Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention relates to optical systems for forming an
image of an object at unit magnification, and more particularly
to such a system having a large spectral range. Optical
systems constructed in accordance with the concepts of this
invention are particularly adapted, among many other possible
uses, for effecting the exposure of photoresist-coated semi-
:
conductor wafers in the manufacture of integrated circuits.
The present invention is closely related to the annular
projection system of the type disclosed in my prior U.S. Patent -`
~; No. 3/748,015 issued July 24, 1973 and assigned to the same
Assignee as the instant application. My prior patent discloses
~ a catoptric system for forming in accurate micro detail an
; image of an object at unit magnification with high resolution,
; characterized by convex and concave spherical mirrors having
their centers of curvature coinciding at a single point. The
. ~ ,
mirrors are arranged to produce at least three reflections
within the systemj and they are used in the system with their
axial conjugates at said point and to provide two off axis
conjugate areas at unit magnification in a plane which contains
the center of curvature, the axis of the system being an axis
normal to the latter plane and through said point. While this
: .
optical system has many features and advantages, the present
invention is directed to improvements thereover, which will
become apparent as the description proceeds.
Other related patents in this field include U.S. Patent
No. 3,821,763, issued June 28, 1974, U.S. Patent No. 3,951,546
issued April 20, 1976 and U.S. Patent No~ 4,011,011 issued
March 8, 1977 . These patents are assigned to the same Assignee
as the present invention.
:
.'' ~ ,
~' ' ' ., ' '~ ' .
3~$~
SUMM~RY OF THE INVENTION
In order to accomplish the ~esired results, the invention
- provides, in one form thereof, a new and improved annular field
optical system for use in apparatus for photographically exposiny
an image receiving surface to a light image of an object. This
;~ optical systPm includes at least one convex and one concave mirror,
which are substantially concentrically arranged along an optical
. .
axis. The system is arranged to form conjugate planes normal to
said axis for which the system is of unit power. The system
further includes at least one pair of symmetrically disposed nearly
. .:
concentric meniscus elements whose convex radii are larger than
., .
their con~ave radii and whose thickness is graater than the differ-
ence between their convex and concave radii.
According to one aspect of the invention, color compensating
means are interposed between the mirroxs and the object and image
locations which, in one form thereof, is a substantially plane
parallel plate mounted normal to the optical axis oE themmirrors.
~referably, one of the faces of the plane parallel plate is made
.
~ aspheric, or the plate is a meniscus element mounted normal t~
..~ ao the optical axis o~ said mirrors.
- In accordance with another aspect of the invention, the con-
: ..
cave and convex mirrors are supported with a distance betwePn their
; centers of curvature of less than about two percent of thellength
;~ of the shorter radius, and according to another aspect thereof, the
,
~' mirrors are arranged so that there are three reflections from the
concave mirror and two reflections from the convex mirror.
-~ ~ In one form thereof, the invention provides a new and improved
i annular field optical system for use in apparatus for photographi-
` cally exposing an image receiving surface to a light image of an
.; .
object which includes: a first half and a second half with each
; half including a unit optical system having an optical axis and
- having conjugate p~nes substantially normal to that axis ~or
-2-
3491~
which the system is of unit power. These two halves are coaxially
disposed in back-to~back relationship so that the conjugate planes
are superposed on at least one side of the optical system to form
an intermediate image locatioh~ and provision is made for providing
spaced object and image locations on the other side of the optical
system. According to one aspect of the invention, each half of the
optical system includes a nearly concentric meniscus element whose
convex radius is larger than its concave radius and whose thickness
is greater ~han the difference between its convex and concave radiiJ~
and according to another aspect at least one color correcting
; element is provided in the system.
.....
In one embodiment each unit optical system includes a concave
spherical mirror and a convex spherical mirror facing the concave
mirror, said mirrors being supported with their centers o~ curva-
ture substantially coincident. Means are provided to define a
location for an object ~he image o which is a real image at a
second location, with said convex mirror ~eing positioned to re-
;~ ~ flect to the concave mirror light from the object location initially
reflected to the convex mirror from the concave mirror ! where~y
light from the object location will be reflected at least twice atthe concave mirror and at least once at the convex mirror before
being focused at the second location. T~e small aberrations aris-
- ing from the departure of the meniscus from concentricity are
-; compensated for by introducing a small departure from concentricity
in the mirror pair. That is, in each half of the optical system,
the concave and convex mirrors are supported with a distance be-
tween their centers of curvature of less than about two percent
of the length of the shorter radius.
Also, preferably, the color correction element is a plane
parallel plate mounted normal to the optical axis of the mirrors.
In some forms of the invention, the plane parallel color correct-
ing plate is mounted at the intermediate image location, and in
., .
~ -3-
, . .
'~
~3~9~
other forms a color correcting plate is mounted in each half of
the system.
In some embodiments of the invention, the two halves of the
optical system are coaxially disposed in back-to-back relationship
so that the conjugate planes are superposed on both sides of the
system to form an intermediate image location on one side and a
superposed object and image location of the other side~ Means,
such as folding mirrors, are provided to form spaced object and
image locations to make them accessible for practical installations.
In other embodiments, the two halves of the optical system are
coaxially disposed in back-to-back relationship to form an inter-
madiate image location on on0 side of the optical system and to
form spaced o~ject and image locations on the other side. For this
purpose, the intermediate image is spaced axially from the other
conjugate in at least one-half of the system. In some embodiments,
the distance from the two mirror components to the intermediate
image is greater than the distance to the o~her conjugate location
in at least one-half, thereby spacing said object and image loca-
tions one from the other. In still further embodiments, the
distance from the two mirror components to the intermediate image
is less than the distance to the other conjugate location in at
-. ~
least one-h~lf, thereby forming crossed object and image planes.
In this case, reflecting means are interposed between the object
and image locations to make them physically accessible.
It will be appreciated that a necessary co~dition for the
~- absence of field curvature (i.e., for a flat image surface) in
--- the resultant system is that the algehraic sum of the quantities
,
;~ obtained by dividing the power of each surface by its index of~ ,.,
refraction be substantially zero, the index of refraction of a
reflecting surface being defined as negative one for this compu-
tation. The meniscus refracting elements, which are essential
components of the optical systems in accordance with the invention,
result in a chromatic variation of the above mentioned sum so that
even though it is substantially zero, there is a resultant varia-
tion of the field curvature with color. In accordance with the
invention, the focal posi~ion is made constant for a broad spectral
range by the introduction of a longitudinal chromatic difference
of focus which compensates for the chromatic variation of field
curvature in the annular field of the optical s~stem.
There has thus been outlined rather broadly the more impor-
tant features of the invention in order that the detailed
description thereof that follows may be better understood, and
in order that the present contribution to the art may be better
appreciated~ ~here are, of course, additional features of the
invention that will be described hereinafter and which will form
the subject of the claims appended hereto. Those skilled in the
; art will appreciate that the conception upon which the disclosure
is based may readily be utilized as a basis for the designing of
,
other systems for carrying out the several purposes of the
invention. It is important, therefore, that the claims be
regarded as including such equivalent systems as do not depart
from the spirit and scope of the invention.
Specific embodiments of the invention have been chosen for
purposes of illustration and description, and are shown in the
accompanying drawings, forming a part of the speci~ication.
BRIEF DESCRIPTION OF THE~DRAWINGS
_____________________________ :
Fig. 1 is a schematic repr sentation of an optical system,
constructed in accordance with the concepts of the present inven-
tion;
Fig. 2 is a schematic representation of a double optical
system, wherein two optical systems similar to the system of
Fig. 1 are mounted in back-to-back relationship;
Fig. 3 is a schematic representation of an optical system
similar to the system of Fig. 2, but simpliied by removing
.
i
.
3~
the requirement that each half of the system be symmetrical,
while keeping the system symmetrical as a whole;
; Fig. 4 is a schematic representation of an optical system
similar to the system of Fig. 3, but simplified by combining
the two color correcting plates into a single element o~ the
same form and by elimina~ing the folding flats required to separate
` the object and final image by making the conjugate distances of
the two-mirror components unequal;
Figs. 5 to 7 are schematic representations of ~ther embodi-
.
ments, reepectively, of optical systems according to the invention;
.
Fig. 8 is a schematic representation of an optical system
similar to Fig. 1, but showing another embodiment of the invehtion;
and
,, .
-~ Fig. 9 is a graphic representation showing the variation of
: .~
`` the focal position as a function of the distance from the axis
and the wavelength of the image forming light.
DETAILED DESCRIPTION OF T~E PREFERRED EMBODIMENTS
In the embddiment of the invention shown in Fig. 1, the new
and improved optical system, indieated generally at 10, comprises
Z0 two spherical mirrors, a convex mirror 12 and a concave mirror 14,
.~ .
; arranged to pro~ide three reflections within the system. The
- mirrors are arranged with their centers of curvature along the
; ~ system axis SA and to have off a~is conjugate areas centered at
- points O and I. The points O and I are each a distance H from
the reference axis SA at opposite sides thereof. In the optical
:,
system illustrated in Fig. 2 of my prior Patent No~ 3,748,015,
the concave m~rror forms an image of the object O at I; the convex
mirror forms a virtual image of point I at the point O which is
reimaged by the concave mirror at I. It is noted that the width
of the corrected annulus attainable with the optical systems of
said patent is limited by the fifth order astigmatism inherent
in the design. The high order astigmatism results from the
... .
-6-
:'
;
'',:`
, .
:
:
sph~rical aberration of the principal rays in this system.
I~ will be appreciated that meniscus elements can be used
to reduce or remove the spherical aberration of principal rays
`~ parallel to the optical axis. If the meniscus elements are
concentric with the mirrors, they introduce no third order aber-
ration except field curvature when the conjugates are at the com-
mon center of curvature.
As seen in Fig. 1, a pair of symmetrically disposed meniscus
elements 16 are provided for reducing the spherical aberration
of the principal rays. It is noted that the meniscus elements
would also be effective for reducing the spherical ab~rration of
the principal rays if they were mounted directly adjacent the
convex mirror 12 so that the surface of the mirror 12 and the
convex surface of the meniscus elements 16 are parts of the same
spherical surface. It will be appreciated that the high order
astigmatism has been greatly reduced with the result that the
width of the corrected annulus is increased by an order of
magnitude.
A necessary condition for the absence of field curvature,
iOe., for a flat image surface, in the resultant system is that
the algebraic sum of the quantities obtained by dividing the
`` power of each surface by its index of refraction be substanitally
` zero, the index of refraction of a reflecting surface being
defined as negative one for this computation. Since the index
of refraction of the meniscus elements varies with the wavelength
of the image forming light, it will be appreciated that the
incoxporation of these elements in the optical system results
in a variation of field curvature with wavelengthO This results
in a variation of the focal position as a function of the distance
from the axis and as a function of the wavelength of the image -
forming light, as shown in Figure '9. In an annular fi~ld optical
system, the variation with distance from the axis is effectively
. , .
3~
removed by restricting the fi~ld to an annulus whose distance
from the axis is constant. The variation of field curvature with
wavelength in such a system becomes a variation of focal position
~ with wavelength and it can be balanced by the introduction of
-- color aberration of the opposite sense. To accomplish this in
` accordance with the invention, the refracting meniscus departs
~i~ from exact concèntricity by having its convex radius of curvature
shorter than the aum of its concave radius and its thickness.
That is, its thickness is greater than the difference between
the radii of its convex and concave surfaces. The way in which
this works can be explained as follows:
The variation of field curvature with wavelenyth introduced
by a nearly concentric meniscus whose power is negative is such
. .
that the back focus is greater for short wavelength than for long
wavelengths. A concentric meniscus with conjugate at its center
` of curvature does not introduce any longitudinal color aberration.
~.'
- The same is substantially true of such a meniscus with a conjugate
near its center of curvature. The acldition of a positive lens
to such a meniscus introduces longitudinal color of the sense
required to balance the variation in focus with wavelength
resulting from the variation of the field curvature (contributed
by the meniscus) with wavelength. This can be accomplished by
making the convex radius of the meniscus shorter than the sum of
its concave radius and its thickness. The meniscus is then equiv-
alent to two lenses, one being a fictitious concentric meniscus
with convex radius equal to the sum of the concave radius and the
thickness, while the second is a zero thicknessppositive meniscus
whose concave radius is the convex radius of the fictitious menis-
cus and whose convex radius is the convex radius of the actual
meniscus. For a nearly concantric meniscus with concave radius
Rl, convex radius R2, thickness t,'and refractive index N, the
longitudinal color compensates for the change in focus due to the
--8--
'
variation of field curv~ture with wavelength in an annulus of
radius H when
R~ ~ Rl
and t~ R2 ~ Rl ~ (H2 / 2N2) (l/Rl - l/R2) (1)
I have found that the introduction of a pair of menisci
whose parameters substantially satisfy equa~ion (1) into an
optical system of the type disclosed in my aforementioned U.S.
Patent No. 3,748,015, together with accompanying modifications
which will be discussed more fully hereinafter, results in a
reduction in the high order astigmatism over a wide spectral band.
The resultant system can be improved considerably by
modifying the menisci so that their thicknesses are greater than
.. . ..
the values giv~n by equation (1). This results in a variation
of focus of an annular field system whose sense is such that
it can be compensated for by the introduction of a plane parallel
plate of appropriate thickness, as indicated at 18 in Fig. 1.
The extra degrees of freedom provided by the additional element
makes possible a much greater degree of correction~
Further improvement can be obtained by modifying the
plane parallel plates in one of two ways:
(l) One of the faces of the plane parallel plate may be
- ` -
made aspheric.
(2) The plane parallel plate may be "bent" resulting in a
-~ meniscus element.
The high~st degree of correetion has been obtained with a
system in which the thickness of the menisci is greater than the
value given by equation (1~ and in which color compensation is
obtained by adding plane parallel plates modified in accordance
with one of the two ways described above. The small aberrations
arising from the departure of the meniscus element 16 from con-
centricity are compensated for by introducing a small departure
from concentricity in the mirror pair 12 and 14. That is, the
_g_
.;~ - .
.:
.
concave mirr~r 14 and the convex mirror 12 are supported with a
: distance between their centers of curvature of less than about
:
two percent of the length of the shorter radius.
Table l is an example, indicating the construction data, of
the annular field optical system of Fig. l. As i5 well known in
- the art, a plus sign is used to denote that a s~rface is convex
` to the object and that distance is measured from left to right
- whereas a minus sign is used to den~te that a surface is concave
to the object and that a distance is measured from right to left.
_ _TABLE 1
: RADIUS OF ANNULUS - 100 mm.
: SURFACE NO. RADIUS (mm) DISTANCE TO NEXT MATERIAL NOTE
.~ FROM OBJECTSURFACE 'Qmm)
TO IMAGE
, ~ ~ ~__ __ _________ _______________ ____~____ __ , . (PLANE) 144.92 AIR OBJECT
1 ~144.96 11.03 FUSED SILICA
2 ~151.75 88.70 AIR
3 -~57.30 16.75 FUSED SILICA
4 -967.84 295.25 AIR
5 -551.15 -279.0'7 AIR MIRROR
6 -267.18 279.07 AIR MIRROR
7 -551.15 -295.25 AIR MIRROR
8 ~967.84 -16.75 FUSED SILICA
9 -957.30 -88.70 AIR .:
-151.75 -11.03 FUSED SILICA
11 -144.96 -144.9~ AIR
,. . .
- 12_ (PLANE) _ _ IMAGE_
Table II is a table of the computed performance of the annular
.,
field optical system of Table 1 over an extended spectral range
`.;`. (2800 A to 5461 A) in terms of the r~s wave aberration at various
- 30 annular radii. The width of the usable annulus is the difference
be~ween the values of the upper and lower radii for which
the p~Efo~mance is adequate for the application. It is noted that
,,
--10--
.
. .
~' .
. .,
a system is usually called "diffraction limited", or more precisely
"aperture limited" when the rms wave aberration is less than ~.07.
For a scanning system, the rms wave aberration may be as high as
0.09 or 0.1 at the edges of the annulus.
TABLE II
N.A. = 0.17 AT OBJECT AND IMAGE
RADIUS OF RMS~WA~E ~ERRATION_(WAVELENGTH ~NITS)
:-~ ANNULU~ (mm)
; WAVELENGTH (ANGSTROM UNITS)
:.. ~ ~ _________________________
~ 38~ 3200 3650 400~ 4358 5461
___~__________________________ _______________
105 .09 .12 .13 .13 .13 .12
104 .05 .08 .09 .09 .09 .08
103 .02 .04 .05 .06 .06 .05
~00 .02 .01 .01 .01 .01 .01
97 .04 .01 .02 .02 .02 .03
;
96 .06 .02 .02 .03 .03 .03
` 95 .08 .04 .04 .04 .04 .04
~ 94 .11 .06 .05 .05 .05 .05
- 93 .13 .08 .07 .07 .06 .06
~: _ ~ ___ _ _ ______ ~________________~ ______
As indicated hereinbefore, the annular field optical system of
the present invention provides at least one convex and dne concave
mirror, which are substantially concentrically arranged along an
optical axis to form conjugate planes normal to said axis for
which the system is of unit power. Fig. 1 shows one suitable
arrangement of the convex and concave mirrors, and other suitable
arrangements of these mirrors are shown and described in my prior
Patent No. 3,748,015. Fig. 8 of the present specification shows an
arrangement which includes a convex mirror 12e and a concave mirror
; 14e arranged substantially concentrically along an optical axis
SA in a manner utilizing a total of five reflections within the
` 30 system, there being three reflections from the concave mirror 14e
and two from the convex mirror 12e. In this embodiment the alge-
braic sum of the powers of the reflecting surfaces utilized is zero
.~ ,
,, , --1 1--
'''
.,
:~ .
3~
when the radius of the convex mirrox 12e is two-thirds that of the
`~ concave mirror 14e. In a manner similar to that described in
connection with the embodiment of Fig. 1, the system of Fig. 8
- includes meniscus elements 16e and a color correcting plate 18e
that function in the aforesaid manner.
It is noted that annular field optical systems of the type
described are usually used in a scanning mode and, for this purpose,
it is highly desirable that the orientation of the object and image
be the same so that their physical supports can be maintained in
fixed relation to each other while being moved relative to the
optical system for scanning and so that the accuracy requirèments
of the scanning motion are minimized. An arrangement that achieves
this by incorporating three flat mirrors in the optical system was
shown in the aforesaid U.S. Patent No. 3,951,546. I have discover-
ed another means of achieving this effect, by using two optical
systems 10 and 10', each being of the type shown in Fig. 1, dis-
posed in back-to-back relationship so that the object and image
planes are superposed, as illustrated in Fig. 2. Thus, the optical
system 10 includes two spherical mirxors 12 and 14, a pair of
-~ 20 meniscus elemen~s 16 and a color correcting plate 18, and the
. .
- symmetrical optical system 10' includes two spherical mirrors 12'
and 14', a pair of meniscus elements 16' and a color correcting
plate 18'. The physical sq~aration between the object and final
.:.
image required for a practical arrangement is obtained by the add-
ition of folding mirrors 20 and 20' shown by broken lines in Fig. 2,
..:
to move the actual object and image to 0' and I', respactively. In
this arrangement the separation between the folding mirrors must be
....
sufficient to provide clearance ~or scanning. It will, of course,
~ . . .
be appreciated that other arrangements of folding flats, which
retain the relative orientation of the object and image, are within~
the scope of this invention.
In the optical system of Fig. 2, the intermediate image, in-
-12-
P8
dicated at 22, i5 highly corrected because it is formed by the
optical system 10 of Fig. 1. In this arrangement, each half 10 and
10', of the system is longitudinally symmetrical and thus revers-
ible. Since for most applications a high degree of correction at
the intermediate image is not requir~, the system can be simpli-
fied by removing the requirements that each half o -the system be
symmetrical, while keeping the system or at least the re~ractive
components thereof symmetrical as a ~hole, and thereby reduce the
number of compensating menisci and correcting plates to two each,
as illustrated in the embodiment of Fig. 3. Thus, a half of the
optical system, indicated at lOa, comprises two spherical mirrors
12 and 14, a meniscus element 16a and a color correcting plate 18a
disposed on the side of the intermediate image 22, all of said
elements being symmetrically disposed about the optical a~is SA~
The other half of the optical system, indicated at lOa', comprises
two spherical mirrors 12l and 14', a meniscus element 16a' and a
color correcting plate 18a' disposed on the side of the int~r-
mediate image 22, all of said elements being symmetrical about the
optical axis SA. For the same reasons indicated hereinbefore,
each half of the optical system is provided with a folding mirror
~; 20 and 20' shown by the broken lines in Fig. 3, to mo~e the actual
object and final image to 01 and I', respectively,
; Referring nex~ to the embodiment of Fig. 4 t it will be appre-
ciated that systems with an intermediate image, indicated at 22,
-~ can be further simplified by combining the two color correcting
;; plates 18a and 18a' of the embodiment of Fig. 3 into a qingle ele-
- ~ ment of the same form as indicated at 18b in Fig. 4. Symmetry is
maintained by placing the single color corrector 18b a~ or closely
adjacent the intermediate image 22. Further, the folding mirrors
20 and 20' of Fig. 3 can also be eliminated by making unequal the
conjugate distance of at least one of the two-mirror components 12b,
14b, and 12b', 14b'. That is, the intermediate image distance to
' . ' ~ t '
-13-
.
.
i3~8
the two-mirror component is made greater than t~e object and/or
image distances, to therebyjyspace the final image I from the
object 0. In this system, the color correcting plate 18b at the
intermediate image can be a true plane parallel.
;~ In a truly afocal system, the magnification is the same for
all conjugate positions~ Howe~er, this desirable feature is not
; achieved in practical applications because real systems do not
in general remain truly afocal for all field positions. In the
unit magnification system of Fig. 4, for example, if the object 0
and image I are moved together longitudinally by lmm., the magni-
fication of a 4mm. radial annulus varies from unity by + .00032.
This variation, which results in ~racking smear during scanning,
can be reduced to + .00001 by aspherizing one of the faces of ths
color correcting plate 18b.
A modification o~ the optical system of Fig. 4 is illustrated
in Fig. 5, wherein the correcting menisci are moved rom the inter- -
~ mediate image side of the system -to the object-image side thereof.
- In the embodiment of Fig. 5, one half of the system includes two
spherical mirrors 12b and 14b and a meniscus element 16c disposed
on the object-image side and symmetrically about the system axis
`-- SA, and the other half of the system includes two spherical mir-
: :
, . ,-- .
j~ rors 12b' and 14b' and a meniscus element 16c' al50 disposed on
; the object-image side and symmetrically about the system axis SA.
A single color correcting plate 18b is disposed symmetrically
."
about the system axis at or closely adjacent the intermediate
image 22. As in the embodiment of Fig. 4, one of the faces of
: .:
this plate is aspherized. Further, as in the embodiment of Fig.
4, the intermediate image distances to the two-mirror components
- are made greater than the object and image distances, to thereby
:
space the final image I from the object 0 for scanning purposes.
Table III is an example, indicating the construction data,
af the annular field optical system of Fig. 5.
~:
-14-
:
.
TABLE IIT
, ~ _____ ____ ___________________________________________________
RADIUS OF ANNULUS = 100 mm.
SURFACE NO. RADIUS (mm~ DIST~NCE TO NEXT MATERIAL NOTE
F~OM OBJECT SURF CE ~prnm)
. TO IMAGE
:~ --_________
O(PLANE) 107.13 AIR OBJECT
1128.18 10.48 ~USED SILICA
.~ 2-135.29 378.48 AIR
~; 3-541~32 -273.56 AIR MIRROR
4-264.61 273.56 AIR MIRROR
5-541.32 -590.28 AIR MIRROR
, . .
: ~ 6-1772.58* -7.01 FUSED SILICA
~: ASPHERIC
7(PLANE) -587.26 AIR
. ~ 8541.32 273.56 AIR MIRROR
~; 9264.61 -273.56 AIR MIRROR
10541.32 378.48 AIR MIRROR
11135.29 10.48 FUSED SILICA
12128.18 107.13 AIR
; 13`,~PLANE) IMAGE
*ASPHERIC SURFACE SYMMETRICAL ABOUT OPTICAL AXIS.
, ~ 20 DEPARTURE ,X, FROM PLANE SURFACE AT DISTANCE r F~OM AXIS:
. ; X= -1772.58~ ~1772.58-r~ ~1.732xl0~8r4~4.210xl0 13r6
~ 8.2-78-x-lo-l8r8_~oo78xlo-2lrlo
- Table IV is a table for the computed performance of the
m annular field optical system of Table III over an extencled
'' spectral range (2800 A to 5461 A) in terms of the rms wave abber-
; ation at various annular radii. The width of the usable annulus
`' is the difference between the values of the upper and lower radii
for which the performance is adequate for the appliaation.
" :
~,
~: -15-
.
"'
, . . .
~3~
TABLE IV
:, :
~ N.A. = 0.17 AT OBJECT AND IMAGE
~; .
,
RADIUS OF RMS WAVE ABBERATION (WAVELENGTH UNITS)
ANNULUS (mm)
: WAVELENGTH (ANGSTROM UNITS)
" _________ ________________
280~ 3200 3650 ~000 4358 5461
______ ___ ________ __________ ______ ________
1~ .0~ .08 .08 .08 .08 .07
102 .06 .05 .05 .05 .04 .04
10~ .05 .04 .04 .03 .03 .03
98 .06 .04 .0~ .04 .0~ .05
. 10 97 .09 . 06 .05 .05 .05 .05
~;~ 96 ___ .13 .09 _ .08 _.07_ .07 .07
An optical system in which the distances from the intermediate
. . .
image 22 to the two-mirror components l2d-14d and 12d'-b4d' are
~ less than the distances from the object O and final image I is
;;; shown in ~ig. 6. In the emhodiment of Fig. 6, one half of the
` ~ system includes two spherical mirrors 12d and 14d and a meniscus
element 16a disposed on the intermediate image side and symmetrically
.. ~. . .
;~`` about the system axis SA, and the other half of the system includes
......
two spherical mirrors 12d' and 14d' and a meniscus element 16a' also
disposed~on the intermediate image side and symmetrically about
the system axis SA. A single color correcting plate 18b is dis-
po9ed at or closely adjacent the intermediate image 22. Parallel
folding flats, as in the systems of Figs. 2 and 3 are introduced
.. ..
'!' between the two crossed conjugate positions ~ and I to make them
accessible for scanning purposes. However, in the embodiment of
S !:
Fig. 6, the two ~olding flats are the front and back surfaces o~
~` a plane parallel plate 20d, whose thickness is determined by
mechanical considerations, in contrast to the arrangements of
Figs. 2 and 3 in which other considerati~ns determinad the separ-
ation between the reflecting surfaces. Thus, in the embodiment
of Fig. 6, the front and back surfaces of the plate 20d serve
to deflect the object 0 to~`70' and the final image I to I', thereby
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:
:
providing the spacing therebetween necessary for scanning.
Another embodiment of the invention utilizing crossed
object and image planes i5 shown in Fig. 7, wherein the single
color correcting plate 18b of Fig. 6 at the intermediate image
has been replaced by two epaced, color correcting plates 18e
and 18e' on the object-image side of the system. In this
embodiment, the substantially plane parallel plates have been
"bent" to form a meniscus element. The remainder of the system
of Fig. 7 is similar to that of Fig. 6. That is, one half of
the system includes two spherical mirrors 12d and 14d and a
meniscus element 16a disposed on the intermediate image side,
and the other half of the system includes two spherical mi~rors
12d' and 14d' and a meniscus element 16a' also disposed on the
intermediate image side. As described hereinbefore in connection
with the embodiment of Fig. 6, plate 20d having mirror front
and back surfaces serves to move the object 0 to 0' and the
final image I to I' to provide physical separation between the
object and image as required for a practical arrangement.
Table V is an example, indicating the construction data,
of the annular field optical system of Fig. 7.
".~
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:
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;13~
TABLE V _ _
RADIUS OF ANNULUS = 100mm.
SU~FACE NO. RADIUS (mm.) DISTANCE TO NEXT MATERIAL NOTE
SURFACE (mm )
~ _____________ _ ____ _____ ~ ___~____ _____~_ ____
(PLANE)151.33 AIR OBJECT
1 -726.8928.69 FUSED SILICA
.2 -730.32410.82 AIR
.:
,
~ 3 -552.06-280.16 AIR MIRROR
.. 4 -267.~18280.16 AIR MIRROR
-552.06363.25 AIR ~RROR
~: 6 -160.78-24.03 FUSED SILICA
7 -145.19-272.77 AIR
.;
.. 8 145.19 -24.03 FUSED SILICA
~ ,
- 9 160.78-363.25 AIR
; 10 552.06280.16 AIR M~RROR
: 11 267.18-280.16 AIR MIRROR
; 12 552.06410.82 AIR MIRROR
13 730.3228.69 FUSED SILICA
~ 14 726.89151.33 AIR
:. 15 (~BaNE~ AGE
~~~ ~~--~~~ ~-
`.: ; Table VI is a table for the computed performance of the
-~:
': annular field op~ical system of Table V~over an extended spectral
range (2800 A to 5461 A) in terms of the rms wave aberration at
various annular rad.ii. The width of the usable annulus is the
'~ difference between the values of the upper and lower radii for
~;
~: ~ which the performance is adequate for the application.
.,,~.; .
. ,,
.,, ~
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, ' ' .
. .
.
-18-
'.
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,
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_ __ _ TABLE VI __ _ _ __ ____
N ~ A . = 0 ~1 7 AT OBJECT AND IMAGE
RADIUS OF RMS WAVE ABERRATION (WAVELENGTH UNITS )
ANNULUS (mm.)
W~VELENGTE tANGSTRoM UN TS)
2800 3200 3650 ~000 4358 5461
___________________ ~ ~_________________________ ~
104 .08 .07 .09.10 .10 .11
103 .06 .04 .05.06 .07 .08
102 .05 .02 .03.04 .05 .06
100 .03 .02 .02.02 .03 .04
,
~:~ 10 98 .07 .04 .03.03 .02 .03
. 97 slO ~06 ~05.04 .04 .03
:; 96 .14 .09 .07_ .06__.06 .05
.r.~ : It is noted that with the arrangement of Fi~. 7 and with the
: configuration of Fig. 5, the refracting or meniscus elements 18a,
:. 18e', 16c and 16c' can be used as windows for sealing the portions
of the optical ~ystem therebetween.
... It will thus be seen that the present invention does indeed
provide a new and improved optical system for use in applica~ions
.~ whose spatial relations must be reproduced with great accuracy,
: ~ 20 and which can be corrected for both a wide annulus and an extended
: ::
spectral ranger Although specific embodiments have been illustrated
~: and described, it will be obvious to those skilled in the art thatvarious modifications may be made without departing from the spirit
and scope of the invention, which is to be limited solely by the
. appended claims.
;',:
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; :
... ~ .
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