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Description  |
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BACKGROUND OF THE INVENTION
1. (Field of the Invention)
The present invention relates to a holographic interferometer for precisely
measuring the surface configurations of optical elements such as lenses
and mirrors, in particular, those of various aspherical optical devices.
2. (Description of the Prior Art)
Various proposals have heretofore been made with respect to a method of
precisely measuring the surface configurations of aspherical optical
elements, and, in particular, holographic interferometers are well known
to those skilled in the art. A typical holographic interferometer utilizes
a hologram standard including a hologram pattern formed by the
interference between the wave front of a reference beam and the wave front
of a beam reflected or transmitted by an aspherical reference surface, or
a hologram standard including a so-called "computer hologram". A computer
hologram is commonly made by an electron beam drawing method or the like,
after obtaining a hologram pattern from the optical design value of an
aspherical reference surface through an electronic computer. The beam
reflected or transmitted by an aspherical optical element being measured
is diffracted by one of these types of hologram standard, and the
diffracted beam is made to interfere with a reference beam, thereby
obtaining interference fringes. Finally, based on the physical number and
the shapes of the thus-obtained interference fringes, precise measurement
is made of the error, from the aspherical reference surface of the
aspherical optical element being measured.
These holographic interferometers are typically classified into the
following types: Twyman-Green type; Mach-Zehnder type; and Fizeau type. A
Twyman-Green interferometer is generally arranged such that light rays
supplied from a light source (a laser) are split into two light beams by a
beam splitter, one of the beams being used as a reference beam while the
other is made to pass through the optical element being tested and a
hologram standard, so as to obtain light in diffraction. The diffracted
beam light is made to interfere with the reference beam. A Mach-Zehnder
interferometer commonly has a construction wherein light rays supplied
from a light source (a laser) are split into two light beams by a beam
splitter, one of the beams being converted into a reference beam by
diffraction by a hologram standard while the other is shone onto the
optical element being tested, thereby forming an object light beam, and
both beams are made to interfere with each other.
A Fizeau interferometer has the construction shown in FIG. 18. As shown, a
light beam emanating from a light source (laser) LS is collimated by a
collimator lens C and is then reflected from a beam splitter BS which
consists of an inclined half mirror disposed between a focusing lens
L.sub.1 and a divergent lens L.sub.2. The reflected beam is then made to
be incident upon the divergent lens L.sub.2 as an incident light beam
l.sub.1. After the incident beam l.sub.1 has been diverged by the
divergent lens L.sub.2, the diverged beam is made to be incident upon a
spherical reference surface R. The incident beam is partially reflected by
the surface R, returns along the same optical path as that of the incident
light beam l.sub.1, and passes through the beam splitter BS, a hologram
standard H and the focusing lens L.sub.1. Finally, the light beam passes
through the opening of a spacial filter SF in the form of a zero-order
reference beam.
In the meantime, the incident light beam transmitted through the spherical
reference surface R is refleted by an optical element T being tested (or
aspherical concave mirror), to obtain an object light beam. The object
light beam is then made to travel in the reverse direction and is
transmitted through the beam splitter BS. The component of the transmitted
light beam which is not diffracted by the hologram standard H, that is,
the zero-order light, is cut off by the spacial filter SF. On the other
hand, the component of the transmitted light beam which is diffracted by
the hologram standard H, for example, the first-order diffracted light
beam, is passed through an opening in the spacial filter SF and forms
interference fringes on an interference screen or photographic film as it
is combined with the zero-order reference beams thereon.
(Problems to be Solved by the Invention)
The above-described dual optical path types of Twyman-Green and
Mach-Zehnder interferometers have the following drawbacks. Since it is
necessary to use a large number of optical components such as lenses and
mirrors for each optical path, the structure becomes inevitably
complicated and the manufacturing cost is increased. In addition, any
difference or error in the production or the optical arrangement of the
individual optical components directly affects the interference fringes
which can be observed, thereby lowering the precision of the measurement.
The Fizeau interferometer described previously with reference to FIG. 18
has no drawbacks such as those of the dual optical path type of
interferometers. However, the hologram standard H and the collimator lens
C require substantially the same diameter, thereby raising a problem in
that it is difficult to produce a hologram standard as a computer
hologram.
In addition, since the inclined beam splitter BS is disposed within the
parallel pencil of rays defined between the focusing lens L.sub.1 and the
divergent lens L.sub.2, the diameter of the splitter BS becomes close to
1.5 times as large as that of the collimator lens C, thereby making it
difficult to produce this type of interferometer with a high accuracy.
The Fizeau interferometer further involves the following disadavantage. If
the optical piece to be tested is shaped in a greatly aspherical form, a
high-density hologram pattern (or interference pattern) is formed on the
hologram standard H. In this case, if a computer hologram is to be made by
an electron beam drawing method, then huge quantities of calculations and
drawing data are necessary.
As described above, prior-art interferometers have the disadvantage in that
it is impossible to increase the size of each optical component and the
density of the hologram pattern on the hologram standard. In consequence,
only a test piece with a small diameter and a small degree of
asphericality can be measured, owing to the small effective diameter of
the conventional interferometer and the low density of the hologram.
While, a measurement method using an interferometer normally includes
on-axis and off-axis measurement methods, the on-axis method is a method
in which the object light reflected from a test piece is made to be
coaxial with respect to the reference light from a reference surface. In
this method, since the spacial frequency of the hologram standard used for
measurement can be reduced, it is possible to measure a test piece which
has a greatly aspherical surface. However, zero order and higher order
lights diffracted by the hologram standard are superposed on the optical
axis, although their focal lengths differ from one another. Even if a
spacial filter is located at the focal position of the first-order
diffracted light in order to select that light, the spacial filter allows
the passage of part of the zero-order and second-order, and higher-order
diffracted light. For this reason, the on-axis method has the disadvantage
in that the central portion, including the optical axis, cannot be
measured.
On the other hand, unlike the on-axis method, the off-axis method does not
have the above-described portion which cannot be measured. However, when
compared with the on-axis method, since the spacial frequency of the
hologram standard is increased, only test pieces having small
asphericality can be measured, because of limitations on alignment and the
production of the hologram standard.
Since the off-axis angle depends on the kind and asphericality of each test
piece, an optimum off-axis angle is determined before the hologram
standard is produced. The holographic interferometer is so constructed
that it is incapable of measuring by both the on-axis and off-axis
methods, with the off-axis angle being variable.
Furthermore, in conventional interferometers the hologram standard must be
precisely located at a predetermined position in order to carry out the
measurement precisely. Location error of the hologram standard causes
measurement error or lowers the measurement precision.
Conventionally, the adjustment for locating the hologram standard is
carried out so that the optical piece to be tested is supported by holders
and the interference pattern that appears is reduced the smallest one.
However, it takes a long time to perform the above-mentioned adjustment
and every time that the optical piece is to be tested, and both it and the
hologram standard are exchanged for new ones, it is necessary to perform
this adjustment.
(Summary of the Invention)
Accordingly, it is an object of the present invention to provide a
holographic interferometer capable of measuring the surface configuration
of even a test piece which has a large diameter and a large degree of
asphericality.
It is another object of the present invention to provide a holographic
interferometer which comprises a small number of components and wherein
the individual components, in particular, a beam splitter and a hologram
standard, have a low production cost so that high-precision measurements
can be performed even with a low-precision hologram standard.
Another object of the present invention is to provide a holographic
interferometer that is capable of measuring by both the on-axis and
off-axis methods, with the off-axis angle being variable.
It is, furthermore, another object of the present invention to provide a
method of setting a measurement hologram standard, and an apparatus for
setting a measurement hologram standard that does not require an
adjustment to be carried out for locating it every time the test piece and
the hologram standard are exchanged for new ones.
According to the invention, there is provided a holographic interferometer
comprising: a source of laser beams; a condenser lens for converging a
laser beam supplied from said laser-beam source; a pinhole which allows
the passage of said converged laser beam; a collimator lens having a focal
point which lies on said pinhole; a beam splitter disposed in an inclined
manner between said pinhole and said collimator lens; a spacial filter
disposed on the optical axis of a light reflected from said beam splitter
and at an optically conjugate position relative to said pinhole; and a
hologram standard disposed between said spacial filter and said beam
splitter.
According to another aspect of the invention there is provided a
holographic interferometer comprising: a collimator lens for collimating
light from a light source and projecting said collimated light on a piece
being tested; a reference-beam producing element disposed on the side of
ah exit of said collimator lens so as to reflect part of light from said
collimator lens, thereby obtaining a reference beam; a beam splitter
disposed on the side of an entrance of said reference-beam generating
element, said beam splitter being inclined with respect to the direction
normal to the optical axis of said collimator lens in order to reflect
said reference and object light scattered from said test piece; a hologram
standard disposed on the optical axis of light reflected by said beam
splitter; and an observation optical system for observing interference
fringes between said object light diffracted by said hologram standard and
said reference beam, wherein said reference-beam generating optical
element can be made to be inclined with respect to the direction normal to
said optical axis of said collimator lens, and said observation optical
system can be swivelled through a given angle about a substantial center
of an exit pupil of said collimator lens.
In still another aspect of the present invention, there is provided a
method of setting a measurement hologram standard comprising the steps of:
locating an adjustment hologram standard with a first alignment mark at a
predetermined position; aligning an index on a reticle of an alignment
optical system with said first alignment mark; and locating a measurement
hologram standard having a second alignment mark so that said second
alignment mark agrees with said index, said second alignment mark being
formed at a position geometrically equivalent to that of said first
alignment mark.
Another aspect of the present invention provides an apparatus for setting a
measurement hologram standard comprising: an adjustment hologram standard
having a first alignment mark; holder means for selectively holding an
adjustment hologram standard and a measurement hologram standard having a
second alignment mark geometrically equivalent to the position of said
first alignment mark; alignment means for locating said adjustment
hologram standard at a predetermined position by said holder means; and
alignment optical systems respectively having a movable reticle which is
disposed on said holder means and has an index so formed as to correspond
to said first and second alignment marks.
The above and other objects and features of the present invention will
become apparent from the following descriptions of the preferred
embodiments and referring to the accompanying drawings.
(Brief Description of the Drawings)
FIG. 1 is a schematic illustration of the general construction of a
preferred embodiment of a holographic interferometer in accordance with
the present invention;
FIG. 2 is a plan view, schematically showing the construction of the
hologram standard incorporated in the embodiment shown in FIG. 1;
FIG. 3 is a schematic illustration of a black/white-ratio inspection
pattern formed on the hologram standard shown in FIG. 2;
FIG. 4 is a schematic illustration of the first quadrant of one example of
the hologram pattern shown in FIG. 2;
FIG. 5 is a schematic, perspective view of the optical arrangement of the
alignment optical system for the hologram standard holder;
FIG. 6 is an illustration of one example of the ocular visual field of the
alignment optical system for the hologram holder of this invention;
FIG. 7 is a schematic, fragmentary view of the relationship between the
optical arrangements of the on-axis and off-axis measurement methods in
accordance with the present invention;
FIG. 8 is a schematic illustration of one example of a pinhole reticle
plate incorporated in this embodiment;
FIG. 9 is a schematic illustration of the optical arrangement of an
off-axis adjustment microscope adapted to be incorporated in this
embodiment;
FIG. 10 is a schematic illustration similar to FIG. 9, showing the optical
arrangement of the on-axis adjustment microscope adapted to be
incorporated in this embodiment;
FIG. 11A is a fragmentary, enlarged schematic illustration of the
arrangement of a reference lens, an adjustment mirror, an adjustment lens
and an automatic collimator which are incorporated in the embodiment of
this invention;
FIG. 11B is a schematic of one example of the visual field of the
observation eyepiece of the automatic collimator shown in FIG. 11A;
FIG. 12 is a schematic illustration of the optical arrangement of one
modification of the pinhole reticle plate in accordance with the present
invention;
FIGS. 13 to 16 are respectively schematic illustrations of modifications of
the alignment marks of the hologram standard and the alignment optical
system which can be applied to this embodiment;
FIGS. 17A to 17B are schematic views of other methods of setting the
adjustment hologram standard;
FIG. 18 is a schematic illustration of the optical arrangement of a
conventional Fizeau type of interferometer.
(DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS)
A preferred embodiment of the holographic interferometer of the present
invention will be described in detail below, with reference to the
accompanying drawings.
A. General Optical Construction
FIG. 1 is a diagram of the general optical construction of a preferred
embodiment of the holographic interferometer of this invention. A light
beam emanates from a laser 101 which acts as a light source, and, after
the optical path of the beam has been changed by mirrors 102a and 102b,
the beam is converged into the form of a spot by a condenser lens 103. A
reticle plate 104 having a pinhole 104a is disposed in the vicinity of the
thus-converged spot. When the converged light beam passes through the
pinhole 104a, it diverges, thus acting to make the pinhole 104a function
as a secondary source of light. A quarter wavelength plate 105 is disposed
between the mirrors 102a and 102b.
A collimator lens 106 is disposed in such a manner that its focal point
lies on the pinhole 104a. The light beam which has diverged through the
pinhole 104a to form a secondary light source is collimated by the
collimator lens 106. A reference flat plate 107 is disposed behind the
collimator lens 106 (on the right side of the lens 106 is viewed in FIG.
1), and a front surface 107a of the flat plate 107 (the surface closer to
the collimator lens 106) is disposed perpendicularly to an optical axis
O.sub.1 (the optical axis of the collimator). On the other hand, a rear
surface 107b of the plate 107 (the surface opposite to the surface 107a)
is inclined at slight angles to the front surface 107a, to prevent the
results of measurements from being affected by the interference formed
between the lights reflected from the surfaces 107a and 107b.
If a test piece T is a concave object such as an aspherical concave mirror,
a reference lens 109 mounted on a lens barrel 108 is disposed behind the
reference flat plate 107 (on the side of the plate 107 nearer the test
piece T). The collimated light beam transmitted through the reference flat
plate 107 is focused on a point P by the reference lens 109, and the
focused beam again diverges, and is incident on the test piece T, such as
an aspherical concave mirror.
Object light and a reference beam are reflected from the test piece T and
the front surface 107a of the reference flat plate 107, respectively. The
object and reference light rays are incident upon a prism beam splitter
110 with a half-mirror surface 110a which is inclined relative to the
optical axis O.sub.1 and is located between the pinhole reticle plate 104
and the collimator lens 106. The incident object and reference lights are
reflected by the half-mirror surface 110a, and enter a hologram standard
300 supported by a hologram standard holder 200, described later.
The laser 101, the mirrors 102a, 102b, the quarter wavelength plate 105,
the pinhole reticle plate 104, the beam splitter 110, the collimator lens
106, the reference flat plate 107, the reference lens 109, the test piece
T and the hologram standard holder 200 are arranged on a single common
optical bench 100.
The light transmitted through the hologram standard 300 is then focused on
a spacial filter 113 through a focusing lens 111 and a half mirror 112.
The spacial filter 113 is provided in order to selectively pass either the
light diffracted by the hologram standard 300 or that which is not
diffracted by the standard 300. More specifically, in the same manner as
that of the prior-art Fizeau interferometer shown in FIG. 18, the spacial
filter 113 selectively enables the passage of either the zero-order
reference beam formed by the reference light from the reference flat plate
107, which is not diffracted by the hologram standard 300, or the
first-order object light formed by the object light which is reflected
from the test piece T and diffracted by the hologram standard 300.
However, the filter 113 cuts off a reference beam in diffraction and the
zero-order and second-order or higher diffracted lights obtained from the
object light.
The object and reference light beams selected by the spacial filter 113 are
passed through a zoom lens 114, a half mirror 115, and a focusing lens
116, and form an interference pattern on an image pickup plane 117a of a
TV camera 117. The image picked up by the TV camera 117 is sent to an
interference analysis device 119 which includes a monitor 118 and a
personal computer (not shown). The reference and object light beams
transmitted through the half mirror 115 form an interference pattern on
the surface of a piece of photographic film 120a placed within an
instant-development type of camera 120 for recording purposes, the
interference pattern being the same as that formed on the image pickup
plane 117a.
Part of the light beam passing through the focusing lens 111 is transmitted
through the half mirror 112 and is then focused onto a reticle plate 121
which is provided with a cross-shaped pattern aligned with the optical
axis. The light image formed on the reticle plate 121 is picked up by a TV
camera 123 through an image pickup lens 122, and the picked-up image is
displayed on the monitor 118 via a switching circuit 124. The reticle
plate 121, image pickup lens 122, TV camera 123, and monitor 118
constitute an alignment optical system 125 for setting the test piece T
within a measurement optical path.
The focusing lenses 111, 116, the half mirrors 112, 115, the spacial filter
113, the zoom lens 114, the image pickup lens 122, the TV cameras 117, 123
and the photographic camera 120 are arranged on an optical bench 130. The
optical bench 130 is secured to an arm 131 to enable the adjustment of an
off-axis angle, as described later, and the arm 131 is driven by the
motion of a known micro-feed mechanism to enable free swivelling movement
about the axis of a point LC which has a conjugate relationship with
respect to an exit pupil EP of the optical system composed of the
collimator lens 107 and the reference lens 109. It should be noted that,
if the test piece T is a flat plate, the reference lens 109 is not needed.
In this case, the point LC about which the arm 131 swivels is located in a
conjugate relationship with respect to the center of the exit pupil of the
collimator lens 106.
As described above, the present holographic interferometer is of the Fizeau
type, so it is possible to greatly reduce the number of optical components
when compared with Twyman-Green and Mach-Zehnder interferometers. The
Fizeau interferometer disclosed in this application differs from the
prior-art Fizeau ones in that the beam splitter is disposed within the
divergent light beam defined between the collimator lens and the pinhole
104a, whereby the area of the half-mirror surface can be reduced to one
quarter of that of a conventional interferometer having a beam splitter
placed within the light beam collimated by its collimator lens.
Moreover, the reduced size of the half-mirror surface enables a prism type
construction of the beam splitter. Therefore, as described later, it is
possible to reduce the amount of computation and drawing information
required for the drawing of hologram patterns. Since the size of the beam
splitter is reduced, the accuracy of production can be greatly enhanced
and its production costs reduced.
According to the present invention, the hologram standard is also arranged
to be disposed within the convergent light beam reflected from the beam
splitter. This arrangement permits a reduction in size and cost, and also
enables high-precision drawing. The present invention can realise a
holographic interferometer capable of accurately measuring a test piece of
a large diameter or an aspherical test piece of a greatly aspherical
surface.
B. Hologram Standard
FIG. 2 is a schematic plan view of the construction of the hologram
standard incorporated in the holographic interferometer of this invention.
As shown, the hologram standard 300 includes a central hologram pattern
portion 301 constituted by a computer hologram.
Conventional types of interferometers have employed a mirror type of beam
splitter to split and combine reference and object beam lights. In such a
mirror type of beam splitter, if the obverse side of the mirror and the
reverse side (or half-mirror surface) thereof are parallel to each other,
the lights reflected from both sides interfere with each other, thereby
adversely affecting the results of measurements. For this reason, in
prior-art mirror-type beam splitters, the obverse side of the mirror is
normally inclined so that it is not parallel, that is, at a slight angle
to the reverse side thereof. In consequence, since there is no symmetry
with respect to the optical axis, even if a hologram pattern is obtained
by means of an on-axis type of hologram standard, it is necessary to
obtain drawing data by calculating from data on each of the four
quadrants.
However, as described above, since the illustrated embodiment of this
invention adopts the beam splitter 110 of a prism type, symmetry with
respect to the optical axis can be maintained. Thus, a hologram standard
of an on-axis type provides concentric hologram patterns having point
symmetry. Therefore, pattern calculations and drawing-data operations as
shown in FIG. 4 are made with respect to the first quadrant (x, y). Data
on the second quadrant (-x, y), the third quadrant (-x, -y), and the
fourth quadrant (x, -y) can be obtained through simple coordinate
transformations of the first quadrant's data. This enables a reduction in
the costs and time required for these operations. Furthermore, if the cost
and time required for the calculation and operation of data on quadrants
are applied to the operation of hologram patterns and the production of
drawing data, it is possible to obtain more accurate data.
A plurality of cross-shaped distortion inspecting patterns 302 are formed
around the hologram pattern portion 301, these patterns 302 being drawn
during the step of drawing the hologram pattern by electron-beam scanning.
Each of the distortion inspecting patterns 302 is compared with a
corresponding preformed reference pattern, so that it is possible to
obtain the degree of the distortion of the hologram pattern from the
offsets between the patterns 302 and the corresponding reference patterns.
As shown, two black/white-ratio inspection patterns 303 are formed outside
of the distortion inspecting patterns 302. Each of the black/white-ratio
inspection patterns 303 has a checkboard pattern in which a plurality of
black segments 304 and white segments 305 of the same areas are
alternately arranged in a flat manner, as shown in FIG. 3.
The black/white-ratio inspection patterns 303 are drawn during the step of
drawing the hologram pattern 301 by an electron beam. Accordingly, if the
ratio of black to white of each black/white-ratio inspection pattern 303
is measured by a densitometer, it is possible to indirectly obtain the
ratio of white to black of the hologram pattern itself.
Cross-shaped alignment marks 306 are formed at the four corners of the
hologram standard 300 so as to enable alignment when the hologram standard
300 is mounted on the hologram standard holder 200. As shown in FIG. 2,
L-shaped top/bottom identification marks 307 are formed under the two
lower alignment marks 306.
C. Hologram Standard Holder and Optical Alignment System
FIG. 5 is a perspective view, diagrammatically illustrating the
construction of an optical system 250 for aligning the hologram standard
300 which is supported by the hologram standard holder 200.
The optical alignment system 250 comprises: a first optical path 253
consisting of the light-emitting diode 248, the heat-absorbing filter 249,
an objective lens 240, a mirror 251 and a beam splitter 252; a second
optical path 255 consisting of the light-emitting diode 248, the
heat-absorbing filter 249, the objective lens 240, a mirror 254 and the
beam splitter 252; and an ocular optical path 256 wherein the first and
second optical path 253, 255 are combined by the beam splitter 252.
The ocular optical path 256 comprises: reticle plates 257, 258 which are
moved in the plane perpendicular to an optical axis O.sub.3 (the X-Y
plane) by means of a known moving means (not shown), and an eyepiece 259.
As shown in FIG. 6, the reticle plates 257, 258 have circular indexes 260,
261, respectively.
D. Device for Measuring Off-Axis Angle
FIG. 7 diagrammatically shows an optical arrangement for performing the
on-axis and off-axis measurement methods. For the on-axis measurement
method, the reference flat plate 107 is disposed perpendicularly with
respect to the optical axis O.sub.1 of the collimator lens 106. The
optical observation system for observing the interference pattern formed
between the object and reference beams, namely, the spacial filter 113,
the zoom lens 114, the focusing lens 116 and the image pickup tube 117, is
disposed on the optical axis 02 perpendicular to the optical axis O.sub.1.
The spacial filter 113 is placed at the focal point of the first-order
diffracted light formed by the object and reference beams which are
diffracted by the hologram standard 300. The interference fringes between
the first-order diffracted light of the object and reference lights are
picked up by an image pickup tube or recorded by photographic means.
On the other hand, for the off-axis measurement method, as shown by broken
lines in FIG. 7, the reference flat surface 107 is inclined at an angle of
.alpha. to the norm of the surface 107. (This angle .alpha. is called an
off-axis angle). The optical observation system together with the optical
bench 130 is caused to swivel through an angle of .beta. about the axis of
the swivelling center LC. The angle of swivel is determined by the
following equation:
tan.beta.=2 f tan .alpha./L
(where f is the focal length of the collimator lens.)
As a result, a spacial filter 113' (the spacial filter 113 shifted to an
off-axis position) allows only the passage of the first-order diffracted
light and the zero-order diffracted light which are obtained from the
object light and the reference light through the hologram standard 300,
respectively.
The off-axis angle .alpha. can be detected from the focal position of the
pinhole reticle plate 104 on which a spot light S is focused, the spot
light S being formed by the light (the reference beam light) reflected
from the reference surface 107 which is transmitted through the
half-mirror surface 110a of the beam splitter 110. Specifically, the
offsets between the focal position of the spot S and the optical axis
O.sub.1 are in proportion to the minute off-axis angle .alpha..
FIG. 8 is a diagrammatic plane view of the structure of the pinhole reticle
plate 104. The pinhole 104a is formed in one end of the reticle plate 104
and a scale 104 extends longitudinally from the pinhole 104a to the other
end. The scale 401 has graduations indicative of the offsets between the
optical axis O.sub.1 (the center of the pinhole 104a) and the spot S
corresponding to the off-axis angle .alpha., and measurement numbers 402
indicative of the off-axis angles .alpha.are formed along the graduations
(below the graduations as viewed in FIG. 12).
FIG. 9 is a schematic illustration of the optical arrangement of a
microscope 410 used for adjusting the off-axis angle .alpha.. The
microscope 410 includes an objective lens 411, a mirror 412, a focusing
lens 413, a diaphragm 414 and an eyepiece 415. The light reflected from
the scale 401 on the reticle 104 and the measurement numbers 402
representative of the off-axis angle .alpha. is formed into a parallel
pencil of rays through the objective lens 411. The mirror 412 reflects the
parallel rays directed toward the focusing lens 413, and the rays are
focused on the diaphragm 414 through the focusing lens 415. An operator
observes the light image projected on the scale 401 and a corresponding
one of the measurement numbers 402 representative of the off-axis angle
.alpha..
FIG. 10 is a schematic illustration similar to FIG. 9, showing the optical
arrangement of a microscope 420 used for the on-axis adjustment. The
microscope 420 differs from the above-described microscope 410 in the
following respects. The mirror 412 is replaced with a half mirror 421, and
the objective lens 411 is removed. The focusing lens 413 forms an image in
cooperation with the condenser lens 103 of the interferometer.
The off-axis measurement device incorporated in the illustrated embodiment
of this invention further possesses the following advantage. Since the
scale 401 on the pinhole reticle 104 extends in the same direction as that
of the rotation of the flat reference plate 107, even if the projected
spot S is vertically offset from the scale 401, it is possible to judge
that the reference plate 107 is irregularly inclined or is rotated about
the optical axis O.sub.1. As a result, the status of the reference plate
107 can also be checked.
E. Method of Adjusting Interferometer
(a) Regarding setting for the on-axis measurement method
(The on-axis arrangement of the measurement optical system)
a-1: As shown by a circle made of a two-dot chain line in FIG. 1, the
microscope 420 used for the on axis adjustment is positioned on the
optical axis O.sub.1 of the collimator lens 106.
a-2: The light beam transmitted through the half mirror 421 of the
microscope 420 is focused on the pinhole 104a by the condenser lens 103.
Observations are made, through the eyepiece 415, of whether or not the
spot image S reflected from the reference flat plate 107 is correctly
refocused on the pinhole 104a.
Only when the spot S agrees with the pinhole 104a, the corresponding spot
light can be observed through the eyepiece 415. In this case, the position
of the reference flat plate 107 is adjusted so that the spot S can be
correctly observed through the eyepiece 415. When the spot S is observed,
the reference plate 107 is perpendicular to the optical axis O.sub.1, and
the interferometer is set in the on-axis mode.
a-3: The switching circuit 124 is switched so that the image sent from the
TV camera 123 of the optical alignment system 125 is displayed on the
monitor 118.
The monitor 118 displays a state wherein the intersection point of each of
the cross-shaped marks on the reticle plate 121 agrees with the spot image
which is reflected by the reference plate 107 and has a conjugate
relationship with respect to the pinhole 104a.
a-4: The reference lens holder 109a having the reference lens 109 is
mounted on the barrel 108 of the interferometer by known holding means
(not shown).
a-5: A holder 500 having an adjustment mirror 501 is mounted on the
reference lens holder 109a by a mounting screw 503 in such a manner that a
reference surface 502 adjoins the reference surface 109b of the reference
lens holder 109a (see FIG. 11A).
a-6: The automatic collimator 510 is placed on the optical bench 100, and,
as shown in FIG. 11B, the automatic collimator 510 is correctly positioned
with respect to the adjustment mirror 501 so that a cross-shaped target
511 is made to agree with a circular index 512a on a reticle 512.
Subsequently, the automatic collimator 510 is fixed to the optical bench
100.
a-7: The holder 500 with the adjustment mirror 501 is removed from the
reference lens holder 109a.
a-8: An adjustment lens 520 held by a known five-axis holder (not shown)
(including the five axes of X, Y, Z, .phi..sub.A, .phi..sub.B ;
.phi..sub.A and .phi..sub.B indicate lateral and vertical inclinations) is
disposed between the reference lens 109 and the automatic collimator 510.
In order to generate a spherical wave 520a of the adjustment lens 520, the
adjustment lens 520, as shown in FIG. 11A, is arranged in such a manner
that a curvature centure Q.sub.1 of a spherical surface 521 is made to
agree with a focus F of the reference lens 109 and an optical axis 05 of
the automatic collimator 510 is orthogonal to a flat surface 522 of the
adjustment lens 520.
Firstly, in order to roughly position the adjustment lens 520, the
interference fringes between the reference beam reflected from the
reference flat plate 107 and the object light (or spherical wave) from the
spherical surface 521 of the adjustment lens 520, which are displayed on
the monitor 118, are adjusted to be changed into a monochromatic pattern
Secondly, the target image 511 is made to correspond to the reticle image
512a while observing a visual image through the eyepiece of the automatic
collimator 510, thereby precisely positioning the adjustment lens 520.
(Setting of Adjustment Hologram Standard)
a-9: When an aspherical surface 523 for producing an aspherical wave 523a
of the adjustment lens 520 is located in the above-described position
where the adjustment lens 520 is held in position, an adjustment hologram
standard is closely vacuum-fixed to the X-Y axis movable stage 211 of the
hologram standard holder 200. The adjustment hologram standard has either
the interference fringes produced by the interference between the object
light from the aspherical surface 523 and the reference light from the
reference flat plate 107 or the computer hologram which is produced by an
electron-beam drawing method on the basis of the interference pattern
obtained by a computer operation.
a-10: switching circuit 124 is switched so that the monitor 118 may display
the image picked up by the TV camera 117 of the optical system for
observing interference fringes.
a-11: The feed screws 208, 216, 217 and 218 are turned so as to adjust to a
monochromatic pattern the interference fringes which are produced on the
spacial filter 113 by the interference between, for example, the
first-order diffracted light of the object light (or aspherical wave)
reflected from the aspherical surface 523 of the adjustment lens 520 and
the zero-order diffracted light of the reference light reflected from the
reference flat plate 107. In consequence, the adjustment hologram standard
is adjusted in the X, Y, Z and .theta. directions. This completes the
positioning of the adjustment hologram standard.
(Regarding the setting of a measurement hologram standard)
a-12: A reticle-moving knob (not shown) is adjusted and the circle indexes
260, 261 on the reticle plate 257 are made to correspond to the alignment
mark 306 on the adjustment hologram standard which is aligned at the
above-described step (a-11), as shown by one example of the visual image
which is observed through the eyepiece 259.
a-13: While observations are being made with the automatic collimator 510,
reconfirmation is made as to whether or not the target image 511
corresponds to the reticle index 512a, that is, the adjustment lens 520 is
held in position.
If the lens 520 is held in position, it is judged that the setting process
described in the steps (a-9) to (a-12) is correctly performed. The
automatic collimator 510 and the adjustment lens 520 are removed because
they are not needed in the following steps.
a-14: The adjustment hologram standard is | | |
