Infrared optical systems often work in environments with a relatively large temperature range. The thermal expansion coefficient of optical materials and mechanical materials and the change in the refractive index of optical materials with temperature will seriously affect the performance of the optical system.
Compared with optical materials in the visible light band, the refractive index of most infrared optical materials changes with temperature gradient dn/dt relatively large, so the thermal effect of the infrared optical system is more obvious. In addition, some documents and material manufacturers sometimes give material refractive indexes, especially thermal expansion coefficients, which are quite different. As a rigorous designer, these coefficients must be verified or checked.
In order to obtain satisfactory image quality, many systems use athermalization technology to focus the system. Temperature compensation is also a complex technology. We can use mechanical (electromechanical) methods or optical methods to achieve athermalization of the system. For example, use a manual or closed-loop servo system to adjust the distance between optical parts to realize the system in a new temperature environment. Under refocusing or selecting appropriate optical materials and rationally distributing the optical power of each optical component, the athermalization in the optical sense can be achieved.
Optical Athermalization Technology
The primary starting point of optical athermalization technology is to use the temperature characteristics of different optical materials, such as linear expansion coefficient, refractive index temperature gradient, and more, while meeting the system's imaging quality requirements, appropriately selecting materials, and reasonably distributing the optical power of each lens. So that the defocus amount of the entire optical system itself is consistent with the thermal expansion of the lens barrel. The optical athermalization design belongs to passive temperature compensation.
In order to obtain an optical system that eliminates not only chromatic aberration but also eliminates chromatic aberration, the following three conditions must be met: optical power, achromatic aberration, and heat dissipation. The optical system needs to contribute at least three optical powers to achieve the simultaneous elimination of thermal and chromatic aberrations.
Special attention: The optical system contributes at least three optical powers does not mean that the optical system needs at least three lenses. For example, a diffractive surface can be used to contribute optical power, thereby reducing the number of lenses.
Problems that should be paid attention to in the design of optical athermalization.
In the process of athermal design of the infrared optical system, the following issues should be paid attention to:
With the change of temperature, the original aberration compensation relationship is destroyed, and the best focus position of the system may change nonlinearly with the change of temperature.
For the reflective system, if the material of the reflector is the same as the material of the lens barrel (or the material has the same thermal expansion coefficient), when the temperature changes, the system will only zoom in or out to a certain extent, and the temperature has little effect on the performance of the system. In principle, no heat dissipation design is required.
Since the mechanical lens barrel for installing the lens is complex in most cases, the way of expansion (or contraction) of the lens barrel with different structures is not necessarily the same when the temperature changes. The athermalization design should be based on the athermalization equation given above. Based on the specific problems of the structure of different lens barrels, specific analysis to ensure a good heat dissipation effect.
In view of the above uncertain factors, optical systems designed based on optical athermalization technology should also be equipped with adjustment links to ensure the practicability and safety of the design.
Mechanical (Electromechanical) Temperature Compensation
Mechanical (electromechanical) temperature compensation can be divided into active compensation and passive compensation. Active compensation uses manual, mechanical, or electromechanical methods to adjust the compensation mechanism; passive compensation uses mechanical and electronic methods to achieve automatic refocusing of the image surface.
Mechanical (Electromechanical) Active Temperature Compensation
From the basic theory of optics, we know that when the axial position of a lens (or lens group) in the optical system is changed, the focal plane position of the system will change accordingly. Active temperature compensation uses this principle to reproduce the system. The focus and adjustment method can be manual or electric.
In order to improve the sensitivity of adjustment and maintain the stability of the optical axis, this temperature compensation method generally requires a precise mechanical transmission mechanism. At the same time, the required stroke of the temperature compensation lens should be investigated, and the most sensitive lens affected by the focusing surface should be selected as the temperature compensation element. The basic mechanical transmission structure of this compensation method is basically the same as the focusing mechanism of the optical system. The method is simple in principle and easy to implement, but it increases the weight of the optical instrument, and at the same time, easily brings about aiming errors.
Mechanical (electromechanical) passive temperature compensation
The principle of mechanical (electromechanical) passive temperature compensation is basically the same as that of active temperature compensation, except that the way of changing the lens group has changed. It uses certain materials or mechanisms with special functions to achieve automatic adjustment.
The following are two commonly used methods:
A mechanical passive temperature compensation method that uses two materials with different expansion coefficients as the inner barrel of the lens barrel. When the ambient temperature changes, the expansion or contraction of the inner barrel drives the compensation lens to move axially to achieve focal plane stability. This compensation method requires a reasonable selection of materials with appropriate expansion coefficients, lengths, and matching with the optical system.
An electromechanical passive temperature compensation method. In this method, after the temperature sensor measures the ambient temperature, it transmits the signal to the controller. The controller obtains the required amount of movement from the database according to the temperature value and drives the motor to complete the temperature compensation. The data in the database needs to be rigorously calibrated in advance. Mechanical (electromechanical) passive temperature compensation will also bring additional aiming errors.
Comparison of different athermalization methods
Error tolerance of temperature compensation
I mentioned the following temperature compensation methods for infrared optical systems, but the actual optical system cannot achieve strict temperature compensation. That is, in a certain temperature range, the image surface of the system cannot be consistent with the changes of the lens barrel with complex structure, which requires an Error tolerance for temperature compensation. In a broad sense, the image defocuses caused by temperature can be regarded as a kind of aberration. According to the Rayleigh law of optical system aberration, the temperature compensation error should be controlled at its maximum wave aberration less than 1/4 wavelength.
Special attention: If the optical system allows focusing during use, then the system does not need a thermal design at all, so be sure to understand the user's requirements before designing.