between the mirrors are lost.

[0010] In FR2953017B1, with different curvature radii ρ Cyl1 and ρ Cyl2 Two rows of cylindrical mirrors are used for vertical focusing and deflection out of the deflection plane. According to equation (1), the two rows of cylindrical mirrors are offset from each other and the focus curve, and a standard deflection angle of 90° is selected from the deflection plane. As shown in the patent specification, the center points of the cylindrical reflectors belonging to a row can be considered to be located on the so-called folium curve. However, this is only because their longitudinal axes are still installed in the scattering plane along the respective polygonal figures described in the previous paragraph. This means that in this arrangement, each row of cylindrical reflectors is installed in exactly the same way as in DE19853754B4 and US6614528B1, except that deflection occurs only in one direction. In this arrangement, the reflector edges cannot collide because the reflectors covering adjacent parts of the spectrum are located in different rows. In order to also ensure a collision-free arrangement in the "third dimension" (i.e., deflection direction) of the optical system, the radius of curvature is selected in such a way that the mounting plane of the designated detector of the front row of mirrors (i.e., the reflectors in the row close to the grating) is located much higher than the mounting plane of the designated detector of the rear row of mirrors. In this example, the radius of curvature selected to avoid collisions between the reflector edges and the detector bracket is 27.5mm and 75mm. Since the mirrors from rows 1 and 2 are offset from the focal curve by distances d1 and d2, respectively, the spectral lines are again not focused on the surfaces of those cylindrical mirrors. Thus, in a region of a certain width around the edge of the front row of mirrors, the spectral lines may fall partly on one mirror and partly on the other, and thus be detected by both detectors simultaneously, or they may fall into the gap between the edges of adjacent mirrors and thus not be detected at all. The width of this region (transition zone) again depends on the distance d from the focal curve of the cylindrical mirrors involved. i In particular, the larger the radius of curvature of the front mirror (here 75 mm), the wider the transition region. If the optical system is operated in the direct field of view of the plasma, spectral lines that fall into this region in their field of view of the plasma may also be compromised. If applicable, using the combined signal of lines from adjacent detectors can be problematic due to variations in detector characteristics. Therefore, spectral lines that fall into the transition region must be considered lost.

[0011] The use of aspherical (off-axis parabolic) mirrors as another example for deflection and vertical focusing described in DE 10 2007 027 010 A1 and US Pat. No. 8,649,009 B2 does not solve the gap problem of DE 198 53 754 B4, it only improves the vertical focusing.

[0012] The prior art is also reflected in several spectrometers that use no or only limited vertical focusing and no deflection, or only partial deflection out of the deflection plane: Almost gapless wavelength coverage can be achieved by using an alternating arrangement of: a) plane mirrors that deflect the light out of the deflection plane at an angle of 1 / 2 relative to the specified detector b) Direct light detector units (i.e., detectors without any reflectors), with a and b arranged in two rows, with the reflector edges overlapping the receiving areas of adjacent direct light detectors (in the line of sight from the center of the grating). When the direct light detectors are located on the focal curve itself, the width of the transition zone here depends solely on the position of the deflection mirror relative to the focal curve. Vertical focusing at isolated locations can be achieved by replacing the plane mirrors with matching cylindrical reflectors, or by attaching, for example, cylindrical or rod lenses with appropriate imaging properties to the detectors themselves and appropriately correcting their position relative to the focal curve. However, the imaging properties of cylindrical or rod lenses are inferior to those of cylindrical reflectors or aspherical mirrors. Another way to achieve gapless coverage is to alternately mount the detectors slightly above and slightly below the deflection plane tangential to the focal curve, as long as the uncompressed spectral line is sufficiently high (=extends in the vertical direction), and again by overlapping the receiving areas of the detectors. This arrangement can be combined with non-planar vertical focusing, for example by applying cylindrical lenses to the detectors. However, in non-planar areas, the light output decreases and the field of view of the plasma may be affected. This setup can also be combined with methods that collect light from an area closer to the deflection plane than the detector's receiving area, for example, by using a window with a slanted edge in front of the detector to act as a periscope. In either of these setups, the overlapping area allows selection of which detector to measure the line in question. Again, combining the signals from two detectors for a spectral line is not recommended due to potentially different detector characteristics and plasma field of view issues when the optical system is looking directly into the plasma.

[0013] However, it is desirable to detect the spectrum quasi-continuously over the entire usable wavelength range of the optical system with a minimized transition region width and at the same time have the option of in-plane vertical focusing. Summary of the Invention

[0014] An object of the present invention is to provide an optical system that enables quasi-continuous detection of the spectrum within a desired wavelength range by minimizing the extent of the transition regions between adjacent, overlapping, and possibly perpendicularly focused mirror elements. Another object of the present invention is to reduce the overall size (height) of the optical system.

[0015] The object is solved by an optical system comprising: at least one entrance aperture for admission of light to be analyzed, at least one diffraction grating for spectral deflection of said light, at least two detectors for measuring the spectrum of said light, comprising a front row of detectors and a rear row of detectors, wherein said detectors are arranged on the same side of a deflection plane of said optical system, and at least two vertical focusing mirror elements for focusing said light onto said detectors assigned to said vertical focusing mirror elements, said vertical focusing mirror elements being arranged along two polygonal figures offset from each other and from a focal curve as a front row of mirror elements and a rear row of mirror elements, wherein said polygons are arranged as a front row of mirror elements and a rear row of mirror elements. Each part of the polygonal figure is parallel to a dedicated local tangent to the focal curve of the grating, so that the deflection angle of the front row mirror element is <90°, thereby minimizing the offset of the front row mirror element relative to the focal curve, minimizing the distance between the front row mirror element and the front row detector assigned to the front row mirror element, while still avoiding collision between the front row detector and the bracket of the front row detector and the bracket of the rear row detector, and at least one end of the front row mirror element overlaps with an adjacent rear row mirror element, or overlaps with the receiving area of ​​an adjacent direct light detector if viewed from the grating.

[0016] The inventive optical system comprises at least one entrance aperture for admitting light to be analyzed, at least one diffraction grating for spectrally deflecting the light, at least two vertical focusing mirror elements for focusing the light onto detectors assigned to the vertical focusing mirror elements, and at least two detectors for measuring the spectrum of the light, wherein each light focusing mirror is assigned to one detector. The detectors are arranged on the same side of the deflection plane of the optical system to reduce the height of the optical system. The vertical focusing mirror elements are arranged as a front row of mirror elements and a rear row of mirror elements offset from each other and from the focal curve along two polygonal patterns, wherein each portion of the polygonal patterns is parallel to a dedicated local tangent to the focal curve of the grating. In order to provide an optical system that enables quasi-continuous detection of the spectrum over a desired wavelength range by minimizing the extent of the transition region between adjacent, overlapping, and possibly vertically focusing mirror elements, the offset of the mirror axes of the front row of mirror elements from the focal curve is minimized. This is achieved by selecting the deflection angles of the vertical focusing mirror elements arranged as the front row of mirror elements. This is achieved so that its designated detector and its bracket are folded away from the rear detector and its bracket that may collide.

[0017] It is particularly advantageous if the length of the mirror element is shorter than the length of the receiving area of ​​the detector to which it is assigned and if adjacent ends of the mirror elements overlap with each other or with the receiving area of ​​an adjacent detector to which no mirror element is assigned, so that the receiving areas of adjacent detectors and their illumination areas also overlap. Furthermore, it is very advantageous if the width of the transition zone is minimized by offsetting the focusing mirror elements as little as possible with each other and by minimizing the maximum occurring distance d1 between the detector and the mirror element. This is achieved by selecting a deflection angle \f\ < 90° for the front row of mirror elements. As a result, the detectors to which they are assigned and their supports can be folded away from possible collisions with the rear row of detectors and their supports. This is in contrast to FR2953017B1, in which only ρ is increased. Cyl , in order to create the necessary installation space for the specified detector. Minimizing the maximum occurrence distance will also reduce the overall height of the optical system, which is the second purpose of the present invention. In addition, by selecting the deflection angle for the front mirror element Compared to existing optical systems in which the detector is arranged only on one side of the deflection plane, the distance between the front mirror element and the focus curve can be reduced. In the optical system of the present invention, the reduced distance from the focus curve reduces the out-of-focus line area on the front mirror element and reduces the width of the transition zone.

[0018] In order to minimize the maximum occurrence distance d1 between the detector and the mirror element, we have to estimate the resulting width of the transition region for two different, exemplary spatial arrangements of the vertical focusing mirror and the specified detector in the above-mentioned Paschen-Runge spectrometer and show the most advantageous setting, see Figure 5 .

[0019] In a preferred embodiment, two or more detectors are equipped with mirror elements which are located adjacent to each other, the mirror elements having different distances to the focal curve in order to achieve overlap. Depending on the shape of the detector bracket, the rear row has the same deflection angle or a larger angle, e.g. Figure 1 shown.

[0020] It is advantageous if the curved surfaces of adjacent vertical focusing mirror elements have different radii of curvature and different deflection angles, so that the distances between the vertical focusing mirror elements and the assigned detectors and thus the overall size of the system can be optimized.

[0021] In a further preferred embodiment, if the optical system comprises a detector arrangement covering a continuous wavelength range, it is particularly advantageous if at least each second detector of the detector arrangement is equipped with a mirror element and if each mirror element overlaps the receiving area of ​​any adjacent reflector or any adjacent direct light detector, respectively. In this way, the receiving areas of the detectors and their illumination areas overlap. By selecting the deflection angles for the (front row) mirror elements Their designated detectors and their brackets can be folded away from the (rear row) direct light detectors, with which they could collide, creating the necessary installation space for the designated detectors. The front row can therefore be closer to the direct light detectors, as the detector brackets cannot collide.

[0022] In another preferred embodiment, the reflecting surface of at least one of said vertical focusing mirror elements is shaped as a portion of a cone, ie the surface has a variable radius of curvature which increases along the mirror axis to achieve overlap.

[0023] In another preferred embodiment, the reflective surface of at least one of the vertically focusing mirror elements is cylindrical in shape, for example, the surface has a constant radius of curvature along the mirror axis.

[0024] In another embodiment, the light focusing element is a cylindrical reflector and / or a conical reflector. Additionally or alternatively, the vertical focusing element is a focusing lens, such as a rod lens or a cylindrical lens in combination with a flat deflection mirror element. The choice of the vertical focusing element can be determined according to the required task, the resolution, the grating used, or the size of the optical system. In particular, the combination of different vertical focusing elements increases the flexibility of the optical system. Furthermore, it is possible to construct the vertical focusing element or the optical system so that the vertical focusing element can be exchanged, thereby creating a flexible and upgradeable optical system.

[0025] In another embodiment, at least one detector is equipped with a lens for vertical focusing, eg a rod lens or a cylindrical lens.

[0026] In another embodiment, the construction principle of the optical system is a Paschen-Runge setup or a flat-field setup or a Czemy-Tumer setup or an Ebert-Fastie setup.

[0027] In a very advantageous embodiment, the detector of the detector arrangement is a line detector, preferably a CCD or CMOS detector.

[0028] The spectrometer device according to the invention comprises at least one optical system according to the invention and further comprises an operating unit connected to at least one detector arrangement for operating the detector arrangement and analyzing the measured spectrum of the diffracted light.

[0029] The embodiments listed above may be used alone or in any combination to provide apparatus and methods according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] These and other aspects of the invention are described in detail in the following figures.

[0031] Figure 1 : A schematic diagram of a first embodiment of an optical system according to the present invention when measuring diffracted light in an isometric view;

[0032] Figure 2 : Schematic and detailed views of a front view of a vertical focusing element and a detector in a first embodiment of an optical system;

[0033] Figure 3 : Schematic and detailed illustrations of an isometric view of a vertical focusing element in a second embodiment of the optical system;

[0034] Figure 4 : Schematic and detailed illustrations of isometric views of a vertical focusing element and a detector in a third embodiment of an optical system;

[0035] Figure 5 : Out-of-plane deflection of the spatial arrangement of the mirror-detector arrangement and the mirror-detector arrangement in the different methods according to the prior art (a) and the invention (b);

[0036] Figure 6 : the minimum line width of the spectral line at a certain distance from the focal curve;

[0037] Figure 7 : The cross section perpendicular to the deflection plane at the normal point to determine the favorable geometry, achieving the minimum safety distance s between the two mirrors and resulting in the minimum transition zone width wT;

[0038] Figure 8 : Schematic diagram of light paths and angles of an optical system according to the present invention, as Rowland circle optics. DETAILED DESCRIPTION

[0039] Figure 1The general arrangement of an optical system 1 in a Paschen-Runge setup is shown, comprising an entrance aperture 2, the optical system 1 through which the light L to be analyzed enters, a grating 3 for spectral scattering of the light L, a vertical focusing mirror element 4 for focusing the light L, and a detector 5 for measuring the spectrum of the light L. In order to focus the scattered light, the vertical focusing mirror elements 4 are arranged along two polygonal figures 6a, 6b, which are offset from each other and from the focus curve. Each part of the polygonal figure is parallel to its dedicated local tangent to the focus curve of the grating 3 and can then be measured by the detector 5 assigned to each vertical focusing element 4. Since the vertical focusing mirror elements are arranged along the polygonal figures 6a, 6b, the detector 5 or its center line and the focus curve can be superimposed on each other. In the optical system, it is particularly important that the adjacent ends 7a, 7b of the vertical focusing elements 4 overlap and that the maximum distance d between the vertical focusing mirror element and the detector, calculated according to formula (1), is minimized. By choosing the deflection angle φ for the front row of mirror elements, the maximum distance d between the vertical focusing mirror element and the detector is minimized. In this example, this is 60°, which ensures virtually gapless measurement of the covered wavelength range.

[0040] Figure 2 Shown in Figure 1 Details of the arrangement of the vertical focusing mirror element 4 and detector 5 used in the embodiment shown. Figure 1 and Figure 4 The vertical focusing mirror element 4 of the embodiment of FIG. 1 is arranged along two polygonal figures 6a, 6b which are offset from each other and from the focus curve. Figure 2 In the viewing angle, the two polygonal figures 6a, 6b fall on top of each other, which is why only one polygonal figure can be seen. Each part of the polygonal figure is parallel to a dedicated local tangent of the focal curve of the grating 3. In this embodiment, the rear row mirror element 4a is assigned to the rear row detector 5a, while the front row mirror element 4b is assigned to the front row detector 5b. In order to make it easier to handle these elements, each rear row detector 5a and rear row mirror element 4a is mounted on a substrate 9. This also applies to the front row detector 5b with its corresponding front row mirror element 4b. The rear row and front row mirror elements 4a, 4b are arranged within the distances d(4a) and d(4b) to the focal curve calculated according to formula (1), and the center lines of the rear row and front row detectors 5a and 5b overlap with the focal curve.

[0041] Usually, the deflection angle Can be less than or equal to 90°. Here, the deflection angle of adjacent vertical focusing elements are not equal, and the deflection angle of the rear mirror element 4a is 90°, while the deflection angle of the front mirror element 4b is 60°. The vertical focus is further influenced by the curved surface 8 of the rear and front mirror elements 4a, 4b. The radius of curvature ρ(4a)=27.5mm of the curved surface 8 of the rear mirror element 4a is a cylindrical surface, and the radius of curvature ρ(4b)=35mm of the curved surface 8 of the front mirror element 4b is also a cylindrical surface. This embodiment allows the adjacent ends of the vertical focusing mirror elements to overlap while minimizing d(4b). This embodiment also allows the vertical focusing element 4 to be easily replaced, so that the mirror element can be easily replaced by a plane mirror element to suppress the focusing, thereby reducing the intensity level when necessary. In addition, the focusing mirror element can also be replaced by a combination of a plane deflecting mirror element and a focusing lens.

[0042] exist Figure 3 In the second embodiment shown, the vertical focusing mirror elements are conical mirrors. The longitudinal axis A of each vertical focusing mirror element 4 intersects the polygonal pattern 6, and each part of the polygonal pattern 6 is parallel to its dedicated local tangent to the focus curve. The distance between the polygonal pattern parts and the local tangent is given by formula (1), where p Cyl Must use ρ Avg Instead, ρ Avg is the average radius of curvature of the focusing conical mirror elements characterized in this embodiment. Adjacent vertically focusing conical mirror elements 4 have equal deflection angles, in this case 90°. Since the radius of curvature of the conical mirror element is different at each point along its longitudinal axis, formula (1) will provide a different distance d from the focal curve for each point along the longitudinal axis of the mirror element. Therefore, the longitudinal axis of the mirror element intersects the polygonal figure and the axis cannot be on the figure itself. Since the radius of curvature at one end of the conical mirror element is smaller than the radius of curvature at the other end, the corresponding distances are different, so that adjacent and identical conical mirror elements can overlap like tiles on a roof, thereby providing quasi-continuous coverage of the wavelength range while minimizing the width of the transition zone between adjacent conical mirror elements. The tangential focus of the wavelength of the spectral line falling on a specific point of the conical mirror element is of course not affected by deflection away from the deflection plane or vertical focusing in a direction perpendicular to the deflection plane. Therefore, the detector cannot be parallel to the deflection plane as in other embodiments, but must be centered around the angle from ρ Avg The point where the beam hits it is tilted while remaining in the same plane as the longitudinal axis of the conical mirror element. By properly tilting, the correct tangential focus is achieved for each wavelength at each point along the longitudinal axis of the conical mirror. Avg and / or different deflection angles of adjacent conical mirror elements.

[0043] Figure 4The third embodiment in FIG has a greater number of detectors 5 than vertical focusing mirror elements 4. It is necessary to assign each vertical focusing mirror element 4 to a detector 5, but it is not necessary for each detector 5 to be assigned a vertical focusing mirror element 4 in order to detect light. These direct imaging detectors 5 c that do not have an assigned vertical focusing mirror element 4 are arranged directly along a portion of the focal curve of the grating 3. In any case, these direct imaging detectors 5 c must be similar to be considered as adjacent vertical focusing elements 4, so that adjacent ends 7 a, 7 b of the direct imaging detectors 5 c overlap adjacent ends of adjacent vertical focusing mirror elements 4. The use of direct imaging detectors 5 c that do not have a vertical focusing mirror element 4 enables a portion of the spectrum to be imaged directly onto the detector 5. Figure 4 Embodiment 3 also uses vertical focusing mirror elements 4 with different radii of curvature ρ for the curved surface 8. The front mirror element 4b has an infinite radius of curvature ρ, making the curved surface 8 appear flat. The rear mirror element 4a, on the other hand, has a smaller radius of curvature ρ than the front mirror element 4b, making the curved surface 8 appear cylindrical. The difference in the radius of curvature ρ of the rear and front mirror elements 4a, 4b means that vertical focusing is effectively achieved with the rear mirror element 4a, while the front mirror element 4b does not perform any focusing at all. In order to use the same substrate for both detector units, the same deflection angle is chosen, i.e. in this example

[0044] Figure 5 Shown are out-of-plane deflections and mirror-detector arrangements for the spatial arrangement of mirror-detector arrangements in different methods according to the prior art (a) and the present invention (b). Figure 5 a shows an optical system 1s according to the prior art disclosed in FR2953017B1, wherein A deflection angle of 1 is used for two rows of mirrors and detectors (front row and rear row mirrors), which are arranged along two polygonal figures widely offset from each other. Here, the mirror elements 10s have a large distance between each other so as to deflect the incident light L from the scattering plane DP to the detector 11s as deflected light LD. If the distance is smaller, the detectors 11s of the rear row and front row mirrors 10s or their supports will collide. In order to match the long distance between the front row mirrors 10s and the detectors 11s, the vertical curvature of the mirror element 10s is adjusted. However, the position of the front row mirror element 10s away from the focal curve results in a larger area of ​​spectral lines at the defocused position of the front row mirror 10s, resulting in an overlapping effect between the adjacent front and rear row mirrors, resulting in part of the light near the edge of the mirror 10s not being reflected by the dedicated front row mirror, but unintentionally penetrating to the rear row mirror 10s. Figure 5b shows an optical system 1 according to the invention, characterized by a minimized transition zone at the edge of the front mirror element 4b. The deflection mirror used at the optical pickup is cylindrical in both methods in order to provide a vertically focused mirror element 4. When the deflection angle is inserted When , calculate the distance d1,2 from the mirror axis to the focal curve FC according to formula (1) (see Figure 7 ). When using a deflection angle less than 90° When, with Figure 5 Compared with a, the distance Δ=d1-d2 between the two reflectors is much smaller, resulting in a shorter distance d1 between the front mirror element 4b and the front detector 5b, which provides a smaller height of the optical system 1 (for better comparison, the distance Δ=d1-d2 between the front mirror element 4b and the front detector 5b is much smaller). Figure 5 The prior art detector position shown in a is added as a dotted line Figure 5 b). On the other hand, a closer distance d1 from the focal curve FC leads to a minimization of the transition area at the edge of the front mirror element 4b. 90° is just an example and may be different for other embodiments.

[0045] Figure 6 The minimum linewidth t of a spectral line at a distance d from the focal curve is shown. Here, the Rowland grating has a radius of curvature (ROC) RG, and an illumination area of ​​width W is applied to the grating 3, using an optical pickup around a normal point N, which is also the focus of the focal curve FC at a normal wavelength λ at β = 0°. The angle ξ corresponds to the half-illumination angle of the grating. The width t of the spectral line at β = 0° at a distance d from the focal point N is determined by the following formula:

[0046] (2)t=2d tanξ

[0047] Where, (3) sinξ=W / 2R G .

[0048] Figure 7 A cross section perpendicular to the deflection plane at the normal point is shown to determine the advantageous geometry for obtaining a minimum safety distance S between two vertically focusing mirror elements 4, one of which is the front row and the other is the rear row, resulting in a minimum width of the transition zone for the spectral lines illuminating the two rear and front rows of mirror elements 4a, 4b. The following equation can be derived from Figure 7 get.

[0049] (4)h 1,2 =ρ cy1-1,2 (1-cos(arcsin(b / 2ρ cy1-1,2 )))

[0050] (5)

[0051] (6)

[0052] (7)Δ=d1-d2

[0053] (8)

[0054] (9)

[0055] (10)

[0056] (11)

[0057] where dm denotes the center thickness of the mirror element and b denotes the height of the mirror element. For further considerations, a reasonable maximum mirror height b can be estimated. In a spark OES, the distance between the electrode tip and the sample (= counter electrode) is typically between 2 mm and 5 mm. A coupling lens or an imaging coupling mirror or a mirror arrangement images the light from the generated plasma onto the entrance aperture, illuminating its entire height of up to 5 mm. In the optical system of the spectrometer, imaging errors of the diffraction grating 3 will result in the lines (in particular) becoming longer than the height of the entrance slit. As a rule of thumb for Rowland circle gratings, an average length of 7 mm can be assumed. In order to use a rear row of mirror elements 4a as implemented in the present invention The cylindrical reflector of the deflection angle focuses the light from the entire length of the spectral line onto the sensor of the detector 5, and the height of the cylindrical reflector needs to be b=10 mm, high (=7 mm / cos(45°)).

[0058] Only spectral lines that completely fall on the vertical focusing mirror element 4 are considered usable, which makes the width of the transition region WT between adjacent mirrors usable:

[0059] (12)WT=2t(β)

[0060] Since the two methods (existing technology and according to Figure 5 The width of the transition zone WT in the invention of a and 5b) is determined only by di, so the distance from the front mirror (closest to the diffraction grating) to the focal curve FC. Figure 5 In the methods a and 5b, it is assumed that the curvature radius of the rear mirror elements 4a, 10s is Cyl-2 Similarly, the task is to minimize Δ, that is, the distance between the two polygonal figures 6a, 6b of the two methods. Here, we set the closest safety distance s between the rear and front mirror elements 4a, 4b to S = 1mm to find ρ Cyl-1 , and Assuming dm = 2.2 mm and b = 10 mm, and taking values ​​of dm and b for both the rear and front mirror elements 4a, 4b, and further assuming RG = 400 mm and W = 40 mm such that sinξ = 0.05 (Formula 2+3), we will obtain Figure 5 The setup shown in 5a (prior art optical system 1s) and 5b (optical system 1 of the present invention):

[0061] Optical system 1s: ρ Cyl-1 =75mm,ρ Cyl-2 =27.5mm Optical system 1: ρ Cyl-1 =35mm,ρ Cyl-2 =27.5mm

[0062] for Figure 5 a (prior art optical system 1s) and Figure 5 b (optical system 1 of the present invention) at the normal point N, using these values ​​the following results are obtained for the width WT of the transition zone:

[0063] Optical system 1s: d1 = 26.517 mm and d2 = 9.723 mm WT = 2.6517 mm

[0064] Optical system 1: d1 = 15.155 mm and d2 = 9.723 mm WT = 1.5155 mm

[0065] According to the present invention, a smaller transition region WT of the optical system 1 shows an improved performance of the optical system 1 compared to a larger transition region WT of the optical system 1 .

[0066] Figure 8 A schematic diagram of the optical paths and angles of the optical system according to the present invention is shown as Rowland circle optics, where β represents the diffraction angle that occurs, α represents the angle of incidence, ω represents the angle of incidence of the intermediate light beam on the focusing lens element relative to the vertical direction, O represents the offset that increases with β, t represents the line width at a distance d that increases with β, N represents the normal point, W represents the width of the illuminated grating, RG represents the radius of curvature of the grating and the measurement of ξ. Figure 8 It can be seen that, despite the diffraction angle β>0°, the distance d and the angle ξ along the course of the focal curve (in this case the Rowland circle) remain constant, but the optical path length from the axis of the cylindrical mirror element 4 to the pixel strip lp of the detector 5 increases with increasing β according to the formula:

[0067] (13)l p =d / cosω

[0068] The axis of the cylindrical reflector is offset from the center of the pixel strip by an amount O:

[0069] (14)O=d tanω

[0070] In order to calculate the width t(β) of the spectral line of β other than 0 on the mirror, it is necessary to use (l p / cosω) to replace d, we get:

[0071] (15)t(β)=2d tanξ / cos 2 ω

[0072] Here, ω is specific to β and depends on the course of the focal curve FC. For Rowland circle optics, ω=β applies.

[0073] To determine the diffraction angle and the focal point on the focal curve, the grating equation:

[0074] (16)nGλ=sinα+sinβ

[0075] Back focus equation:

[0076] (17)

[0077] Where: n diffraction level

[0078] G grating constant (number of grooves per mm at the grating center)

[0079] λ diffraction wavelength

[0080] α angle of incidence

[0081] β diffraction angle

[0082] LA distance between the incident aperture and the grating center

[0083] The distance between the focus of LB diffraction λ and the center of the grating

[0084] R is the curvature radius of the grating substrate

[0085] Exposure wavelength used in λ0 grating production

[0086] Cf flat field constant;

[0087] is used; if Cf=0, then all elements comprising the optical system lie on a circle called the Rowland circle.

[0088] Figures 1 to 8The vertical focusing mirror element of the embodiment shown in FIG is primarily described as a cylindrical mirror, but it is also possible to have a mirror element having a curved surface 8 with an infinite radius of curvature ρ, which is therefore flat, or a curved surface 8 having a conical surface formed by several radii of curvature ρ. Thus, flat, cylindrical, or conical mirrors can be used. Combinations of these are particularly advantageous if used in the optical system 1. A focusing lens, such as a rod lens or a cylindrical lens, can be used in all of the above-described embodiments, for example in combination with a flat deflecting mirror, rather than as a vertical focusing mirror element for the vertical focusing element 4.

[0089] The arrangement of the optical system 1 in the Paschen-Runge arrangement described above is not restrictive; Figures 1 to 8 The embodiments also apply to a flat-field setup or a Czerny-Turner setup or an Ebert-Fastie setup.

[0090] Furthermore, in all the above-described embodiments, the detector 5 as well as the direct imaging detector 5 c are line detectors, such as CCD or CMOS detectors.

[0091] At least one of the optical system 1 and the operating unit described above can be used in a spectrometer device, which will not be described further. The operating unit is thereby connected to at least a detector arrangement of at least two detectors 5 in order to operate these at least two detectors 5 and analyze the measured spectrum L.

[0092] The embodiments shown here are merely examples of the present invention and therefore should not be understood as limiting. Alternative embodiments considered by those skilled in the art are also covered by the scope of protection of the present invention.

[0093] Reference Signs List

[0094] 1 Optical system according to the present invention

[0095] 1s Optical system according to prior art

[0096] 2 Entrance aperture

[0097] 3 Grating

[0098] 4 Vertical Focusing Mirror Element (Present Invention)

[0099] 4a Rear mirror element

[0100] 4b Front mirror element

[0101] 5 Detector (present invention) - general term, which can be rear row detector (5a) or front row detector (5b)

[0102] 5a Rear row detector (an example of detector (5))

[0103] 5b Front row detector (an example of detector (5))

[0104] 5c Direct Imaging Detector

[0105] 6, 6a, 6b Polygonal figures, part of which are parallel to the local tangent of the focal curve behind the mirror element

[0106] 7a, 7b Adjacent ends of vertical focusing element

[0107] 8 Surface

[0108] 9 substrate

[0109] 10s vertical focusing mirror element (invention)

[0110] 11s State-of-the-art detectors

[0111] 100 Spectrometer Device α angle of incidence β diffraction angle

[0112] Deflection angle

[0113] Deflection angle of the front vertical focusing mirror element 4b Deflection angle of rear vertical focusing mirror element 4a ξ sinξ=W / 2R G The resulting angle ω is the angle of incidence of the vertical intermediate beam to the vertical focusing mirror element relative to the vertical A major axis

[0114] A′ 1,2 The obtained distance

[0115] A 1,2 The obtained distance

[0116] B′ 1,2 The obtained distance

[0117] B 1,2 The obtained distance

[0118] B′1(A2) The obtained distance

[0119] b Figure 3 The length (height) of the vertical focusing mirror element 4 perpendicular to the A axis is shown as follows:

[0120] d is the distance to the focal curve according to formula (1)

[0121] d1 Distance between the focal curve and the axis of the deflection surface of the vertical focusing front mirror

[0122] d2 is the distance between the focal curve and the axis of the deflection surface of the vertical focusing rear mirror

[0123] Δ The difference between d1 and d2, wherein the difference corresponds to the distance between the two polygonal figures 6a, 6b

[0124] Δ s The distance between the rear row of vertical focusing elements and the front row of vertical focusing elements in the deflection plane of the optical system according to the prior art

[0125] D A The distance between the entrance aperture and the grating center

[0126] D B The distance between the focus of the deflection wavelength λ(β) and the center of the grating

[0127] DP Deflection Plane

[0128] dm Center thickness of vertical focusing mirror element

[0129] FC Focus Curve

[0130] h 1，2 h 1，2 =ρ cy1-1，2 (1-cos(arcsin(b / 2ρ cy1-1，2 The curvature height of the vertical focusing mirror element obtained by

[0131] Pixel bands of IP detectors

[0132] L Light from the sample

[0133] LD deflected light

[0134] N Normal point, focus of normal wavelength β=0

[0135] O Offset between the center point of the cylindrical mirror axis and the pixel strip of the detector

[0136] ρ Cyl Curvature radius of surface 8

[0137] ρ Cyl-1 The curvature radius of the curved surface 8 of the front vertical focusing mirror element 4b

[0138] ρ Cyl-2 The curvature radius of the curved surface 8 of the rear vertical focusing mirror element 4a is

[0139] R G Curvature radius of grating 3

[0140] S The resulting safe distance

[0141] t Line width at distance d from the focal point

[0142] W is the illumination width of grating 3

[0143] X-axis

[0144] Y axis perpendicular to the X and Z axes

[0145] Z vertical axis

Claims

1. An optical system (1), comprising: - at least one entrance aperture (2) for entry of the light (L) to be analyzed, - at least one diffraction grating (3) for spectral deflection of said light (L), - at least two detectors (5) for measuring the spectrum of the light (L), comprising a front row of detectors (5b) and a rear row of detectors (5a), wherein the detectors are arranged on the same side of the deflection plane of the optical system, and - at least two vertical focusing mirror elements (4) for focusing said light (L) onto said detectors (5) assigned to said vertical focusing mirror elements, Characterized in that the vertical focusing mirror elements (4) are arranged as a front row of mirror elements (4b) and a rear row of mirror elements (4a) along two polygonal figures (6a, 6b) offset from each other and from the focus curve, wherein each portion of the polygonal figures (6a, 6b) is parallel to a dedicated local tangent to the focus curve of the grating (3), so that - the deflection angle of the front mirror element (4b) thereby minimizing the offset (d1) of the front mirror element (4b) relative to the focal curve, - the distance (d1) between the front row mirror element (4b) and the front row detector (5b) assigned to the front row mirror element (4b) is minimized while still avoiding collisions between the front row detector (5b) and its support and the rear row detector (5a) and its support, and - at least one end portion (7a) of the front row mirror element (4b) overlaps with an adjacent rear row mirror element (4a), or, if viewed from the grating (3), with the receiving area of ​​an adjacent direct light detector (5).

2. The optical system (1) according to claim 1, characterized in that The reflective surface of at least one of the vertical focusing mirror elements (4) is shaped as a section of a cone.

3. The optical system (1) according to claim 1 or 2, characterized in that The reflective surface of at least one of the vertical focusing mirror elements (4) is cylindrical in shape.

4. The optical system (1) according to any one of the preceding claims, characterized in that At least one of the vertical focusing mirror elements (4) is a plane mirror element which deflects the light onto one of the detectors (5) assigned to the plane mirror element.

5. The optical system (1) according to any one of the preceding claims, characterized in that Deflection angles of at least two vertical focusing mirror elements (4) The absolute values ​​are different.

6. The optical system (1) according to any one of the preceding claims, characterized in that The average or constant radius of curvature (ρ) of at least two vertical focusing mirror elements (4) Cyl )different.

7. The optical system (1) according to any one of the preceding claims, characterized in that With different deflection angles At least two mirror elements of the absolute value of the vertical focusing mirror element (4) have the same average or constant radius of curvature (ρ Cyl ).

8. The optical system (1) according to any one of the preceding claims, characterized in that At least one detector (5) has no assigned vertical focusing mirror element (4), so that the detector (5) is positioned along a section of the original focal length curve of the grating (3) so that a portion of the spectrum is imaged directly onto the detector (5).

9. The optical system (1) according to any one of the preceding claims, characterized in that At least one detector (5) is equipped with a lens for vertical focusing.

10. The optical system (1) according to any one of the preceding claims, characterized in that The construction principle of the optical system (1) is a Paschen-Runge setup, a flat-field setup, a Czemy-Tumer setup, or an Ebert-Fastie setup.

11. The optical system (1) according to any one of the preceding claims, characterized in that The detector (5) is a line detector.

12. A spectrometer device (100), characterized in that Comprising at least one optical system (1) according to one of the preceding claims, further comprising an operating unit connected at least to the detector (5) for operating the detector (5) and for analyzing the measured spectrum of the light (L).

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