Freeform concave grating imaging spectrometer

Abstract

The application discloses a free curved surface concave grating imaging spectrometer, which comprises a slit, a free curved surface concave grating and an image plane. Light rays incident from the slit are irradiated onto the free curved surface concave grating to be dispersed and reflected to form a reflected light beam, which is irradiated onto the image plane so that different wavelength light rays incident from the slit are separated and respectively imaged on the image plane. The application also relates to a design method of the free curved surface concave grating imaging spectrometer.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optics, in particular to a free-form concave grating imaging spectrometer. BACKGROUND

[0002] Spectral information is very important and has a wide range of applications in the fields of biological science, medical health, pharmaceutical, energy, food and agriculture. The acquisition of spectral information depends on spectrometer and imaging spectrometer. The key component in the imaging spectrometer is the optical system with dispersion function. The optical system with dispersion function has a larger field of view, a wider spectral range and a higher resolution, which can promote the development of related application fields and has been the goal pursued by people for a long time.

[0003] Free-form surface refers to a non-traditional surface that cannot be represented by spherical or aspherical coefficients. Free-form surface is a complex optical surface with no symmetry. In the past decade, the rapid development of free-form surface optics not only brings all-round improvement in the performance of optical systems, but also realizes many optical systems that were difficult to design or never existed before, bringing revolutionary breakthroughs in the field of optical design.

[0004] The existing imaging spectrometer rarely uses free-form surface for design. The reason is that the free-form surface has too many variables and too much freedom, and there are many factors to consider, such as the volume, weight and number of lenses of the system, so it is difficult to design an imaging spectrometer with simple structure, low cost and good imaging effect. SUMMARY

[0005] Therefore, it is necessary to provide a free-form concave grating imaging spectrometer with simple structure, low cost and good imaging effect.

[0006] A free-form concave grating imaging spectrometer, comprising a slit, a free-form concave grating and an image plane, the light incident from the slit is dispersed and reflected on the free-form concave grating to form a reflected light beam, and the reflected light beam is incident on the image plane, so that different wavelengths of light incident from the slit are separated and imaged on the image plane.

[0007] Compared with the prior art, the free-form concave grating imaging spectrometer provided by the present application only contains one optical element, i.e. the free-form concave grating, which simultaneously serves as a dispersion element and an imaging element, so that the free-form concave grating imaging spectrometer has simple structure, small volume, low cost and good imaging effect. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1The optical path diagram of the free-form concave grating imaging spectrometer is provided for the first embodiment of the present application.

[0009] Figure 2 The structural schematic diagram of the free-form concave grating imaging spectrometer is provided for the first embodiment of the present application.

[0010] Figure 3 The wave aberration of different fields of view and different wavelengths of the system 1 is provided for the first embodiment of the present application.

[0011] Figure 4 The flowchart of the design method of the free-form concave grating imaging spectrometer is provided for the second embodiment of the present application.

[0012] Main element symbol explanation

[0013] Free-form concave grating imaging spectrometer 100

[0014] Slit 102

[0015] Concave grating 104

[0016] Detector 106

[0017] Image plane 108

[0018] The following specific embodiments will further illustrate the present application in combination with the above-mentioned drawings. Specific embodiments

[0019] The free-form concave grating imaging spectrometer provided by the present application will be further described in detail below in combination with the drawings and specific embodiments.

[0020] Please refer to Figure 1 and Figure 2 The first embodiment of the present application provides a free-form concave grating imaging spectrometer 100, which comprises a slit 102, a concave grating 104 and an image plane 108. Light is incident on the concave grating 104 through the slit 102 to be dispersed and reflected to form a reflected light beam, which is incident on the image plane 108 to be imaged. The concave grating 104 has a free-form surface, so the concave grating 104 is a free-form concave grating 104. The free-form concave grating 104 simultaneously serves as a dispersion element and an imaging element of the free-form concave grating imaging spectrometer 100, i.e. the free-form concave grating 104 simultaneously has the functions of dispersion and imaging. That is, the light incident from the slit 102 is incident on the free-form concave grating 104 to be dispersed and reflected, and the reflected light beam is incident on the image plane 108, so that the light of different wavelengths incident from the slit 102 is separated and imaged on the image plane 108 respectively.

[0021] The free-form concave grating imaging spectrometer 100 only contains a single optical element, which is the concave grating 104. That is, the free-form concave grating imaging spectrometer 100 only contains one optical element, which is the concave grating 104. The slit 102 and the image plane 108 are located on the same side of the concave grating 104.

[0022] The slit 102 is a narrow, elongated aperture with adjustable width. The slit 102 can be a fixed slit 102, a single-side adjustable asymmetric slit 102, and a double-side adjustable symmetric slit 102. In this embodiment, the slit 102 is a double-side adjustable symmetric slit 102.

[0023] The line distance of the concave grating 104 can be fixed or variable. When the line distance of the concave grating 104 is fixed, it means that the concave grating 104 has a fixed line distance, and the concave grating 104 is a fixed-line-distance concave grating 104. When the line distance of the concave grating 104 is variable, it means that the concave grating 104 has a variable line distance, and the concave grating 104 is a variable-line-distance concave grating 104. In one embodiment, the concave grating 104 is a fixed-line-distance free-form concave grating 104. In another embodiment, the concave grating 104 is a variable-line-distance free-form concave grating 104.

[0024] The line distance of the grating on the free-form surface is fixed or variable, and the shape of the grating line is curved or straight, which corresponds to the distance and shape of the projection line of the grating line on the xy plane of the local coordinate system of the grating. The variable-line-distance concave grating 104 refers to a straight-line variable-line-distance grating, and the line distance d at a point (x, y) on the mirror surface satisfies:

[0025] d(x, y) = d0+ d1y + d2y 2 +d3y 3 +… (1)

[0026] The imaging optical system further includes a detector 106 located at the image plane 108 for recording the information of the image plane 108. The detector 106 can also be a film, etc. In this embodiment, the free-form concave grating imaging spectrometer 100 is composed of the slit 102, the concave grating 104, the image plane 108, and the detector 106.

[0027] Table 1 is the performance parameters of the free curved concave grating imaging spectrometer 100 when the concave grating 104 is different constant line spacing concave grating 104 and different variable line spacing concave grating 104. In Table 1, Constant specifies the constant line spacing concave grating 104, Variable refers to the variable line spacing concave grating 104, System No. refers to the system number (the system is the free curved concave grating imaging spectrometer 100, abbreviated as system), Grating line-space refers to the line spacing of the concave grating 104, Spectral bandwidth refers to the spectral bandwidth, Spectral dispersion refers to the spectral dispersion, Slit length refers to the length of the slit 102, Numerical aperture refers to the index of aperture, System length refers to the system length (the system length refers to the distance from the slit 102 to the concave grating 104), Spectral resolving power refers to the spectral resolving power, the unit of spectral bandwidth is nm (nanometer), the unit of spectral dispersion is nm / mm (nanometer per millimeter), the units of slit length and system length are both mm (millimeter).

[0028] Performance parameters of the free curved concave grating imaging spectrometer 100 in Table 1

[0029]

[0030] As can be seen from Table 1, when the concave grating 104 is a constant line spacing concave grating 104, the spectral range (spectral bandwidth) of the free curved concave grating imaging spectrometer 100 is 400nm to 1000nm, the spectral dispersion is greater than or equal to 240nm / mm, the slit length 102 is less than or equal to 2.1mm, the numerical aperture is less than or equal to 0.13, the system length is greater than or equal to 61mm, and the spectral resolving power can be up to 342. When the concave grating 104 is a variable line spacing concave grating 104, the spectral range of the free curved concave grating imaging spectrometer 100 is 400nm to 1000nm, the spectral dispersion is greater than or equal to 100nm / mm, the slit length 102 is less than or equal to 2.3mm, the numerical aperture is less than or equal to 0.16, the system length is greater than or equal to 37mm, and the spectral resolving power can be up to 820.

[0031] In Table 1, the spectral range of system 1 (System No. 1) is 400nm to 1000nm, the slit length 102 is 1mm, the numerical aperture is 0.1, the system length is 100mm, the maximum spectral dispersion is 340nm / mm, and the spectral resolving power is 241. At this time, the wavefront aberration of each field of view and each wavelength of system 1 is as shown in Table 2. Figure 3

[0032] ​Figure 3 In the figure, the horizontal axis represents the length of slit 102, and the vertical axis represents the wavelength. The length of slit 102 and the wavelength are both in mm. Figure 3 In the WFE RMS (wavefront error, root mean square), the minimum value is 0.0081λ, the maximum value is 0.0745λ, the average value is 0.0422λ, the standard deviation is 0.0065, the maximum keystone distortion is 0.175μm, and the maximum smile distortion is 0.109μm.

[0033] Depend on Figure 3 It can be seen that, within the entire working spectral range, the maximum RMS value of the full-field aberration of System 1 at each wavelength λ does not exceed 0.075λ. Therefore, the image quality (imaging quality) of System 1 at each wavelength reaches the diffraction limit, and System 1 has good image quality. The absolute values ​​of spectral line curvature and color distortion at each field point of System 1 at each wavelength do not exceed 1μm. The perpendicular distance from the image point on the image plane 108 of System 1 to the principal ray of the central field of view is not less than 1.4mm, avoiding obstruction. Furthermore, System 1 is object-side telecentric.

[0034] Furthermore, in Table 1, the maximum RMS wavefront aberration of systems 2 to 21 within the entire spectral range does not exceed 0.075λ, indicating that the image quality reaches the diffraction limit. Therefore, systems 2 to 21 all possess good image quality. The absolute values ​​of chromatic aberration and spectral line curvature in systems 2 to 21 are all less than 1 μm, indicating that systems 2 to 21 are unobstructed and are all object-side telecentric. Thus, the freeform concave grating imaging spectrometer 100 is object-side telecentric, unobstructed, and possesses good image quality.

[0035] Comparing systems 1 to 21 using fixed-pitch concave grating 104 and variable-pitch concave grating 104 in Table 1, the percentage improvement of various performance parameters of the entire system by using variable-pitch concave grating 104 can be obtained, as shown in Table 2. In Table 2, Maximum improvement refers to the maximum improvement value, and From Constant To Variable refers to the change from fixed-pitch concave grating 104 to variable-pitch concave grating 104.

[0036] Table 2 shows the percentage improvement in each performance parameter for each system when using the variable-pitch concave grating 104.

[0037]

[0038] As can be seen from Table 2, when the spectral range is 400nm to 1000nm, the variable-line-space concave grating 104 is used, only the slit 102 length is increased from 1.0mm to 1.8mm at most, increased by 80%; only the numerical aperture is increased from 0.10 to 0.14 at most, increased by 40%; only the system length is reduced from 100mm to 52mm at most, increased by 92%; only the spectral dispersion is increased from 340nm / mm to 140nm / mm at most, increased by 143%; the spectral resolution can reach 585 in system 13 at most, increased by 143% compared with system 1.

[0039] When the spectral range is 600nm to 1000nm, the variable-line-space concave grating 104 is used, only the slit 102 length is increased from 2.1mm to 2.3mm at most, increased by 10%; only the numerical aperture is increased from 0.13 to 0.16 at most, increased by 23%; only the system length is reduced from 61mm to 37mm at most, increased by 65%; only the spectral dispersion is increased from 240nm / mm to 100nm / mm at most, increased by 140%; the spectral resolution can reach 820 in system 17 at most, increased by 140% compared with system 5.

[0040] When the spectral range is 400nm to 800nm, the variable-line-space concave grating 104 is used, only the slit 102 length is increased from 1.6mm to 1.8mm at most, increased by 12%; only the numerical aperture is increased from 0.11 to 0.14 at most, increased by 27%; only the system length is reduced from 73mm to 52mm at most, increased by 40%; only the spectral dispersion is increased from 260nm / mm to 120nm / mm at most, increased by 117%; the spectral resolution can reach 683 in system 21 at most, increased by 117% compared with system 9.

[0041] From the above results, it can be seen that the variable-line-space concave grating 104 can improve the performance parameters of the system as a whole, and the effect of improving the spectral dispersion and spectral resolution and reducing the system length is particularly obvious. When the spectral range is 400nm to 1000nm, the improvement range of the performance parameters of the system is higher than that when the spectral range is 600nm to 1000nm and 400nm to 800nm, and the difference between the improvement ranges is the slit 102 length. The wavelength range of 400nm to 800nm has the relatively worst improvement effect due to the shorter average wavelength, which conforms to the design rule of imaging optical systems.

[0042] Table 3 shows the decrease of the best performance of the system after the spectral range is reduced. In Table 3, the Decrease of best performance refers to the decrease of the best performance of the system.

[0043] Table 3 shows the decrease of the best performance of the system after the spectral range is reduced. In Table 3, the Decrease of best performance refers to the decrease of the best performance of the system.

[0044]

[0045] From Table 3, it can be seen that the decrease of the spectral dispersion and the spectral resolution of the system is larger after the spectral range is enlarged. Moreover, the decrease of the performance of the system using the concave grating 104 with variable line spacing is larger than that of the system using the concave grating 104 with constant line spacing. In addition, since the average wavelength in the spectral range of 400nm to 800nm is smaller, the design of the system is more difficult than that in the spectral range of 600nm to 1000nm. When the spectral range is enlarged to 400nm to 1000nm, the system using the concave grating 104 with variable line spacing does not have improvement in the length of the slit 102, the numerical aperture and the length of the system. Therefore, compared with the system using the concave grating 104 with constant line spacing, the system using the concave grating 104 with variable line spacing is not sensitive to the enlargement of the spectral range, and thus the use of the concave grating 104 with variable line spacing is beneficial to the enlargement of the spectral range of the system.

[0046] The free-form concave grating imaging spectrometer 100 has the following advantages: first, using the high degree of freedom of the surface shape of the optical free-form surface, only one optical element is used to achieve high spectral imaging, that is, only the free-form concave grating 104 is contained, which simultaneously serves as a dispersion element and an imaging element, and the optical path is simple, the structure is simple, the volume is small, and the cost is low; second, the imaging effect is good; third, compared with using a fixed-line-spacing free-form concave grating 104, using a variable-line-spacing free-form concave grating 104 can improve the performance of the free-form concave grating imaging spectrometer 100, and is particularly effective in improving spectral dispersion and spectral resolution, reducing system length, and is also beneficial to expanding the spectral range. The second embodiment of the present application provides a design method of the free-form concave grating imaging spectrometer 100, which is: first, determining the initial structure of the free-form concave grating imaging spectrometer 100, and then optimizing the initial structure. In the design method, a point-by-point design method for an optical system with a dispersion device is used to solve the initial structure, the point-by-point design method is: first, selecting a series of light rays incident from different positions of the slit 102 as characteristic light rays, then calculating the coordinates and normals of the characteristic data points at the intersection points of the characteristic light rays and the concave grating 104 surface, then obtaining the free-form surface shape of the concave grating 104 by fitting, and obtaining the initial structure. Finally, the imaging quality of the initial structure is improved by optimization to obtain the free-form concave grating imaging spectrometer 100.

[0047] Please refer to Figure 4 The design method of the free-form concave grating imaging spectrometer 100 includes the following steps:

[0048] S1, selecting a series of light rays incident from different positions of the slit 102 as characteristic light rays;

[0049] S2, calculating the coordinates and normals of the characteristic data points at the intersection points of the characteristic light rays and the concave grating 104 surface;

[0050] S3, obtaining the free-form surface shape of the concave grating 104 by fitting to obtain an initial structure; and

[0051] S4, optimizing the initial structure.

[0052] In step S2, the concave grating 104 is a free curved concave grating 104. When the surface shape of the concave grating 104 is calculated using the point-by-point design method, first, the coordinates of each feature data point on the surface of the concave grating 104 are calculated according to the principle of the nearest light. Then, the normal of each feature data point on the surface of the concave grating 104 is calculated according to the object-image relationship of the system. According to the imaging requirements of the system, the light emitted from different positions on the slit 102 is finally converged at the target point (i.e., the ideal image point) on the image plane 108. Therefore, for a certain feature light, when the wavelength is λ, the direction vector S of the incident light at the corresponding feature data point on the surface of the concave grating 104 and the direction vector S' of the outgoing light should satisfy the grating equation. According to the general grating tracing equation given by Ludwig, S and S' should satisfy

[0053]

[0054] where n represents the diffraction order, λ represents the wavelength of the light, G represents the normal direction of the grating generating surface, R represents the normal of the feature data point, and d represents the grating line distance at the light incident point. Among them, n, λ and G are known quantities, and R and d are unknown quantities.

[0055] The calculation method of the normal of each feature data point on the surface of the concave grating 104 is different according to the type of the concave grating 104. That is, the calculation method of the normal of each feature data point on the surface of the constant-line-spacing concave grating 104 is different from the calculation method of the normal of each feature data point on the surface of the variable-line-spacing concave grating 104. The following is a specific description.

[0056] When the surface shape of the constant-line-spacing concave grating 104 is calculated, the grating line distance d0 is first calculated. According to the size of the spectral dispersion of the system and the distance from the surface of the concave grating 104 to the image plane 108, the size of the dispersion angle of the chief ray at the center of the slit 102 can be calculated, and according to the size of the dispersion angle and the spectral range, the grating line distance d0 that meets the requirements can be calculated. At this time, all the known quantities in formula (2) are known except R, so the value of R can be solved to obtain the normal of each feature data point on the surface of the constant-line-spacing concave grating 104. That is, the method for calculating the normal of each feature data point on the surface of the constant-line-spacing concave grating 104 includes the following sub-steps:

[0057] S21', according to the size of the spectral dispersion of the system and the distance from the surface of the constant-line-spacing concave grating to the image plane, the size of the dispersion angle of the chief ray at the center of the slit is calculated;

[0058] S22', according to the size of the dispersion angle and the spectral range, the grating line distance d0 that meets the requirements is calculated;

[0059] S23', according to formula Solve the value of R, wherein n represents the diffraction order, λ represents the wavelength of the light, G represents the normal direction of the grating generating surface, R represents the normal of each feature data point on the ruled surface concave grating surface, d represents the grating line distance at the light incident point, S represents the direction vector of the incident light at the corresponding feature data point on the ruled surface concave grating 104 surface, and S' represents the direction vector of the outgoing light at the corresponding feature data point on the ruled surface concave grating 104 surface.

[0060] When calculating the surface shape of the variable-line-spacing concave grating 104, the grating line distance at different feature data points is different, and the following method is adopted for solving:

[0061] S21, the surface shape of the grating is set to a spherical surface, the intersection of the characteristic light and the spherical surface is the feature data point, the normal of the spherical surface at each feature data point is known, at this time, all known quantities in formula (2) except d can be solved, so the grating line distance d at this feature data point can be solved, when the grating line distance at all feature data points is solved, the function d(y) of the grating line distance with the coordinate y is fitted according to formula (1); and

[0062] S22, the normal R of the feature data point of the ruled surface concave grating 104 is solved by the method of solving the normal of the feature data point of the ruled surface concave grating 104 (i.e. the step S23'), but the difference is that the value of the grating line distance d at each feature data point is not solved according to the dispersion of the central chief ray of the slit 102, but is given by the function d(y).

[0063] In step S3, after the coordinates and normals of all feature data points are solved, the free surface surface shape of the ruled surface concave grating 104 or the free surface surface shape of the variable-line-spacing concave grating 104 is obtained by fitting.

[0064] The method of fitting the free surface surface shape of the concave grating 104 is not limited. In the embodiment, the method of fitting the free surface surface shape of the concave grating 104 includes the following sub-steps:

[0065] Step S1', an initial surface and a first three-dimensional rectangular coordinate system are established;

[0066] Step S2', K characteristic light rays R i (i=1, 2, …, K) are selected;

[0067] Step S3', the multiple intersection points of each characteristic light and the to-be-solved free surface are solved point by point according to the object-image relationship or the light ray mapping relationship and Snell's law, and then multiple feature data points P i (i=1, 2, …, K) are obtained;

[0068] Step S4', fitting the plurality of feature data points into a spherical surface in the first three-dimensional rectangular coordinate system, defining the feature data point corresponding to the center sampling view main light as the vertex of the spherical surface, and establishing a second three-dimensional rectangular coordinate system with the vertex of the spherical surface as the origin and the straight line between the center of curvature and the vertex of the spherical surface as the z-axis; and

[0069] Step S5', transforming the coordinates (x i ,y i ,z i ) and normal vectors (α i ,β i ,γ i ) of the plurality of feature data points in the first three-dimensional rectangular coordinate system into coordinates (x' i ,y' i ,z' i ) and normal vectors (α' i ,β' i ,γ' i ) in the second three-dimensional rectangular coordinate system, fitting the plurality of feature data points P i (i=1,2…K) into a quadric surface in the second three-dimensional rectangular coordinate system, removing the coordinates and normal vectors of the feature data points on the quadric surface in the second three-dimensional rectangular coordinate system from the coordinates (x' i ,y' i ,z' i ) and normal vectors (α' i ,β' i ,γ' i ) to obtain residual coordinates and residual normal vectors, and performing surface fitting on the residual coordinates and residual normal vectors to obtain a free surface, and adding the equation of the free surface to the equation of the quadric surface to obtain the equation of the free surface of the concave grating 104.

[0070] In step S4, the purpose of optimizing the initial structure is to improve the imaging quality of the initial structure. The method of optimizing the initial system to improve the imaging quality of the initial system is not limited, and can be realized by using optical design software. The optical design software includes CODE V or ZEMAX OPTIC STUDIO, etc.

[0071] Further, the design method of the free-form concave grating imaging spectrometer 100 can further include a step of processing according to the parameters output in step S4 after step S4, so as to obtain a physical element of the free-form concave grating imaging spectrometer 100, which has a shape, is made of a material, and is a physical element entity.

[0072] In addition, for the variable-line-spacing concave grating 104, a function of the grating spacing changing with coordinates is obtained while the free-form surface profile is solved. In order to further improve the solving effect of the variable-line-spacing concave grating 104 and the initial system image quality, the step S3 of the design method of the free-form concave grating imaging spectrometer 100 further comprises the following steps:

[0073] S31, fixing the solved free-form surface profile, repeating the step S21 in the variable-line-spacing concave grating 104 surface profile solving process, and calculating a new grating spacing change function;

[0074] S32, repeating the step S22 in the variable-line-spacing concave grating 104 surface profile solving process to obtain a corresponding new free-form surface profile; and

[0075] S33, iteratively repeating the step S31 and the step S32 to further improve the imaging quality of the calculated variable-line-spacing free-form concave grating imaging spectrometer 100.

[0076] In the embodiment, the design method of the free-form concave grating imaging spectrometer 100 is used to obtain the system 1 to the system 21.

[0077] The design method of the free-form concave grating imaging spectrometer 100 has the following advantages: first, an initial solution meeting the given conditions can be quickly generated, and a high-image-quality system can be obtained through subsequent optimization, which greatly facilitates the design of systems with different spectral ranges and different spectral resolutions; second, the method is simple.

[0078] In addition, those skilled in the art can make other changes within the spirit of the present application, and of course, these changes made according to the spirit of the present application should be included in the scope of protection claimed by the present application.

Claims

1. A freeform concave grating imaging spectrometer, comprising an image plane, characterized in that, The freeform concave grating imaging spectrometer further includes a slit and a freeform concave grating. The freeform concave grating has a freeform surface. Light incident from the slit illuminates the freeform surface, undergoes dispersion, and is reflected to form a reflected beam. This reflected beam illuminates the image surface, causing light of different wavelengths incident from the slit to be separated and imaged on the image surface respectively. The freeform concave grating is a variable-pitch freeform concave grating. The intersection points of characteristic light rays incident from different positions of the slit with the freeform surface are defined as characteristic data points. The grating line spacing is different at different characteristic data points on the freeform surface. The spectral dispersion of the freeform concave grating is greater than or equal to 100 nm / mm.

2. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The freeform concave grating imaging spectrometer contains only one optical element: the freeform concave grating.

3. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The freeform concave grating imaging spectrometer further includes a detector for recording information about the image plane.

4. The freeform concave grating imaging spectrometer as described in claim 3, characterized in that, The freeform concave grating imaging spectrometer consists of the slit, the image plane, the freeform concave grating, and the detector.

5. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The slit and the image plane are located on the same side of the freeform concave grating.

6. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The freeform concave grating has both dispersion and imaging functions.

7. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The slit length is less than or equal to 2.3 mm, the numerical aperture is less than or equal to 0.16, and the system length is greater than or equal to 37 mm.

8. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The spectral range of the freeform concave grating imaging spectrometer is 400 nm to 1000 nm.

9. The freeform concave grating imaging spectrometer as described in claim 1, characterized in that, The slit can be a fixed slit, a single-sided adjustable asymmetrical slit, or a double-sided adjustable symmetrical slit.

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