Note: Descriptions are shown in the official language in which they were submitted.
1~90183
BACKGROUND OF THE INVENTION
The invention relates to illumination systems, and
more particularly to a system for illuminating an annular field.
Illuminating systems constructed in accordance with the concepts
of this invention are particularly adapted, among other possible
uses, for use in optical projecting and scanning apparatus such
as, for example, the one disclosed in U.S. Application Serial
No. 671,653 filed March 29, 1976 (and which issued as U.S.
Patent No. 4,068,947), that is a continuation of U.S. application
Serial No. 339,860, filed March 9, 1973, which
corresponds to Canadian Patent No. 1,011,979 issued June 14,
1977, and is assigned to the same assignee as the present
application. The apparatus therein uses an annular field
optical system of the type disclosed in U.S. Patent No. 3,748,015
issued July 24, 1973 which, in turn, utilizes the annular
field imaging principle disclosed in U.S. Patent No. 3,821,763
issued June 28, 1974. Both of these patents are assigned to
said common assignee. While a number of different types of
illuminating systems have been employed heretofore with moderate
success, our contribution to the art is a new system, which is
an improvement over such prior art systems, as will become
apparent as the description proceeds.
B -1-
83
SUMM~RY 0~ Nvr,NTIoN
1 ` In order to aecoml)lish the desired results we provide, in one
2 form of the invention, a new and improved system for illuminating
3 an annular field disposed about an axis which includes, in eom-
~` bination, an areuate souree of radiation, preferably disposed eon-
S eentrieally about the axis, and an annular field imaging system
6 having an axis of symmetry eoincident with said axis for forming
7 an image of the areuate source on the annular field.
8 Aeeording to an aspeet of the invention, the field imaging
9 system inelùdes a reflective field element, which in one form
o thereof has a refleetive eoneave toroidal surface with an axis
1 of symmetry eoincident with the axis of the system. In some in-
2 stallations it may take a eatadioptric form, and further, the exit
3 faee thereof may be a spherical surface with its eenter on the axis
~ of the system. In another form the field element has a reflective
surface of cylindrical configuration disposed with its cylindrical
6 axis eoineident with the axis of the system. That is, in the last
7 mentioned form, the field element may be a solid cylindrical seg-
ment oriented so that its axis and the optical axis of the system
9 are essentia~ly coincident, with the element being formed of trans-
o parent refractive material. Moreover, in some installations, a
I field stop ma~ be disposed proximate said field element on the side
toward the arcuate source of radlation.
3 Aeeording to another feature of the invention, the areuate
source of radiation is a high pressure vapor, preferably mercury,
S lamp of eapillary dimensions and of areuate eonfiguration.
Further, in aeeordanee with an aspect of the invention, means
7 are provided for formlng an image, preferably magnified, of the
s arcuate source at the field stop, or at the refleetive field elemenl ,
9 whieh ineludes a eoneave primary mirror facing the areuate source
D of radiation and having an axis of`symmetry c~incident with the
l axis o he 5ys tem, and a eonvex secolldary mirror locatcd letween
Il 10901~3 - I
. . ,
l the arcuate source and the primary mirror to receive radiation or
2 light from the primary mirror and direct it toward the field stop.
3 In one en~odiment, the primary mirror has a centrally disposed aper
~ ture and in another en~odiment the primary mirror has an arcuately-
S shaped slit, concentric with respect to the axis of the system,
6 through which the radiation passes from the secondary mirror to the
7 field stop. In still another embodiment there is no aperture in th~
8 primary mirror, but the secondary mirror is in the form of an aspher-
9 ic lens having one portion of the convex surface thereof reflectively
o coated, to serve as the secondary mirror, and a second portion is
1 transparent so that the radiation from the source is transmitted
2 therethrough to the primary mirror.
3 In addition, according to another aspect of the invention,
the annular field imaging system includes first and second field
lenses having axes o~ symmetry coincident with the axis of the
6 system, and the field stop is interposed therebetween. In this
7 er~odiment a transfer mirror is provided for forming a real image
Ei of the field stop, and an aperture stop is located one focal length
9 ahead of the transfer mirror. Preferably the field lenses are
O plano-convex spherical lenses.
Still fu~ther, in another embodment of the invention, the
field imaging system includes, in addition to the reflective field
3 element proximate the field stop, a transfer mirror for forming
4 a real image of the field stop and an aperture stop so located with
s respect to the transfer mirror that the exit pupil appears at a
6 desired location, when viewed in the focal plane of the annular
field. In one form thereof, an aspheric element, having an axis
3 of symmetry coincident with the axis of the system, is interposed
9 between the reflective fleld element and the aperture stop. In one
3 embodiment the aperture stop is in the form of a weak convex mirror
~ ¦¦to th eby make the relay system inverse telephoto, and preferably
~ . I
` 3. ll
ll 109018;~
I ¦ this convex mi-u~n~ l~c disposed one focal length in front o~ the
2 ¦ trans~er mirror.
3 ¦ There has thus been outlined rather broadly the more
~ ¦ important features of the invention in order that the detailed
S ¦ description thereof that follows may be better understood, and in
6 ¦ order that the present contribution to the art may be better
~ ¦ appreciated. There are, of course, additional features of the
O ¦ invention that will be described hereinafter and which will form
9 ¦ ` the subject of the claims appended hereto. Those skilled in the
o ¦ art will appreciate that the conception upon which the disclosure
¦ is based may readily be utllized as a basis for the designing of
2 ¦ other systems for carrying out the several purposes of the in-
¦ vention. It is important, therefore, that the claims be regarded
~ I 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,
7 purposes of illustration and description, and are shown in the
8 accompanying drawings, forming a part of the specification.
g BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of a system for illuminating
an annular field, constructed in accordance with the concepts of
2 the present invention;
3 Fig. 2 is a schematic representation of a second embodi-
ment of the system for illuminating an annular field according to
s the invention;
6 Fig. 3 is a schematic representation, similar to Fig. 2,
7 but showing still another embodiment of the invention. ¦~
9 Fig. 4 is a schematic representation, similar to Fig. 3,
9 but showing an embodiment including an inverse telephoto relay
system;
Fig. 5 is a schematic representation, similar to Fig. 3,
2 but showing an alternate form of field element;
` 4 _
.
~1 ` 109U183 I :
t ~ Fig. 6 is an enlarged schematic representatiorl of a field ¦
1 element, constructed according to the invention, in the form of a
3 reflective surface of cylindrical configuration;
~ Fig. 7 is an enlar~ed schematic representation of a field
S element similar to Fig. 6, but showing a catadioptric form there-
6 of;
7 Fig. 8 is an enlarged schematic representation of a field
;8 element, constructed according to the invention, in the form of a
9 reflective concave toroidal surface;
Fig. 9 is an enlarged schematic representation of a field
element similar to Fig. 8, but showing a catadioptric form
l2 thereof;
Fig. 10 is an enlarged schematic representation of a field
t~ element similar to Fig. 9, but showing an element having an exit
face in the form of a spherical surface; and
~6 Fig. 11 is an enlarged schematic representation showing
l7 still another field element.
1~ DETAILED_DESCRIPTION OF THE PREFERRED EMBODI.~ENTS
l9 In the èmbodiment of the condenser or illumination system
of the present invention illustrated in Fig. 1, the condenser
Zl~ system includes, as a light source, a mercury capillary lamp 502
of arcuate shape, a concave primary mirror 504 centrally apertured
23 at 506, and a convex secondary mirror 508 supported on a plate 510
2~ of suitably transparent material capable of passing both the visi-
ble and ultraviolet light produced by the lamp.
~ An arcuate image of the lamp 502 is formed by the mirrors 504
27 and 508 ( and with the aid of a plane mirror 511) at the plane
28 of an arcuate slit 512 disposed between field lenses 51~ and 516. Z
29 These lenses are segments of plano-convex spherical lenses. The
illumination system is so disposed that the il;umination slit at .
31 512, as reimaged by plane mirrors 513 ànd 515 and by a transfer
32 mirror 518, appears in the plane of a mask 220; and mo~e
.
lO901B;~
particularly in the plane in which the surface of the mask near
the reader is moved on the carriage as described in conjunction
with Figs. 9 to 11 of the aforementioned Canadian Patent No.
1,011,979. The fraction of the mask thus illuminated is
indicated at 223. The arcuate illuminated area 223 is fixed in
position with reference to the image forming system disclosed in
said application. As disclosed therein, the image forming sys-
tem is in accordance with the teaching of U.S. Patent 3,748,015
referred to hereinbefore and has an axis of symmetry which
passes through the center of curvature of the arcuate area 223.
The area 223 of the figure thus constitutes an extra axial
annular object for that image forming system and for that extra
axial object the imaging system processes optimum correction as
described in the aforementioned U.S. Patent.
The high pressure mercury capillary arc 502 is made to
operate as an annular source by properly shaping the envelope.
A suitable lamp is described in U.S. Patent No. 3,878,419, which
is assigned to the same assignee as the present invention. This
patent matured from an application, which was copending with
the parent application of the present application. Light from
this lamp is imaged by the above described reflecting objective
comprising the concave mirror 504, the convex mirror 508 and
the field lenses 514 and 516. This reflecting objective has an
entrance pupil imaged substantially at the convex mirror 508
which partially obscures the aperture of the condenser system.
However, the obscuration is substantially the same for the
imagery of all parts of the light source. The field lenses 514
and 516 form an image of the entrance pupil at the aperture
stop 519, which is one focal length ahead of the mirror 518.
30 The transfer mirror 518 forms a collimated image of the stop 519,
which is the exit pupil of the condenser. At the same time, the
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11 1090183
I ¦annular slit 512 is ima~ed by the mirror 51~ at 223 to form a
2 ¦ field stop on the mask 220. The elements of the condenser are
3 ¦disposed to obtain a common axis of symmetry 503 with the center
4 ¦of curvature of the arcuate light source 502 on this axis. In
S ¦ this case, the condenser system is an annular field optical system
6 ¦in accordance with the principle of off-axis annular field optical
7 ¦imagery disclosed in the above-mentioned U.S. Patent No. 3,821,763.
8 ¦ The size of the stop 519 can be changed, if desired, to alter the
9 ¦degree of partial coherence of the illumination at the mask 220,
lo ¦which is provided to the subsequent imaging system.
¦ It will thus be seen that this embodiment of the invention
12 ¦provides a new and improved optical condenser system for use in an
u ¦ annular field optical system, which permits efficient uniform ill-
i~ lumination of the instantaneous annular field of an optical system
i5 lin which the object is transilluminated.
6 ¦ In the embodiment of the invention illustrated in Fig. 2,
¦ the condenser system for illuminating an annular field comprises,
8 1 in combination, high pressure mercury capillary lamp 502' of
9 ¦arcuate conf`iguration, which is the same as the lamp 502 described
!0 ¦ hereinbefore in connection with the embodiment of Fig. 1. The
!1 lamp 502 ~ iS disposed with its arcuate plasma concentric with
respect to the optical axis 503 1 of the system. In order to
~ - present a magnified image of the lamp plasma (hereinafter referred
'A . to as the "arc") in a plane indicated at 15 in Fig. 2, the arc is
reflected from a concave primary mlrror 504' and a convex secondary
~ mirror 508 1, which together form a reflecting objective.
7 The primary mirror 504' is preferably fabricated of Pyrex
8 glass and the secondary mirror 508' of aluminum, for example.
9 The secondary mirror is mounted on a window 510' preferably of
o fused silica, which in addition to supporting the secondary mirror
¦¦ serv to isolate the remainder of the optical system from air jet
_ 7 _
Il 1090183 ` ~ '
(not shown), which are normally employed to cool the mercury lamp
2 502'. The primary mirror 504' contains an aperture, which is in
3 the form of a slit 506', and through which the magnified image
passes to reach the plane 15. It will be appreciated that the
S pupil stop of the reflective objective is sufficiently near the
6 secondary mirror that obscuration is minimal and nearly identical
7 for all points along the lamp arc.
~ - To obtain an accurately delineated annular illuminated
9 field at a position where it is undesirable to put a field stop, ,
o the illumination system must provide an intermediate image
position, at which an arcuate slit may be placed. In the embodi-
ment of the invention illustrated in Fig. 2, a member 19, con-
3 taining an arcuate slit 512', concentric with the axis, is
positioned at the plane 15 and serves as a field stop. The final
s image is determined by the size of this sli~.
s Field elements are required to direct the light from all
7 portions of the svurce into the desired pupil stop of the system.
9 ` We have invented a new form of field element, which is reflecting
or catadiop`tric. Since it is to be used in a ring field system,
J the new field element is a portion of a surface of revolution
- about the optical axis. Thus, after passing through the slit,
the light is reflected by total internal reflection in a cata-
dioptric, cylindrical element 21, which has its cylindrical
surface 22 disposed in concentric relationship with respect to
the axis 503'. This element is a solid cylindrical segment
i generated by a plane passed through a cylinder containing or
parallel to its cylinderical axis. It i5 formed of transparent
; refractive material, preferably fused silica, so that it will
be highly transparent to ultraviolet actinic light. The light
coming to a focus at each point of the annular slit 512' is propa-
gating away from the optical axis 503' with the central rays alon~ j
a cone with its apex on the axis at a point slightly to the left, s
- 8 - I
1090183
viewed in Fig. 2, of the convex mirror 508'. The element 21
redirects this light so that the central rays travel along a
cone with its vertex on the axis at a circular pupil stop 519'.
Although the refracting material introduces some var-
iations of imagery with color, in many applications this is
quite~tolerable. It will be particularly appreciated that the
cylindrical element 21 is a new form of field element, which
functions as a field lens, and is particularly useful in annular
field systems. Other alternative forms of this field element
are described hereinafter in connection with the embodiments of
Figs. 3 to 10.
Light from the field element 21 is refracted through
an aspheric element 23, which is also preferably of fused silica,
and then passes through the circular pupil stop 519', mounted
concentrically about the axis 503'. The aspheric element 23
corrects for aberrations of the imagery of the slit 512' and
gives a much sharper slit image than would otherwise be the case.
The image, after passage through the pupil stop 519',
is reflected by an aspheric or spherical mirror 518', prefer-
trc~d ~ r~) Q r k-
ably of Pyrex~ The combination of the cylindrical field element
21, the aspheric element 23 and the concave mirror 518' act to
image the slit 512' at 223' in a focal plane 220', where the
illumination is required. It will be particularly appreciated
that the pupi~ stop 519' is disposed at one focal length in ad-
vance of the spherical mirror and is so positioned that the
exit pupil appears at infinity when viewed in the focal plane
220'. The focal plane 220' can be the plane of a microcircuit
mask in a system such as the one described in the aforementioned
Canadian Patent No. 1,011,979.
It is noted that, if desired, the light reflected from
the mirror 518' may be passed through an actinic filter 25,
which is usually inserted during the ali~nt process to re~ove the ultra-
violet light, thereby preventing exposure of the photosensitive
B
ll 109U183
member at that time. In addition, the pupil stop 519' can be made
Z adjustable or can be varied to alter the degree of partial coherence~
3 of the illumination at the focal plane 220'. ¦ i
It is noted that the system of Fig. 2 is particularly desirable
S where space considerations are important. For example, the system
6 of Fig.- 2 only requires a seven inch diameter primary mirror,
? whereas the system of Fig. 1 would require a ten inch diameter
primary mirror to achieve comparable results.
g In the embodiment of the invention illustrated in Fig. 3, the
o condenser system for illuminating an annular field comprises, in
~1 combination, a high pressure mercury capillary lamp 502" of arcuate¦
2 configuration, which is the same as the lamps 502 and 502' described
3 hereinbefore in connection with the embodiments of Figs. 1 and 2.
~ This lamp is also disposed with its arcuate plasma concentric with
5 . respect to the optical axis 503" of the system. In order to pre-
6 sent a magnified image of the lamp arc in a plane indicated at
15" in Fig. 3, the arc is reflected from a concave primary mirror
8 504" and a convex secondary mirror 508", which together form a
reflecting objective. In this embodiment there is no aperture
through the concave primary mirror 504" and the convex secondary
;1 mirror 508" i-; included as part of an element 27, which is in the
2 form of an aspheric lens having one-half of the convex surface
3 thereof reflectively coated. The radiation from the lamp 502" is
transmitted to the primary mirror 504" through the transparent half
S 29 unobscured by the reflective half, which serves as the secondary
6 mirror 508". That is, the secondary mirror 508" presents no ob-
7 scuration for an arc subtending 90 at the center line and contain-
8 ing an f/0.8 divengence angle, larger angles and corresponding
9 higher f/Numbers are possible up to an arc subtending 180 at f/~ .
o A member 19", containing an arcuate slit 5i2", concentric with
the axis, is positioned at the plane 15" and serves as a field stop
.
. ,~
., _ _ . . . ~
I 1090~83
l As indicated hereinbefore in connection with the embodiment of
2 Fig. 2, the final image is determined by the size of this slit.
After passing through the slit 512", the light is reflected by a
~ concentrically disposed reflective field element 21", which has a
S concave toroidal surface 22" that reflects radiation from the im-
6 age at the annular slit 512". The concave toroidal surface is
7 contoured by radii, which both are of finite length. Light from
8 the field element 21" passes through a circular pupil or aperture
9 stop 519", mounted concentrically about the a~is 503", and thence,
o it is refl-ected by a spherical or aspheric mirror 518". The
1 combination of the field element 21" and the concave mirror 158"
act to image the slit 512" as at 223" in a focal plane-220",
3 where the illumination is required. As in the embodiment of Fig.
4 2, the pupil stop 510" is disposed at one focal length in advance
s of the transfer mirror and is so positioned that the exit pupil
6 appears at infinity, when viewed in the focal plane 220".
7 An example of suitable design parameters for the optical
8 apparatus of Fig. 3, with an f/3.5 image subtending an 80 angle
g about the optical axis thereof, is given in the following table:
Radius- Separation-
l Componentmillimeters millimeters Comments
2 Lamp plane C~O 10.15 (1)
3 Aspheric lensC~O
front 8.1 Fused Silic
back (2) 110
S Primary mirror131.5 -110
6 Aspheric mirror (2) 108.6
,7 Slit c~ 20 - Slit radius
97 n~.
Toroid (3) 149.2
g Stop C~ 89.2 30 mm.
diameter
o Aspheric
1 Transfer mirror (4) 128.6
2 Focal plane C~ ,
.. . .
, ~ `
` 1090183
.
(1) The lamp bore has a 1 mm. radius, and the fused
2 silica lamp envelope has a 3 mm. radius. The lamp is conccntric
3 with the optical axis, the center of the bore being 14 mm. from
the axis.
S (2) Equation of aspheric is:
6 Saj = y2 (57.05 +/57.052 - 1;57186y2 ) -1 + 6.403 x 10-7y4
7 (3) Toroid has a radius of 112 mm. normal to the optical
8 axis and a radius of 310.775 parallel to the optical axis.
9 (4) Equation of aspheric is:
~o Saj = y2 (188.826 ~ ~8.8262 - .31297y2 ) -1 + 6.924 x 10-9y4 +
Il 1.391 x 1o~12y6 -
l2 Referring next to the embodiment of the invention illus-
l3 trated in Fig. 4, this embodiment is similar to the embodiment of
l~ Fig. 3, but is employed when a greater distance is required be-
ls tween the aspheric mirror 518" and the image plane 220". In this
16 embodiment, the aperture stop 519" of Fig. 3 is replaced by a
I~ weak convex mirror 30 to thereby make the relay system inverse
l8 telephoto. The convex mirror 30 also serves as the aperture stop
l9 : and is disposed one focal length in front of the mirror 518" so
that the pupil as seen from the image plane 220" is at infinity.
21 The distance between the mirror 518" and the image plane 220" is
22 controlled by varying the power of the convex mirror 30. The
23 portion of the system of Fig. 4 ïn front of the convex mirror 30
2~ is the same as the portion of the system of Fig. 3 in front of the
2S stop 519". Thus, an image of the slit 512" is formed at 223" in
26 the focal plane 220", where the illumination is required.
27 Fig. 5 illustrates still another embodiment of the inven-
tion. This embodiment is the same as the embodiment of Fig. 3,
29 except for the field element 21. The field element 21"' of Fig. 5is a catadioptric reflective toroidal field element, which has a
31 ~ ¦ conca toroidal surface 22"' that reflects radistion from the
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ll lO9U183
1 image at the annular slit 512". The concave toroidal surface is
2 contoured by radii, which are both of finite length. That is, the
3 toroidal surface is generated by rotating the circular section
4 shown about the optical axis 503" so that it is a portion of a
S surface of revolution about this axis. The center of curvature
6 of the circular section is above the axis in the illustrated
7 embodiment. This field element is formed of transparent refractiv
material, preferably fused silica, so that it will be highly
transparent to ultraviolet actinic light. The toroidal field
O element 21"' has a plane entrance face 34 and a spherical exit
1 face 36. That is, the exit face is a spherical surface with its
center of curvature on the optical axis 503". As a result, in
3 this catodiaptric field element, variations of imagery with color
4 are minimized, when the center of curvature of the exit face is
S substantially at the center of the pupil image at 519".
6 It will be appreciated that any one of the field elements
7 21 described hereinbefore in connection with each of the embodi-
8 ments of Figs. 2 to 5 may equally well be employed with any other
9 one of thesè embodiments. In addition, Figs. 6 to ll illustrate
o additional forms of the field element, any one of which being
applicable t~ any one of the embodiments of Figs. 2 to 5. In
2 these embodiments, the entrance pupil of the system seen by the
3 field element, as described in connection with Figs. 2 to 5, is
4 schematically illustrated at 38, and the pupil image is schemati-
S cally illustrated at 40. The pupil and the pupil image are both
6 concentric about the optical axis 503 of the system.
7 Fig. 6 illustrates the field element in its simplest form.
In this embodiment the field element 21a is a reflecting cylindril
9 cal surface 22a disposed wlth its cylinder axis coincident with the
optical axis 503. As seen in Fig. 6, all points in a ring that
1 pass through the pupil to form a ring image at or near the reflec~-
- 13 - I
. .. , _. . . . _. 1
1~9018~
1 ing surface pass through an image of the pupil. The field elemen~
t 21b of Fig. 7 is a catadioptric form of the field element 21a of
3 Fig. 6. In Fig. 7 surface 22b is a reflecting cylindrical surface
~ disposed'with its cylindrical axis coincident with the optical
S axis 503. It is noted that the embodiment of Fig. 7 is similar
6 to the embodiment of Fig. 2, except that in Fig. 7 the center of
7 the annular illuminated area is in the cylindrical surface where-
8 as in'Fig. 2 an arcuate slit 512' is provided whereat the center
9 of the annular illuminated area is located. This distinctlon will
lo be discussed more fully hereinafter. The elements 21, 21a and
21~ of Figs. 2, 6 and 7 serve as field elements only along the
i2 length of the annular field. In applications in which the annulus
3 is narrow, this is acceptable. However, for installations requir-
-~ ing a wide annulus, there will not be a unique pupil image loca-
tion for radiation passing through radially separate portions of
6 the annular field, as shown by the broken-line rays in Fig. 6.
7 This problem is overcome by substituting the toroidal elements
8 22", 22c and 22d of Figs. 3, 8 and 9 for the cylindrical elements
9 21, 21a and''21b of Figs. 2, 6 and 7, respectively. In these
!O toroidal elements, sufficient power is introduced in the planes
!l of the drawin~s to also image the pupil in these planes. -
' ' The toroidal surface 22c and 22d of Figs. 8 and 9, respec-
3 tively, are each generated by rotating the ci~cular section shown
!~ about the optical axis 503 so that it is a portion of a surface of
~ revolution about this axis. The center of curvature of the circu-
!6 lar section is above the optical axis in this case. It is noted
7 that the cylindrical field element 21a of Fig. 6 is a special case
8 of this toroidal element in which the center of curvature of the
circular section is at infinity so that the circular section be-
o comes a straight line. Fig. 9 illustrates a catadioptric version c f
1 the field element of Fig. 8 wherein a reflecting surface 22d,corres-
11 ' .
109()183
1 ponding to surface 22c of Fig. 8, is obtained by putting a ref]ect-
2 ing surface on transparent refracting material. It is noted that
3 the embodiment of Fig. 8 is~similar to the embodiment of Fig. 3,
~ except that in Fig. 8 the center'of the annular illuminated area
S is in the toroidal surface whereas in Fig. 3 an arcuate slit 512"
6 is provided whereat the center of the annular illuminated area is
7 located. `
8 In the field elements illustrated in Figs. 2-4 and 6-9, the
9 angle between the optical axis and the converging bundle of rays
lo - at the pupil is the same as that between the'axis and the diverging
:1 bundle at the pupil image. A field element 21e which alters this
2 angle is shown in Fig. 10. The fleld element 21e is a modification
3 of the catadioptric field element 21d shown in Fig. 9. The toroi-
4 dal surface 22e is the same as the toroidal surface 22d. In the
field element 21e, a spherical surface 36e with its center on the
6 optical axis 503 is substituted for the plane exit face of the
7 field element-21d of Fig. 9. In this catadioptric field element,
8 variations of imagery with color are minimized when the center of
9 curvature of the exit face is substantially at the center of the
o pupil image 40. '
I However, it is also-desirable in some installations, depend-
2 ing on the space available, to reverse the system of Fig. 10 so
3 that the pupi:l image and pupil are interchanged, and the exit and
entrance faces of the field element are interchansed, whereby the
S angle between the imaging forming bundle and the optical axis be-
6 comes larger at the pupil image.
7 It is noted that the embodiment of Fig. 10 is similar to the
B embodiment of Fig. 5, except that in Fig. 5 the center of the
g annular illuminated area is in the toroidal surface whereas in Fig.
o 5 an arcuate slit 512" is provided whereat the center of the
I annular illuminated area is located.
. '' . :
15.
I1 1090183
l Another means of obtaining a difference between the angles !
2 whi~h the converging and diverging bundles make with the axis is
3 ShOWII in Fig. 11. As in the other ~ield elements, the reflecting
~ surface 22f is a toroid which is concentric about the axis. It
S differs from the other field elements in that the normal from its
6 middle point is not perpendicular to the axis.
7 As indicated hereinbefore, the field elements of Figs. 6
8 to ll have been shown with the center of the annular illuminated
g area in the toroidal surface. However, it will be appreciated
lo that their operation is substantially the same when the annuLar
Il image is external to the field element, but close to it as shown
12 in Figs. 2 to 5, wherein the phystical slits 512' and 512" are
13 placed at the annular illuminated area. With the arrangements of ¦
i~ Figs. 2 to 5, astigmatism is introduced into the imagery of the
s annular area. That is, the focus of the radial elements is not
6 at the same position as the focus of the tangential elements, but
~7 ~his is desirable in a ring field illuminating system since at the
8 - tangential focus of the image of the physical slit, the delimiting
9 edges are in sharp focus while residual variations in illumination
O along the arcuate area are averaged out.
:l It will thus be seen that the present invention does indeed
provide a new and improved system for illuminating an annular
3 field, which has a high degree of field illumination uniformity,
reduced aberrations, and a simplified optical arrangement. In
addition, the new system provides uniform illumination over a wide
6 spectral band including the ultraviolet region by means of reflec-
7 ting optics, and delimits the annular illuminated field accurately
a when the field is at a location where it is undesirable to put a
9 field stop. Although specific embodiments have been illustrated
~ and described, it will be obvious to those skiiled in the art that
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ll 109018;~
various modif.ications may be made without departing from the
2 spirit and scope of the invention, which is to be limited solely
by th~ appended l~i-
ll ~ '
