fig1 is a schematic view of an illumination device and a projection exposure apparatus using the same , according to an embodiment of the present invention . in this embodiment , the invention is applied to a reduction projection type exposure apparatus , called a stepper . this exposure apparatus can be used for manufacture of microdevices such as semiconductor memories , ccds , liquid crystal panels or a magnetic head , for example . denoted in the drawing at 1 is a light source such as a high luminance ultra high pressure hg lamp , for example , for emitting ultraviolet light or deep uv light . the light source 1 has its light emitting portion 1a disposed adjacent to the first focal point of an elliptical mirror 2 . the light emanating from the light source 1 is reflected and collected by the elliptical mirror 2 , and then it is reflected by a cold mirror 3 by which an image 1b of the light emitting portion 1a ( an image of the light source ) is formed in the neighborhood of the second focal point 4 of the elliptical mirror 2 . the cold mirror 3 has a multilayered film and it mainly serves to transmit infrared light but to reflect ultraviolet light . denoted at 101 is an imaging system having two lens systems 5 and 9 . it serves to image the light source image 1b , formed in the neighborhood of the second focal point 4 , upon an entrance plane 10a of an optical integrator 10 through the cooperation of an optical device 8 . the optical device 8 comprises a prism member 6 having a conical prism , for deflecting received light to a predetermined direction , and a parallel flat plate 7 for projecting received light directly . denoted at 8a is a holding member by which the prism member 6 and the parallel plate 7 of the optical device 8 can be alternately and selectively placed on the light path . when the parallel plate 7 is on the path , the imaging system 101 is defined as a telecentric system on the exit side . the optical device 8 is disposed in the neighborhood of the pupil plane of the imaging system , where the opening of the elliptical mirror is imaged . the optical integrator 10 comprises a plurality of small lenses which are arrayed two - dimensionally , and it serves to define a secondary light source 10c in the neighborhood of the exit surface 10b thereof . denoted at 11 is a stop member having a plurality of apertures . the stop member is provided with a mechanism by which the shape of an aperture on the light path can be changed . to the secondary light source 10c , the stop member 11 is disposed in such a region in which discrete secondary light source elements do not overlap . denoted at 14a is a lens system for collecting the light from the exit surface 10b of the optical integrator 10 and for illuminating , through the stop member 11 and a mirror 13 as well as a collimator lens 14b , a reticle ( surface to be illuminated ) 15 placed on a reticle stage 16 . the lens system 14a and the collimator lens 14b constitute a condensing lens system 14 . denoted at 17 is a projection optical system for projecting , in a reduced scale , a pattern formed on the reticle 15 upon the surface of a wafer 18 which is placed on a wafer chuck 19 . denoted at 20 is a wafer stage on which the wafer chuck 19 is mounted . in this embodiment , with the condensing lens system 14 , an image of the secondary light source 10c formed in the neighborhood of the exit surface 10b of the optical integrator 10 , is formed in the neighborhood of a pupil 17a of the projection optical system 17 . next , description will be made of the manner of changing the light intensity distribution of the image of the secondary light source , which image is formed at the pupil plane 17a of the projection optical system 17 , in accordance with the present embodiment by changing the light intensity distribution at the light entrance surface 10a of the optical integrator 10 through the optical device 8 . fig2 a and 3a each illustrate the light path from the elliptical mirror 2 ( fig1 ) to the optical integrator 10 , the path being extended . the mirror 3 is not shown in fig2 a or 3a . the illustrations of fig2 a - 2c and 3a - 3c explain that the light intensity distribution upon the entrance surface 10a of the optical integrator 10 is changed by alternately selecting the elements 6 and 7 of the optical device 8 . fig2 a - 2c correspond to a case where the parallel plate 7 of the optical device 8 is disposed on the light path , whereas fig3 a - 3c correspond to a case where the prism member 6 of the optical device 8 is disposed on the light path . the illumination system of fig2 a is adapted for a first state of projection in which a very high resolution is not required but in which a larger depth of focus is assured . the illumination system of fig3 a is adapted for a second state of projection in which a high resolution is required mainly . fig2 c and 3c each is a schematic representation of a light intensity distribution upon the light entrance surface 10a of the optical integrator 10 . the zone depicted by hatching in the drawings denotes the region of higher light intensity , as compared with the remaining region . fig2 b and 3b illustrate a distribution of light intensity i along the x - axis direction ( fig2 c or 3c ). in fig2 a , the parallel plate 7 of the optical device 8 is disposed on the light path , and the light source image 1b as formed at the second focal point 4 of the elliptical mirror 2 is imaged by the imaging system 101 upon the light entrance surface 10a of the optical integrator 10 . here , as seen in fig2 b , the light intensity distribution in section upon the light entrance surface 10a of the optical integrator 10 has an approximately gaussian shape which is rotationally symmetric . in fig3 a , the prism member 6 of the optical device 8 is placed on the light path , and the light source image ( point image ) 1b formed at the second focal point 4 of the elliptical mirror 2 is imaged , into a ring - like shape , on the light entrance surface 10a of the optical integrator 10 by the imaging system 101 . the light intensity distribution on the light entrance surface 10a of the integrator 10 has a ring - like shape such as shown in fig3 b or 3c wherein the light intensity is lower at the optical axis portion but is higher at the peripheral portion . why this is so will now be explained below . fig4 schematically illustrates the disposition of the parallel flat plate 7 , the lens system 9 and the light entrance surface 10a of the optical integrator 10 of fig2 a . in this embodiment , these elements are so disposed that the optical distance between the parallel plate 7 and the forward principal point of the lens system 9 as well as the optical distance between the backward principal point of the lens system 9 and the light entrance surface 10a of the integrator 10 are , if the focal length of the lens system g is denoted by f 0 , both equal to a distance f 0 . here , the incidence height t 1 , from the optical axis , of the light emanating from the parallel plate 7 with an angle α 0 and impinging on the light entrance surface 10a is expressed as follows : if the height , from the optical axis , of the outermost light passing through the parallel plate 7 is denoted by s 0 , then the angle β of incidence upon the light entrance surface 10a of the integrator is given by : it is seen therefrom that , by deflecting the angle of light at the position of the parallel plate 7 ( i . e ., the forward focal plane of the lens system 9 ), it is possible to change only the position of incidence light upon the entrance surface 10a of the optical integrator without changing the angle of incidence . based on the optical principle described , in the present embodiment , by interchanging the parallel plate 7 by the prism member 6 comprising a conical prism , the light intensity distribution on the entrance surface 10a of the integrator 10 is changed into a ring - like intensity distribution having a lower intensity at the optical axis portion and a higher intensity at the peripheral portion . since the light intensity distribution on the entrance surface 10a of the integrator 10 corresponds to the light intensity distribution of an effective light source which is defined at the pupil plane 17a of the projection optical system 17 , by using the prism member 6 in place of the parallel plate 7 such an effective light source having a light intensity at distribution having a lower intensity at a central portion ( optical axis portion ) and a higher intensity at a peripheral portion is defined on the pupil plane of the projection optical system . in this embodiment , the stop member 11 is provided in the neighborhood of the exit surface 10b of the optical integrator 10 . this stop member has a plurality of apertures and is provided with a mechanism for changing , as desired , the aperture shape thereof . the aperture shape which is variable is predetermined and it corresponds to the shape of the secondary light source to be formed at the pupil plane 17a of the projection optical system 17 . for example , the stop member may have a ring - like aperture of a property for passing a larger quantity of light at its peripheral portion than at its central portion . in this embodiment , the selection of the prism member 6 of the optical device 8 singley or the selection of the prism member 6 together with the changing of the aperture shape of the stop member 11 in combination , assures a desired shape of effective light source while attaining a high efficiency of light utilization . it is to be noted that the provision of the stop member 11 is not a requisition in this embodiment . with the arrangement of this embodiment as described above , for a reticle 15 pattern having a relatively large minimum linewidth , the structure shown in fig2 a is selected ( like an illumination system of conventional type ), whereby a gaussian shaped light intensity distribution is provided at the entrance surface 10a of the optical integrator 10 ( first state ). on the other hand , for a pattern having a relatively small minimum linewidth , the structure shown in fig3 a is selected to provide a ring - like light intensity distribution at the entrance surface 10a of the integrator 10 . also , the aperture shape of the stop member 11 is changed . by this , an illumination device for high resolution projection is assured ( second state ). the insertion of the parallel plate 7 in the first state of fig2 a is to minimize the difference in optical path length between the lens systems 5 and 9 , as compared with that in the case where the prism member 6 is inserted in the second state . if the prism member 6 has a small thickness or if a slight change in optical path length between the lens systems 5 and 9 does not influence the optical performance of the optical integrator 10 or of any other optical elements following it , the parallel plate 7 may be omitted . fig5 and 6 are schematic representations for explaining the relationship of the incidence height ( heights t 1 end t 2 from the optical axis ) at the entrance surface 10a of the optical integrator 10 , relative to the position ( exit heights s 1 and s 2 ) and deflection angle ( α 1 and α 2 ) of light passing through the parallel plate 7 , where in the present embodiment the focal length f of the lens system 9 constituting the imaging system 101 is changed . if in fig5 the focal length of the lens system 9 is f 1 , then t 1 = f 1 tan α 1 applies . also , in fig6 if the focal length of the lens system 9 is f 2 , then t 2 = f 2 tan α 2 applies . it is seen from these equations that if the focal length of the lens system 9 is made large then it is possible to obtain , at the entrance surface 10a of the optical integrator 10 , an incidence position t 1 of desired height with a small deflection angle α at the position of the parallel plate 7 . this means that if the focal length f of the lens system 9 is made large then it is possible to make small the angle of the prism member 6 ( prism angle ) in the second state . this assures an imaging system 101 of smaller aberration . practically , in consideration of the size of the prism member 6 , the focal length of the lens system 9 may be so set to define a prism angle of 5 - 20 deg . the prism member 6 of the optical device of the present invention is not limited to a conical prism . it may have any shape provided that it has a function for deflecting received light in a predetermined direction . for example , pyramidal prisms such as shown in fig8 a and 8b may be used . fig7 b and 8b schematically illustrate a light intensity distribution on the entrance surface 10a of the integrator 10 when the prism member such as shown in fig7 a or 8a is used . the zones depicted by hatching denote regions of higher intensity as compared with the remaining region . it is possible in the present invention to use three or more types of interchangeable optical members such as prisms and a parallel plate , rather than only two optical members of the prism 6 and the parallel plate 7 are interchanged as in the present embodiment . further , the pyramidal prism such as shown in fig7 a may be rotated about the optical axis , for smoothing with respect to time , to provide a ring - like light intensity distribution such as shown in fig3 c . still further , the light source 1 may be displaced along the optical axis concurrently with the interchange of the prism member , to change the size of the higher light intensity region . fig9 is a schematic view of a main portion of a second embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 a half mirror 30 is disposed on the light path at the position before ( light source 1 side of ) the optical integrator 10 , so that a portion of the light from the imaging system 101 is directed to a photodetector 31 which may comprise a ccd or a quadrant sensor . the remaining portion is of the same structure as that of the first embodiment . in this embodiment , the light intensity distribution at the light entrance surface 10a of the optical integrator 10 is measured indirectly to monitor the same . this allows adjustment of the imaging system 101 while monitoring changes in light intensity and / or light intensity distribution at the entrance surface 10a . in this embodiment , a mechanism 60 for rotating the optical member 6 about the optical axis or for shifting the same with respect to the optical axis , may be used . this provides the ability to change the light intensity distribution at the entrance surface 10a of the integrator 10 into a desired shape easily . fig1 a is a schematic view of a main portion of a third embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 in addition to the insertion of the prism member 6 into the light path , the lens system 9 is replaced by a lens system 33 of a different focal length which is disposed at the entrance face 10a side of the optical integrator 10 . the remaining portion is of the same structure as that of the first embodiment . in this embodiment , light is collected to a region narrower than the entrance surface 10a of the integrator 10 , and light intensity distribution of a desired shape is obtained . referring now to fig1 a , 11b , 12a and 12b , the optical function of this embodiment will be explained . fig1 a - 12b schematically illustrate the light path from the optical device 8 ( prism 6 and parallel plate 7 ) to the integrator 10 . fig1 a , 13b , 14a and 14b show a light intensity distribution at the entrance surface 10a of the integrator 10 , defined by using the prism member 6 or the parallel plate 7 . fig1 a shows the arrangement where , in the first embodiment , a conventional type of illumination is to be done . generally , the angle of light rays that can enter the optical integrator is determined and , in the example of fig1 a , the angle is θ 1 . thus , the optical system before the integrator 10 is designed so that the angle of incidence upon the integrator 10 becomes not greater than the angle θ . here , in the light intensity distribution at the entrance surface 10a of the integrator 10 , the degree of convergence is limited due to lagrange &# 39 ; s invariant . for example , it is not possible to improve the degree of convergence beyond that of fig1 a . an attempt to obtain a higher degree of convergence simply ends in that the angle of incidence upon the integrator 10 goes beyond the angle θ . fig1 b shows the state where , in the first embodiment , the prism member 6 is inserted into the light path . fig1 b shows a corresponding light intensity distribution at the entrance surface 10a . here , the maximum incidence angle of the light upon the entrance surface 10a , at the point s 1 , is θ 1 the same in the fig1 a example . however , the effective light that angle of the light actually enters is θ 2 . as seen from fig1 a , with the provision of an optical device 32 ( which may comprise a prism or a field lens ) in front of the entrance surface 10a , it is possible to reduce the maximum incidence angle . fig1 a shows a corresponding light intensity distribution at the entrance surface 10a . here , since the maximum incidence angle is loosened , by shortening the focal length of the optical system from the prism 6 to the optical integrator it is possible to obtain a higher degree of convergence . fig1 b shows an example wherein the degree of convergence is improved on the basis of such an optical principle just described . a corresponding light intensity distribution is shown in fig1 b . in the example of fig1 b , the prism member 6 has an enlarged prism angle so as to obtain a light intensity distribution of a ring - like shape . in this embodiment , the insertion of the prism member 6 as described causes shift of the angle of incidence at the entrance surface 10a of the integrator 10 with the maximum incidence angle being unchanged . by correcting such shift and optimizing the incidence angle , the incidence angle is loosened . thus , it becomes possible to increase the degree of convergence to the limit where the incidence angle becomes equal to the critical incidence angle . practical means for this end may be using a zoom system for the optical system from the prism member 6 to the integrator 10 ; using interchangeable optical systems ; provision of a prism ( conical prism where the prism member 6 comprises a conical prism ; a pyramidal prism where the prism member 6 comprises a pyramidal prism ) in front of the integrator 10 ; insertion of an aspherical lens ; or an appropriate combination of them . fig1 a - 15c are schematic views of a portion of a fourth embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 the position of the optical device 8 ( the position of prism member 6 and / or parallel plate 7 ) is shifted from the pupil of the imaging system 101 and the focal length of the optical system 9 is changed , to thereby converge the light intensity distribution at the entrance surface 10a of the optical integrator 10 . the remaining portion is of the same structure as that of the first embodiment . in fig1 a - 15c , reference character p denotes the pupil plane of the lens system 9 . fig1 a shows the first state of illumination in the first embodiment . the angle of incidence upon the integrator 10 is θ . fig1 b shows the second state of illumination in the first embodiment , and the incidence angle is θ the same as in the fig1 a example . here , if the prism member 6 is shifted from the pupil plane p and the beam diameter on the plane p is reduced such as shown in fig1 c , then it is possible to make the incidence angle θ &# 39 ; smaller than the angle θ 2 of the fig1 a and 11b examples . in the present embodiment , on this occasion , the focal length of the lens system 9 is changed so as to assure that the light intensity distribution at the entrance surface 10a of the integrator 10 is collected and converged locally . fig1 is a schematic view of a main portion of a fifth embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 the lens system 5 constituting the imaging system 101 is omitted and the opening 2a of the elliptical mirror 2 is imaged by the lens system 9 upon the entrance surface 10a of the optical integrator 10 . also , the optical device 8 is disposed in the neighborhood of the second focal point of the elliptical mirror 2 . the remaining portion is of the same structure as that of the first embodiment . more specifically , in the embodiment of fig1 an image of the light emitting portion 1a of the light source 1 is formed on the entrance surface 10a of the integrator 10 , and the optical device 8 is disposed in the neighborhood of the imaging position of the opening 2a of the elliptical mirror 2 ( the position of the image of the opening 2a ) which is between the light source 1 and the integrator 10 . in the present embodiment , as compared , the image of the opening 2a of the elliptical mirror 2 is formed on the entrance surface 10a of the integrator 10 , and the optical device 8 is disposed at the imaging position of the light emitting portion 1a ( the second focal point position of the elliptical mirror 2 ) which is between the light source 1 and the optical integrator 10 . thus , in this embodiment , the forward focal point position of the lens system 9 is placed substantially at the second focal point position of the elliptical mirror 2 , and by the lens system 9 , the light from the light source image 1b at the second focal point is transformed into substantially parallel light which is then directed to the entrance surface 10a of the integrator 10 . when the prism member 6 is being inserted and if it is of the type such as shown in fig7 a , four parallel lights from the lens system 9 are projected on the entrance surface 10a of the integrator 10 . fig1 is a schematic view of a main portion of a sixth embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 the optical device 8 comprises at least two prism members 6a and 6b disposed along the optical axis and , for changing the light intensity distribution at the entrance surface 10a of the integrator 10 , namely , for rendering the illumination system into the second state , the optical device 8 ( prism members 6a and 6b ) is mounted on the optical axis and , additionally , a portion of the lens system 9a constituting the imaging system 101 is replaced by another lens system 9b so as to reduce the incidence angle of an off - axis principal ray to the entrance surface 10a . this is done for efficient utilization of light . in the illumination method of this embodiment , in the first state the lens system 9a is placed on the light path ( the optical device 8 is not used ), so at to provide a light intensity distribution at the entrance surface 10a of the integrator 10 , that is , a light intensity distribution at the pupil plane 17a of the projection optical system 17 , which distribution is of a rotationally symmetric shape wherein the intensity is higher at the central portion than at the peripheral portion . the second state is defined by placing the optical device 8 ( prism members 6a and 6b ) on the light path and by replacing the lens system 9a by the lens system 9b having a different focal length . this makes smaller the angle of incidence of the principal ray upon the entrance surface 10a of the integrator 10 , whereby at this entrance surface 10a , namely , at the pupil plane 17a of the projection optical system 17 , such a light intensity distribution in which the intensity is higher in the peripheral portion than in the central portion is provided . structural features of this embodiment over the first embodiment will be explained in more detail . in fig1 , lens system 5 collects the light from the light source image 1b formed in the neighborhood of the second focal point 4 , and it emits parallel light . the imaging system 101 ( lens systems 5 and 9a ) is telecentric on the exit side . at least a portion of the collecting lens 14 is made movable along the optical axis , to adjust the light intensity distribution on the reticle 15 . in this embodiment , in accordance with the orientation and / or the linewidth to be resolved of the pattern of the reticle 15 , for example , the lens system 9a which is a constituent element of the imaging system 101 is replaced by the optical device 8 ( including two prisms 6a and 6b ) and the lens system 9b , to change the light intensity distribution at the entrance surface 10a of the integrator 10 . additionally , if necessary , the aperture shape of the stop member 11 is changed to change the light intensity distribution of the image of the secondary light source which image is formed at the pupil plane 17a of the projection optical system 17 . next , the manner of changing in this embodiment the light intensity distribution on the entrance surface 10a of the integrator 10 as well as the light intensity distribution of the image of the secondary light source to be formed on the pupil plane 17a of the projection optical system 17 , on the basis of the optical device 8 , will be explained . fig1 through 19b each schematically illustrate the light path from the elliptical mirror 2 to the optical integrator 10 of fig1 , the path being extended . the mirror 3 is not shown in fig . these figures . the illustrations of fig1 a and 19a explain that the components of the optical device 8 are interchanged to change the light intensity distribution on the entrance surface 10a of the integrator 10 . fig1 a shows the state in which the lens system 9a is placed on the light path . fig1 a shows the state in which the lens system 9a is removed and , in place thereof , the prism members 6a and 6b of the optical device 8 and the lens system 9b are placed on the light path . the illumination system of fig1 a is in the first state of projection in which a very high resolution is not required but a large depth of focus is assured , as in the illumination method of a conventional type . the illumination system of fig1 a is in the second state of projection , according to the present invention , mainly for attaining high resolution . fig1 b and fig1 b each schematically shows a corresponding light intensity distribution on the entrance surface 10a of the integrator 10 . the zone depicted by hatching denotes the region of higher intensity as compared with the remaining region . in these illustrations , the distribution of light intensity i along the x - axis direction are depicted . fig2 a , 20b and 20c are schematic representations , for explaining the light rays impinging on the optical integrator 10 , in the systems of fig1 a and 19a . reference characters + θ and - θ each denotes the range ( angle ) of light rays that can enter the optical integrator 10 ( that can emerge from the integrator without being eclipsed ). a grid portion in each illustration depicts the zone in which the light intensity is higher than that of the light entering the integrator 10 . fig1 a shows the optical arrangement in ordinary illumination . here , the light intensity distribution at the entrance face 10a of the integrator 10 is like a gaussian distribution such as shown in fig1 ( b ). the incidence angle thereof is such as shown in fig2 a . when in this state the illumination for high resolution is to be done , there may be a method in which a stop 121 having an aperture 121a such as shown in fig2 is inserted at the back of or in front of the integrator 10 . however , with this method , only the light in the hatched zone of the light intensity distribution of fig1 a , can be used and , therefore , the illuminance decreases considerably . in this embodiment , in consideration thereof , as shown in fig1 a , the lens system 9a is replaced by the lens system 9b of smaller focal length ( the optical components are disposed so that , if the focal length of the lens system 9b is f 9b , the optical distance between the prism 6a and the lens system 9b and the optical distance between the lens system 9b and the entrance surface 10a of the integrator 10 are both equal to f 9b ), such that the light intensity distribution as shown in fig1 b , is provided at the entrance surface 10a of the integrator 10 . additionally , the prism member 6b having an appropriate prism angle is inserted in front of the integrator 10 , by which the incidence angle of light rays ( incidence angle of off - axis light ) is made smaller such as shown in fig2 c . this assures efficient impingement or entrance of light into the integrator 10 . thus , almost all the input light can be used for the illumination . on the basis of the optical principle described above , the present embodiment uses the optical arrangement such as shown in fig1 a , by which illumination for high resolution is assured without a substantial loss of illuminance at the surface to be illuminated . the prism member 6a or 6b of pyramidal shape provided within the imaging system 101 may have a shape such as shown in fig8 for example . of course , it may be a conical prism . while the embodiment has been explained with reference to an example wherein the lens system 9a of fig1 a for ordinary illumination is replaced by the lens system 9b of fig1 a for high resolution illumination , the lens elements of the lens system 9a may be displaced ( like a zoom lens system ) to define the same condition as by the lens system 9b . only some of the lens elements may be moved like a zoom lens system or , alternatively , some lens elements may be replaced by different lens elements . the stop member 121 for high resolution such as shown in fig2 may be used as desired , or it may be omitted . further , while in this embodiment the focal length of the lens system 9a is changed to change the magnification of the imaging system 101 , the focal length of the lens system 5 may be changed . alternatively , both of the focal lengths of the lens systems 5 and 9 may be changed . in this embodiment , there are cases wherein , in response to the interchange of the ordinary illumination ( first state ) and the illumination for high resolution ( second state ), uniformness in illuminance ( non - uniformness of illuminance ) upon the surface being illuminated changes into axial symmetry . on that occasion , a portion of the optical system 14 may be displaced along the optical axis to change aberration such as distortion to thereby correct for the axially symmetric non - uniformness of illuminance upon the surface ( reticle 15 surface ) to be illuminated . while in the preceding embodiment the reticle 15 ( surface to be illuminated ) is disposed just after the optical system 14 , an additional imaging system may be disposed between the optical system 14 and the reticle such that a plane which is optically conjugate with the reticle 15 surface with respect to the additional imaging system may be illuminated . fig2 is a schematic view of a main portion of a seventh embodiment of the present invention . in this embodiment , as compared with the first embodiment of fig1 a half mirror 43 is disposed between the integrator 10 and the surface 15 to be illuminated , so as to allow detection of the amount of exposure of the surface being illuminated . the remaining portion is of substantially the same structure as that of the first embodiment . in fig2 , denoted at 44 is the reticle pattern surface or a plane which is optically conjugate with the reticle pattern surface . denoted at 45 is a pinhole member which is disposed at a position optically conjugate with the plane 44 . denoted at 33 is a photosensor ( e . g ., a ccd or quadrant sensor ). with this arrangement of the present embodiment , it is possible to monitor the effective light source distribution at the center of the surface being illuminated . also , in this embodiment it is possible to concurrently monitor , with the photodetector 31 , the amount of exposure of the surface being illuminated . while in this embodiment the half mirror 43 is placed between the lens system 13a and the collimator lens 14b , it may be disposed at any position between the integrator 10 and the surface 15 . in accordance with the present invention , in consideration of fineness and / or orientation of a pattern of a reticle to be projected and transferred , an illumination system suited to such pattern can be selected . while the invention has been described with reference to the structures disclosed herein , it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims .