Basic Test Plates: Two New and Relevant Uses in Optical Technologies
New possibilities are proposed for using the basic test plates (BTP) to solve two pressing problems in modern optical technologies – improving methods and means of metrological support for monitoring optical elements and increasing the productivity of manufacturing lenses and mirrors with spherical working surfaces. The solution to the first problem is based on the use of a reference set including a couple of BTP (convex and concave) and a reference on-axis computergenerated hologram optical element (CGHOE). To solve the second problem, it is proposed to use the BTP as reference masters with the subsequent production of copies of submatrices of specified sizes from them using precise replication.
A. V. Lukin , A. N. Melnikov
JSC “State Institute of Applied Optics”, Kazan, Republic of Tatarstan, Russia
Article received: 13.02.2020
Article accepted: 20.02.2020
In modern domestic and foreign optical production, a paradoxical situation is emerging. On the one hand, the classical “block” technology of serial production of lenses and mirrors with spherical surfaces is still being used using working test plates (WTP) for the technological control, in the manufacture of which control test plates (CTP) or BTP are still used [1, 2]. At the domestic enterprises that have traditional optical production, over many years of practice, a huge number of BTP of various standard sizes  has accumulated with the highest optical quality of spherical work surfaces.
On the other hand, the share of optical elements manufactured on modern precision optical machines with numerical control (CNC), as well as by hot molding and sagging [1, 4–6] without the use of test plates, is rapidly growing.
Currently, there is a growing trend in the world in the use of contact (profilometers) and non-contact (interferometers) measuring instruments to ensure technological and certification control of the processes of forming optical surfaces [1, 2, 5–8]. Therefore, the use of BTP is limited or eliminated altogether, since working test plates, if necessary, can be made directly, without the use of CTP and BTP. An urgent problem when using contact and non-contact measuring instruments at the stage of certification of finished products is the lack of their metrological support (calibration and verification) in the required range of optical parameters. A similar situation takes place in foreign traditional optical industries.
SETTING OF THE PROBLEM
The reference set consisting of an BTP pair of the first accuracy class and the first pair of mates with the selected nominal value of the radius of curvature in the range from 1 to 40 m together with the reference on-axis computer-generated hologram optical element (CGHOE) opens up the possibility of solving this problem.
It should be noted that BTP are always made only in pairs (convex and concave) in accordance with the requirements of  with a high conjugation class that guarantees the same radii of curvature of convex and concave surfaces. The “lapping” technology used in this case guarantees the spherical shape of their working surfaces (convex and concave), and also ensures the equality of their radii of curvature . Note that the manufacture of BTP requires highly qualified opticians.
In this case, the reference on-axis CGHOE is calculated, manufactured and certified [10, 11] based on the actual value of the radius of curvature of this pair of BTP, previously measured using the control on-axis CGHOE [1, 12]. The peculiarity of measuring the radii of curvature of spherical surfaces with the help of CGHOE is that it is not the radius itself that is measured, but its deviation from the nominal value reproduced by such CGHOE.
The on-axis CGHOE used in the ±1st diffraction operating orders is equivalent to both a convex and concave BTP with a given nominal radius of curvature.
Apparently, for verification and calibration work, it is advisable to use reference set of several pairs BTP and related CGHOE within the range of nominal values of the measured radii of curvature. The required number of elements in the reference set is determined by the type of the measuring instrument to be verified and the permissible measurement errors in a given range of radii of curvature and deflection arrows of the optical surfaces of revolution.
In particular, to perform verification of a contact profilometer, it is possible to use on three to five pairs of BTP from the reference set within working range of measuring the profilometre deflection arrow, as well as the profile shape of the optical surfaces of revolution. The difference in the measurements of the radii of curvature of the convex and concave BTP for each pair, as well as the deviation of their measured profiles from the circle, will obviously characterize the accuracy parameters of the verified profilometer.
Apparently, three to five OPS couples and related CGHOE are also sufficient to carry out calibration and calibration work on interferometric measuring equipment constructed according to the Fizeau or Twyman-Green scheme. Using the convex and concave reference BTP and the reference on-axis CGHOE used in the ±1st operating orders, four interferograms for each of the nominal radius of curvature are obtained and decoded. Thus, up to 20 interferograms can be recorded and decoded, each of which characterizes the deviation of the wave-front formed in the working branch of the interferometer from a given spherical shape , which is an exhaustive quantitative characteristic of wave aberrations of verified interferometric measuring equipment. At the same time, the measured deviations of the radii of curvature of the spherical surfaces of the convex and concave reference BTP and the radii of curvature of the geometric wave front reconstructed by the standard SHOE in the ±1st orders uniquely characterize the accuracy of the measuring system of this interferometer.
The proposed technical solution will make it possible to calibrate and verify measuring instruments for the radii of curvature of spherical surfaces, and interferometers (optional) according to local and general errors of the controlled wave-front, which, in turn, will improve the methods and means of metrological support for the needs of optical technologies.
An equally important and urgent problem of modern optical technology is the need to increase the productivity of serial and mass production of lenses and mirrors with spherical working surfaces.
Estimates show that the cost of the process of forming spherical surfaces with “block” technology and using optical CNC machines remains high. Therefore, the practical implementation of the possibilities of shaping optical working surfaces of any shape, including spherical, by the method of precise replication based on the use of low-shrink polymer compositions  can significantly increase productivity compared to the methods used in practice and realize, in particular, the conveyor principle of shaping, moreover, in this case, the laboratory assistant level of workers is sufficient.
It is proposed to use the thousands of available BTPs that are released at the same time as reference masters in the process of implementing precise replication technology  for mass and mass production of lenses and mirrors with spherical working surfaces. In this case, the most important and most expensive stage is the manufacture and certification of the primary master a priori already performed with the highest accuracy  and does not require additional costs.
At the first stage, the traditional “block” technology and CNC optical machines under these conditions are mainly proposed to be used to produce lens and mirror blanks with finely ground, medium and low precision work surfaces. In the future, to increase productivity, these same operations can be performed using the technology of hot molding or sagging.
At the stage of the final shaping of the spherical working surfaces of lenses and mirrors, submatrices of specified sizes are used as part of the implementation of the precise replication process . Moreover, these copy-submatrices are also made by precise replication with BTP, performing the function of reference masters.
This approach opens up the possibility of realizing the presented proposal with minimal costs for the preparation of serial and mass production of spherical optical elements. It is important to note that in this case, as experimentally established in our practice, the cleanliness class and roughness parameters of replicated surfaces practically coincide with the corresponding parameters of the working surfaces of the masters, and the retention of elements of replicated optics, in particular in heated rooms, exceeds 15 years. Note that the replication process has a low probability of damage to the working surfaces of the masters (in this case, BTP) due to the lack of solid abrasive particles in the used polymer compositions, as well as the simplicity of the organization of “clean” production due to the compactness of the technological equipment used.
It is very useful, from the point of view of expanding the range of functional capabilities of replicated optical elements and components, in particular, single lenses and double-glued glues, to use thin adjustment layers preliminarily applied to the workpiece surfaces during replication .
Thus, the implementation of these proposals on the use of BTPs to perform additional and previously inappropriate functions with their help opens the way to solving urgent and interrelated problems:
providing calibration and verification of measuring instruments used in optical technology for the control of spherical surfaces, based on the manufacture and use of reference sets containing the required number of BTP and corresponding reference axial CGHOE for nominal values of the radii of curvature, while the required number of reference sets to meet the needs of domestic metrological services can be made by replication using the method of precise replication from primary reference BTP and CGHOE;
increase productivity and guarantee the identity of the spherical surfaces of optical elements; in this case, the entire accumulated BTP base can be used as reference masters, and the traditional “block” technology and modern CNC optical machines can be used more rationally for shaping lens and mirror blanks with spherical working surfaces of medium and low accuracy, moreover, precise replication does not require the involvement of highly skilled workers and can be implemented in a conveyor form.
From the foregoing, it can be concluded that it is advisable to organize a branch bank of reference BTP of the first accuracy class and a high conjugation group that contains all their nomenclature provided for by the standard  (2712 pairs of convex and concave spherical BTP). This will ensure the unity of measurements and control in the industry of the basic optical parameters of spherical surfaces.
At the same time, we propose to use the same bank at the same time as a bank of reference masters when organizing serial and mass production of lenses and mirrors with spherical working surfaces using precise replication methods [13, 14].
And all this as a whole, apparently, is inevitable, will lead to a significant change in the infrastructure of modern optical production, which will become significantly more productive, naturally, provided that it is staged and as a result of the implementation of the corresponding research and experimental work.
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Patent RU2019129406 (state pending acceptance). A method of manufacturing a combined optical element / Lukin A. V., Melnikov A. N.
A. V. Lukin, Doctor of Engin. Sciences,
JSC “State Institute of Applied Optics”, Kazan, Republic of Tatarstan, Russia, firstname.lastname@example.org.
A. N. Melnikov, Candidate of Engin. Sciences, JSC “State Institute of Applied Optics”, Kazan, Republic of Tatarstan, Russia.