A refraction-insensitive fundus camera based on free-form surfaces

Abstract

The present invention proposes a fundus camera based on a free-form surface that is insensitive to diopter, which does not require the introduction of additional optical elements, can reduce the difficulty of system adjustment, and can still clearly image human eyes with large refractive errors. In the fundus camera of the present invention, the aspherical front mirror has the ability to image the fundus once and the ability to image the pupil of the human eye, which ensures an effective imaging field of view, relieves the pressure of the relay lens group to correct aberrations, and compresses the system volume.

Description

Technical Field

[0001] The invention relates to the technical field of imaging optics, and in particular to a fundus camera based on a free-form surface and insensitive to diopter. Background Art

[0002] Fundus camera is an important instrument commonly used in the diagnosis and treatment of ophthalmic diseases. Based on fundus images, doctors can diagnose a variety of diseases, including diabetes, hypertension, and glaucoma. Clear fundus images are essential for the correct diagnosis of diseases. However, the refractive error of the human eye will cause a certain degree of defocus in the fundus imaging, resulting in blurred image plane, which affects the doctor's judgment of the disease. In addition, there are great individual differences in the degree of refractive error. With the promotion of electronic products, myopia is widely present in the youth group; and the vast majority of elderly people suffer from hyperopia. The huge difference in refractive error among patients with ophthalmic diseases has brought great difficulties to the clear imaging of the fundus. Traditional fundus cameras often use focusing mechanisms to solve the defocus problem caused by refractive error, but this will introduce additional focusing lens groups and focusing motors, greatly increasing the complexity and production cost of the system. Moreover, in the actual shooting process, since the patient needs to dilate the pupil, the illumination light of the fundus camera brings great discomfort to the patient, making it difficult for the patient's eyes to stare at one point without moving, and frequent eye movements also increase the difficulty of clear focus. In addition, diffuser plate imaging is also used for fundus imaging to alleviate the impact of refractive error. However, diffuser plate imaging has high requirements on the detector's readout noise, maximum well capacity, and dynamic range.

[0003] The patent (CN 2017104311408) discloses a lens consisting of an eyepiece objective lens, a collimator lens, and a wavefront coding element, which is mainly used for clear imaging with a fundus camera connected to a mobile phone, wherein the wavefront coding element is used to correct unclear imaging caused by ametropia of the human eye. The system introduces an additional wavefront coding element, namely a phase mask, but the use of the phase mask causes the system to only be able to encode light of the designed wavelength well. When the wavelength of the incident system deviates from the designed wavelength, the diffraction efficiency of the phase mask will be severely reduced. In addition, the current fundus cameras mostly use white light to illuminate the fundus, and the phase mask will limit the imaging capability of the system in actual use; and the patent does not simulate human eyes with different refractive powers, and it is not clear within which refractive power range the system can obtain a clear image. Summary of the invention

[0004] In view of this, the present invention proposes a fundus camera based on a free-form surface that is insensitive to refractive error. It does not require the introduction of additional optical elements, can reduce the difficulty of system installation and adjustment, and can still produce clear images for human eyes with large refractive errors.

[0005] To achieve the above object, the technical solution of the present invention is:

[0006] The present invention discloses a fundus camera based on a free-form surface and insensitive to diopter, comprising a front mirror, a first double-cemented lens, a first positive lens, a second double-cemented lens, a third double-cemented lens and a detector; the first double-cemented lens comprises a free-form surface lens and a first negative lens; the front mirror is an aspherical positive lens, which is used to collect parallel light emitted by the human eye and image the parallel light on a primary image plane; at the same time, the front mirror images the pupil on the free-form surface lens in the first double-cemented lens, so that the free-form surface lens is located at the pupil of the system; the first double-cemented lens, the first positive lens, the second double-cemented lens and the third double-cemented lens image the fundus primary image on the primary image plane to the detector. .

[0007] The pupil is modulated by changing the cubic coefficient of the surface profile of the free-form lens, so that the system has defocus invariance.

[0008] The system aberration is corrected by changing the surface coefficient of the free-form surface lens.

[0009] Here, low-order terms are used for aberration correction.

[0010] The first double-cemented lens, the first positive lens, the second double-cemented lens and the third double-cemented lens form a relay lens group, and the relay lens group is used to correct the aberration of the system.

[0011] Beneficial effects:

[0012] 1. In the fundus camera of the present invention, the aspherical front mirror has the ability to image the fundus once and the pupil of the human eye, which ensures an effective imaging field of view, relieves the pressure of the relay lens group to correct aberrations, and compresses the system volume.

[0013] 2. The present invention introduces a free-form surface lens on the basis of a traditional fundus camera to replace the phase plate in the traditional wavefront coding. The free-form surface lens is set at the conjugate position of the pupil of the fundus camera. By setting the cubic coefficient of the free-form surface lens, the system can be phase modulated to achieve defocus invariance without moving any components. The present invention does not require the introduction of additional optical elements, overcomes the problem that traditional fundus cameras require complex focusing structures to solve the problem of image blur caused by refractive error, reduces the difficulty of system adjustment, and can still clearly image human eyes with large refractive errors.

[0014] 3. The free-form surface lens of the fundus camera of the present invention is the first lens of the relay lens group, which not only plays the role of phase modulation but also plays the role of aberration correction, thus greatly reducing the volume of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the structure of the present invention.

[0016] Among them, 1-fundus, 2-human eye, 3-pupil, 4-front mirror, 5-primary image plane, 6-relay lens group, 7-first double glued lens, 8-first positive lens, 9-second double glued lens, 10-third double glued lens, 11-detector; 7-1-free-form surface lens, 7-2-first negative lens; 9-1-second positive lens, 9-2-second negative lens; 10-1-third negative lens, 10-2-third positive lens DETAILED DESCRIPTION

[0017] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0018] The present invention provides a fundus camera based on a free-form surface and insensitive to diopter, the structure of which is as follows: Figure 1 As shown, it includes a front mirror 4, a first double-cemented lens 7, a first positive lens 8, a second double-cemented lens 9, a third double-cemented lens 10 and a detector 11.

[0019] Among them, the first double-cemented lens 7 includes a free-form surface lens 7-1 and a first negative lens 7-2, the second double-cemented lens 9 includes a second positive lens 9-1 and a second negative lens 9-2; the third double-cemented lens 10 includes a third negative lens 10-1 and a third positive lens 10-2; the front mirror 4 is an aspherical positive lens, which is used to collect parallel light emitted by the human eye 2 (i.e., light reflected from each point on the fundus 1 is emitted as parallel light after passing through the pupil 3), and image the parallel light on the primary image plane 5; at the same time, the front mirror 4 images the pupil on the free-form surface lens 7-2 in the first double-cemented lens, so that the free-form surface lens 7-1 is located at the pupil of the system. The first double-cemented lens 7, the first positive lens 8, the second double-cemented lens 9 and the third double-cemented lens 10 image the fundus primary image on the primary image plane 5 to the detector 11.

[0020] Specifically, by changing the cubic coefficients in the surface profile of the free-form surface lens 7-1, the pupil can be modulated, so that the system has defocus invariance. The free-form surface lens 7-1 has the function of correcting system aberrations, and the system aberrations are reduced by changing the even-order coefficients of the free-form surface lens. In order to reduce the difficulty of processing the free-form surface, only low-order terms are used for aberration correction. The general expression of the 12th-order aspheric surface is:

[0021]

[0022] In the formula, z is the surface vector height, c is the surface curvature, k is the quadratic surface coefficient, α iIt is the coefficient of the i-th (i=2,3,4,5,6) term in the polynomial, x, y represent the position coordinates of the sampling point in the rectangular coordinate system, and r represents the position coordinates of the sampling point in the polar coordinate system.

[0023] The free-form surface shape is generally expressed by an XY polynomial:

[0024]

[0025] In the formula, c j is the jth coefficient in the polynomial, m, n represent the coefficients of higher order terms (m, n ≥ 0 and m, n are integers). The free-form surface lens 7-1 adopts x 3 Item and y 3 The phase modulation is performed using x 2 Item and y 2 The aberration correction is performed by the term, so the actual free-form surface shape expression is:

[0026]

[0027] Furthermore, the first doublet lens 7, the first positive lens 8, the second doublet lens 9 and the third doublet lens 10 form a relay lens group 6, and the relay lens group 6 is used to correct the aberration of the system.

[0028] Preferably, the surface parameters of the fundus camera based on free-form surface and insensitive to diopter disclosed in this embodiment are as follows: the radius R, i.e., the inverse of the curvature c, thickness d, and glass material of the aspheric front mirror 4, the first double cemented lens 7, the first positive lens 8, the second double cemented lens 9, and the third double cemented lens 10 are shown in Table 1, the aspheric surface coefficient α is shown in Table 2, and the free-form surface coefficient is shown in Table 3. The radius r, thickness d, glass material, and surface coefficient are also not limited to those described in Tables 1 to 3, and those skilled in the art can adjust them according to actual needs.

[0029] Table 1 Fundus camera structure parameters

[0030]

[0031]

[0032] Table 2 Aspheric surface parameters

[0033] surface R(mm) k <![CDATA[α2]]> <![CDATA[α3]]> <![CDATA[α4]]> <![CDATA[α5]]> <![CDATA[α6]]> 2 33.57 -1.36 2.7e-5 6.2e-7 -7.3e-9 6.5e-11 -3.7e-13 3 -53.91 -2.04 4.9e-5 2.3e-8 3.3e-9 2.5e-11 1.1e-13

[0034] Table 3 Free-form surface parameters

[0035] surface R(mm) k <![CDATA[c4]]> <![CDATA[c6]]> <![CDATA[c7]]> <![CDATA[c 10 ]]> 4 18.26 -2.46 1.34e-5 -2.3e-6 9e-5 9e-5

[0036] The optical path of the fundus camera based on free-form surface that is insensitive to diopter is as follows: the parallel light reflected by the fundus 1 and emitted through the pupil 3 is imaged onto the primary image plane 5 via the aspheric front mirror 4, and the pupil 3 is imaged onto the free-form surface lens in the first double-cemented lens 7 via the aspheric front mirror 4, that is, the free-form surface mirror 7-1 is at the conjugate position of the pupil of the fundus camera. The surface coefficient of the free-form surface 7-1 is set, wherein the cubic term assumes the phase modulation effect, and the quadratic term assumes the aberration correction effect. At this time, the fundus camera has defocus invariance. The relay lens group 6 images the primary image formed by the aspheric front mirror 4 to the detector 11, and the relay lens group 6 assumes the effect of correcting the aberration of the entire system. At this time, the detector 11 collects a blurred image after phase modulation, but this blurred image is not affected by the refractive error of the human eye, and a clear fundus image can be reconstructed using the Wiener filtering algorithm and the deconvolution algorithm.

[0037] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A refractive insensitive fundus camera based on a free-form surface, characterized in that: It includes a front mirror, a first double-cemented lens, a first positive lens, a second double-cemented lens, a third double-cemented lens and a detector; the first double-cemented lens includes a free-form surface lens and a first negative lens; the front mirror is an aspherical positive lens, which is used to collect parallel light emitted by the human eye and image the parallel light on a primary image plane; At the same time, the front mirror images the pupil onto the free-form surface lens in the first double-cemented lens, so that the free-form surface lens is located at the pupil of the system; the first double-cemented lens, the first positive lens, the second double-cemented lens and the third double-cemented lens image the fundus primary image on the primary image plane onto the detector; By changing the cubic coefficient of the free-form surface lens surface, the pupil can be modulated, so that the system has defocus invariance. The free-form surface lens has the function of correcting the system aberration. By changing the even-order coefficient of the free-form surface lens surface, the system aberration is reduced. In order to reduce the difficulty of processing the free-form surface, only low-order terms are used for aberration correction. The general expression of the 12th-order aspheric surface is: In the formula, z is the surface vector height, c is the surface curvature, k is the quadratic surface coefficient, α i is the coefficient of the i-th (i=2,3,4,5,6) term in the polynomial, x,y represent the position coordinates of the sampling point in the rectangular coordinate system, and r represents the position coordinates of the sampling point in the polar coordinate system; The free-form surface shape is generally expressed by an XY polynomial: In the formula, c j is the jth coefficient in the polynomial, m, n represent the coefficients of higher-order terms, m, n ≥ 0 and m, n are integers; the free-form surface lens 7-1 adopts x 3 Item and y 3 The phase modulation is performed using x 2 Item and y 2 The aberration correction is performed by the term, so the actual free-form surface shape expression is:

2. A fundus camera based on a free-form surface and insensitive to diopter as claimed in claim 1, characterized in that: The first double-cemented lens, the first positive lens, the second double-cemented lens and the third double-cemented lens constitute a relay lens group, and the relay lens group is used to correct the aberration of the system.

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