The article deals with the concerns of industrial production of thermal imaging lenses for IR-range. They relate both to a stage of production of individual optical components, and a stage of assembly and alignment.
The article deals with the concerns of industrial production of thermal imaging lenses for IR-range. They relate both to a stage of production of individual optical components, and a stage of assembly and alignment.
Currently, you can see the rapid leaps of the fundamental, applied researches and development in the field of thermal imaging optical systems. First of all, this is due to the increased relevance of a number of technical problems, the solution of which is in the interests of the military departments of various countries, including the Russian Federation, as well as the major achievements in the field of photodetectors with high output characteristics [1–6]. A qualitative leap in the preparation and organization of industrial production of photodetectors operating in the spectral range of 3–5 µm is done. It must provide the required quality of the complexed optoelectronic devices and systems for various purposes [7, 8]. Modern thermal imaging devices (TID) operate in one of two spectral ranges (3–5 µm and 8–14 µm). Some types of TID operate in both bands simultaneously, i. e. they are combined devices. Each type has certain merits. The lenses working in the range of 3–5 µm (Fig. 1) possess a number of advantages compared with the lenses for the area of 8–14 µm:
• they provide better image quality due to smaller diffraction limit resolution (ε = 1,22 · λ / D); • they have smaller dimensions of the optical systems and input windows of optoelectronic complexes (OEC) by allowing the use of large aperture values (this is also determined by diffraction due to smaller wavelengths in the working spectral range); • when calculating the TID optical systems, a wider product range of optical materials can be used: monocrystalline germanium, silicon, fluorite, chalcogenide materials, polycrystalline sulfide and zinc selenide (this variety in turn allows the calculation of an optical systems with higher output optical parameters in comparison with the systems operating in the range of 8 to 14 µm); • they have enhanced lighting parameter of the antireflective coating and, therefore, higher light transmission properties of the optical system; • they provide greater operation efficiency in the conditions of high humidity and by air targets . The peculiarity of optical systems for TID is determined by the condition that the wavelength decrease simultaneously with a decrease in the diffraction limit leads to increased requirements to manufacturing precision parts. It should be taken into account, that the refractive index of the materials with the wavelength changes does not change so quickly, and for the individual materials the values are close to n = 4 (germanium). Therefore, deviations from the given form of surfaces for optical parts should not exceed N / ΔN ≤ 3 / 0.5. The amount of decentering should be not more than 10 µm, and the tolerance for the thickness of the center should be no more than 0.05 mm on average. These circumstances, in turn, require the development and implementation of fundamentally new technologies for manufacturing optical elements using modern technological equipment, with the required tolerances on the above optical media, with a low manufacturing cost. The authors carried out theoretical studies in the field of optimizing the calculations of the above optical systems. Calculations confirmed that in order to guarantee the high quality of the lens, in addition to manufacturing optical elements with the above accuracy parameters, it is necessary to set the refractive indices of the optical blanks used with an accuracy of at least a third decimal place in the calculation of the optical scheme. However, regulatory and technical documentation for the said optical material lacks the requirements of the certification works to measure the refractive index and dispersion of materials specific to each material batch produced. For this reason, suppliers of optical blanks do not perform certification work on measuring the refractive index and dispersion in each batch of blanks. And the computing designer of the optical systems uses the nominal values of the refractive index in the calculation, which are specified in the regulatory and technical documentation on the materials: OSTs, TS, etc. and the accuracy of which is limited to the first decimal point. Thus, in a number of cases, systematic errors are introduced in the calculation of sets of optical parts to determine the exact air gaps for specific batches of lenses. In order to solve this undoubtedly important issue, you must carry out a number of operations to adjust standards, technical specifications for optical media. Moreover, it is necessary to emphasize the need for the spectral range from 3 to 5 µm to measure the index of refraction and dispersion up to a fourth decimal place. Also, it is necessary to develop special techniques and to conduct the above measurement procedures, preparation of the necessary metrological base, development and construction documentation for special detecting prisms, etc. A noticeable trend in the field of creating lenses for this spectral range has been the minimization of mass-size parameters with consistently high quality of the image being formed. As a rule, the use of traditional spherical optical elements has exhausted its potential for improving the output parameters of lenses. Optical systems have a large number of components, large weight and size characteristics and, as a rule, very high production costs. As a way out, optical systems use non-traditional optical elements (aspherical, kinoform, hybrid aspherical-kinoform elements) (Fig. 2). As a result, the optical system, which includes these optical elements, becomes compact. It has better aberrational characteristics and their manufacturing cost is low . Such optical elements can be manufactured using two technologies: precision diamond turning technology and manufacturing technology using the vacuum ion-plasma etching method and functional masks synthesized using computer programs. For a number of reasons, the second method is not currently used. The use of precision diamond turning technology involves the use of super-precision CNC module (Nanoform series machines, by AMETEK PRECITECH, USA) (Fig. 3), as well as special diamond cutters, but the range of the firms producing such equipment is limited. The production of optical elements from various optical materials by the method of precision diamond turning implies the carrying out preliminary optimization and technological modes of cutting, as well as individual selection of cutting fluids for each specific material. An important factor is the mandatory use of a precision system for active control of the geometry of the machined surface in the process of real-time cutting. Unfortunately, the production of such technological systems is not available in the Russian Federation, as well as special cutting tools. Application of kinoform, aspheric-kinoform optical elements requires currently a system solution of a number of problems. The state standards of the design for kinoform (aspheric-kinoform) elements shall be developed with a mandatory list of structural parameters, accuracy recommendations, various developing methods of measuring these parameters, and the establishment of appropriate metrology base. One of the most important design parameters of an optical system, irrespective of the spectral range where it operates, is the transmittance of radiation . This parameter is directly related to the range of detection and recognition of the object. And in the case of a thermal imaging lens, working in one part of the infrared spectrum, it affects the optical signal level input to the photodetector device, registers guide radiation from the object. To increase the signal, you need to reduce radiation losses in the working spectral range (3–5 µm). This circumstance requires the synthesis of special multilayer optical coatings that would provide the lowest possible residual reflection from each optical surface. Naturally, an increase in the number of layers leads to an increase in the magnitude of the scattering loss and absorption in the layers themselves. This in turn leads to inevitable losses of the signal passing through the optical system. This problem can be solved only one way: by replacing the multilayer optical coatings on single-layer structure films with a gradient refractive index which is equivalent to the multilayer interference film. Experimental work in this area was carried out by the employees of the department of optical coating of JSC "NGO "GIPO" (Kazan) in conjunction with PJSC "S. A. Zverev Krasnogorsky Plant" using optical substrates from monocrystalline germanium and polycrystalline zinc selenide and showed a good result . A feature of optical coatings applied to optical elements for the IR-range is their relatively large optical thickness. The total optical coating thickness is obtained with the highly refractive indices of the film-forming materials used, as well as the thickness of each layer separately. In the case of deposition of an optical layer on an optical surface with varying thickness on the entire surface leads to the effect of a deterioration of the surface correspondence to the actual requirements of its geometric design. This in turn leads to a phase delay in the passage of radiation through each surface and deterioration in the quality of the lens as a whole. The greatest negative effect is described on the steep concave surfaces which are generally characteristic for the optical schemes of the thermal imaging lenses. To ensure the deposition with the uniform thickness over the entire surface of each element, planetary mechanisms for rotating optical components are used, as well as specially designed functional masks . The development and production of lenses with the required output parameters will ensure the creation of high-quality new-generation domestic thermal imaging devices. The tendencies of the last decade, which require constant minimization and miniaturization, were not ignored by optoelectronic instrumentation. This trend leaves its imprint on the development of modern thermal imaging devices. This problem is solved by innovative design solutions. One of them is the development of optical systems with broken optical axes (which allows you to change the direction of the sighting axis to 180є) using non-traditional optical components. But the implementation of this solution in turn complicates the process of assembling and monitoring the output parameters of the lenses. Tightening requirements for decentering of optical components, including lateral and angular decentering, leads to improved quality indices of the IR optical systems. In particular, decentering of the lens component intended for operation in the infrared range should not exceed 3–5 µm. Such methods of lens centering in frames, like an autocollimation point with the use of a floating cartridge or its improved version, a two-sided autocollimation point, do not fully achieve the required allowable lens decentering values. To solve such problems the staff of PSJC "KMZ" developed a special booth layout: laser autocollimation microscope, using laser radiation. The employees of N. E. Bauman MSTU from the department RL‑2 of the Radioelectronics and laser technology faculty are involved to its completion and elimination of flaws of the structure found at the stage of experimental work. During the final assembly of the lens to achieve the required parameters, it is necessary to highly accurately maintain gaps between the optical elements. To monitor the resulting air gaps with an allowable error of about 0.01 mm, precision metrology equipment is required. As one variants of metrological equipment, OptiCentric IR laboratory bench by TRIOPTICS (Germany) may be used which is specifically produced to control optical parts intended for operation in the infrared radiation range. When assembling lenses with broken optical axes, special methods of aligning and exposing the lenses in the frames and mirrors in the lens case are required. The alignment of optical components relative to each other should not exceed 0.01 mm in the systems without moving components and 0.03 mm in the systems with a moving optical component. Furthermore, it is necessary to ensure the installation of mirrors with an angular error of not more than 20 degrees with respect to the required angle with the simultaneous adherence to the air gaps with an accuracy of not more than 10 µm. To solve these problems, it is required to create a certain nomenclature of specialized control and adjustment equipment, development of alignment techniques for this class of optical systems. All these works require a systematic approach and compulsory funding of theoretical and experimental work. What are the trends related to the development and production of the lenses for TID that we observe? Currently, the market of unmanned aerial vehicles for various purposes and security systems generates a great demand for the creation of panoramic instruments with a circular view. In connection with this, the customers highly demand the development and production of lenses for the spectral range of 3–5 µm with a field of view of not less than 180°. This is where the introduction of the listed technological developments is required the most. When creating TID operating over a wide range varying external conditions, including at unacceptably low temperature contrasts "object-background", the semiconductor quantum cascade lasers (QCL) are commonly applied, operating in the range 3 to 5 µm. They can create the basis for small-sized IR-illuminators for TID kitting. Details on the possible application of QCL in TID are given in . But the QCL-based IR-illuminator requires suitable optics for the formation of radiation. It must be calculated in accordance with the requirement of collimation of coherent radiation, taking into account the requirement to form an illumination spot of circular or rectangular, in the form of a cross or other shape acceptable for creating laser target designators for thermal imaging devices. Another important area of the TID application is medicine, and it is still one of the directions of their development. TID create a thermal image of the body, thermograms, due to own IR-radiation of the human body. If the human body emits less than 1% of the total energy in the spectral region, then in the spectral range 5–9 µm, 9–16 µm and above 16 µm, the radiation is 20, 38 and 41%, respectively . However, it is in the spectral region of 3–5 µm that the greatest resolution of the IR-image on the thermogram is provided. This opens up a wide horizon for their use as an effective medical control. Lack of energy, radiated by the human body in this region of the spectrum, can be compensated by the use of more sensitive 3-generation modern TID, and also by additional illumination of the radiation by the QCL. In all healthy people, the temperature distribution patterns over the body surface have much in common, including the manifestation of image symmetry relative to the midline. However, the absolute values of the temperature of the skin, depending on several factors (ambient temperature and humidity, time of day and year, the individual characteristics, etc.) can vary within wide limits, so the most interesting are the comparative temperature distribution and detection of violations of its symmetry. These disturbances arise due to abnormalities of the vasculature (e. g., vascular tumor), vascular tone changing, local circulatory disorders (injury, thrombosis, multiple sclerosis) and venous blood flow, local overheating (tumors, inflammatory lesions) and tissues thermal conductivity change. Technical basis of the thermography and the respective apparatus is described in [15, 16]. All anomalies are mainly detected as the appearance of regions with a higher temperature than in the symmetric point as in local areas and in large areas of the human body. Exceptions are burns, frostbites and injuries, which areas are always colder than the surrounding tissue. IR-thermography has many important advantages in comparison with X-ray tomography and nuclear magnetic resonance [16–19]. Let’s consider the examples of TID for medical diagnostics. Fig. 4 shows TID IRTIS2000 ME for medical thermography, Fig. 5 shows typical thermogram, and Fig. 6shows an example of thermal imaging diagnostics medical office layout, Fig. 7 shows the minimum size of an object based on the distance [16–19]. TID Thermovision ®870 System by AGEMA Infrared Systems (Sweden)  is CdHgTe range photodetector-based device with a number of elements 100, workspace range 2–5 µm, the temperature resolution 0.1 °C, a frame rate 25 Hz, weight 2.5 kg (TID device itself) and 4.5 kg (TV monitor), with the dimensions, respectively, 92 Ч 150 Ч 205 mm and 252 Ч 129 Ч 322 mm, operating temperature range 15–50 °C. Digital thermal CCD cameras TVC300 and TVC400 by "SILAR"  made based on IR-CCD with Schottky barriers, use cryogenic cooling. It is provided by either pouring liquid nitrogen into a cryostat (ML modification), or by means of Stirling microcooler (ST modification). It is possible to work autonomously without a computer, with the computer, the possibility of recording images in real time (over 1000 frames without gaps) is also provided for. The number of CCD pixels is 320 Ч 240 (TVC300 model) and 480 Ч 320 (TVC400 model), the size of the pixel is respectively 40 Ч 40 µm and 28 Ч 28 µm, the frame rate is 25 Hz, the working spectral range is 1.8–5.3 µm, the dynamic range is over 56 dB, the temperature sensitivity is not more than 0.1 °C, operating temperature range is – 15 to 35 °C . TID "IRTIS‑2000 ME" (LLC "IRTIS", Russia) [16–19], based on single-element InSb or HgCdTe IR- photodetector, operates in two regions of spectrum: 3–5 µm or 8–12 µm. The instrument has a sensitivity 0,02єC (temperature resolution over the entire field) at 30 °C, the range of its measurements is from – 60 to 300 °C (IRTIS21000 CB) and from – 60 to 1700 °C (2000 IRTIS CH), the measurement accuracy is ±1 °C, operating temperature range is – 40 to 85 °C, the spatial resolution is 1 mrad, the angle of view is 25 Ч 20°, frame resolution is 256 Ч 256 or 640 Ч 480 pixels, weight 1.4 kg, dimensions 92 Ч 120 Ч 200 mm, voltage U = 6 V, power consumption is 1.2 W, continuous operation time is more than 8 hours, the time of frame format is switchable: 0.8 s, 1.6 s and 3,2 s. Camera IRTIS2000 ME operates with any modern computer, and can be connected to the computer of the customer. Also, the camera can be supplied with a desktop computer or laptop. Due to the peculiar design of the camera, a high repeatability of the measurement results from frame to frame, which allows for dynamic infrared thermal mapping (multiple recording of the same portion of a patient’s body, at predetermined intervals), and then view the thermogram obtained as a dynamic thermal film. The devices operating in the spectral region of 3–5 µm, may be produced as modules, embedded in a multi-channel observation and targeting system consisting of modules, and thermal devices operating in the spectral region of 8–14 µm, and eventually 30–100 µm, composed of TV modules for spectral region 0,1–0,25 µm, 0.4–1.1 µm, 0.8–1.7 µm, laser range finders, target designators, and other modules. Thus, thanks to the microprocessor, the images of all modules are summed into a single integrated picture [1, 15]. In this variant of the thermal imaging device, the 3–5 µm region of the spectrum module need to be designed and manufactured to such specifics of its application, the constructive execution of the entire system and, in particular, with the maximum degree of standardization and unification. We would like to mention one more perspective technological direction of creating optics for TID – the use of 3D-printing for the manufacture of various components of the instrument. This is not a utopia, but a reality today. There are serious preconditions for its implementation not only abroad but also in the Russian Federation. With the help of 3D-printing, it is possible to create microminiature thermal and television systems, laser designators illuminators for aiming missiles with remote image transmission. These sensors can be used for covert surveillance in security systems. 3D-technology will allow in the future creating a micro-miniature (the size of a bee), unmanned aerial vehicles and ground robotic mobile devices to carry out covert intelligence and special operations, with the corresponding optics, but instead of contemporary TID and GPS sensors, the new micro-miniature sensors of reconnaissance, targeting, navigation, measuring coordinates and target range. 3D-printing technology will create new camouflage materials to ensure secrecy in all regions of the spectrum: ultraviolet, infrared, radio-frequency range. 3D-printing is effective in creating large scale integrated circuits, microminiature personal computers and microprocessors, as well as MEMS, microelectric motors and primary power sources in an integrated design (including solar panels) in order to develop promising TID. Creation of new optical materials with high resistance to mechanical, climatic and special effects (ionizing radiation, corrosive media, etc.) is also impossible without 3D-printing. 3D-printing technology opens a wide scope for design of TID and devices with high performance, the creation of information storage systems with truly unlimited scope. The factors preventing widespread 3D-printing in this technical direction will eventually be eliminated. CONCLUSION All TID application directions reflect the future prospects for the development of manufacturing technology of thermal imaging lenses. Undoubtedly, the trend towards an increase in production volumes of thermal optical devices and systems in the near future will only grow. All of the above concerns to some extent will affect the stability, cost of production, as well as operational parameters of manufactured products. It requires a systematic approach to solving these problems under condition of attracting leading specialists of enterprises and organizations directly or indirectly involved in the production of optoelectronic devices in this profile. The authors express their sincere gratitude to Kolyuchkin V. Y., Sklyarov S. N. for reviewing and correcting the article.