The development of optical instrument making entails the need to continuously improve and develop methods and means of controlling products at all technological stages of production. High quality products are ensured not only through the use of modern materials and components, but also through the development of progressive control and measurement methods implemented in the devices. A brief review of modern samples of autocollimators and goniometers is given in the article and their brief metrological analysis is carried out.
INTRODUCTION Numerous measurements of optical parts and devices are reduced to determining the lengths and angles . The measurements, depending on the method of obtaining the values of the measured value, are divided into direct, indirect and cumulative measurements. Furthermore, depending on the presence or absence of direct contact between the measuring instruments and the object of measurement itself, there are contact and non-contact methods. In problems related to angular measurements, autocollimators and goniometers are widely used. These devices provide direct angle measurement in a non-contact way. From the metrological point of view, the non-contact method is preferable, since it excludes the possibility of damage to the surface of the monitored or measured object, has increased aaccuracy, allows monitoring not only individual local zones but also the entire surface of the part.
AUTOCOLLIMATORS Autocollimators (Fig. 1) are used to measure small angular deviations of normals to the reflective surfaces from the measuring axis of the instrument. At industrial enterprises, autocollimators are used in various methods for controlling the angular parameters of parts and machines. In the field of optical production, these devices are used to control the shape of the surface of parts, the correspondence of dimensions and other geometric parameters of the elements being manufactured. Furthermore, autocollimators are used in metrological laboratories and certification and metrology centers for calibration and verification of angle standards and various associated systems. Angular measurements in the latter case are characterized by high accuracy, reaching hundredths of an angular second.
According to the manner of processing and presentation of measuring information, all these devices can be visual or digital. In the visual autocollimator, the operator uses the eyepiece to determine the displacement of the image of the luminous mark, obtained from the reflecting surface of the measurement object, relative to the measuring grid. In the domestic market, this group of devices is represented by models AKU‑0.2, AKU‑0.5 and some other produced by Novosibirsk-based company JSC "Novosibirsk Instrument-Making Plant" (Fig. 2) . Due to their relatively low cost, visual autocollimators are still very popular devices. Although, it should be noted, they have a number of significant drawbacks, the main of which is low measurement accuracy and low efficiency. Visual autocollimators are still produced by large foreign producers, such as Trioptics and Moller Wedel (Fig. 3) [3, 4]. However, the models proposed by these companies are designed rather for a special consumer with specific tasks. Devices are of high cost and also have all those disadvantages that accompany the procedures of visual reading of the measuring information. With the development of computer and computation technology, the digital instruments began to acquire greatest popularity. The image of the mark is registered using a CCD array or matrix there. Measuring information is digitized and then fed to a personal computer. The operation principle of the digital autocollimator is as follows (Fig. 4). The light flux from the source 1 passes through the mark 2, gets reflected from the beam splitting cube 3 and falls on the lens 4. The mark is placed in the focal plane of the lens, with which its image is projected to infinity. Reflecting from the surface of the light-reflecting surface 5, the mark image falls on the photosensitive element 6, which is also in the focal plane of the lens. In the case where the light-reflecting surface 5 is perpendicular to the axis of the autocollimator (α = 0), the image is formed at the center of the photosensitive element 6, and this position of the image is considered to be zero. When the light-reflecting surface is inclined by an angle α, the mark image in the plane of the photosensitive element will receive a certain linear displacement x, which is determined by the expression: x = f·tg 2 α, where f is the focal length of the autocollimator lens.
Digital autocollimators have a number of undeniable advantages over visual instruments: high measurement accuracy, ease of operation and maintenance, compact dimensions and weight, automation of measurements, etc. From the manufacturer’s point of view, the design of such devices does not impose strict restrictions on optical and mechanical components, which allows the creation of inexpensive devices with relatively high accuracy. Analysis of modern digital autocollimators of foreign manufacturers, Moller Wedel (Elcomat 3000), Taylor Hobson (Ultra) and Trioptics (TA 1000–140) (Fig. 5) , showed that the most popular devices today are autocollimators with a measurement error of 0.2–0.25 angular seconds. Among domestic manufacturers, the devices of this level of accuracy are offered only by INERTECH. The errors in measurements of the autocollimators AK‑0.1 and AK‑0.25 produced by INERTECH (Fig.6) do not exceed 0.1 and 0.25 angular seconds, respectively  (see Table 1). Autocollimators of AK series are designed and manufactured in our own modern enterprise at the St. Petersburg State Electrotechnical University "LETI", taking into account modern trends in the development of the world instrument making industry. The use of sensitive CCD arrays and high-precision optical elements in the AK autocollimators allows the creation of reliable and convenient high precision instruments. Thus, one of autocollimators, AK‑0.25, is included in the register of approved measurement standards as the I echelon working standard of the plane angle unit. Specially developed software GonioScan AC (Fig. 7) allows to implement both single and multiple measurements, create scripts with different profiles of settings, save measurement results in various formats, etc. The intuitive interface of the software does not require special training from the operator. The interface is designed in such a way as to maximize the efficiency of the measurement procedure with an autocollimator. It should be noted that there are several other manufacturers of similar devices of lower accuracy class in Russia. Table 2 is a summary table of technical and operational characteristics of digital autocollimators of all major manufacturers, represented in the domestic market. GONIOMETERS Goniometers are used to measure the angles formed by the flat surfaces of various objects capable of reflecting light rays. Goniometers are modern precision angle measuring devices, which are currently used in a number of large optical and instrument-making enterprises in Russia, as well as in metrological institutes of other countries.
Like autocollimators, all goniometers are divided into visual and digital according to the manner of processing and presentation of measuring information. In a visual goniometer, angles are measured by an absolute method, i. e. by comparison with a precisely graduated arc (circular scale), rigidly connected to the stage where the optical part is mounted. The error of the instruments of this type is due mainly to the features of manufacturing the arcs, namely, the uneven application of a circular scale. An additional contribution to the error in measuring the angle by a visual instrument is the error of the operator associated with the subjective sensing of the operator (the person’s eye serves as the receiver of information on the angular readout). In the domestic market, visual goniometers are represented by GS2 and G5 (Fig. 8) produced by plant "Arsenal" (Kiev), which ceased to exist several years ago; despite the liquidation of the plant, most enterprises, laboratories and certification and metrology centers still use mass devices of these models. Foreign companies also produced visual goniometers (e. g., Moller Wedel devices Gonio II–VIS) (Fig. 9), however, to date almost all large manufacturers produce only digital goniometers. It should be noted that visual instruments, as a rule, are goniometer-spectrometers and, in addition to their objective function, can measure the refractive index of transparent solid materials. But with more extensive functionality, these obsolete products are very large, difficult to operate, not satisfying the modern consumer. Therefore, the digital devices replaced the visual instruments. In digital goniometers, a digital autocollimator is used as a telescope, which provides optical reference to the reflecting facets of the object by measuring the angle between its own optical axis and the normal to the reflecting face. As an arc mounted on the axis of rotation of the rotating stage, a photoelectric transducer of the angle converter is used, and the stage itself is usually provided with a drive that rotates it in accordance with the required measurement scenario. The use of such technical solutions makes it possible to virtually eliminate errors inherent in visual instruments. The current level of development of electronic computers provides a significant increase in the accuracy of measurements due to the application of various error compensation algorithms associated with the technological features of the manufacture of angle sensors (encoders). Finally, due to the complete automation of the measurement process, modern digital goniometers fully satisfy the user requirements in terms of usability and ease of use. Taking into account all the advantages of digital goniometers comparing to visual ones, large manufacturers of optical angle measuring equipment today prefer to develop and create devices of the former type. Among foreign companies, it is worth highlighting Trioptics (PrismMaster series devices) and Moller Wedel (Goniomat series devices) (Fig. 10). The analysis of the accuracy characteristics of goniometers of foreign production showed that instruments with an error of measuring the angle not more than one angular second are in demand (Table 2). Among domestic manufacturers, digital goniometers with this level of accuracy are produced and supplied only by INERTECH (SG‑1) (Fig. 11). Goniometer SG‑1 is equipped with a high-precision digital autocollimator with a wide range of measurements, insensitive to disturbing light effects and is a fully automated instrument with the multifunctional software GonioScan SG. The software GonioScan SG (Fig. 12) has the following features: • display in real time of the image from the camera – angular position of the reference cross in two coordinates; • registration of absolute and relative angular position in several dimensions; • registration of the angular position of the mobile object in the continuous recording mode, with the recording frequency determined by the operator; • automatic or manual adjustment of camera parameters and creation of various settings profiles for working with different types of objects and reflective surfaces; • recording measurement results in a file; • control of the servo drive of the goniometer table; • algorithmic processing of measurement data; • formation of measurement results files; • creation of test scenarios, i. e. automatic sequence of table turns. Goniometer SG‑1 is a modern high-precision device that meets all the requirements of the world’s angle measuring instrumentation. Table 2 shows the technical and operational characteristics of digital goniometers of all major manufacturers represented on the domestic market. CONCLUSION A review of the current state of the market of angle measuring devices has shown that not only expensive imported products are available to domestic consumers today, but also relatively inexpensive, reliable and not inferior in terms of accuracy to their foreign counterpart autocollimators and Russian-made goniometers. INERTECH serially produces digital autocollimators (AK series) and static goniometers (SG series), which do not have domestic analogues. Devices produced with INERTECH trademark are awarded with diplomas and medals of various specialized exhibitions and are in great demand both in the Russian market of measuring instruments in Russia and abroad.