Method for air mounting of underwater planar window telecentric objective rear intercept

Abstract

The present application relates to the rear intercept adjustment, specifically relates to the rear intercept air adjustment method of underwater plane window fixed focus objective, in order to solve the rear intercept adjustment workload of prior art, influence the service life of watertight connector, underwater measurement object distance is difficult, and the resolution of underwater shooting resolution target is not high, resulting in the precision of rear intercept adjustment is reduced, the equivalent object distance obtained by calculation makes the whole rear intercept adjustment process can be carried out in air, and the pressure cylinder encapsulation of lens and camera is not needed after changing the adjustment spacer thickness each time, the workload in the rear intercept adjustment process is greatly reduced, and the process of underwater objective rear intercept adjustment is simplified.

Description

TECHNICAL FIELD

[0001] The present application relates to the rear intercept adjustment, in particular to the rear intercept air adjustment method of the underwater planar window fixed focus objective. BACKGROUND

[0002] Underwater optical imaging technology is an important technical means of underwater observation and detection. Compared with traditional underwater acoustic detection technology, it has the advantages of high imaging resolution, rich detection information and intuitive imaging effect. The core component of underwater optical imaging is underwater imaging objective. Considering the backscattering and underwater platform vibration, underwater imaging objective is mostly fixed focus objective, and the rear intercept between the lens and the camera is generally fixed and connected, thereby increasing the reliability and stability of the system. After the lens assembly of the fixed focus objective is completed, the rear intercept between the lens and the camera needs to be adjusted to reach the best imaging position. This process is called rear intercept adjustment of fixed focus objective. For fixed focus objective in air, the rear intercept adjustment can place a resolution target at a fixed object distance. By adding adjustment spacers of different thicknesses between the lens and the camera mounting interface, the rear intercept can be accurately adjusted, and the error can be reduced to 10 μm, thereby minimizing the defocus aberration on the camera target surface and ensuring the imaging resolution.

[0003] Because the deep sea water quality is good, the water attenuation coefficient can reach 0.05m -1 Therefore, the underwater imaging range can even reach 50m-100m, that is, the design object distance of the underwater fixed focus objective for deep sea can be greater than 50m. However, during laboratory adjustment, although the size of the large pool can reach 50m in length, the water in the pool is generally tap water, and the attenuation coefficient is generally 0.2m -1The target light energy of 50m in the water quality has been attenuated and cannot be detected by the camera, so the rear intercept adjustment of the underwater telephoto lens cannot be realized. The technical route of the rear intercept adjustment of the existing underwater plane window telephoto lens is to directly apply the adjustment method in the air to underwater. That is, a resolution target is placed at a fixed object distance underwater, a design given initial thickness of the adjustment spacer is added between the lens and the camera, the lens and the camera assembly are sealed in the pressure-resistant cylinder and placed underwater to take pictures of the resolution target. The resolution of the taken pictures is judged, if the resolution meets the design requirements, the rear intercept adjustment process is completed. If the taken pictures do not meet the design requirements, the thickness of the adjustment spacer needs to be increased or decreased. However, the process becomes very cumbersome and the work load is huge underwater. The basic conditions required for debugging are extremely high, and even difficult to achieve. Because the underwater telephoto lens and the camera must be water-tightly sealed first, that is, they are packaged in a pressure-resistant cylinder composed of a plane window, a metal side wall, a rear end cover and a water-tight connector, and then placed underwater to take pictures and detect the imaging resolution at a specific object distance underwater. The lens and camera assembly need to be taken out of the pressure-resistant shell each time the thickness of the adjustment spacer is increased or decreased. After changing the thickness of the spacer, the lens assembly is reassembled into the pressure-resistant cylinder through the above water-tight sealing process, and then placed underwater for the next round of shooting and resolution judgment after pressure sealing, until the resolution of the taken pictures meets the design requirements. This process will require tens or even hundreds of times of disassembly and assembly of the pressure-resistant cylinder and plugging of the water-tight connector, which not only has a large amount of work, but also reduces the service life of the water-tight connector. When adjusting the rear intercept underwater, the object distance between the underwater resolution target and the lens needs to be accurately measured, which is difficult and low in precision. In addition, the direct rear intercept adjustment underwater also faces the influence of back scattering on resolution judgment, because the clarity of the underwater resolution target pattern is affected by two factors: one is the insufficient imaging resolution due to the incorrect adjustment of the rear intercept, and the other is the target resolution decline caused by the scattering of the water body. Therefore, when the image is blurred, it is difficult to determine whether it is caused by back scattering or by the incorrect adjustment of the rear intercept, resulting in insufficient accuracy of the rear intercept adjustment. SUMMARY

[0004] The purpose of the present application is to solve the problems in the prior art, such as large amount of work for underwater rear intercept adjustment, affecting the service life of the water-tight connector, difficulty in measuring the object distance underwater, and low accuracy of the rear intercept adjustment due to low resolution of the underwater shooting resolution target, and to provide an air adjustment method for the rear intercept of the underwater plane window telephoto lens.

[0005] To achieve the above-mentioned purpose, the technical solution provided by the present application is as follows:

[0006] An air adjustment method for the rear intercept of an underwater plane window telephoto lens, characterized in that:

[0007] Step 1, calculate the equivalent object distance L in air by the following formula air ;

[0008]

[0009] where L water is the underwater object distance, d is the flat window thickness, d air is the air gap between the back surface of the flat window and the front surface of the first lens of the lens, n water is the refractive index of water, n win is the refractive index of the flat window.

[0010] Step 2, place the resolution target at a position L air in air from the front surface of the first lens of the lens without the flat window.

[0011] Step 3, adjust the back focal length of the lens to optimize the resolution of the resolution target at the position L air in air, and the back focal length at this time is the optimal back focal length of the underwater design object distance L water .

[0012] Further, it further comprises step 4:

[0013] Put the adjusted lens into the pressure-resistant chamber and place it in water to image the resolution target at the object distance L water and verify the imaging resolution under the back focal length.

[0014] Advantages of the present application:

[0015] 1. The present application proposes a method for adjusting the back focal length of an underwater flat window fixed focus objective lens in air based on the basic theory of optical imaging. The equivalent object distance obtained by calculation enables the entire back focal length adjustment process to be carried out in air, and there is no need to reseal the lens and camera in the pressure-resistant cylinder after changing the thickness of the adjustment spacer each time, which greatly reduces the workload in the back focal length adjustment process, and there is no need to accurately measure the resolution target object distance underwater, and there is no need for a water pool, which simplifies the process of adjusting the back focal length of the underwater objective lens, and can completely eliminate the influence of water scattering on the resolution of the imaging lens, ensuring the adjustment accuracy of the back focal length of the underwater flat window fixed focus objective lens and the imaging resolution.

[0016] 2. The present application places adjustment work in the air, which can reduce the adjustment difficulty and adjustment workload of the underwater flat window optical lens, avoid the multiple disassembly and assembly of the sealed shell during the adjustment process, and improve the assembly efficiency of the underwater optical lens; at the same time, the problem of insufficient water quality conditions of the experimental pool during the assembly of the deep-sea long-distance imaging lens is solved, the equivalent object distance adjustment in the air is avoided, the high requirement for water quality is avoided, the problem of inaccurate measurement of resolution target distance under water is avoided, and the adjustment precision of the underwater lens is improved; the adjustment in the air can also avoid the influence of water scattering on the resolution, the rear intercept adjusted is completely from the change of the object distance, and the adjustment precision of the rear intercept is improved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a position diagram of the object side main plane B when the flat window is not arranged in the embodiment of the present application;

[0018] Figure 2 is a position diagram of the object side main plane D when the flat window is arranged in the embodiment of the present application;

[0019] Figure 3 is a structure diagram of the underwater lens with the flat window arranged in the embodiment of the present application;

[0020] Figure 4 is a transfer function diagram of the underwater lens with the flat window arranged in the embodiment of the present application;

[0021] Figure 5 is a structure diagram of the lens without the flat window placed in the air at the equivalent object distance in the embodiment of the present application;

[0022] Figure 6 is a transfer function diagram of the lens without the flat window placed in the air at the equivalent object distance in the embodiment of the present application. DETAILED DESCRIPTION

[0023] The underwater flat window fixed focus objective rear intercept air adjustment method provided by the present application is described in detail below in combination with specific embodiments and drawings.

[0024] From the paraxial optical Gauss formula, it can be known that:

[0025]

[0026] Wherein, l' is the image distance, indicating the distance from the image-side principal plane to the image plane, l is the object distance, indicating the distance from the object-side principal plane to the object plane, f' is the image-side focal length, n is the object-side refractive index, and n' is the image-side refractive index. When the paraxial parallel light is incident, the intersection of the reflection extension line of the light ray exiting from the last lens of the lens and the parallel light and the plane perpendicular to the optical axis is the image-side principal plane. The parallel light is incident from the image side, exits from the front surface of the first lens of the lens, and the reverse extension line of the exiting light ray intersects with the incident parallel light at a point, and the plane perpendicular to the optical axis passing through the point is the object-side principal plane.

[0027] When the parallel light is incident to the lens from the object side, since the parallel light is perpendicular to the interface of water and the plane window when passing through the water and the plane window, no light deflection occurs. For the two cases that the object space is water plus a plane window and the object space is air without a plane window, the incident angle of the incident parallel light on the front surface of the first lens of the lens is the same, the incident height is also the same, the same lens is passed through, the angle and the exit height of the light ray exiting from the last lens are also the same, so that the intersection position of the parallel light and the reverse extension line of the light ray exiting from the last lens is also the same. Therefore, the image-side principal plane position H' and the image-side focal length f' in the case that the object space is water plus a plane window and the case that the object space is air without a plane window are unchanged, so that the lens is not provided with a plane window in front of the lens during the air adjustment.

[0028] The rear intercept adjustment is to find the best rear intercept L', so that the optical lens can clearly image on the camera chip. The rear intercept L' is the difference between the image distance and the image-side principal plane position, that is:

[0029] L' = l' - H'

[0030] In order to reduce the underwater optical lens rear intercept adjustment workload and achieve air adjustment, firstly, the equivalent object distance of the air lens without a plane window which is the same as the underwater lens with a plane window is calculated. Then, the lens without a plane window is placed in the air, the resolution target is placed at the equivalent object distance position, the rear intercept is adjusted, the resolution target at the equivalent object distance position is clearly imaged on the camera chip, and then the lens with a plane window is placed underwater, which can clearly image the objects at the designed object distance under water.

[0031] In combination with Figure 1 and Figure 2 , the rear intercept L' of the fixed-focus objective lens under water water and the rear intercept L a ' ir of the fixed-focus objective lens in air respectively satisfy the following relationships:

[0032] L' water = l' water -H' water

[0033] La ′ ir = a ′ ir -H a ′ ir

[0034] wherein, l′ water is the underwater image distance, H′ water is the underwater image side principal plane E position, l a ′ ir is the image distance in air, H a ′ ir is the air image side principal plane C position.

[0035] The condition for clear imaging under water after the completion of the air adjustment is that the back intercepts in air and under water are the same, i.e. L′ water = L a ′ ir From the foregoing analysis, the air image side principal plane C and the underwater image side principal plane E position are unchanged, i.e. H′ water = H a ′ ir . Therefore, the image distance in air and under water is the same, i.e.:

[0036] l′ water = l a ′ ir

[0037] The Gaussian formulas in air and under water are respectively:

[0038]

[0039] wherein, n water is the water refractive index, n air is the air refractive index, l water is the underwater object plane to object side principal plane D distance, l air is the air object plane to object side principal plane B distance, f w ′ ater is the underwater image side focal length, f a ′ ir is the air image side focal length;

[0040] From the above two formulas, we can get:

[0041]

[0042] wherein, L water is the underwater object distance, i.e. the plane window front surface to the object plane distance, H water is the underwater object side principal plane D position, i.e. the plane window to the object side principal plane D distance;

[0043] At the same time, according to l air =L air -H air The calculation yielded:

[0044]

[0045] Among them, L air H is the equivalent object distance in air, which is the distance from the first lens element to the object plane. air The position of the object principal plane B in the air is the distance from the first lens to the object principal plane B.

[0046] like Figure 1 and Figure 2 As shown, according to paraxial trigonometric relations and the law of refraction of light, we know that:

[0047]

[0048] Among them, u air Let f be the angle between the ray in air and the optical axis, h be the incident height of the parallel ray incident from the image side, and f be the angle between the ray in air and the optical axis. air U is the object-side focal length in air without a plane window. win Let n be the angle between the light ray in the glass and the optical axis. win For the refractive index of the planar window, u water The incident angle of parallel light underwater;

[0049] The longitudinal deflection Δh of the incident light within the plane window is:

[0050]

[0051] Where d is the thickness of the planar window;

[0052] The ray deflection h1 of the incident light between the object-side focal plane A and the rear surface of the plane window is:

[0053]

[0054] Where, d air The air gap between the rear surface of the plane window and the front surface of the first lens element;

[0055] The total ray deflection h of the underwater incident light from the principal plane D behind the plane window to the front surface of the window all for:

[0056]

[0057] The distance from the main object plane D to the front surface of the planar window after setting the planar window can be calculated using the following formula, i.e., the position H of the main object plane D underwater. water ;

[0058]

[0059] H water The equivalent distance L brought into the air air From the expression, the equivalent object distance L in air after removing the planar window from the underwater imaging lens can be calculated. air for:

[0060]

[0061] Therefore, for an underwater object distance of L water The thickness of the planar window is d, and the air gap between the rear surface of the planar window and the front surface of the first lens element is d. air For underwater objectives, during back focus adjustment, the distance between the objective lens and the front surface of the first lens element can be directly measured in air. Place the target at the desired resolution, adjust the lens back focus to achieve the optimal resolution at that position, and the back focus at that position will be the optimal back focus corresponding to the designed underwater target distance.

[0062] Specifically, in this embodiment, the planar window is a sapphire window with a thickness of 25mm. The air gap between the rear surface of the planar window and the front surface of the first lens is 2mm. The designed underwater objective lens has an underwater object distance of 6401.763mm and a back focal length of 13.991mm. The refractive index of the planar window is 1.771763, and the refractive index of water is 1.341467. Its lens structure and transfer function are as follows: Figure 3 , Figure 4 As shown. By Figure 4 As shown in the transfer function graph, the lens designed can clearly image a target at a distance of 6401.76mm underwater, with an imaging transfer function of 0.8@200lp / mm.

[0063] Following the steps in this invention, the equivalent object distance in air is first calculated. Then, with the sapphire window removed and the object placed in air, the equivalent object distance is calculated using the following formula.

[0064]

[0065] Based on the above calculations, removing the planar window, a high-resolution target is placed at an equivalent object distance of 4788.32 mm in air, and the back intercept is adjusted to achieve the clearest image. The back intercept at this point is the same as the back intercept for achieving a clear image of a 6401.76 mm target underwater with the planar window added. It should be noted that this method is applicable to various types of incident light in practical applications and is not limited to parallel light.

[0066] For ease of explanation, the object medium in the optical simulation software is set to air, the window is removed, and the object distance is set to 4788.321 mm. The optical structure diagram at this time is as follows. Figure 5 As shown, the back intercept at this time is 13.991 mm, as... Figure 6 As shown, the transfer function of the imaging can still reach 0.8@200lp / mm, thus the simulation verifies the practicality of the present invention.

Claims

1. An air adjustment method for the back focal length of an underwater flat window objective lens, characterized in that: Step 1, the equivalent distance L in air is calculated by the following formula air ; where L water is the underwater object distance, d is the flat window thickness, d air is the air separation between the back surface of the flat window and the front surface of the first lens element of the lens, n water is the refractive index of the water, n win is the refractive index of the flat window; Step 2, place a resolution target at a distance L from the first lens front surface in air without a flat window in place; air Step 2, place a resolution target at a distance L from the first lens front surface in air without a flat window in place; Step 3, adjust the back focal length of the lens, the resolution of the resolution target in the air L air position is adjusted to the best, then the back focal length position at this time is the optimal back focal length of underwater design L water .

2. The method for adjusting the back intercept of an underwater planar window fixed-focus objective lens in air according to claim 1, characterized in that, Further comprising step 4: The adjusted lens is mounted in the pressure-proof housing, placed in water, and at object distance L water The imaging resolution at the back intercept is verified by imaging the resolution target at the L position.

Images