 Welcome you all for module 9 lecture 2 on interferometry. In this second lecture we will discuss about the different types of interferometers. We will discuss the construction and working of NPL flatness interferometer, bitter NPL gauge interferometer and also we will study about laser based interferometer and then we will see some commercially available gauge block interferometers. Now let us discuss on NPL flatness interferometer. We can see the schematic diagram of NPL flatness interferometer. We have a base plate on which the workpiece to be inspected is placed and there is a provision for keeping the optical flat and at the other end we have a light source, so normally mercury vapor lamp is used with a lambda of 0.5 micrometer and we can see there is a condensing lens to condense the light to a point and the condensed light will pass through a green filter so we get green monochromatic light source with wavelength of 0.5 micrometer and at the focal point we have a pinhole through which the light will pass so here we get an intense point source of monochromatic light. The pinhole is placed in the focal plane of the collimating lens so we have a collimating lens here and pinhole is placed at the focal point of this collimating lens so we get a parallel beam of monochromatic light which will fall on the workpiece to be inspected and then the light will pass through the optical flat it will fall on the workpiece and the light will get reflected and at this place we have a glass plate reflector so reflected light will get deflected and it will fall at the detector so through the eyepiece we can observe the fringe pattern now the entire optical system is enclosed in a metallic cover you can see here the mercury vapour lamp and then the entire optical system is enclosed in a metal cover we can always adjust the height of this optical system to accommodate the workpieces of different heights and then the optical flat is mounted on adjustable tripod stand and its angle can be adjusted to get the required fringe pattern the base plate can be rotated so that the fringes can be oriented as per the requirement so through the eyepiece we can observe the fringe pattern here we can see a display unit is interfaced with the interferometer so that we can observe the fringe pattern in the monitor now when the surfaces of the workpiece say we have a workpiece which is to be inspected like this and this top surface of the workpiece is parallel to the bottom surface that means these two working surfaces of the workpiece they are parallel and if this is the case we can see here we can observe the fringe pattern like this now we can observe that these fringes are obtained by the base plate and these fringes are obtained from the surface of the workpiece if the two surfaces of the workpiece are parallel to each other and the surface of the base plate is parallel to these two surfaces then we obtain an equal number of fringes from the base plate as well as the workpiece and the width of the fringe will also be equal and the number of fringes will also be equal if the workpiece surface is not parallel now we can observe here that this surface of the workpiece or the gauge is not parallel to this then the number of fringes obtained will be different from the number of fringes obtained from the base plate which indicates that the two surfaces of the gauge workpiece are not parallel when thinner slip gauges are used which are thinner than 25 millimetre then the interference fringes are formed both on the gauge surface and the base plate as we observed in the previous slide as the gauge is wrung on the base plate its underside is parallel with its base plate that means this is the base plate and this is the workpiece the underside is parallel with the base plate this means that if the gauge faces are parallel this surface and the bottom surface of the gauge if they are parallel the fringes on the base plate should be equally spaced and parallel with the fringes on the gauge surface now if the gauges or workpieces are thicker than 25 millimetre now we can observe here we have placed a gauge which is thicker than 25 millimetre this is the rotary base plate on which we have placed the thicker gauge plate then the fringe pattern on the base plate is difficult to observe because of this large distance but the base plate is rotary and its underside is lab truly parallel with its working surface so if a non-parallel gauge is viewed now you can see here this surface of the gauge is not parallel with this with this surface if a non-parallel gauge is viewed the angle it makes with the optical flat so that is this angle angle it makes with the optical flat will be as in case A that means this is the angle that is made if the table is turned through 180 degrees now we can observe here the table is completely turned through 180 degrees the surface is now less parallel with the optical flat that means we have a greater angle between the gauge surface and optical flat and a large number of fringes is observed when the angle increases we know that the number of fringes will increase now the error in parallel that means the error in parallel between this surface and this surface can be calculated using this relationship N2 minus N1 times lambda by 4 where N1 and N2 are number of fringes obtained in the first and second position by inserting N1 and N2 we can find what is the amount of error in parallelism between the two surface of the workpiece now let us try to solve some numerical problems so that we can understand the basis in a better manner now how do we check the parallelism error the error in the parallelism of the workpiece surfaces a slip gauge is being inspected using an NPL interferometer the gauge exhibits 10 fringes along the width in one position now we know the gauge is placed on the rotary base plate in one position we observe 10 fringes and it is rotated through 180 degrees and we observe 16 fringes in the second position if the cadmium light source is used in the interferometer determine the parallelism error over its width now wavelength of cadmium light source is 0.5 micrometer number of fringes in the first position that is N is equal to N1 is equal to 10 number of fringes in the second position N2 is equal to 16 now error in parallelism is obtained using this relationship N2 minus N1 times lambda by 4 so N2 minus N1 is 16 minus 10 that is 6 and lambda is 0.5 micrometer divided by 4 which gives 0.75 micrometer is the error parallelism error between the two working surfaces the gauge that means we use the NPL interferometer to count how many fringes are there in the first position how many fringes are there in the second position and by knowing the lambda value of the source we can find the error in parallelism now let us see how we can check the height of slip gauges using interferometers the problem is like this and an optical flat is used to check the height of slip gauge against a standard gauge of height 20 millimeter that means the given slip gauge is compared with the height of standard gauge of height 20 millimeter cadmium light is used in the NPL interferometer if the number of fringes on the gauge width of 15 millimeter is 12 and the distance between the two blocks is 13 millimeter calculate the difference in height of the gauge being inspected that means we have a gauge of 20 millimeter height and then another gauge is placed on the base plate so this is the base plate so this height we have to check the width of this gauge is 15 millimeters and distance between the two blocks is 30 millimeter that means this distance is 30 millimeter now the difference in height h can be calculated using this relationship N lambda by 2 times N by L where N is the number of fringes observed by the interferometer in this case number of fringes on the gauge width of 15 millimeter is 12 so N is equal to 12 distance between gauges that is L is 30 millimeter and L small L is width of gauge inspected is 15 millimeter so N lambda by 2 times L upon L is N is 12 lambda is 0.5 times capital N is 30 small L is 50 divided by 2 so this gives 6 micrometer that means the difference in height of the given gauge when compared with the standard gauge of 20 millimeter height is 6 micrometer now different fringe patterns we get depending upon the surface condition of the workpiece you can observe here we have different fringe patterns A to F and the corresponding surface condition is also mentioned here we get the straight parallel fringes when the block, the surface of the workpiece or the gauge block is nearly flat along its length we get straight fringes parallel to one edge now in this case we have convex fringes so this is case B fringes curve towards the line of contact so here this is the line of contact fringes curve towards the line of contact showing that the surface is convex and high in the center the surface is high in the center like this now the case C is surface is concave and low in the center we can observe here this we get concave fringes and they are low in the center this is the case then surface is concave in case of D surface is flat at one end so here the surface is flat and becomes increasingly convex at the other end now here we are observing convex fringes indicating that the surface is convex at the other end and in case of E surface is progressively lower towards the bottom left hand corner and here we observe that there are two peak points case F there are two points of contact which are higher compared to the other area so like this depending upon the surface condition we get different fringe patterns now here we are observing the fringe pattern we have the work surface whose surface is convex and then we have placed an optical flap over the surface of the work piece and when we observe and when this pair is kept in the area where we have the monochromatic light source we can observe circular fringes like this now so this is the angle between the surface of the work piece and the surface of the optical flap as the angle between the surface and the optical flap increases the number of the fringes become narrower see this angle increases we can observe here the fringes are narrower and they are closely back as the angle increases the fringes become very closer now how do we test whether the surface is convex or concave here is a method to show that the surface is convex so initially the contact when the optical flap is placed like this there will be contact here and then we get the concentric fringes as shown in the previous slide so contact is here and we get the concentric fringes like this now we have to apply a slight pressure as shown here then the contact point will move and now this is the fresh contact point that means the center of fringes will move towards left when the slight pressure is applied so this indicates that the surface is convex now let us start discussion on pitter NPL gauge interferometer I can see the schematic diagram we have the base plate over which the slip gauge which is to be inspected is placed and then the optical flap is placed like this and we have a monochromatic light source normally cadmium lamp is used so we get the monochromatic light source and using this condenser less the monochromatic light source is condensed at this point where an illuminating aperture is placed and from here we get the light source monochromatic light source and here by using this columnatic lamps we obtain a parallel beam of light which will fall on the tilt table a constant deviating prism and then the light is deflected and it falls on the slip gauge via the optical flap and the reflected light will follow this almost the same path and then from here it deviates because of the slight inclination of the optical flap the reflected light will slightly get deflected and it will fall on the weaving the aperture and because of the presence of this reflecting prism the reflected light is deflected and will fall on the detector and then we can observe the fringes now this type of interferometer is used to measure the actual length of the slip gauge and these interferometers are used in highly controlled physical conditions like the temperature will be 20 degrees Celsius and the working pressure and the chamber where the workpiece is placed is of 760 mm mercury and water vapor pressure will be 7 mm containing 0.33% by volume of carbon dioxide light from the monochromatic source normally cadmium lamp is condensed and focused on the aperture and this aperture provides concentrated light source at the focal point of a collimating lens so we can see here this is the collimating lens and this is the focal point so we get the condensed light at this particular point the collimated light falls on the constant degrading prism which splits the incident light into light rays of different wavelengths and hence different colors and now we can always select a desired color by varying the angle of reflecting the phases of the prism so we can observe here this constant deviating prism can be tilted to obtain the monochromatic of a desired color the light reflected from the optical flat and slip gauge and base plate will travel back and fall on the viewing aperture at this place is the viewing aperture where we obtain the fringe pattern now how do we measure the height of the slip gauge so when the fringe pattern is obtained if depending upon the height of the slip gauge the fringe pattern is obtained like this this is the fringe pattern obtained by the surface of the gauge and this is the fringe pattern obtained by the base plate now we can see height of the slip gauge is obtained by using this relationship n lambda by 2 plus a times b times lambda by 2 where b is the pitch of the fringe pattern obtained by the base plate and this a is the gap between the fringe of the base plate and fringe obtained from the gauge surface so here n is the number of fringes observed lambda is the wavelength of the light source and a by b is the observed fraction so there is provision to measure a and b and then we can calculate this fraction and then by inserting these values in the relationship we can find the height of slip gauge now let us start the discussion on laser interferometer nowadays laser is used in the interferometer normally the gas lasers consisting of helium, neon provide perfect monochromatic light and we can have an intensity thousand times greater than the other light sources so that we can observe the fringes in a better fashion the drawback of laser interferometer is its high initial investment and other drawback is since the laser beam will have a small diameter the spread of the laser on the work surface will be very small in order to cover the larger area of the workpiece we need to have additional optical elements to spread the beam to cover larger area of the workpiece these laser interferometers can be used to measure the small diameters and also larger displacements can be measured so these laser interferometers are used to measure the machine tool slide movement we can observe here we have the machine tool slide on which the corner cube the part of the laser interferometer is fixed so when the table moves its movement is recorded in the interferometer like this the movement of the machine tool slides is measured now coming to the construction of laser interferometer mainly it has a laser head and then the corner cube and then the electronic unit and the distal counter the corner cube is the moving part of the laser interferometer which is mounted on the sliding member of the machine tool we can see here the light ray which falls on the corner cube it gets deflected by 180 degree now coming to the laser head it has the laser source HEN laser source and we get the laser light and then we have the two photo diodes and then two semi reflectors are housed in the laser head and then we have the amplifier to amplify the signal and then there is a counter to count the number of fringes and the working of this laser interferometer is like this we get the laser light for the source which will fall on the semi reflector first semi reflector so light ray falling on the first semi reflector part of it is deflected which moves in this direction again it is deflected by the second semi reflector and then finally it falls on the first photo diode part of the light falling here will be transmitted and it moves in this direction it gets deflected by 180 degree and then it falls on the semi reflector part of it is deflected and it falls on the second photo diode and part of the reflected light will pass through the semi reflector and it gets combined here now when the corner cube moves along with the machine tool slide we can observe here the first light path is p s and the second light path is p q r s so again depending upon the OPD optical path difference between these two rays if it is equal to r number of half wavelength we get the dark band and if the OPD optical path difference is equal to even number of half wavelength we get bright band so like this depending upon the movement we get the light band and bright band by counting these bands like in the measure the machine tool slide movement so normally these laser interferometers are used to calibrate the axis movements of CNC machine tools now this diagram shows the arrangement of a commercial interferometer which uses a gas laser a CNC 630 nanometer stabilized laser and this is a dual laser system the other one is a 543 nanometer stabilized laser and then we have the unit which houses the optical elements and in this chamber we have the reference flat surface the table surface on which the gauge to be inspected is placed and then we have a moving mirror and a tilting mirror we get the laser source and then the light will move in this fashion we get the collimated light which will fall on the gauge surface and then it is reflected back and then finally it falls on the CCD television camera and on the monitor we observe the fringes now it is very important that this chamber where the gauge is placed we should control the working environment like pressure, temperature, humidity so those things we need to control and another advantage of these interferometers is at the time we can load the 12 gauge blocks so that the measurement throughput is more and length of gauge block that can be accommodated in the working chamber is 0.5 millimeter to 300 millimeter long but gauges can be placed here now this shows a photograph of the gauge block, NPL gauge block interferometer which can house gauge blocks up to 300 millimeter long gauges and uncertainty values in these gauge block interferometer starts at 0.02 micrometer so such a fine accuracy can be achieved in these gauge block interferometers I can see the chamber where we can keep the gauge blocks to be inspected and in this chamber we need to control the working environment and on the monitor we can observe the fringes and we can process the signals received from the interferometer now the features of the dual wavelength measurement of the gauge block interferometer are like this dual wavelength laser source is used and the pre-classification of sizes of the gauge block is possible with such an arrangement and two frequencies stabilized laser light sources are used phase stepping optical system is used for high precision length measurement and detailed analysis of gauge block surface geometry is possible and large area interchangeable platens are provided in such interferometer so that we can have optimum throughput in one platen we can mount the gauges to be inspected while the other platen will be inside the chamber where the measurement will be carried out and once the measurement of the gauge blocks placed on the platen which is inside is over it is turned and the outside platen will move in so like this we can have optimum throughput and full and automatic measurement is possible without intervention of the operator once the gauge blocks are loaded in the chamber all the gauge blocks are measured for the length aspect as well as for surface aspect and the result is printed and then the compensation for ambient condition if there is a change in the temperature automatic compensation will be provided integrated and rapid gauge block flatness and length variation measurement is possible so these two frequency stabilized laser interferometers can be used for analysis of the flatness of the gauge block surface as well as the length variation can be measured and automatic calculation and printout of measurement is possible in inch as well as the metric units now the other features of the laser interferometer are listed here flexible image processing software for measurement of rectangular gauge blocks square gauge blocks and round bars as possible with such a comparator interferometers and after appropriate corrections have been applied a typical measurement uncertainties of 20 nanometer of 1 mm gauge and 40 nanometer for 100 mm gauge can be achieved at 95% confidence level and calibration of internal optics using calibrated reference flats is possible and it is also possible to have automatic background temperature monitoring variation in the temperature and it can be compensated automatically and 3D topography of the lower surfaces can be had a result printout of entire set so if we keep some 10 or 12 pieces at a time the result printout of all the gauges is possible or we can have result printout of selected gauge blocks automatic storing of results in measurement file is possible so that they can be retrieved at a later date for analysis purpose now the temperature measurement of the gauge blocks which are placed in the working chamber is possible using calibrated platinum resistance thermometers and a resistance bridge and the temperature adjustment or control of temperature is possible using software phase stepping technique is a very powerful technique for processing interference patterns and such a system is used the instrument to obtain 3 dimensional topographic measurements of the gauge block surfaces and plate and surfaces with nanometer resolution the product specifications are listed here the measurement capacity of the interferometers are gauge blocks up to 300 millimeter long blocks can be loaded into the interferometer for measurement purpose gauge blocks which can be inspected or like this rectangular square and round gauges made of steel, ceramic tungsten carbide, chrome carbide can be inspected with these interferometers length measurement uncertainty is like this plus or minus 0.02 plus 0.2 times L micrometers where L is in meters such an uncertainty is possible when the interferometer is located within the specified working environment and what are the features that can be measured with these interferometers gauge length can be measured flatness of the gauge block surface can be measured length variation over the measuring phases can be measured and measurement results can be displayed in metric units or inch units and the software like FLAM for window validated by NBL for precision computation so FLAM software is used. Now here we can see another commercially available interferometer made by JGO interferometer we can see the table, FLAM plate and surface on which the completed interferometer is placed this is the unit which can be mounted on the moving element of the machine tools and this is the measuring yet which houses the laser sources optical elements etc and then we can have a monitor which will show the shape of interference and then the three dimensional topography of the surfaces etc the details of such a interferometer can be obtained by JGO doc. Now let us conclude the lecture number two in this lecture we discussed some of the interferometers like NBL flatness interferometer Pinter NBL gauge interferometer and then interferometer based on laser monochromatic light sources and some commercially available gauge block interferometers are also discussed we discussed about the constructional features and then how they can be used for measurement of various features like the gauge block surface parameters and what are the various applications of these interferometers that also we discussed with this we will conclude the module number nine on interferometry Thank you.