 Welcome back to the metrology lecture series. Now, we will start module 12 lecture number 8. In this lecture, we will be covering the following topics. We will continue the discussion on the optical system design in which we will be studying lens assembly, measuring and aligning of lenses, cementing and bonding of lenses in which we will cover the manual cementing of lenses as well as automated bonding of lenses and then how to mount the mirrors in the optical systems and then we will discuss about some complex opto-mechanical assemblies and finally, we will be discussing about testing and certification services connected with the metrology. Now, we will start the discussion on lens assembly, we can see here an optical assembly. So, this assembly consists of two play seven lenses, lens 1, 2, 3, 4, 5, 6 and 7 lenses. Seven lenses is plano concave lens and lens number 3 is also plano concave lens. Now, lenses 1 to 4, they form sub-assembly number 1. So, I can observe here this is the barrel, it is a split barrel you can see here. So, this portion is sub-assembly number 1, it consists of 4 lenses and you can see the retainer rings and here also we have retainer ring to retain the lens and then between the two retainer rings we have shim plates for adjusting the axial alignment of lenses. Now, sub-assembly number 2 consists of again three lenses, lens number 5, 6 and 7. So, these three lenses are assembled in another split barrel. So, this is the second barrel in which these three lenses are mounted and now you can observe in sub-assembly number 2 we have stray light baffle, interior stray light baffle which will reduce the stray light inside the barrel and since the stray light is reduced the contrast of the image will increase. Now, we can see the sub-assembly number 3 consists of the electronic circuitry these CCDs attached to the mosaic plate that we can observe here and also we can observe focal plane electronic circuit boats. Now, all the three sub-assemblies are assembled using appropriate mechanical fasteners. Now, we will see the measuring and aligning of lenses. Now, when the different lenses are assembled in the barrel, now we have to align them properly so that the optical axis of all the lenses aligned with the barrel axis so that we get a quality image. We can see an assembly of lens here which is properly mounted, aligned and mounted. We can see the barrel body here and we have placed the double convex lens in its seat. Now, we can measure the alignment by rotating the lens we can measure whether there is any wobbling so that for measuring the wobbling we can either use a mechanical measurement system or optical measurement system. A dial indicator is a simple mechanical measuring unit we can use the feeler gauge also for measuring the alignment and wobble. So we should have a rotating lens system normally mounted on the ear bearing so that the out of plane moment is reduced. So we have to slowly rotate the rotating lens system and the measurement system will measure the wobble and if there is any wobble we have to adjust lens saturation either by tilting or shifting the lens as we can see here. We can tilt the lens or we can shift the barrel otherwise we can shift the lens and tilt the barrel for aligning the lenses with the barrel. Once they are aligned we have to apply the polymer and we should fix the lenses in the place. Now, we can see the measurement and alignment of the lenses on the lens rotation device. So this is the barrel mounted on the rotation device which is mounted on ear bearing. So we have to rotate this cell and you can see here now in the first picture the optical axis of the lens C1 C2 the line joining C1 C2 is the optical axis of the lens that we have to determine using the measurement device. And then we have this is the mechanical axis of the barrel now there is some misalignment. So we have to adjust the mount or cell mount is aligned to the optical axis lens we have to tilt the mount so that the axis of the mount coincides with the optical axis and then we have to machine the edge of the mount edge of the mount is machined so that it coincides with the optical axis. Now you can see in the third picture with the optical axis of the lens is coinciding with the mount axis. So this measurement and alignment either we can do manually or we can always use some automatic aligning machines which uses the software for measurement of centering error and for alignment purpose. Now you can see a set of pictures here. So in picture A we have this is the axis of rotation this is the cell and this is the seat for lens and lens is placed the seat and then after proper alignment that is either by shifting the lens or by tilting the lens we have to see that optical axis of the lens coincides with the cell axis and then after adjustment after centering we have to apply the adhesive UV adhesive can be used so that the lens is fixed in its seat. Now the for adjustment we can always use a PGO actuator for tilting and shifting of the lens. So we can then picture C and PGO we can see how the alignment is carried out using the PGO actuator once the lens is centered we have to apply the adhesive so that it is fixed to the cell body. Now we can always use a lens alignment software. So if we use automatic centering and bonding machines it uses a software for measurement of centering error and for adjusting the for eliminating the error for adjusting the lenses. I can see the video display which shows the centering error we can always measure the radius of lens thickness of lens and then we can calculate software will calculate what is the amount of centering error and then alignment is carried out so that centering error becomes zero. Now sometimes two or three lenses are bonded together to make a doublet or triplet. Now we should understand why this bonding of lenses is required. So now we know that chromatic operation is a sort of error that happens in optical system. If we use a single lens as shown here this lens will split the white light into its component colors. We can see here red light is focused at a longer distance from the lens surface and blue light is refracted more and it is focused at a little distance from the lens. So because of this the chromatic operation occurs and the image will not be sharp. So to eliminate this chromatic operation two or three lenses are bonded together and now we can see in this picture two lenses are cemented or bonded together. Now in this case white light it enters the first lens and again they are refracted in the second lens and finally the components different components of white light they are focused at the same focal point and then we get a very sharp image. So to eliminate chromatic operation cementing of lenses is needed. So different adhesives are available in the market polyester, epoxy, acrylate and puritine based adhesives are available, ultraviolet, visible light curable and thermal curing adhesives are also developed for bonding the optical elements. So curing is possible in a few seconds. So the cement should be optically clear with excellent light transmission capability and the shrinkage of the cement should be very-very less in order to prevent stresses in the lenses. Now let us understand the manual cementing of lenses we can see in this picture we have set of lenses, two lenses are there which ought to be cemented and they are placed in a lens holder and then we have an arrangement, a rotation arrangement which rotates the lenses and this rotation arrangement is placed on a bearing for precise rotation of the lenses. The cementing of two to three lenses to a doublet or triplet is a very hard process, time consuming and cumbersome process. It requires an accurate rotation device whose axis serves as a reference axis, a sample or lens holder is needed which must be very accurate and precisely aligned to the reference axis. Now we should align the lens holder until the axis that means we have to place the first lens in the lens holder and we have to rotate the lens holder and we have to align the holder, lens holder until the axis of the bottom lens coincides with the rotation reference axis that means by tilting or shifting of this bottom lens we have to make sure that the axis of first lens coincides with the axis of reference axis and then we have to place the second lens which is to be cemented, before placing we have to apply, we have to apply the cement on the surface of the first lens and it should be spread properly and then the upper lens is placed and again it should be tilted and shifted so that the optical axis of the second lens coincides with the optical axis of first lens and we should see that the combined axis of the doublet coincides with the reference axis so that way we have to manually adjust the lenses, the shifting and tilting of lenses we should make and finally we should we will get a doublet or triplet so this is a very time consuming process and it largely depends on operator skill. Now in order to make the cementing easy and effective automated bonding systems are developed these systems automatic bonding systems accurately measure the position of the center of curvature of the lenses to be cemented relative to a reference rotation axis. So this process takes 1 or 2 seconds and the accurate measurement is possible with sub micron accuracy. The measured data is transferred to an automatic alignment device which is equipped with appropriate lens manipulators such as Bejo actuators. So these you can in the diagram you can see the Bejo actuators. These actuators shift and tilt the lenses so that the optical axis of lenses coincides with rotation axis. The upper lens is aligned to the bottom lens with an accuracy of 1 micron and then cement is applied and then second lens is placed. The doublet axis that means axis of the two lenses is aligned with the reference axis again it takes a few 1 or 2 seconds and they are now let us discuss about automated bonding of lenses. The automated bonding systems accurately measure the positions of the centers of curvature of the lenses relative to a reference rotation axis. So they can measure the positions in 1 or 2 seconds with sub micron accuracy. So in this diagram we can see the arrangement of automated bonding system. The measurement data is transferred to an automatic alignment device equipped with appropriate lens manipulators such as Bejo actuators. So you can see the arrangement here. So the two lenses are placed on a rotation device which is mounted on a bearing. So the lenses are rotated and the optical axis of the lenses are determined and by using these manipulators the lenses are shifted and tilted so that the optical axis of lenses coincide with the rotation reference axis. The alignment of lenses is carried out to an accuracy of 1 micron and cement is applied and cured. The complete cycle including measurement, alignment, bonding and curing takes place in less than 10 seconds. Now I can see a close view of a bonding station with three actuators. We can see here we have one actuator here, the second actuator here and the third actuator here. By using three actuators normally Bejo actuators the lenses can be moved in XY plane and then they can also be tilted so that the axis of lenses coincides with rotation axis. So in this view we can see an arrangement for rotating the lenses. This is the stage, rotating the stage on which the lenses are placed and this is the motor to rotate the rotation device. Now the position of lenses is determined by using electronic auto columnators. So the lenses are rotated through 360 degree once around the reference axis and then center of curvature of lens surfaces are determined with respect to the axis of rotation and these data are transferred to a software. The software will calculate what is the amount of saturation error and signals are sent to the lens manipulators and lenses are adjusted to optical axis cemented and cured. So you can see the arrangement of auto columnator and this is the lens rotation device and this is the lens placed on the lens rotation device and this is the axis of rotation. Also sensors are provided to determine the shift and inclination of cell. So if necessary the cell is also shifted and tilted for aligning the rotation axis or the cell axis with lens axis. So here we can see a lens alignment and cementing station. The alignment of lenses takes as little as four seconds with a centering accuracy of 2 micro meters. Now we will start the discussion on complex optomechanical systems. Optomechanical design is a sub-discipline of optical engineering in which optical elements such as lenses, mirrors and prisms are integrated into mechanical structures such as cells, housings, tosses etc. so has to form an optical instrument. So the designing of optical instrument needs a co-operative efforts from different team members, members from lens designers, optical engineers, mechanical engineers, electrical and electronic engineers and software engineers. The input from the various experts in fabrication, assembly, alignment and testing as well as input from specialists on light sources, detectors, focal plane arrays, electronic systems and distal signal processing is very essential to build an optomechanical instrument. When we want to design an optical instrument we have to consider the manufacturing aspects and design for manufacturing and assembly and design for maintenance of the instrument and also we should consider the ergonomical aspect so that the instrument becomes easy to handle and then we shall also see aesthetic aspects and it should be compact so that it can be made portable. So when we consider all these aspects the system becomes very complex. Now in the case of optical instrument design it starts with a need statement for a particular instrument and it undergoes designing the undergoes with several phases such as feasibility study. So we should see whether it is really economically and practically feasible. We should see the preliminary design in which we fix the broader dimensions and then we should go for the detailed design of individual components. We should plan for production, distribution, consumption and finally we should plan for the retirement of the instrument. So along this way the team should consider the operational and survival environments that is the working conditions such as temperature, humidity, contamination whether there are any chances of vibration shock and is it necessary to seal the instrument so such things one should consider. So knowledge of the cost, projected cost of fabricating the device and how to maintain what is the maintenance cost that also we should consider. What is the total life cycle cost of the instrument we should properly assess and proper choices in instrument configuration, materials and dimensional tolerances are very essential to control the cost of the instrument and then we should always use the intuition and experience with unknowns verified through analysis and testing and team members must make proper decisions in the five basic design categories such as materials aspect, what is the material of optical system, what are the materials used for the structure, mechanical structure and then structural design aspect and then how to mount the lenses, the lens to mount interfaces and then the mountings for prisms and mirrors and how to assemble and how to align the instrument so in these basic areas team membership properly decide. Now we will discuss about the material decisions. So we should select the material based on density because that tends to reduce the total weight of the instrument. We should select the material of barrel and lenses with this weight and then another important thing is we should match the coefficient of thermal expansion of materials used in the mechanical and optical parts to minimize the differential expansion or contraction due to temperature changes. So in this diagram we can see how the lens behaves at different temperatures. At 20 degree centigrade the focal point is at this place and due to the rise in temperature, when the temperature rises to 80 degree centigrade there is deformation of the lens and the focal point moves nearer to the lens. So because of this the image becomes blurred. So the selection of optical materials and mechanical materials is very very important to have clear image at different temperatures and we should heat treat all the metal parts to maximize their dimensional stability and we should always use adhesive and sealants with low outgassing properties for vacuum applications and choosing elastomers is also very important. We should always choose elastomers with minimal coefficient of thermal expansion and low shrinkage during curing because the shrinkage is more that will lead to internal stresses in the lenses. And then the structural design aspect is our important thing in designing the optomechanical systems. The design should be very much stable enough to control the effect of gravity and other external forces such as shocks and thermal effects and then the structure must constrain the optics such that they are not damaged or irreversibly moved when they are exposed to extreme in environment conditions such as extreme temperatures and very high shocks etc. And to minimize the effects of temperature changes we should make this structure to be passively or actively a thermal that means structure should become insensitive to temperature such a design should be used. And we can see here one example of how we can make the structure passive a thermalization. So this diagram shows cell the barrel mechanical structure with lens the top of of the picture is not made passive thermalization that is normal structure. So due to say we can see two working temperatures 20 degree and 80 degree centigrade. So when the temperature rises there is expansion of lenses and structure because that lens moves towards left in the left direction and then image becomes blurred. Now how we can make this passive a thermalization we can see here there is a spacer the length and material of the spacer are so chosen that they attain the system attains the spacer attains thermal expansion equal and opposite to the combined effect of the lenses and housing over the temperature range. Now when the temperature increase you can see the structure the passive a thermalization structure has an external cell and inside there is inner cell and then there is a spacer and the lens is mounted on the inner cell when the temperature rises the spacer expands in the opposite direction. So the spacer pushes the inner cell back to the right position this spacer pushes the inner cell back to the right position. So the effect of thermal expansion or contraction is nullified. So this way we can make the structure passive a thermalization. So this needs extra mechanical complexity that means we have to provide a inner cell and then spacer. So this results in a bigger and heavier and more costly solution than simple mount. Now we should always select a proper lens material for a thermalization that means normally germanium, zinc, selenide and zinc sulphide can be used to maximize performance without excessively tight tolerances or dimensions. We can design a carefully optimized number of mechanical adjustments into the instrument and design structure should have maximum stiffness within the weight and packaging constraints to reduce deflection from external forces such as gravity. And we should always isolate the supported optical components from mechanical response effects under vibration conditions. Proper lens to mount interfaces should be used while assembling lenses in the barrel. For best results we should design the metal reference surfaces to interface with most accurately polished surfaces. We can observe here that this is the optical surface and this is another optical surface which are made more accurately than the rim of the lens. So we should use the optical surface for mounting purpose and provision should be made for pre-loading the lenses. Lenses should be pre-loaded axially for the maximum expected acceleration loads at extreme anticipated temperatures. There are chances that pre-load changes with temperature so this point also should be considered. And always we should avoid over pre-loading since it will cause the contact stress. Lens to mount interfaces should be designed for low axial contact stress and we should use appropriate arrangement for interface of retainer ring with the lens. Conical metal surface can be used to touch convex lens surface tangentially as shown here or we can use convex toroidal interfaces to touch concave lens surface as shown here and we can use flat metal surfaces to interface with flat surface and the lenses as shown in this picture. And mounting of mirrors and prisms is also very important when we design the optical system, optometallic system. Flat pads touching flat surfaces of the optic must be lapped co-planar prior to assembly proper lapping should be made so that contact is good. We should use the hinge type mounds using multiple levers and arrays of pneumatic and hydraulic chargers to support large mirrors. Multiple point support should be used to support the localized portion of the mirrors weight at the support points. We should support larger mirrors at multiple points around their rims and their backs to minimize the gravitational distortion. That means the mirror should be support large mirror should be supported at multiple points on the surface and also support should be given at rims. You can see the different arrangement mounting arrangements for mirrors angle mirror mounts. So we have adjusting screw for adjusting the angle of mirror the gimbal mounting is also possible. Micrometers are provided for finer adjustment of the angle of mirror and here you can see the kinematic mirror mounts. So this is the actuator screw by rotating this the angle of the mirror can be changed. Now this picture shows the hindle mount. Now multiple support points are available. We can see the triangular plates. Each triangular plate has three supporting points. So large mirrors can be supported by using the hindle mount as shown here. You can see a photographic view of the hindle mount. So tetrahedrons are there made of iron to have the lighter structure. So here we can see spring clip mounted flat mirror assembly. You can see the co-planar support pads here. So depending upon the size of the mirror multiple supporting can be made and this is a 45 degree mirror adapter and this is a kinematic mirror mount. So you can see the actuator for adjusting the inclination. We can also have a smart structure to support very big mirrors. So you can see here supporting points. So active damping can be used to support the mirror big mirrors. And we should use appropriate prism holders. You can see the prism mounted on a flat surface and there is a vertical pillar and there is a clamp. We can see the weakened clamp here made of scratch resistant delrin material. So because of this the damage to the prism is prevented using this screw. We can clamp the prism properly and we can also have rotary prism mount which allows smooth manual 360-degree rotation of the prism by operating this rotary device. We can change the inclination. The fifth important design decision is about assembly and alignment of optics inside the mechanical structure. We should clean all the parts, all optical parts and mechanical parts thoroughly and then we should carry out the assembly process in a very clean and dry environment. We should always use approved lubricants and we should apply them very carefully to avoid contamination. And for multiple lens assemblies we should rotate the lenses and make adjustments so that axis of lenses coincides with cell axis. We can always go for a manual lens assembly process or we can use automated alignment stations if necessary. And then the adjustment should be logged after the optics are aligned by means of mechanical clamping or epoxy pinning or laser welding or by soldering so that the position of optics is fixed. So we should seal the optical instruments after the assembly is forward to protect optics from moisture and particulates. We can always purge the instrument with dry nitrogen or helium. Now let us see some complex up-to-mechanical assemblies. So we have optical microscope. The external appearance of the optical microscope is shown here. So we have the mechanical structure, the C-shaped structure which supports all the other optical elements. We have a stage on which a workpiece to be inspected can be placed. We have various screws for adjustment, for focusing of the workpiece. And then we have a turret, objective lens turret. So different objective lenses are provided on the turret. So depending upon the application we can select appropriate objective lens with required magnification. And then we have eyepiece sub-assembly and then there is a camera attachment. So this is photo camera attachment. And then we have another sub-assembly that is light source sub-assembly. We have a light source for surface elimination and we have another light source for contour elimination. Now all these sub-assemblies are carefully aligned and assembled. And then each sub-assembly is mounted on this C-shaped body by taking proper care so that we can have accurate image of the workpiece and we can inspect the workpiece easily. Now we have another optomechanical assembly here. This is a power focaling microscopic objective. Now power focaling, it refers to the objectives that can be changed with minimal or no refocusing. Now power focaling objectives allows us to adjust each objective lens to remain in relative focus with other objective lenses when switching from one magnification to the other magnification. We can see here we have a turret assembly on which multiple objective sub-assemblies are mounted. So if we have power focaling microscopic objective, so when we change the objective to get a different magnification, we do not have to do the refocusing. Here we can see a very complex nature of the mechanical structure. We have the knurled surface on the structure that we can see here this is knurled surface. And then we have the main barrel in which lens assembly is mounted. And then we have power focality adjustment sleeve. By adjusting this sleeve we can make the objective power focaling. And then we can see the lens assembly here we have doublets and we have another doublet and we have connex surface lens here. And then we have a centering screw for coma removal in the assembly by operating this we can remove the coma. And then we have a spacer here. So this spacer is selected to remove spherical aberration in the assembly that means properly spacer is chosen to remove aberration. And then we can see a power focality lock nut after adjusting the sleeve to have proper focusing. This lock nut is used to lock the sleeve in its position. Now alignment accuracy of this objective is 5 micron and then 5 micron alignment accuracy is possible. Now we have another eyepiece of a military telescope. We can see the cemented lenses lens doublets lens doublets all the three are lens doublets. They are properly aligned optical axes are aligned and then they are cemented and then they are placed in the mechanical structure. The unique feature of this design is use of rubber bellow that is static and dynamic seal is accomplished with a rubber bellow here and the tube is sealed to prevent air entry. So in this precision optical assembly we can see precision lens mounting. You can see the arrangement how the lenses are mounted and the mounting of precisely machined lenses in the lens mounts. And then we can also see the electrical drives for making adjustment in optical elements and mechanical elements. And in this we can see lithography projection lens system. We can see very complex nature of the lens assembly. Some lenses are cemented and some individual lenses are mounted in the mechanical structure. We can see the planocon base, planocon cave and other types of lenses properly aligned axially as well as radially. We can make the electronic circuits with very precise features and features sizes of as small as 250 nanometers can be made with this lithography projection lens assembly. And we have another important system here. This is extreme ultraviolet lithography system EUV lithography system. So this lithography system is an advanced technology for making microprocessors a hundred times more powerful. So this system helps to develop a microchip with etched circuit lines smaller than 0.1 micrometer in width. In other manufacturing methods line widths of greater than 180 nanometers are possible. Whereas in this system the circuit line width can be as low as 100 nanometer or even lesser than 100 nanometer. You can see here in these pictures we have lines, circuit lines which are smaller than 100 nanometer. Here it is 31 nanometer, 29 nanometer line width, 28 nanometer line widths are possible with this sophisticated system. A microprocessor made with this technology is a hundred times more powerful than microprocessor made by other methods. Memory chips would be able to store 1000 times more information if they are made by this technology. We can see a very complex nature of optomechanical system here. Now let us discuss on testing and certification services. Many organizations are available throughout the world offering testing and certification services related to metrology. They test the various components precision components traceable to NIST standards and they are also offering the calibration services. They calibrate various metrological instruments according to international standards and they shoot calibration certificates. So one such organization is JIGO testing and JIGO Corporation offering testing and certification services. Throughout the world they have many centers with testing facilities. JIGO Corporation specializes in optical metrological systems. JIGO's metrology systems are based on optical interferometry measuring displacement, surface figure and optical wave friendly. State of the art equipment for testing the following parameters are available with JIGO. Roughness measurement of precision surfaces, air bearing surface geometry of magnetic heads, optical components and systems such as flats, spears and prisms can be tested. Reflective and anti-reflective coating facility and testing facility, surface angle measurement and an absolute calibration of spears and flats as well as large aperture planar surfaces is possible with the testing facility available with JIGO. Now here we can see JIGO's 3D optical surface profiler. They have different facilities, 3D optical surface measurement facility based on white light interferometer systems offering the fast non-contact high precision 3D metrology of surface features. So different surface profilers are produced and used by JIGO for testing purpose with different configurations like bench type configuration, workstation configuration and portable configuration. These profilers have surface topography repeatability of 3.5 nanometers and 0.15 nanometers and 0.018 nanometers such a fine repeatability these instruments have and they have XY stage automated XY stage fixed to the profilers and then vertical scan speed varies from 15 micrometer per second up to 96 micrometer per second. Based on the customer's application demands, testing and calibration services are offered selecting appropriate profilers. All these profilers are non-contact profilers. So these profilers can be directly mounted on work pieces. Since they use non-contact technology there is no risk of heart damage. Operation in virtually any environment without the need for vibration isolation is possible. Another important organization offering testing and certification services is Central Manufacturing Technology Institute situated at McEluru. The metrology laboratory attached to CMTI is accredited by the National Accreditation Board for Laboratories and is equipped with precision calibrating equipment in precisely controlled environmental conditions. Services offered by this laboratory are mentioned here. Calibration of gauges and measuring instruments with traceability to international standards and then dimensional form and surface finish measurements of components. Supply of high precision grade granite surface plates and straight edges. Calibration and supply of reference masters like precision spears, roughness masters and master cylinders. The CMTI has very good nanometrology facilities. They have con focal microscopy for 3D imaging and surface topology studies. They have flatness interferometer which can be used for calibration of optical flats and measurement of radius of curvature of optical lenses. A ultra precision coordinate measuring machine is available with CMTI which can be used for dimensional measurements of very complex micro components and also for tactile and optical proving system. A gauge block interferometry is also available with CMTI which can be used for calibration of slip gauges to accuracy of 20 nanometers. CMTI has an optical profiler which can be used to study the dynamic behavior of MEMS devices and to study the surface measuring surface roughness at nanoscale study of coating film thickness and study of wear on inserts. They have plasma enhanced chemical vapor deposition system which can be used to deposit thin films of carbon nanotubes and graphene on surfaces and for the application of coatings for tool inserts. And then they have very good characterization facilities such as spectroscopic ellipsometer which can be used to measure the coating thickness in the range of 1 nanometer to 10 micrometer and it can be used to measure the refractive index of the coated surface and then there is a Raman microscope which can be used for identification of materials and phases. Field emission scanning electron microscopy is also available with CMTI with a resolution of 1.1 nanometer and this can be used for high resolution imaging and microstructural studies. Atomic force microscope is also available which can be used for surface morphological studies and to measure the electric and magnetic properties of materials and for the measurement of surface roughness at nanoscale. X-ray diffractometer is also available which can be used for identification of material, identification of structure of material and for measurement of residual stresses. Nano indenter is also available which can be used to measure the fracture toughness, friction coefficient, Young's modulus of materials, hardness of materials, crack propagation and thin film testing. Now let us summarize the module 12 lecture number 8. In this lecture we discussed about lens assembly, measuring and aligning of lenses in optical systems, cementing and bonding of lenses, automated bonding of lenses and then how to move mirror in optomechanical systems and then we discussed about some of the complex optomechanical assemblies like lithography systems and then we also discussed about testing and certification services offered by many organizations throughout the world related to the metrology. With this we will stop this lecture. Thank you.