The super field of view infrared optical lens is mainly used in the military to warn and indicate incoming missiles or infrared threat targets. It can be deployed on weapon equipment platforms (such as airborne, vehicle-mounted, and ship-borne platforms) at the same time to realize the panorama It can view 360°alarm and possess Omni-directional situational awareness. It is an important military optoelectronic product.
Ultra-large field of view infrared optical lens has some characteristics of wide-angle lens and infrared optical lens, but at the same time, it is different from the ordinary infrared optical lens, specifically in the following aspects.
(1) There is a large negative distortion
When the field of view of the optical system exceeds 60°, when the ideal imaging mode is adopted, the image height will increase sharply with the increase of the field of view. For a detector with limited image size, a large amount of negative distortion must be introduced to obtain enough field of view.
(2) Has a large negative vignetting
The illuminance of the image plane of the system is proportional to the fourth power of the cosine of the incident light angle.
Because of the relatively large field of view of the optical system. For example, when Half the field of view angle is 65°, the edge illuminance is only about 40% of the center illuminance. Therefore, in the optical design, a large negative vignetting must be introduced to the edge field of view to increase the luminous flux and improve illuminance.
(3) Non-thermal design requirements
Like other military infrared optical lenses, it needs to meet the military's wide temperature range (usually a typical temperature range is -55 ~ 70 ℃) of excellent imaging quality.
(4) The important factors of image quality evaluation are different
When used to warn the incoming missile, compared with the conventional infrared optical lens, the infrared optical lens with a super field of view mainly focuses on the indication accuracy and range of the target. This is reflected in the uniformity of single-pixel angular resolution and single-pixel energy convergence.
In recent years, many scholars at home and abroad have conducted research on the super field of view infrared optical lenses, and the research content is mainly concentrated on the part of optical lenses.
Among them, the detector area used in the optical lens is small; there are many optical elements; the length of the lens is longer; the optical and mechanical structural parts are realized by titanium alloy with small thermal expansion coefficient, but high density, low thermal conductivity, and poor workability; it has no heat The technical means of athermal design is realized by refraction diffraction.
In this article, starting from the application requirements, the design characteristics of the superfield of view infrared optical lens are analyzed.
The detector used in the optical system is a 1 024×1 024@15 μm large area array mediumwave refrigerated infrared detector, which uses only 4 lenses and does not contain a diffractive surface to realize an optical passive athermal design. The material of the optomechanical structure is aluminum alloy, the lens structure is compact, the total length is less than 69 mm, and the optical field of view reaches 116°.
In combination with actual engineering application requirements, the design features of the superfield of view infrared optical lens are analyzed in terms of projection mode, optical configuration, image surface illuminance, the field of view, thermalization, and evaluation mode.
For imaging of objects at infinity, an ideal optical lens with an imaging distortion of 0 has an image height and field of view in accordance with the following formula, namely:
h = f tanθ (1)
Where: f is the focal length of the object side; θ is the half-field angle of the object side.
It can be seen from formula (1) that when the half-field angle is large, the image height will increase sharply. Therefore, the super-large field-of-view optical lens must introduce a certain amount of negative distortion can design a sufficient imaging field of view on limited image size.
The design of the super field of view lens adopts the "non-similar" imaging principle, and the object area that the ideal optical lens cannot imagine is deformed and compressed by introducing negative distortion to the image. The imaging image height and field of view usually conform to the following projection methods:
h = 2 f tan(θ/2) (2)
h = 2 f sin(θ/2) (3)
h = f sinθ (4)
h = f θ (5)
Taking the derivation on both sides of formulas (2)~(5) respectively, the relationship between angular resolution and field of view is obtained as follows:
dh/dθ = f / cos (θ/2)2 (6)
dh/dθ = f cos(θ/2) (7)
dh/dθ = f cos(θ) (8)
dh/dθ = f (9)
The relationship between image height and field of view corresponding to different projection methods is shown in Figure 1(a), and the relationship between angular resolution and field angle corresponding to different projection methods is shown in Figure 1(b).
In the actual use of the super field of view infrared optical lens, the infrared target is usually a point target after being imaged by the optical system. Compared with the geometric feature information of the target, the system is more concerned about its angular position information in order to obtain higher angular position indication accuracy.
It can be seen from the figure that the h = f θ projection method, the image height is proportional to the field of view, the single-pixel angular resolution does not change with the field of view, and the overall field of view is consistent. Therefore, the military super field of view infrared optical lens should choose the h = f θ projection method.