Neutral density optical filter and low-light night vision imaging system

Abstract

The invention discloses a neutral density optical filter and a low-light night vision imaging system. One side face of the optical filter is plated with a metal film of 400-630 nm, and OD of the comprehensive passing rate of 400-630 nm meets the following conditional expression: 0.1 < = OD < = 4.0; in the formula, OD is the optical density value of the optical filter; and the other side surface is plated with an IR infrared cut-off film, and the infrared cut-off film forms a cut-off band in a 700-1000nm infrared region of a transmission band of a 420-630nm visible light region. The neutral-density optical filter provided by the invention is in the shape of a flat plate, can be used in a visible light broadband, has a spectral range of 400-630nm, and is uniform in wavelength attenuation and low in power consumption. The light intensity energy in a daytime scene can be effectively reduced, and normal exposure is realized. One side of the neutral density optical filter is plated with a visible light metal film of 400-630 nm, one side of the neutral density optical filter is plated with an IR CUT film, spectrum light within the range of 700-1100 nm is cut off, the influence of infrared light on imaging is filtered out, the image color is normal, and the reddish phenomenon is avoided.

Description

Technical Field

[0001] The invention relates to the technical field of filters for low-light-level night vision imaging, and in particular to a neutral density filter and an optical system. Background Art

[0002] Low-light-level night vision imaging system is an imaging technology used at night or in very weak lighting conditions. It mainly relies on low-light-level image intensifiers to enhance the weak light in the environment through photoelectric conversion, electron multiplication and other means to achieve nighttime observation and target recognition. Existing low-light-level night vision imaging systems have at least one of the following disadvantages:

[0003] 1) The low-light-level night vision imaging system does not have a neutral density filter. Due to the high-light design of the low-light-level night vision system, it is easy to produce overexposure when used in a well-lit environment such as daytime or dusk. The strong light damages the low-light-level night vision camera sensor, greatly reducing the service life of the sensor and reducing the imaging effect;

[0004] 2) The low-light-level night vision imaging system does not have a neutral density filter. Since the low-light-level night vision lens uses a wide-spectrum coating, the image is prone to reddish color when used during the day or in the evening, which seriously interferes with target observation;

[0005] 3) The existing neutral density filter coating range is large and cannot effectively match the daytime use environment of the low-light night vision imaging system;

[0006] 4) Existing neutral density filters are designed individually without considering the comprehensive impact of low-light-level night vision imaging systems. Summary of the invention

[0007] In view of this, the object of the present invention is to provide a neutral density filter and an optical system. The filter can solve at least one technical disadvantage mentioned in the background technology.

[0008] According to one aspect of the present invention, a neutral density filter is provided, wherein one side of the filter is plated with a 400-630 nm metal film to achieve a comprehensive pass rate of 400-630 nm OD that satisfies the following conditional formula:

[0009] 0.1≤OD≤4.0

[0010] Wherein, OD is the optical density value of the filter;

[0011] The other side is coated with an infrared cut-off film, which forms a cut-off band in the transmission band of the visible light region of 420 to 630 nm and the infrared region of 700 to 1000 nm.

[0012] In the above technical solution, the neutral density filter is in the shape of a flat plate, and is used for broadband visible light, with a spectral range of 400 to 630 nm, uniform wavelength attenuation, and low power consumption. It can effectively reduce the light intensity energy in daytime scenes and achieve normal exposure. One side of the neutral density filter is coated with a visible light 400 to 630 nm metal film, and the other side of the neutral density filter is coated with an IR CUT film, which cuts off the spectrum light in the range of 700 to 1100 nm, filters out the influence of infrared light on imaging, and the image color is normal, avoiding the reddish phenomenon. The neutral density filter can be a visible light metal film or an absorption type. In terms of hardware, the low-light night vision system is designed to cover a wavelength of 400 to 1100 nm, which can support daytime imaging. In some daytime occasions, it is necessary to use a low-light night vision system to detect a longer distance and save pictures and video information in real time. However, in order to adapt to the use of weak light at night, the existing low-light night vision is designed to have a large light transmission, which is prone to overexposure when used during the day. Therefore, in response to the all-weather use requirements of the low-light-level night vision system, this case proposes a filter that is mainly used in the use scenarios of the low-light-level night vision system during the day, so as to avoid the phenomenon of overexposed images with a reddish cast when using the device. There is also an existing solution that uses the object side of the CMOS to stack protective lenses, low-pass filters, and infrared cutoff filters in sequence, but this solution needs to consider the aberration effect of the filter in the design stage, and the optical system and the filter need to be optimized and designed together. Therefore, the versatility of the filter is limited, and different optical systems cannot directly use the filter, otherwise the image will become blurred. If software is used for correction, digital image processing requires a certain amount of time. The effect is poor for some occasions with strange shapes and different strengths, and the software stability is not higher than the hardware stability. In some embodiments, the metal film adopts a visible light metal film, and the base material of the filter satisfies the following conditional formula:

[0013] 1.4 <Nd<1.6;40<Vd<70

[0014] Wherein, Nd is the refractive index of the filter, Vd is the dispersion coefficient of the filter; the base material is float glass or D-263T.

[0015] In some embodiments, the metal film is an absorption type metal film, and the base material is neutral gray glass.

[0016] In the above technical solution, the visible light metal film neutral density filter is a reflective neutral density filter, which adopts the principle of thin film interference, transmits part of the light of 400-630nm, and reflects the light after 700nm, and the material is optical float glass. When the filter is an absorption neutral density filter, the material itself or after certain elements are mixed in the material, absorbs some specific wavelengths of light, and has no or little effect on other wavelengths of light. The application wavelength range is 400-630nm, and the material is neutral gray glass.

[0017] In some embodiments, the filter has a light clearance greater than 90%.

[0018] In the above technical solution, the effective aperture of the neutral density filter is greater than 90%, and the working area is wide, which can meet the needs of incident light for the low-light-level night vision optical system.

[0019] According to another aspect of the present invention, a neutral density filter device is provided, the device comprising: a mounting seat and a neutral density filter as described above mounted on a lens frame.

[0020] In the above technical solution, the advantages of the neutral density filter device depend on the neutral density filter, which will not be described in detail here.

[0021] In some embodiments, a filter mounting slot is provided inside the filter frame, the filter is installed in the filter mounting slot, an inner pressure ring is used to tighten and fix the filter, and glue is dispensed around the left edge of the pressure ring and cured.

[0022] In the above technical solution, it can better adapt to various harsh and strong vibration environments, effectively solve the possibility of loosening of the filter, and greatly improve the vibration resistance, stability, and reliability of the filter.

[0023] In some embodiments, the filter satisfies the following condition:

[0024] 20mm <D<90mm;0.5mm<T<4.0mm

[0025] Where D is the outer diameter of the neutral density filter and T is the core thickness of the neutral density filter.

[0026] In the above technical solution, the external dimensions of the neutral density filter satisfy the following relationship, which further reduces the weight and volume while ensuring the strength of the lens.

[0027] According to another aspect of the present invention, there is provided a filter switch, wherein the filter switch comprises at least one neutral density filter as described above.

[0028] In the above technical solution, the advantages of the neutral density filter device depend on the neutral density filter, which will not be described in detail here.

[0029] According to another aspect of the present invention, there is provided an optical system including an imaging lens, and an image sensor that receives light from the imaging lens and performs photoelectric conversion;

[0030] The optical system also includes a neutral density filter device or a filter switch.

[0031] In the above technical solution, the advantages of the neutral density filter device depend on the neutral density filter, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 It is a structural schematic diagram of a neutral density filter of the present invention;

[0034] Figure 2 It is a schematic diagram of the film system structure of a neutral density filter of the present invention;

[0035] Figure 3 is a simulation diagram of a spectral transmittance curve of Example 2 of a neutral density filter of the present invention;

[0036] Figure 4 It is a structural schematic diagram of a neutral density filter device of the present invention;

[0037] Figure 5 It is a structural schematic diagram of a filter switcher of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be further described in detail below in conjunction with the accompanying drawings and examples. It is particularly noted that the following examples are only used to illustrate the present invention, but are not intended to limit the scope of the present invention. Similarly, the following examples are only partial embodiments of the present invention rather than all embodiments, and all other embodiments obtained by those of ordinary skill in the art without creative work are within the scope of protection of the present invention.

[0039] The object of the present invention is to provide a neutral density filter and an optical system with high optical performance. Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0040] Embodiment 1

[0041] See also Figure 1 A neutral density filter, the filter 1 is made of float glass, and the use of this optical material is reliable, stable and low-cost. One side of the filter 1 is coated with a 400-630nm metal film 2, and the other side is coated with an infrared cut-off film 3; the neutral density filter is in the shape of a flat plate, and is used for broadband visible light, with a spectral range of 400-630nm, uniform wavelength attenuation, and low power consumption. It can effectively reduce the light intensity energy in scenes with sufficient light such as daytime and evening to achieve normal exposure. One side of the neutral density filter is coated with a visible light 400-630nm metal film, and the other side of the neutral density filter is coated with an IR CUT film, which cuts off the spectrum light in the range of 700-1100nm, filters out the influence of infrared light on imaging, and the image color is normal, avoiding the reddish phenomenon. The neutral density filter can be made of a visible light metal film or an absorption type.

[0042] See also Figure 2 , the metal film is composed of alternating low refractive index layers and high refractive index layers, and a metal layer is arranged in the middle of the film layer; metal materials such as Ti and other absorbent materials are added in the middle of the metal layer to achieve a high absorption and low surface reflection effect film structure. The advantage of the film layer is that it can achieve specific transmittance requirements of 400-630nm. By adding the metal material titanium, the absorption of light is increased. The more layers of metal material titanium and the thicker the thickness, the more light energy is absorbed by the film layer. Different transmittance requirements for visible light are achieved by different film layers and thicknesses. The refractive index of the low refractive index layer is 1.43-1.49; the refractive index of the high refractive index layer is 2.0-2.5; the refractive index of the metal layer is 1.9-2.3. The infrared cut-off film structure is composed of alternating low-refractive index layers and high-refractive index layers. The refractive index of the panel is equal to 1.45-1.55, the refractive index of the low-refractive index layer is equal to 1.43-1.49, and the refractive index of the high-refractive index layer is equal to 2.0-2.5. By optimizing the thickness of each layer of the film structure and matching 30-50 layers, the optimized film structure forms a cut-off band in the transmission band of the visible light region of 420-630nm and the infrared region of 700-1000nm.

[0043] A neutral density filter in this embodiment can satisfy at least one of the following setting conditions 1) to 5):

[0044] 1) 0.1≤OD≤4.0;

[0045] 2)1.4 <Nd<1.6;40<Vd<70;

[0046] 3) The filter aperture is greater than 90%;

[0047] In the above conditional formula, OD is the optical density value of the filter; Nd is the refractive index of the filter, and Vd is the dispersion coefficient of the filter.

[0048] Conditional formula 1) defines the optical density value of the filter. By setting the conditions appropriately, the filter can achieve the light intensity transmittance requirements of different scenes. If the optical density value is higher than the upper limit, the light energy is too low, and the low-light night vision detector cannot obtain enough energy for exposure. If the optical density value is lower than the lower limit, the light energy is too high, and there is no need to use a neutral filter. In addition, in order to reliably obtain the effect of conditional formula 1), it is more preferred to set the value of conditional formula 1) to 0.2.

[0049] Conditional formula 2) defines the refractive index and dispersion coefficient of the filter. By setting the conditions appropriately, the filter can achieve good processing performance, good material process stability, low price and easy acquisition. If the two parameters are higher than the upper limit, the cost will increase and the stability will deteriorate. If the two parameters are lower than the lower limit, the light will be greatly deflected and the processing will be complicated. In addition, in order to reliably obtain the effect of conditional formula 2), it is more preferred to set the value of conditional formula 2) to Nd = 1.5231Vd = 58.57.

[0050] Conditional formula 3) defines the aperture of the filter. By setting the conditions appropriately, the filter can achieve a good optical window to ensure that the light of the optical system is not blocked. If it is less than 90%, the effective light of the optical system is easily blocked, affecting the brightness of the edge of the optical system and easily reducing the imaging field of view of the optical system. In addition, in order to reliably obtain the effect of conditional formula 3), it is more preferred to set the value of conditional formula 3) to 92%.

[0051] A detailed description will now be given of optical filters according to two examples.

[0052] Example 1

[0053] The specific parameters of Example 1 are shown in Table 1. In Example 1, the neutral density filter uses a visible light metal film, and the conditional formula is as follows:

[0054] 1) 0.1≤OD≤4.0;

[0055] 2) Nd, Vd see Table 1;

[0056] 3) The filter aperture is greater than 90%;

[0057] Table 1 Visible light metal film neutral density filter

[0058] Example 1 Type Radius Thickness 1 S1 side infinity 2 2 S2 side infinity Nd 3 Finish 60-40 1.5231 4 Application wavelength 400~630nm Vd 5 Effective caliber >90％ 58.57 6 Material Optical float glass

[0059] Example 2

[0060] In Example 2, the neutral density filter uses an absorption-type metal film and the material is neutral gray glass, and the remaining conditions are the same as those in Example 1.

[0061] The corresponding transmittances of different OD values ​​of Example 1 and Example 2 are shown in Table 2 below:

[0062] Table 2 Transmittance of neutral density filters at different OD values

[0063]

[0064]

[0065] Please refer to the filter transmittance curve. It can be seen from the figure that the filter of this embodiment has an OD of 0.2 and an average transmittance of 60%±10% at 400-630nm, and OD of 0.2 (T=63%) at 550nm.

[0066] See also Figure 3 The spectral transmittance curve simulation diagram can achieve specific transmittance value requirements, which can effectively reduce the light intensity energy in daytime scenes and achieve normal exposure. The spectrum light in the range of 700-1100nm is cut off to filter out the influence of infrared light on imaging, and the image color is normal to avoid the reddish phenomenon. According to the lighting conditions in the daytime scene, select the appropriate OD value to achieve the best exposure. Figure 3 The spectral transmittance curve corresponds to the coating requirements as follows:

[0067] Metal film: 400-630nm Tave=60%±10%; 550nm OD=0.2 (T=63%).

[0068] IR infrared cut-off film: 420~630nm Tave>92%; 650±10nm T=50%; 690~700nm Tmax<3%; 700~1000nm Tmax<1%.

[0069] Based on the above examples, the filter proposed in this case has the following beneficial effects:

[0070] 1. This solution sets a neutral density filter at the front end of the optical system, which can effectively reduce the light intensity energy in daytime scenes, so that the system can achieve normal exposure imaging;

[0071] 2. This solution sets a neutral density filter at the front end of the optical system, so that the image color is normal in daytime scenes without color cast;

[0072] 3. This solution sets a neutral density filter at the front end of the optical system, which is screwed in with a thread, has high reliability, good stability, convenient replacement, and convenient placement. Neutral filters of different specifications and models can be replaced according to the light intensity of the use environment, and have good expansion performance;

[0073] 4. This solution sets a neutral density filter at the front end of the optical system, which will not introduce aberrations. The low-light night vision system does not need to be redesigned and has good compatibility.

[0074] Embodiment 2

[0075] See also Figure 4 , a neutral density filter device, the device comprises: a mounting seat B and a neutral density filter A as described in one of the embodiments mounted on the frame B.

[0076] In this embodiment, the advantages of the neutral density filter device rely on the neutral density filter described in one of the embodiments, which will not be described in detail here.

[0077] In this embodiment, the lens frame B is connected to the front end of the lens (not shown, referring to the object side of the lens) via a thread B1. The neutral density filter adopts a modular design, uses threaded engagement, and is designed with the same external dimensions, which facilitates the replacement of neutral density filters with different optical density OD values.

[0078] In this embodiment, the filter satisfies the following condition:

[0079] 4) 20mm <D<90mm;0.5mm<T<4.0mm

[0080] In the above conditional formula, D is the outer diameter of the neutral density filter, and T is the core thickness of the neutral density filter.

[0081] Conditional formula 4) defines the size of the filter. By setting the conditions appropriately, the weight and volume can be further reduced while ensuring the strength of the filter. If the two parameters are higher than the upper limit, the filter will be too large and heavy, which is not conducive to lightweight requirements. If the two parameters are lower than the lower limit, the filter will be too small and cannot meet the effective aperture of the optical system. In addition, in order to reliably obtain the effect of conditional formula 4), it is more preferred to set the value of conditional formula 4) to D = 54mm and T = 2mm.

[0082] Embodiment 3

[0083] See also Figure 5 , a filter switcher, the filter switcher O includes at least one of the above-mentioned neutral density filters A, and other filters G.

[0084] In this embodiment, the advantages of the neutral density filter device depend on the neutral density filter, which will not be described in detail here. It should be noted that the remaining structure of the filter switch can refer to the prior art, which will not be described in detail here.

[0085] Embodiment 4

[0086] An optical system includes an imaging lens, and an image sensor that receives light from the imaging lens and performs photoelectric conversion;

[0087] The optical system further includes a neutral density filter device as described in the second embodiment, or a filter switch as described in the third embodiment.

[0088] In this embodiment, the advantages of the neutral density filter device depend on the neutral density filter, which will not be described in detail here.

[0089] Various examples may provide an optical system having high optical performance.

[0090] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments.The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

1. A neutral density filter, characterized in that: One side of the filter is coated with a 400-630nm metal film, and the OD of the comprehensive pass rate of 400-630nm satisfies the following conditional formula: 0.1≤OD≤4.0 Wherein, OD is the optical density value of the filter; The other side is coated with an IR cut-off film, which forms a cut-off band in the transmission band of the visible light region of 420 to 630 nm and the infrared region of 700 to 1000 nm.

2. A neutral density filter as claimed in claim 1, characterized in that: The metal film is composed of alternately stacked low-refractive index layers and high-refractive index layers, and a metal layer is arranged in the middle of the film layers.

3. A neutral density filter as claimed in claim 2, characterized in that: The metal layer is made of absorbent material.

4. A neutral density filter as claimed in claim 2, characterized in that: The refractive index of the low refractive index layer is 1.43 to 1.49; The refractive index of the high refractive index layer is 2.0 to 2.5; The refractive index of the metal layer is 1.9 to 2.

3.

5. A neutral density filter as claimed in claim 1, characterized in that: The metal film is a visible light metal film, and the base material of the filter satisfies the following conditional formula: 1.4 <Nd<1.6;40<Vd<70 Wherein, Nd is the refractive index of the filter, Vd is the dispersion coefficient of the filter; the base material is float glass or D-263T.

6. A neutral density filter as claimed in claim 1, characterized in that: The metal film is an absorption type metal film, and the base material is neutral grey glass.

7. A neutral density filter as claimed in claim 1, characterized in that: The optical filter has a light aperture greater than 90%.

8. A neutral density filter device, characterized in that: The device comprises: a mounting seat and a neutral density filter as claimed in any one of claims 1 to 7 which is mounted on a lens frame.

9. A filter switch, characterized in that: The filter switch includes at least one neutral density filter as described in any one of claims 1-4.

10. A low-light-level night vision imaging system, characterized in that: It includes an imaging lens, and an image sensor that receives light from the imaging lens and performs photoelectric conversion; The optical system further comprises a neutral density filter device as claimed in claim 8, or a filter switcher as claimed in claim 9.

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