Object fixturing in interferometer

An object fixturing system applies to an interferometer having a pair of diffraction gratings arranged to produce and recombine test and reference beams. The fixturing system positions an object between the diffraction gratings so that a test beam is incident on a surface of the object at a grazing incidence angle. A positioning fixture engages the object and a reference surface of the interferometer in moving the object to a measurement position on a window platform that transmits the test and reference beams. The fixture is then removed for a simultaneous measurement of an entire surface of the object, which can be clamped in place if necessary by a clamping window that also transmits the test and reference beams.

TECHNICAL FIELD 
Interferometers that measure the surface of an object positioned between a 
pair of diffraction gratings arranged to separate and recombine test and 
reference beams. 
BACKGROUND 
Interferometers using a pair of diffraction gratings arranged on an optical 
axis can separate and recombine test and reference beams so that a test 
beam can be incident on an object surface to measure that surface. The 
arrangement works readily for cylindrical and conical surfaces and can 
also be applied to other surfaces that depart from cylindrical or conical 
configurations. The object to be measured is positioned between the 
diffraction gratings so that the test beam is incident on the object 
surface at a grazing angle; and for object surfaces of revolution, which 
are most easily measured in such interferometers, the axis of the object 
surface is positioned on the optical axis of the diffraction gratings. 
An operator of such an interferometer may need to place a succession of 
similar objects into the same measurement position so that a number of 
objects can be measured. It is also desirable for objects to be placed 
quickly and accurately into the measurement position so that measurements 
can be made as rapidly as possible. This is especially important when 
interferometers of this type measure an entire surface of an object 
simultaneously in one brief operation so that the time required for 
positioning objects for measurement becomes a significant portion of the 
total time required for a series of measurements. 
SUMMARY OF THE INVENTION 
We have devised an object-fixturing system that enables an operator to 
successively place objects rapidly into the same measuring position within 
the interferometer. Our system accommodates different sizes and shapes of 
objects and different ways of holding objects in place, once positioned. 
It aims at speeding up the process of positioning objects for measurement 
and improving the accuracy of object positioning. It also does this 
compatibly with the requirements of the interferometer. 
Our object-fixturing system applies to an interferometer that has a pair of 
diffraction gratings arranged to produce and recombine test and reference 
beams. An object to be measured is positioned between the diffraction 
gratings so that a test beam is incident on a surface of the object and a 
reference beam passes by the object. This allows an entire surface of the 
object to be measured by a single interference pattern. 
We use a fixture that can move in and out of engagement with a reference 
surface of the interferometer so that the fixture can return repeatedly to 
a known position. The fixture has an object-positioning surface that can 
engage and position an object in a measurement location within the 
interferometer. Object positioning occurs accurately when the object 
engages the positioning surface of the fixture and the fixture engages the 
reference surface of the interferometer. This allows a succession of 
objects of the same nominal size to be rapidly placed in the same 
measurement position by repeatedly deploying the fixture. 
A preferred way of accomplishing this is with an object supporting platform 
arranged movably between the diffraction gratings. The object platform has 
a window that transmits test and reference beams and supports the object 
for measurement. The window platform is mounted on a stage that is movable 
to adjust the tilt of a plane in which the stage moves in X and Y 
directions. This movement allows an object positioned on the movable 
platform to be brought accurately into alignment with the optical axis of 
the diffraction gratings, by observing the interference patterns. Once a 
fixture-positioned object is properly aligned with the optical axis, then 
a succession of similar objects can be placed in the same position by 
reusing the fixture. The window platform can provide the reference surface 
for the fixture, and the fixture can provide a positioning surface for the 
object while both the object and the fixture are supported on the window 
platform. Objects that cannot stand freely without support are clamped in 
the measurement position. This can be done by a clamping window that is 
vertically adjustable and can be lowered to engage the top of a supported 
object.

DETAILED DESCRIPTION 
An object-measuring interferometer 10 of the type used with our fixture 
system is systematically shown in FIG. 1. A source 11 of preferably 
monochromatic, collimated light is directed through a pair of diffraction 
gratings 12 and 13 arranged on an optical axis 15 shown as a broken line. 
For the simple illustrated case, each diffraction grating 12 and 13 is 
formed with concentric circular lines that diffract transmitted light into 
a reference beam 20 and a test beam 25. The reference beam 20 is a zero 
order beam that passes aside of an object 30 to be measured, and the test 
beam 25 is preferably a first order beam that is incident on a surface of 
object 30 at a grazing incidence angle. Test beam 25 reflects off the 
surface of object 30 and is recombined with reference beam 20 at 
diffraction grating 13. This produces an interference pattern that is 
viewed by imaging system 14 so that the surface of object 30 can be 
measured. 
In practice, circumstances can be more complicated. Both positive and 
negative first order beams can occur; a zero order beam can be repressed; 
second order beams can become involved; the gratings can be blazed, given 
different pitches, and made in different configurations; and the surface 
of object 30 can depart from cylindrical or conical. The problem of 
fixturing objects 30 into a measurement position between diffraction 
gratings 12 and 13 remains similar, though, regardless of optical 
variations. 
Object 30 is supported between gratings 12 and 13 on a window 16 that 
transmits the reference and test beams. Window 16 is supported by window 
platform 21 that is preferably movable, for reasons explained below. 
Platform 21 includes an adjustable aperture 32 that is preferably an iris 
device for adjusting the diameter of the light transmitted through grating 
12. 
Window platform 21 is mounted on a stage 22 that permits the desired 
movement. Schematically illustrated movement devices 23-27 provide 
movement in X and Y directions and tilt adjustment of a plane in which the 
X and Y motions occur. Movement controllers 23-27 can be micrometer-type 
manual devices or motor driven and computer controlled. 
A clamping window 17 is sometimes used to engage an upper portion of object 
30. This is important for measuring objects 30 that are unstable or are 
unable for some reason to stand reliably upright. Clamping can also be 
used for objects oriented in a horizontal or other non-vertical position. 
Window 17 is supported on a clamping platform 28 that is adjustable 
vertically on guide rods 29 and can be held in an elevated position by set 
screw 31. Many other variations are possible for movement of clamping 
platform 28. The basic requirement is to elevate platform 28 while 
changing objects 30, and lower platform 28 so that clamping window 17 
engages a positioned object 30 for measurement. The weight of window 17 is 
normally sufficient for clamping, although other clamping force can be 
applied. Window 17 also transmits the test and reference beams to upper 
diffraction grating 13. 
A fixture 40 for positioning object 30 on window 16 in alignment with 
optical axis 15 is shown in FIGS. 2-4. A circular periphery of window 
platform 21 forms a reference surface 41 that fixture 40 engages with a 
surface 42. Fixture 40 can be slid on and off window platform 21 to bring 
its surface 42 into and out of engagement with reference surface 41 and 
the periphery of window platform 21. 
Fixture 40 also has a positioning surface or edge 43 that engages object 
30. A cylindrical or conical surface of object 30 can engage positioning 
surface 43 of fixture 40 along two lines or four points, but many other 
alternatives are possible. 
Fixture 40 can be a single piece of material, as schematically illustrated, 
or can be an assembly of several parts. It can also have many shapes and 
can engage objects with positioning surfaces, edges, or points having 
different configurations. It can also engage an interferometer reference 
surface in a variety of ways, including lines, points, and keys. Many 
different forms can be substituted for the simple circular reference 
surface 41 illustrated for window platform 21, and the shape adopted for 
an interferometer reference surface will affect the shape of fixture 40 
and its engagement surface 42 that moves into contact with a reference 
surface. 
Objects 30 can be somewhat barrel shaped or have surfaces of other 
configurations that depart from cylindrical or conical. Such variations 
can be accommodated by different shaped positioning surfaces 43 on 
fixtures 40. A single interferometer can have a variety of fixtures 40 of 
different dimensions and configurations for positioning different shaped 
objects. An example of this is illustrated in FIGS. 5 and 6 where a 
fixture 45 has a conical shaped positioning surface 46 for engaging and 
positioning the correspondingly conical surface of an object 50 supported 
on window platform 21. 
FIG. 7 schematically shows a clamping arrangement for an object 55 that 
does not stand stably upright. Object 55 has rounded ends so that it needs 
to be positioned upright as well as aligned with the optical axis of the 
interferometer. A fixture 45, such as illustrated in FIG. 6, can have a 
positioning surface 46 shaped to engage object 55 and hold it in an 
accurately upright position on window 16 where object 55 is positioned on 
supporting pad 56. Then, upper window 17 is lowered into clamping 
engagement with the upper end of object 55, which is engaged by a clamping 
pad 57 on the underside of clamping window 17. 
Another example of an object 60 that benefits from clamping is 
schematically shown in FIG. 8. Object 60 has end projections 61 that are 
lodged in holes or recesses 62 formed in supporting window 16 and clamping 
window 17. A fixture 40 or 45 can aid in positioning object 60 so that 
projections 61 fit quickly and reliably into receiving openings 62. 
FIG. 9 schematically shows an object 65 that has end projections 64 but 
also stands stably upright. Objects 65 can be positioned on window 16 by 
inserting an end projection 64 into a hole or recess 63 in window 16. 
FIGS. 7-9 illustrate the possibility of using different windows in 
platforms 21 and 28 and giving windows and fixtures different 
configurations for accommodating differently shaped objects. 
Supporting window 16 can also be provided with a magnet 66 for holding an 
object 70, and a magnet 66 can be arranged in many ways at or above the 
upper surface of window 16. In the embodiment schematically illustrated in 
FIG. 10, magnet 66 is mounted on a small universal joint or ball joint 67 
that is frictionally stiff enough to hold a position into which it is 
adjusted. This allows object 70 to be positioned accurately on magnet 66 
and adjusted to an upright position by joint 67. Depending on the 
circumstances, object 70 can be measured as supported by magnet 66 or can 
be clamped in place by an upper window 17 such as illustrated in FIGS. 7 
and 8. A fixture 45 such as illustrated in FIG. 6 can be used for engaging 
a conical surface of object 70 for such positioning. Magnet 66, whether 
mounted on joint 67 or on the surface of window 16, provides holding power 
and movement resistance so that fixturing of objects into a measurement 
position is quick and reliable. Objects supported on a magnet are less 
likely to be dislodged from a measurement position by withdrawal of a 
positioning fixture or a finger of an operator. 
FIG. 11 schematically illustrates the fact that an object 75 can have an 
internal surface 76 to be measured, instead of, or in addition to, an 
external surface 77. The positioning of object 75 on window 16, with the 
aid of fixture 40, is done in the same way regardless of whether an inside 
or outside surface is being measured. 
FIGS. 12 and 13 schematically illustrate locating notches or recesses 71 
and 72 formed in the reference surface periphery 41 of window platform 21. 
These are engaged by projections or keys 73 extending from the engagement 
surface 42 of fixture 40. This allows two positions for fixture 40 on 
window platform 21, to accommodate right-handed and left-handed operators 
of the interferometer. Although object positioning might differ slightly 
between the two positions of fixture 40, if the same position is used 
repeatedly, objects of the same size will be consistently positioned. 
FIG. 13 also illustrates a magnet 80 arranged on fixture 40 adjacent 
positioning surface 43 for magnetically engaging an object to be 
positioned. Use of magnet 80 saves an operator from holding an object 
against fixture positioning surface 43 by finger pressure. An object held 
by magnet 80 against positioning surface 43 can be moved into a 
measurement position by fixture 40 and then clamped by upper window 70 
before fixture 40 is withdrawn. Alternatively, magnet 80 can be movable 
within fixture 40, as indicated by the double-headed arrow, to apply and 
then withdraw magnetic holding power. An object can be positioned while 
magnet 80 is adjacent positioning surface 43, and then magnet 80 can be 
withdrawn from positioning surface 43 so that fixture 40 can be removed 
from the interferometer without moving the positioned object. Many other 
possible uses for magnets can involve window receivers for objects and 
fixtures for positioning objects. 
Assuming that a number of objects with the same nominal size are to be 
measured in sequence, an operator using our fixturing system would proceed 
as follows. A fixture 40 having a positioning surface 43 that fits or is 
compatible with the objects to be positioned would be selected for use, 
and the first object would be positioned on window 16. This would be done 
by moving fixture 40 into engagement with a reference surface while 
holding the object against positioning surface 43, either magnetically or 
by finger pressure. If a fixture of proper dimensions has been selected, 
this should place the object near the center of window platform 21 and 
near optical axis 15 of interferometer 10. If the object needs to be 
clamped in position, window clamp 17 would be lowered to engage a top of 
the object. If the object has end projections or otherwise benefits from a 
special receiver on window 16, this is preselected by mounting a suitable 
window on platform 21 and possibly another suitable window on clamping 
platform 28 in interferometer 10. 
Once the selected object is positioned and the selected fixture 40 is 
withdrawn, stage 22 is adjusted to align the object with optical axis 15. 
This can be done by manually moving adjustment devices 23-27 while 
observing a pattern of fringes produced by imaging system 14. It can also 
be done by computer control of motor-driven adjustment devices that are 
responsive to computer analysis of an interferogram formed in imaging 
system 14. Adjustable aperture 32 may also be set at this time to exclude 
any unnecessary illumination transmitted through diffraction grating 12. 
After the object is adjusted into alignment with optical axis 15, a 
measurement of its surface is made quickly by optical analysis of the 
interferogram, and the measured object is removed from the interferometer. 
This requires unclamping the object, if a clamp was necessary to hold the 
object in an upright position. 
The same fixture that was previously used is then redeployed for 
positioning a second object for a second measurement. Since similar 
objects in a measurement series have the same nominal size, reuse of the 
fixture brings the second object accurately to the same position that the 
first one occupied, without requiring any readjustment of stage 22. The 
second object is then quickly measured and removed, and the process is 
repeated until all the objects in the sequence have been measured. Fixture 
positioning of subsequent objects for measurement proceeds quickly after 
the first object is brought into adjusted alignment with the optical axis 
of the interferometer. Repeated use of the same positioning fixture 
ensures that each subsequent object in a series occupies the same position 
as the first one. 
When a different size or configuration of object is to be measured, the 
process is repeated. Any adjustments in window 16 or 17 are predetermined, 
a proper fixture is selected, the initial object is positioned and 
properly aligned, and then subsequent objects are accurately brought to 
the same position by reusing the same fixture. 
Since interferometer 10 can complete a measurement of a properly positioned 
object in a few seconds, our fixturing system appreciably speeds up the 
process of measuring a sequence of objects of the same size. It also does 
this with simple and inexpensive components that do not interfere with the 
mechanical or optical operation of the interferometer.