A method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer

Abstract

The present invention discloses a method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer. First, a measurement optical path for the transmitted wavefront of the infrared lens is built, and a Mask1 for delineating the fringes of the transmitted wavefront and a Mask2 for the entire field of view are collected. The key lies in the accuracy of delineating the Mask and the resolution of the detector. Parameters such as the image size of the interference fringes and the image size of the field of view are extracted using interference measurement software or image processing methods. This method is implemented on an infrared interferometer and has the characteristics of low additional cost, fast speed, and convenience. The measurement error is better than 2%. It can be applied to the design, manufacture, and use of infrared lenses, providing important support for the development and application of infrared optical technology.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular to a method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer. Background Art

[0002] For the evaluation of the imaging quality of an infrared optical system, not only requirements are put forward for the surface shape and aberration of the infrared lens, but also the actual focal length of the infrared lens has important guiding significance for building an infrared optical system.

[0003] In the visible light range, common methods for measuring the focal length include: magnification method, precision goniometry method, Abbe focal length meter method, etc. The above methods are based on the principles of geometric optics and are for visible light. According to the principles of physical optics, methods such as the Talbot effect and single-slit diffraction can also be used to measure the monochromatic focal length of a lens. Most of these methods use commercially available CCDs, etc. as photodetectors. In the infrared band, especially in the mid- and far-infrared bands, since infrared light is invisible, the visible light methods lose their universality. And most of the instruments used for measuring the focal length of infrared lenses are expensive and not widely used.

[0004] The basic imaging principle of an interferometer is to use the interference phenomenon of light to form an interference image, and then image the interference image into a detector through a telescopic system. When using an interferometer to measure a spherical mirror, common measurement methods can obtain the radius of curvature, surface shape error, etc. of the spherical lens, but there is a certain blank in the measurement of the focal length of the spherical lens, and one-stop measurement cannot be achieved.

[0005] In view of the above problems, and considering that an infrared interferometer is an essential device for testing the aberration of an infrared lens and the uniformity of an infrared material, there is an urgent need for a new infrared focal length measurement method and device based on an infrared interferometer to meet the requirements of low additional cost, fast and convenient measurement of the focal length of an infrared lens. Summary of the Invention

[0006] The present invention proposes a method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer, which is simple and easy to operate, and realizes the measurement of the focal length of an infrared lens without additional cost.

[0007] The technical solution for realizing the present invention is as follows: A method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer, the steps are as follows:

[0008] Step 1: Build a measurement system:

[0009] The measurement system includes a Fizeau infrared interferometer, an infrared standard lens, a linear guide rail, a holder, a five-axis adjustment frame, a standard plane mirror, a two-dimensional adjustment frame, and a diaphragm.

[0010] A linear guide rail is set on the test bench. The Fizeau infrared interferometer is set on the test bench and located at one end of the linear guide rail. The standard plane mirror is fixed on the two-dimensional adjustment frame, and the two-dimensional adjustment frame is fixed at the other end of the linear guide rail.

[0011] Step 2: Turn on the Fizeau infrared interferometer, debug the standard plane mirror, and adjust its pitching and tilting postures to achieve the state where the light spot fills the field of view.

[0012] Step 3: Set the five-axis adjustment frame on the linear guide rail, and then fixedly connect the infrared lens to be measured to the five-axis adjustment frame through a clamp.

[0013] Step 4: Select the infrared standard lens equipped with the Fizeau infrared interferometer, its F number is F1, calculate the cat's-eye position Cp through its F number and aperture, place the infrared lens to be measured at Cp, and adjust the pitch of the infrared standard lens to make the number of fringes the least.

[0014] Step 5: Adjust the pitching and tilting postures of the infrared lens to be measured through the five-axis adjustment frame to keep the interference fringes in the center of the field of view.

[0015] Step 6: If the focal length of the infrared lens to be measured is positive, move backward from Cp, otherwise move forward. If the fringes or light spots shift, return to Step 5 until the phenomena of the fringes bending, straightening, and bending appear in sequence.

[0016] Step 7: Place the aperture closely against the front surface of the infrared lens to be measured, and circle the measured area Mask 1 and the field of view area Mask 2 on the interference measurement software, and obtain the pixel diameter value d 1 of the measured area Mask 1 , and the pixel diameter value d 2 of the field of view area Mask 2 .

[0017] Step 8: Calculate the focal length f of the infrared lens to be measured as:

[0018]

[0019] where f 1 is the aperture number of the infrared standard lens, and D 0 is the aperture diameter of the aperture.

[0020] Compared with the prior art, the remarkable advantages of the present invention are as follows:

[0021] (1) Based on the construction of the transmission wavefront measurement optical path of the optical system, the present invention can measure the quality of the transmission wavefront of the infrared lens while measuring the focal length of the infrared lens by using the infrared Fizeau interferometer.

[0022] (2) After the mirror calibration of the system is completed, the same batch of lenses can be quickly measured. For lenses of different batches, only the front and rear positions of the five-axis adjustment frame need to be moved to measure again. The structure is simple and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flowchart of the method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to the present invention.

[0024] Figure 2 This is a schematic diagram of the self-collimation calibration of the mirror in the embodiment.

[0025] Figure 3 This is a schematic diagram of the calibration of the outgoing light in the embodiment.

[0026] Figure 4 This is a schematic diagram of the positioning of the telescopic system in the embodiment.

[0027] Figure 5 This is a schematic diagram of Mask circle drawing.

[0028] Figure 6 This is a schematic diagram of realizing batch focal length measurement by adding a known aperture diaphragm. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] It should be noted that all directional indications (such as up, down, left, right, front, back...) in the embodiments of the present invention are only used to explain the relative position relationship and movement conditions between components in a specific posture (as shown in the drawings). If the specific posture changes, the directional indications will also change accordingly.

[0031] In addition, the descriptions such as "first" and "second" in the present invention are only for descriptive purposes, and cannot be understood as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of such features. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0032] In the present invention, unless otherwise clearly specified and defined, terms such as "connection" and "fixation" shall be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or integrated; "connection" can be a mechanical connection or an electrical connection. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0033] In addition, the technical solutions between various embodiments of the present invention can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0034] Next, the specific implementation manners, as well as the technical difficulties and inventive points of the present invention, will be further introduced in combination with the design examples.

[0035] Infrared lenses have important applications in infrared imaging, medical diagnosis, and industrial inspection. Generally, the quality information of an infrared lens is judged by separately detecting its surface shape and focal length. Although this can achieve high precision, it is time-consuming and requires a variety of detection devices. Therefore, in order to reduce the additional cost of measurement, the infrared lens to be measured is placed in the optical path of an infrared interferometer, and while measuring the transmitted wavefront of the infrared lens with a spherical wave, its focal length is calculated, reducing the additional cost and operation difficulty of the measurement and realizing one-stop measurement of the infrared lens.

[0036] Combined with Figures 1 to 6 , a method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to the present invention is as follows:

[0037] Step 1: Set up the measurement system:

[0038] The measurement system includes a Fizeau infrared interferometer 1, an infrared standard lens 2, a linear guide rail 3, a gripper 4, a five-axis adjustment mount 5, a standard plane mirror 6, a two-dimensional adjustment mount 7, and a diaphragm.

[0039] The linear guide rail 3 is set on the test bench. The Fizeau infrared interferometer 1 is set on the test bench and is located at one end of the linear guide rail 3. The standard plane mirror 6 is fixed on the two-dimensional adjustment mount 7, and the two-dimensional adjustment mount 7 is fixed at the other end of the linear guide rail 3.

[0040] Among them, the aperture of the two-dimensional adjustment mount 7 is 150 mm, and the center height of the diaphragm matches the measurement system.

[0041] Step 2: Turn on the Fizeau infrared interferometer 1 and debug the standard plane mirror 6, and adjust its pitching and tilting postures to achieve the state where the light spot fills the field of view.

[0042] Step 3: Set the five-axis adjustment frame 5 on the linear guide rail 3, and then fix the infrared lens 8 to be tested to the five-axis adjustment frame 5 through the clamp 4.

[0043] Step 4: Select the infrared standard lens 2 equipped with the Fizeau infrared interferometer 1, whose F number is F1, calculate the cat's eye position Cp according to its F number and aperture, and place the infrared lens 8 to be tested at Cp, and adjust the pitch of the infrared standard lens 2 to minimize the number of fringes.

[0044] Step 5: Adjust the pitch and tilt posture of the infrared lens 8 to be tested by the five-axis adjustment frame 5 so that the interference fringes are maintained in the center of the field of view.

[0045] Step 6: If the focal length of the infrared lens 8 to be tested is positive, move backward from Cp, otherwise move forward. If the stripes or spots are offset, return to step 5 until the stripes are curved, straight, and curved in sequence. This is the difficulty of the design, and the adjustment needs to be completed within the field of view, that is, it is necessary to return to step 5 for adjustment while moving.

[0046] At this point, the entire system is built, which is consistent with the system's transmission wavefront measurement optical path, and can simultaneously measure the transmission wavefront of the infrared lens 8 to be tested.

[0047] Furthermore, when the focal length of the infrared lens 8 to be measured is positive, the measuring system is a Kepler telescope system; when the focal length is negative, the measuring system is a Galilean telescope system.

[0048] Step 7: Place the aperture close to the front surface of the infrared lens 8 to be tested, and circle the tested area Mask on the interferometry software. 1 And the field of view area Mask 2 , get the Mask of the tested area 1 The pixel diameter d 1 , field of view area Mask 2 The pixel diameter d 2 , as follows:

[0049] Statistics Mask 1 The total number of pixels with valid values ​​is sum, which is used as Mask 1 The diameter d is calculated based on the area formula of a circle. 1 :

[0050]

[0051] Similarly, the field of view area Mask can be calculated 2 The pixel diameter value d 2 .

[0052] Step 8: Calculate the focal length f of the infrared lens 8 to be tested as:

[0053]

[0054] Among them, f 1 is the aperture number of the infrared standard lens 2, and D 0 is the aperture diameter.

[0055] Example 1:

[0056] The infrared interferometer is a mid-wave infrared interferometer with a quantum cascade infrared laser (wavelength 3.39 μm) as the light source; the material of the infrared lens to be measured is CaF2, with an aperture of Φ150 mm and a theoretical focal length of 320 mm. Therefore, an F / 3.3 infrared standard lens that can cover its relative aperture requirements is selected.

[0057] Combined with Figures 1 to 6 , a method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer includes the following steps:

[0058] Step 1, set up the test system. Set a linear guide rail 3 on the test bench. The Fizeau infrared interferometer 1 is set on the test bench and located at one end of the linear guide rail 3. The standard plane mirror 6 is fixed on the two-dimensional adjustment frame 7, and the two-dimensional adjustment frame 7 is fixed at the other end of the linear guide rail 3.

[0059] Step 2, calibrate the plane mirror. Turn on the Fizeau infrared interferometer 1 and debug the standard plane mirror 6 to adjust its pitching and tilting postures to achieve the state where the light spot fills the field of view.

[0060] Step 3, set the five-axis adjustment frame 5 on the linear guide rail 3, and then fixedly connect the infrared lens 8 to be measured with the five-axis adjustment frame 5 through the gripper 4.

[0061] Step 4, system adjustment. Select the infrared standard lens 2 equipped with the Fizeau infrared interferometer 1, with its F number being F1 = 3.3 and the aperture being 150 mm. Calculate the cat's eye position Cp to be 495 mm in front of the interferometer, and place the infrared lens 8 to be measured at Cp. Adjust the pitch of the infrared standard lens 2 to minimize the number of fringes.

[0062] Step 5, adjust the pitching and tilting postures of the infrared lens 8 to be measured through the five-axis adjustment frame 5 to keep its interference fringes in the center of the field of view.

[0063] Step 6, move the measured part to the set conditions. If the focal length of the infrared lens 8 to be measured is positive, move it backward from Cp, otherwise move it forward. If the fringes or light spots shift, return to Step 5 until the phenomena of the fringes bending, straightening, and bending appear in sequence.

[0064] Step 7, draw the Mask and set the parameters of the measured lens. Draw the Mask graphics of the field of view and the fringes on the interference measurement software. The field of view diameter d 2= 358 pixels, stripe diameter d 1 = 94 pixels, aperture number f of the infrared standard lens 1 = 3.3 and the actual effective aperture (diaphragm) D of the lens under test 0 = 25.4 mm.

[0065] Step 8. Use the formula f = d 1 / d 2 f 1 D 0 The calculated value is the focal length f = 318.5 mm of the infrared lens to be tested, with an actual error of less than ±2 mm, which has a certain feasibility.

Claims

1. A method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer, characterized in that, the steps are as follows: Step 1, set up the measurement system: The measurement system includes a Fizeau infrared interferometer (1), an infrared standard lens (2), a linear guide (3), a holder (4), a five-axis adjustment frame (5), a standard plane mirror (6), a two-dimensional adjustment frame (7), and a diaphragm; Set the linear guide (3) on the test bench. The Fizeau infrared interferometer (1) is set on the test bench and located at one end of the linear guide (3). The standard plane mirror (6) is fixed on the two-dimensional adjustment frame (7), and the two-dimensional adjustment frame (7) is fixed at the other end of the linear guide (3); Step 2, turn on the Fizeau infrared interferometer (1), debug the standard plane mirror (6), and adjust its pitch and tilt attitude to achieve the state where the light spot fills the field of view; Step 3, set the five-axis adjustment frame (5) on the linear guide (3), and then fixedly connect the infrared lens to be measured (8) with the five-axis adjustment frame (5) through the holder (4); Step 4: Select the infrared standard lens (2) equipped with the Fizeau infrared interferometer (1), with an F-number of f 1 , calculate the cat's eye position Cp based on its F-number and aperture, place the infrared lens to be measured (8) at Cp, and adjust the pitch of the infrared standard lens (2) to minimize the number of fringes; Step 5, adjust the pitch and tilt attitude of the infrared lens to be measured (8) through the five-axis adjustment frame (5) to keep the interference fringes in the center of the field of view; Step 6, if the focal length of the infrared lens to be measured (8) is positive, move backward from the Cp position, otherwise move forward. If the fringes or the light spot shift, return to Step 5 until the phenomena of the fringes bending, straightening, and bending appear in sequence; Step 7: Place the diaphragm closely against the front surface of the infrared lens (8) to be measured, and draw the measured area Mask on the interference measurement software 1 and the field of view area Mask 2 , and obtain the pixel diameter value d of the measured area Mask 1 , and the pixel diameter value d of the field of view area Mask 1 ; 2 ; 2 ; Step 8, calculate the focal length f of the infrared lens to be measured (8) as: Among them, f 1 is the aperture number of the infrared standard lens (2), and D 0 is the aperture diameter.

2. The method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to claim 1, characterized in that: The aperture of the two-dimensional adjustment frame (7) is 150 mm.

3. The method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to claim 1, characterized in that: When the focal length of the infrared lens to be measured (8) is positive, the measurement system is a Keplerian telescope system; when the focal length is negative, the measurement system is a Galilean telescope system.

4. The method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to claim 1, characterized in that: The center height of the diaphragm matches the measurement system.

5. The method for measuring the focal length of an infrared lens using a Fizeau infrared interferometer according to claim 1, characterized in that: In step 7, place the diaphragm closely against the front surface of the infrared lens (8) to be measured, and draw the measured area Mask on the interference measurement software 1 and the field of view area Mask 2 , and obtain the pixel diameter value d of the measured area Mask 1 , and the pixel diameter value d of the field of view area Mask 1 , specifically as follows: 2 2 ​​ Statistical Mask 1 The total number of valid pixels in it is sum, which is used as the Mask 1 area. The diameter d is calculated according to the area formula of a circle 1 : Similarly, the Mask of the field of view area is calculated 2 and the pixel diameter value d 2 is obtained.

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