SPECTROMETER ARRANGEMENT

Abstract

The present disclosure resides in a spectrometer arrangement including a first dispersing element for spectral separation of radiation in a main dispersion direction, and a second dispersing element for spectral separation of radiation in a cross-dispersion direction, which is at an angle to the main dispersion direction, so that a two-dimensional spectrum is producible. The spectrometer arrangement also includes a collimating optics, which directs collimated radiation to the first and/or second dispersing element, a camera optics, which images a two-dimensional spectrum in an image plane, a two-dimensional detector for detecting the two-dimensional spectrum in the image plane, and an off-axis section of a rotationally symmetric, refractive element, which is arranged between the camera optics and the detector. The present disclosure resides likewise in an optical module comprising such a spectrometer arrangement.

Claims

1-14. (canceled)

15. A spectrometer arrangement, including: a first dispersing element for spectral separation of radiation in a main dispersion direction; a second dispersing element for spectral separation of radiation in a cross-dispersion direction, which is at an angle to the main dispersion direction, so that a two-dimensional spectrum is producible; a collimating optics, which directs collimated radiation to the first and/or second dispersing element; a camera optics, which images a two-dimensional spectrum in an image plane; a two-dimensional detector for detecting the two-dimensional spectrum in the image plane; and an off-axis section of a rotationally symmetric, refractive element, which is arranged between the camera optics and the two-dimensional detector.

16. The spectrometer arrangement of claim 15, wherein the refractive element is embodied as a biconvex lens.

17. The spectrometer arrangement of claim 15, wherein the refractive element is embodied as a spherical lens.

18. The spectrometer arrangement of claim 15, wherein at least one lens area is embodied aspherically.

19. The spectrometer arrangement of claim 15, wherein the refractive element includes an anti-reflective coating.

20. The spectrometer arrangement of claim 15, wherein the refractive element is manufactured from a low dispersing material.

21. The spectrometer arrangement of claim 15, wherein the collimating optics and/or the camera optics includes a concave mirror.

22. The spectrometer arrangement of claim 15, wherein the first dispersing element is embodied as an echelle grating.

23. The spectrometer arrangement of claim 15, wherein the first dispersing element is replaced with a mirror arranged perpendicularly to the image plane of the two-dimensional detector, and wherein the two-dimensional detector is replaced by a one-dimensional detector.

24. The spectrometer arrangement of claim 15, wherein the second dispersing element is embodied as a prism.

25. The spectrometer arrangement of claim 24, wherein the prism is reflectively coated on a rear side.

26. The spectrometer arrangement of claim 24, wherein the prism is rotatably seated.

27. The spectrometer arrangement of claim 15, wherein the spectrometer arrangement forms a Littrow spectrometer.

28. An optical module for retrofitting a spectrometer arrangement, including: wherein the spectrometer arrangement includes a first dispersing element for spectral separation of radiation in a main dispersion direction, a second dispersing element for spectral separation of radiation in a cross-dispersion direction, which is at an angle to the main dispersion direction, so that a two-dimensional spectrum is producible, a collimating optics, which directs collimated radiation to the first and/or second dispersing element, a camera optics, which images a two-dimensional spectrum in an image plane, and a two-dimensional detector for detecting the two-dimensional spectrum in the image plane; at least a rotationally symmetric, refractive element, an off-axis section of which is configured for positioning between the camera optics and the two-dimensional detector.

Description

[0036] The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:

[0037] FIG. 1 an embodiment of the spectrometer arrangement of the invention.

[0038] FIG. 2 aberrations measured with an uncorrected spectrometer arrangement.

[0039] FIG. 3 aberrations measured with an arrangement of FIG. 1.

[0040] FIG. 4 an individual spot of FIG. 2.

[0041] FIG. 5 an individual spot of FIG. 3.

[0042] FIG. 6 aberrations measured with an arrangement of FIG. 1 with additional wavelengths.

[0043] FIG. 1 shows an embodiment of the spectrometer arrangement 10 of the invention. The radiation of a radiation source 11 is directed as a combined beam 13 to the entrance slit 15 of a spectrometer arrangement 10, from where it enters into the actual spectrometer. Examples of such a light source 11 include a plasma torch, such as used in ICP-OES (inductively coupled plasma, optical emission spectrometry). In such case, wavelengths between 165 nm and 900 nm are registered with the system.

[0044] The light entering into the spectrometer includes radiation of all wavelengths emitted by the light source. By a collimating optics 17, e.g. a concave mirror, the light beam is parallelized 19. The collimated light strikes then on in the wording the present inventiona second dispersing element, especially a prism 21, which functions as an optical order separating unit. The rear side 23 of the prism 21 is reflectively coated.

[0045] The radiation, predispersed by the prism, by way of example into beams 25, 27 and 29 with wavelengths .sub.1, .sub.2 and .sub.3 respectively, passes from the prism 21 to the main dispersion element, an echelle grating 31. The radiation is diffracted by the echelle grating into a large number of diffraction orders with high order numbers. The orders are, in given cases, spatially still strongly superimposed at the grating 31. In the here illustrated Littrow arrangement, the beams travel from the grating back to the reflectively coated 23 prism 21, where the different wavelengths 25, 27, 29 are again dispersed now crosswise to the main dispersion direction. On the rear side of the prism, the beams of the different wavelengths .sub.1, .sub.2 and .sub.3 (25, 27, 29) are significantly superimposed, because of the large beam diameters, see reference character 33. The relative beam overlap is thus large.

[0046] From the prism 21, the beams are directed to the concave mirror 17, which performs the imaging of the beam on the detector 39. The spectrometer arrangement is shown in a Littrow arrangement, i.e. the concave mirror 17 is also embodied as a camera optics 34.

[0047] Detector 39 is, for instance, a CCD detector in the form of a detector array (2-D detector). The detector has, for example, a resolution of 10001000. Detector 39 is oriented such that the individual columns are approximately oriented in the same way as the diffraction orders. The entire detector 39 is read-out at the same time.

[0048] On the path to the detector 39, the cross-sections of the beams become progressively smaller (because of the camera optics 34). Interposed on such path is a refractive element 35, through which the beams pass. The refractive element 35 is here embodied as a lens body. The two optically effective surfaces of the biconvex lens body are spherically embodied. In order to keep chromatic aberrations induced by the lens body 35 as small as possible, CaF.sub.2 is used as material of the lens body. Additionally, the optically effective surfaces of the lens body 35 are provide with an anti-reflective coating, in order to limit as much as possible false light falling on the detector 39. At the correcting lens 35, the beam diameters are already so small that for the here illustrated wavelength examples a complete beam separation is already present, see reference characters 44. The relative beam overlap is thus small.

[0049] In general, the refractive element 35 is an off-axis section of a rotationally symmetric, refractive element.

[0050] The corrected beams of the wavelengths .sub.1, .sub.2 and .sub.3 (reference characters 25, 27, 29) are directed further to the detector unit 39. Through the correction provided by the prismatic lens body 35, very sharp image points can be produced in the image plane 41 on the detector. The position of the correction-lens body 35 in the immediate proximity of the detector 39 is, because of the good beam separation, suitable in special measure for increasing the image quality. Due to the beam separation, the individual beams are strongly individually correctable, in order to minimize previously occurring aberrations. Also, the increasingly smaller diameter of the group of beams permits a smaller dimensioning of the correcting element. Due to the biconvex embodiment of the lens body, there results in the here proposed example a lessening of the dimensions of the displayed spectrum on the detector. In this way,assuming uniform detector- and pixel sizethe simultaneously registrable wavelength range is enlarged in given cases, however, the spectral resolution lessens only slightly. By reduced optical imaging errors, however, as a whole, a significantly better spectral resolution is achieved.

[0051] Instead of the echelle grating as main dispersion element 31, a mirror standing perpendicular to the spectrometer plane can be applied. There results a pure prism spectrograph, which profits in like measure from the above described aberration correction provided by the prismatic lens body. Due to the strongly reduced aberrations, use of slits with large slit height is possible. The slit orientation changes compared with an echelle spectrometer by 90, so that a strong improvement of the geometric etendue of the spectrometer results.

[0052] FIG. 2 shows aberrations for an uncorrected spectrometer arrangement. FIG. 4 represents a corresponding, single spot 42.

[0053] FIG. 3 shows the achievable image quality throughout the entire image field 41 with a spectrometer arrangement 10 such as above described with correcting optics 35. The optical arrangement is a Littrow spectrograph with a focal length of 400 mm and an aperture ratio of f/12.5. Applied as collimator/camera is an off-axis, parabolic mirror. The grating is an R4 echelle grating. Using a detector with an area of 20.520.5 mm.sup.2, a wavelength range between 380 nm (upper detector edge) and 900 nm (lower detector edge) is simultaneously registered in the illustrated beam calculation simulations. The illustrated spots represent the images of different wavelengths in a single point source. The image points are magnified by a factor of 15 compared with the scale of the detector area. The optical systemespecially the embodiment of the prismatic lens 35 was optimized for the above mentioned wavelength range. The aberrations can be reduced many times throughout the entire image field with the correcting lens 35. In comparison with the uncorrected system, the same spectral region occupies less space in main- and cross dispersion directions. Boxed in FIGS. 2 and 3 is the single spot shown enlarged in FIGS. 4 and 5, respectively.

[0054] FIG. 5 shows the image quality for a single spot 43 coming from a prismatic correcting lens 35, in a setup as illustrated in FIG. 1. The parameters of the correcting lens, such as, for instance, position, orientation, radius of curvature and off-axis distance, were so determined that the spot sizes can be reduced as strongly as possible throughout the entire image field. The expanse of spots can be markedly reduced throughout the entire image field. The benefit of the system for a large part of the covered wavelength region is now limited by the diffraction.

[0055] As mentioned, the design of the correcting lens 35 was for a wavelength range between 380 nm and 900 nm. In the case of echelle spectrometers with a prism as cross dispersion element, optimizing for longwave regions is beneficial, because the diffraction orders lie always closer together with increasing wavelength: in order to achieve a separation of the diffraction orders in the case of as great as possible slit height at the entrance slit 15, a best possible image quality in this wavelength range is desirable. By a rotation of the prism 21, however, also other, especially shorter wave wavelength ranges can be led to the detector 39. FIG. 6 shows that the chromatic aberrations induced by the refractive element 35 are comparatively small. Shown besides selected wavelengths of the longwave region (red; reference character 47) are also 9 points in the shortwave region (blue; reference characters 49) in a wavelength range between 165 nm and 171 nm. The aberrations are, however, likewise significantly smaller than in the uncorrected state. Due to the lesser width of the free spectral region of the diffraction orders for shorter wave light, the spots are located only centrally in the detector in the main dispersion direction. Farther left or right lying detector regions in the shortwave measuring range remain unused.

LIST OF REFERENCE CHARACTERS

[0056] 10 spectrometer arrangement
11 radiation source
13 combined beam from 11
15 entrance slit
17 collimating optics
19 parallel light in the beam path after 17
21 second dispersing element, especially a prism
23 rear side of 21
25 first wavelength
27 second wavelength
29 third wavelength
31 first dispersing element, especially an echelle grating
33 overlapping of the beams of 25, 27, 29 on 23 after passage through 31
34 camera optics
35 refractive element
39 detector
41 image plane on 39
42 single spot uncorrected
43 single spot corrected
44 overlapping of the beams of 25, 27, 29 on 35
47 longwave region
49 shortwave region