Disclosure of Invention
The invention aims to provide an optical lens group, a vision compensation method and a driving method which have simple structure, meet the optical requirements of ultra-wide angle, wide spectrum and high image quality, have a small number of lenses and are suitable for a color fundus image system.
In order to achieve the above object, the present invention provides an optical lens group suitable for a color fundus image system, comprising an entrance pupil, a scanning lens group, an intermediate image plane, an eyepiece group and an exit pupil which are sequentially arranged along a first direction, wherein the entrance pupil is positioned on one focal plane of the scanning lens group far away from the intermediate image plane; the exit pupil is positioned on one focal plane of the ocular group, which is far away from the middle image plane, and the exit pupil coincides with the pupil of the eye when the eye is detected; the curvature radius of curvature of field curvature on the intermediate image plane in meridian plane is smaller than 2 times of the effective focal length of the ocular lens group, and the field curvature is bent to one side of the ocular lens group; the effective focal length of the scanning lens group is larger than that of the eyepiece group, the air interval between the scanning lens group and the eyepiece group is larger than the transverse diameter of the intermediate image plane and smaller than the sum of the effective focal lengths of the scanning lens group and the eyepiece group.
The present invention also provides the above-mentioned vision compensating method for an optical lens group adapted to a color fundus image system, the optical lens group including an entrance pupil, a scanning lens group, an intermediate image plane, an eyepiece group, and an exit pupil sequentially arranged along a first direction, wherein the scanning lens group includes a second cemented doublet lens and a front lens group arranged along the first direction,
the vision degree compensation is performed by changing the air interval L2 between the eyepiece group and the scanning mirror group and the air interval L1 between the front mirror group and the second double-cemented lens in the scanning mirror group.
The present invention also provides a driving method of the optical lens group suitable for the color fundus image system, the optical lens group comprises an entrance pupil, a scanning lens group, an intermediate image plane, an eyepiece group, an exit pupil and a driving device which are sequentially arranged along a first direction, wherein the scanning lens group comprises a second double-cemented positive lens and a front lens which are arranged along the first direction,
the driving device changes the air interval L2 between the ocular lens group and the scanning mirror group and the air interval L1 between the front mirror group and the second double-cemented lens in the scanning mirror group according to the received human eye vision information, so that the L1 and the L2 meet the preset relation.
The optical lens group of the color fundus image system has the following advantages:
1. the optical lens group consists of a group of wide-angle ocular lens groups and a group of scanning lens groups, has a simple overall structure and a small number of lenses, can meet the optical requirements of ultra wide angle and wide spectrum, and is matched with the scanning lens groups to realize complete aberration correction, so that the image quality with low distortion and diffraction limit level is obtained in the full spectrum of the full field of view;
2. the incident angle and the emergent angle of the light rays on the surface of each mirror surface are small, the types of glass materials are few, and only conventional stable glass materials are adopted, so that convenience is brought to the production and processing of the lenses, and great flexibility is brought to the arrangement of light paths;
3. the eyepiece group is provided with only four reflecting surfaces, so that the risk of image center ghost image caused by reflection of lenses is reduced, and the requirements of ultra-wide angle and wide spectrum dispersion correction are met;
4. the positive and negative vision compensation is realized by simultaneously moving two air spaces between the scanning lens group and the ocular lens group and between the front lens group and the second double-cemented lens in the scanning lens group, so that the aberration introduced by the high vision compensation under the ultra-large visual field wide spectrum is effectively reduced;
5. the intermediate image plane is double telecentric imaging, namely, the eyepiece group and the scanning mirror can realize telecentric imaging on the intermediate image plane, and the vision degree compensation is carried out while the vision field range is kept unchanged.
The optical lens design of the invention can be applied to various color fundus imaging devices such as fundus photography, confocal scanning laser fundus oculi, line scanning fundus oculi and wide line scanning fundus oculi, and is also applicable to three-dimensional image modes with depth information such as optical coherence tomography.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The invention provides an optical lens set, a vision compensating method and a driving method suitable for a color fundus image system, and the embodiment of the invention is described in detail below with reference to the accompanying drawings.
Example 1
Fig. 1 is a schematic structural diagram of an optical lens assembly according to a first embodiment of the present invention. As shown in fig. 1, the optical lens assembly 100 includes: an entrance pupil 50, a scanning mirror group 40, an intermediate image plane 30, an eyepiece group and a 20 exit pupil 10 which are sequentially arranged along a first direction, wherein the entrance pupil 50 is positioned on a focal plane of the scanning mirror group 40, which is far away from the intermediate image plane 30; the exit pupil 10 is positioned on a focal plane of the eyepiece group 20 at a side far away from the intermediate image plane 30, and the exit pupil 10 coincides with the pupil of the eye when the eye is detected; the curvature radius of the field curvature of the intermediate image surface 30 in the meridian plane is smaller than 2 times of the effective focal length of the eyepiece group, and the field curvature is towards one side of the eyepiece group 20; the effective focal length of the scanning lens set 40 is larger than the effective focal length of the eyepiece set 20, the air gap between the scanning lens set 40 and the eyepiece set 20 is larger than the transverse diameter of the intermediate image plane 30 and smaller than the sum of the effective focal lengths of the scanning lens set 40 and the eyepiece set 20.
When capturing a fundus image, a parallel beam of illumination light starts at the entrance pupil 50 along the first direction, is focused on the intermediate image plane 30 after passing through the entrance pupil 50 and the scanning mirror group 40, and then enters the pupil of the eye (coinciding with the exit pupil 10 of the eyepiece group 20) through the eyepiece group 20 and is converged at the fundus. After being reflected or scattered by the fundus, a part of the illumination light returns through the original path to reach the entrance pupil 50. If there is a beam splitting means at this location, a portion of the light will be directed in a different direction than the incident illumination light and will be detected by the detector. The light splitting can be realized by coating films, and can also be realized by arranging different transmission and reflection areas on one plane, depending on specific modes of illumination, light splitting and detection.
Based on the design of the optical lens group 100 shown in fig. 1, fundus photography, confocal scanning laser fundus mirror, line scanning laser fundus mirror and wide line scanning slit fundus mirror can be realized. The invention does not limit the application scene of the optical lens group.
Further, the eyepiece unit 20 in the optical lens unit 100 includes: the third lens 23, the second lens 22 and the first lens 21 are sequentially arranged along the first direction, the first lens 21 is an aspheric positive lens, the second lens 22 is a negative lens, the third lens 23 is a positive lens, and the second lens 22 and the third lens 23 form a first double-cemented positive lens 223. The first lens 21 is a single aspherical positive lens or a double aspherical positive lens, one surface close to the second lens 22 is an even aspherical surface, and the other surface is a spherical or aspherical mirror. More preferably, the first lens 21 may be a positive meniscus lens, which has a concave surface facing the exit pupil 10 and a convex surface facing the second lens 22, and is curved toward the exit pupil 10 for disambiguation.
The first lens 21 and the third lens 23 are made of medium dispersion materials, and the second lens (negative lens) 22 is made of high refractive index and high dispersion flint glass materials. Specifically, referring to table one, the first lens 21 is made of a material having a refractive index greater than 1.5 and an abbe number of 40 to 65; the second lens 22 is made of a material with a refractive index greater than 1.7 and an Abbe number less than 40; the third lens 23 is made of a material having a refractive index of greater than 1.6 and an abbe number of between 40 and 65. Preferably, the first lens 21 and the third lens 23 may be made of the same material having a refractive index greater than 1.6 and an abbe number between 40 and 65.
List one
| Lens | Refractive index | Abbe number |
| First lens | Refractive index > 1.5 | An Abbe number of 65 > 40 |
| Second lens | Refractive index > 1.7 | Abbe number 40% |
| Third lens | Refractive index > 1.6 | An Abbe number of 65 > 40 |
In the present embodiment, the focal length of the first lens 21 is smaller than the focal length of the first positive doublet lens 223, and the effective focal length of the eyepiece group 20 is between 20mm and 40 mm; the distance from one side surface of the exit pupil 10 of the first lens 21 to the exit pupil 10 is between 10mm and 40 mm; the effective focal length of the first lens 21 of the eyepiece group 20 is smaller than the effective focal length of the first cemented doublet 223.
In the optical lens assembly 100 of the present embodiment, the focal power of the eyepiece lens assembly 20 is shared by two positive lenses (the first lens 21 and the third lens 23), the first lens 21 bears most of the focal power of the eyepiece lens assembly 20, the meniscus shape is favorable for reducing the emergent angle of the light beam on the edge of the surface of the first lens 21 close to one side of the exit pupil 10, and the first double-cemented positive lens 223 formed by the second lens 22 and the third lens 23 performs the functions of dispersion correction and a small amount of focal power.
In this embodiment, the distance from the surface of the first aspheric positive lens 21 on the side of the exit pupil 10 to the exit pupil 10 is 21.4mm, the distance (working distance) from the front surface of the cornea is 19mm, the ultra-wide angle of 95 ° is realized on the side of the eye, the designed spectrum width is from 445nm blue light to 920nm near infrared, and the paraxial angle magnification is 3.1. For the illumination light path, the entrance pupil 50 has a diameter of 8-10mm, corresponding to an exit pupil diameter of 2.6-3.2mm, and an image space (fundus) numerical aperture of 0.08 to 0.10; for imaging, the entrance pupil 50 (if the fundus is the ray origin, the "entrance pupil 50" should be the exit pupil at this time, for consistency, it is still called the entrance pupil) has a diameter of 4mm, corresponding to an exit pupil diameter of 1.3mm, and the image space (fundus) numerical aperture of 0.04.
Further, the scanning mirror group 40 in the optical mirror group 100 includes: a fifth lens 43, a fourth lens 42 and a front lens group 41 sequentially arranged along the first direction, wherein the front lens group 41 includes at least one positive lens (in other embodiments, the front lens group 41 may be composed of a plurality of lenses), the fifth lens is a negative lens, the fourth lens is a positive lens, and the fourth lens 42 and the fifth lens 43 constitute a second double cemented positive lens 423. In the present embodiment, the fourth lens 42 is a low-dispersion crown glass having a refractive index of less than 1.7 and an abbe number of 50 to 85; the fifth lens 43 is made of flint glass material having a refractive index of more than 1.7 and an abbe number of less than 40. Preferably, in the present embodiment, at least one positive lens of the front lens group (when it is a single positive lens) 41 and the fifth lens 43 are made of the same material as the second lens 21 of the eyepiece lens group 20.
Eyepiece group 20 and scanning mirror group 40 telecentrically image at intermediate image plane 30.
The design of eyepiece group 20 allows for the short focal length required for ultra wide angle field of view, and for the chromatic dispersion correction required for color imaging, while employing as few lenses and reflective surfaces as possible, the number of lenses being much lower than US3390935 and US4286844 for similar field of view. If the number of lenses is lower than that of the first embodiment, the eyepiece group 20 can only use a single positive lens or two positive lenses to satisfy the short focal length required by the ultra-wide angle field of view, so that not only is effective dispersion correction difficult, but also the eyepiece group 20 itself introduces additional dispersion, and brings huge design pressure to the scanning mirror group 40, which makes it extremely complex. Through many design attempts by the inventor, in order to achieve similar image quality requirements, the number of lenses of the scanning lens set exceeds 7, and at most reaches 14, and it is difficult to avoid a large incident angle of more than 45 degrees, and if the number of lenses of the eyepiece set 20 is higher than that of the first embodiment, it is difficult to eliminate the central reflection of each lens surface in the eyepiece set 20. If the first lens 21 is replaced with a double cemented lens, the power is difficult to ensure, and it is difficult to realize an ultra-wide angle field of view. If the first positive doublet 223 combined by the second lens 22 and the third lens 23 is replaced by a triple cemented lens assembly, the diameter of the intermediate image surface 30 will be increased without increasing the number of glass-air interfaces, and the rear surface of the triple cemented lens assembly is closer to the intermediate image surface 30, and the intermediate image surface 30 formed by high myopia will be very close to the rear surface of the triple cemented lens assembly, greatly increasing the risk of central reflection of the lens surfaces.
One gist of the present invention is that the eyepiece group 20 corrects only the central field of view of the intermediate image plane 30, corrects the off-axis lateral chromatic aberration, and simultaneously releases off-axis aberrations such as curvature of field, astigmatism, coma and distortion of the intermediate image plane 30, and the curvature of field and astigmatism of the intermediate image plane 30 are particularly prominent. The scanning lens assembly 40 adopts a structure of single lens (or lens assembly) and double-cemented lens similar to the eyepiece lens assembly 20, and field curvature, astigmatism and dispersion on the intermediate image plane 30 are opposite to those of the fundus formed by the eyepiece lens assembly 20, so that most of off-axis aberration is counteracted. This has the advantage that the design of the overall optical system is greatly simplified. The scanning mirror assembly 40 can use only a minimum of three lenses to just compensate for the uncorrected (or overcorrected) off-axis aberrations of the intermediate image plane. The whole light path can adopt six lenses at least, namely, the diffraction limit imaging within the wide spectrum from 445nm to near infrared 920nm and the full field of view of the ultra-wide angle field of view of 95 degrees is achieved. FIG. 2 is a schematic view of an in-meridian curvature of field (dashed line) on an intermediate image plane according to a first embodiment of the present invention; fig. 3 is a schematic diagram of field curves and astigmatism curves of an intermediate image plane according to a first embodiment of the present invention.
Although the intermediate image plane 30 has a large off-axis aberration, the eyepiece unit 20 and the scanning lens unit 40 can be combined to use only 6 lenses, and a minimum of 3 glass materials, so that the diffraction limited-order image quality is realized in a wide spectral range from blue light to near infrared and in an ultra-large field of view. FIG. 4 is a schematic diagram of an aberration Ray Fan of a Fan of light rays according to an embodiment of the present invention; FIG. 5 is a schematic diagram of a wavefront aberration OPD according to a first embodiment of the invention; FIG. 6 is a diagram illustrating a distribution of root mean square wavefront aberration in a field of view according to an embodiment of the present invention; fig. 7 is a schematic diagram of a field curvature and astigmatism curve according to a first embodiment of the present invention. As shown in fig. 4, the optical Fan aberration Ray Fan of the first embodiment is lower than 10 μm in the full field of view. As shown in fig. 5, the wavefront aberration OPD peak-to-peak value of the first embodiment is about 0.5wave. As shown in fig. 6, the root mean square wavefront aberration is below the diffraction limit of 0.07wave over the entire field of view. As shown in fig. 7, the image side NA is 0.04, whereby the focal plane depth of field can be calculated to be 0.5mm. In fig. 7, the curvature of field, astigmatism, axial chromatic aberration, and lateral chromatic aberration are all much lower than the focal plane depth of field. Fig. 8 is a schematic diagram of an angle-enlarged distortion curve according to a first embodiment of the present invention. Distortion is defined as the change in angle of magnification of the angle of the pre-corneal scan relative to the chief ray of the entrance pupil, and the specific formula is:
where γ is the angle magnification distortion, θ is the angle of the chief ray at the entrance pupil 50 relative to the optical axis, M is the paraxial angle magnification,is the angle of the chief ray at the exit pupil 10 relative to the optical axis corresponding to the entrance pupil chief ray angle θ. As shown in fig. 8, the angular distortion of the entire optical path is within 1.5%.
In addition to refractive changes, off-axis aberrations increase significantly as the eye's refraction deviates from positive vision. The degree to which the off-axis aberrations increase is also different for different wavelengths. This is not a serious problem for conventional field of view or narrow spectrum fundus imaging applications. For example, the invention of application number 202111440377.5, although having a field of view of 87 ° or more, has a spectral bandwidth limited to the near infrared range, has a low dispersion, and still has an added additional aberration within an acceptable range in the case of myopia up to 21D. Therefore, the vision compensation can be realized by only moving the scanning lens group to change the air interval between the ocular lens and the scanning lens group, which is the vision compensation mode of most standard vision or achromatic fundus image equipment.
The visibility compensation of wide-spectrum color wide-angle images is much more complex. According to the Navarro myopia eye model, for a super wide angle field of view of 95 degrees, the variation of off-axis aberration along with the vision is much larger than that of near infrared under the visible light wave band, and the aberration variation degree caused by the variation of different wavelengths along with the vision is also different, wherein the variation of blue light is particularly severe. The dramatic change in off-axis aberrations causes the aberrations to be very different from positive vision in high vision situations, which is further exacerbated by ultra-wide angles. Therefore, the vision compensation of the ultra-wide-angle wide-spectrum fundus image cannot be simply performed by changing the air interval between the eyepiece group and the scanning mirror group, otherwise, the system image quality is drastically reduced with the increase of the vision deviation.
With continued reference to fig. 1, the present embodiment further provides a method for compensating the visibility of an optical lens group suitable for a color fundus image system, where the optical lens group includes an entrance pupil 50, a scanning lens group 40, an intermediate image plane 30, an eyepiece group 20, and an exit pupil 10 sequentially arranged along a first direction, and the scanning lens group 40 includes a second double cemented positive lens 423 and a front lens 41 arranged along the first direction, and the visibility compensation is performed by changing an air interval L2 between the eyepiece group 20 and the scanning lens group 40 and an air interval L1 between the front lens group 41 and the second double cemented lens 423 in the scanning lens group 40.
Further, in the optical lens group vision compensating method, myopia compensation is performed by reducing an air gap L2 between the eyepiece group 20 and the scanning lens group 40 and an air gap L1 between the front lens group 41 and the second double cemented lens 423 in the scanning lens group 40; the distance vision compensation is performed by increasing the air interval L2 between the eyepiece group 20 and the scanning lens group 40 and the air interval L1 between the front lens group 41 and the second double cemented lens 423 in the scanning lens group 40.
The invention solves the problems well by adjusting the air interval L2 between the scanning mirror group 40 and the eyepiece group 20 and simultaneously adjusting the air interval L1 between the front mirror group 41 and the second double-glued mirror group 423 in the scanning mirror group 40 in a matching way, thereby realizing wide-range vision compensation and maintaining good image quality.
By way of comparison, fig. 9 shows the difference in image quality of the two approaches described above when vision compensation is performed for-5D myopia. FIG. 9 (left) is a diagram showing wavefront aberrations when only the scanning mirror set is moved (only L2 is changed) for compensation; fig. 9 (right) is a schematic diagram of wavefront aberration when the air interval L1 between the front lens group and the second cemented positive lens is simultaneously changed to compensate for L2. taking-5D myopia compensation as an example, if only the air gap L2 between the scanning mirror group 40 and the eyepiece group 20 is reduced by 9.75mm to refocus, the imaging image quality of the peripheral region is significantly degraded, as shown in fig. 9 (left). If the air space L2 between the scanning mirror group 40 and the eyepiece group 20 is reduced by 6.15m, and at the same time, the air space L1 between the front mirror group 41 and the fourth lens 42 is reduced by 3.29mm, the image quality is reduced much less. As shown in fig. 9, the image quality of fig. 9 (right) is significantly better than that of fig. 9 (left).
FIG. 10 (left) is a diagram showing field curvature and astigmatism when only the scan mirror set is moved (only L2 is changed) for compensation; fig. 10 (right) is a schematic diagram of curvature of field and astigmatism when the air interval L1 between the front lens group and the second cemented positive lens is simultaneously changed to compensate for L2. The field curvature astigmatism of fig. 10 (right) is significantly lower than that of fig. 10 (left).
Fig. 11 is a schematic diagram illustrating the adjustment of the lens air gap at different myopia according to the first embodiment of the present invention. As shown in fig. 11, as the near vision increases, the interval L2 between the scanning mirror group 40 and the eyepiece group 20 and the air interval L1 between the front mirror group 41 and the fourth lens 42 become smaller. FIG. 12 is a graph showing the L1/L2 adjustment curve during myopia compensation according to the first embodiment.
The compensation of the far vision is achieved by increasing the air space L2 from the front lens group 41 to the third lens 23 and the air space L1 from the second doublet 423 to the front lens group 41, the relative movement direction of the lenses being opposite to the near vision compensation.
In the first embodiment, for ±21D visibility compensation, the air interval L2 between the scanning mirror group 40 and the eyepiece group 20 varies over a range of ±25.5mm, and the air interval L1 between the scanning pre-mirror group 41 and the second double cemented lens 423 varies over a range of ±10.7mm.
The driving device for changing the two air spaces L1 and L2 can be realized by a traditional mechanical wire slot of the zoom lens or other transmission modes, and can also be composed of two stepping motors for respectively driving the positions of the front lens group 41 and the second double-glued lens group 423 or two motors for respectively driving the positions of the scanning lens group 40 and the front lens group 41 to realize vision compensation.
With continued reference to fig. 1, there is further provided a method for driving the optical lens group including an entrance pupil 50, a scanning lens group 40, an intermediate image plane 30, an eyepiece group 20, an exit pupil 10, and a driving device (not shown) sequentially arranged along a first direction, wherein the scanning lens group 40 includes a second cemented doublet 423 and a front lens group 41 arranged along the first direction, and the driving device changes an air interval L1 between the second cemented doublet 423 and the front lens group 41 and an air interval L2 between the front lens group 41 and the eyepiece group 20 according to received human eye visibility information such that L1 and L2 satisfy a preset relationship.
In this embodiment, the specific composition and driving manner of the driving device are not limited, and the purpose of driving the related components is to make L1 and L2 satisfy a preset relationship. The preset relationship of L1 and L2 is a relationship shown by a curve in fig. 12, and may be a relationship of L1 and L2 pre-stored in a formula form or an array form. The relationship between L1 and L2 may be calculated in real time by a specific calculation method, or may be a formula or a plurality of relationship arrays obtained by previous theoretical calculation or experiments, which is not limited herein.
It will be appreciated by those skilled in the art that the scanning lens assembly 40 and the optical lens assembly design thereof in the present invention are equally applicable to non-scanning fundus imaging systems, such as conventional fundus cameras, which only require the addition of an illumination light path and an image acquisition light path before the entrance pupil 50, but the technical details related to the illumination light path and the image acquisition light path of the conventional fundus camera are not important in this patent, and any person skilled in the art can add the required technical details based on the optical design of the above embodiment without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications and variations which are within the spirit and scope of the present invention can be accomplished by those skilled in the art having ordinary skill in the art shall be covered by the appended claims.
Example two
Fig. 13 is a schematic structural diagram of a second optical lens assembly according to a second embodiment of the present invention. As shown in fig. 13, the second optical lens group 200 according to the second embodiment includes: the entrance pupil 50a, the scanning lens group 40a, the intermediate image plane 30a, the eyepiece group 20a, and the exit pupil 10a are sequentially arranged along the first direction, wherein the scanning lens group includes a front lens group 41a and a second cemented doublet 423a, and the front lens group 41a is a positive lens composed of a first lens 411a and a second lens 412 a. The eyepiece group 20a includes a first cemented doublet 223a and a first aspheric positive lens 21a. The distance from the surface of the first aspherical positive lens 21a on the side of the exit pupil 10a to the exit pupil 10 is about 22.7mm, the distance (working distance) to the front surface of the cornea of the eye is 20mm, a super wide angle of 100 ° is achieved on the side of the eye, the spectral width is 445nm to 920nm as in the first embodiment, the paraxial angle magnification is 4.5, the diameter of the entrance pupil 50a is 6mm, the diameter of the exit pupil corresponds to 1.3mm, and the image space (fundus) numerical aperture is 0.04..
FIG. 14 is a diagram illustrating aberration Ray Fan of a light beam Fan according to a second embodiment of the present invention; FIG. 15 is a schematic diagram of a wavefront aberration OPD according to a first embodiment of the invention; FIG. 16 is a schematic diagram showing the distribution of square root wavefront aberration in a field of view according to a second embodiment of the present invention; FIG. 17 is a diagram illustrating a field curvature and astigmatism curve according to a second embodiment of the present invention; fig. 18 is a schematic diagram of an enlarged angle distortion curve according to a second embodiment of the present invention. As shown in fig. 14 to 18, the overall optical performance is similar to that of the first embodiment, and the angle-magnification distortion is increased.
In the second embodiment, the material of the first lens 411a and the second lens 412a may be flint glass. However, the number, material, position and power distribution of the lenses constituting the front lens group 41a may be very flexibly configured, the material may not be limited to flint glass, the number of lenses may be more than 2, and the lenses may be positive lenses or negative lenses, and may be single lenses or cemented lenses, as long as the total effective focal length of the front lens group 41 is positive.
In the invention, the first and second embodiments realize good chromatic aberration correction from blue light to near infrared in the spectrum range of 475nm, the lens materials are common stable glass materials, and the coefficients of acid resistance, alkali resistance, phosphorus resistance, pollution resistance, environment resistance and the like are all within 10. By relaxing the off-axis aberrations of the intermediate image plane 30, the use of erodible materials such as ultra-low dispersion glass, anomalous dispersion glass, lanthanide LAF and LAK glasses, etc. can be avoided.
The examples of the present invention are merely illustrative of the principles of the present invention and its efficacy, and are not intended to limit the invention. Modifications and variations may be made to the above-described embodiments by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is intended that all equivalent modifications and variations of the invention be covered by the claims, which are within the ordinary skill of the art, be within the spirit and scope of the present disclosure.