the present invention and the problems it addresses will be best understood by first referring to a prior art instrument employing a bidirectional optical path steering system , and a consideration of its different light signal paths , as illustrated in fig1 . fig1 schematically illustrates an instrument 1 for illumination and observing the eye fundus . instrument 1 includes an instrumentation section 200 having the primary instrumentation for generating illumination and for analyzing or displaying collected reflected light , and a steering section 100 for aiming at the object to be inspected . section 200 provides illumination from one or more light sources which may , for example , be several laser sources s 1 , s 2 of different wavelengths , and a broadband source s 3 . a plurality of beamsplitters bs 1 and dichroic beamsplitters bs 2 , bs 3 are positioned to direct the light from all the sources along a common optical input path 7 to the steering mechanism 100 , and to split off return light passing in the opposite direction along the same general path into a plurality of filtered output beams 17a , 17b , 17c which pass to direct viewing , electrical image forming or analysis , and eye motion image tracking subsystems or the like . the steering system 100 redirects or steers the optical path 7 to a deviated path 8 by reflection from the surface of x - and y - galvanometer - controlled steering mirror assemblies 21 , 22 , each of which pivots about one orthogonal axis , and is controlled by feedback signals from a tracking system . such an instrument is described in greater detail in the aforesaid u . s . patent . path 8 leads , via an optional image rotator 50 and objective optics 23 , 24 , to the fundus f of the eye of a subject the objective optics 23 , 24 , together with various relay lenses 25 , 26 , 27 and 28 , are set up such that the eye fundus is imaged in the intermediate planes designated ip 2 and ip 3 conjugate to the fundus , while the center of rotation 19 of the eye is conjugate to the plane containing the axis of rotation of each mirror 21 , 22 , and is centered on the pivot axis of each mirror . in the above - described apparatus , all input and output light is reflected from the faces of the two mirrors 21 , 22 , and the separation of the relatively intense illumination or laser surgical light from the various viewing , treatment or analysis beams is addressed primarily by spectral and spatial light separation techniques . while different , e . g ., adjacent , pupils may be set up for different beams , and may be required , for example , for a two - beam doppler analysis instrument , nonetheless the use of the same mirror surfaces for light traveling in two directions degrades the low - level signals by introducing significant retroreflection and scattering noise . these problems are addressed in accordance with the present invention by providing a bidirectional steering system with isometric but disjoint optical paths for the two directions . a basic embodiment of such a steering system 100a according to the present invention is shown in fig2 . this system may be substituted for the steering system 100 of fig1 to overcome the above mentioned limitations . system 100a is operative to direct a first light path 7a , illustratively of a laser beam , from x - and y - steering elements 60 , 70 , respectively , to a deviated or steered output path 8a leading to an objective optical assembly 30 , and to receive back along path 8b from the same assembly collected light which is also directed at the first and second steering elements 60 , 70 and thereby undergoes an angular transformation closely corresponding to that of input beam 7a , 8a . the steering elements are two sided elements , which are each centered at a pivot axis , and the light paths in opposite directions are reflected from , or are redirected after being incident upon , opposite sides of the elements . suitable elements are , for example , thin planar mirrors such as a 0 . 2 mm thick sheet silvered on at least one side and preferably both sides , or possibly specially arranged acousto - optic modulator cells . for clarity of discussion , these elements , which are operative to redirect light incident upon the element from each of two opposite sides , will be referred to simply as &# 34 ; mirrors &# 34 ; or &# 34 ; two sided mirrors &# 34 ; below . in the case of a conventional mirror , i . e ., a sheet of glass with a metallized reflective surface , it is not essential that the two opposite surfaces each be metallized , provided the sheet is sufficiently thin to avoid introducing refractive errors . however , such two - sided reflection coating is preferred to reduce scattering , and for applications in which low - level signal collection is carried out through the steering system , both surfaces are metallized . in the embodiment shown , the input beam on path 7a is relayed from the outside face 61a of mirror 60 to the outside face 71a of mirror 70 by a fixed arrangement of mirrors m1 , m2 , m3 which operate as an optical relay to translate path 7a at the central pivot axis of mirror 60 to path 8a at the central pivot axis of mirror 70 . as illustrated , mirrors m1 , m3 have lens elements 63 , 64 on their front surfaces which are telecentric to the pivot axes of the mirrors 60 , 70 , and thus place the outer sides of the mirrors in conjugate relation . as discussed further below in connection with preferred instrumentation , separate curved mirrors , or lenses located between the planar mirrors are preferably substituted for lenses 63 , 64 to reduce scattering and allow greater flexibility in positioning the various components . the optical relay system preferably is of unit magnification , and this property is required for certain stereoscopic instrument embodiments discussed further below . continuing with a description of the optical paths in fig2 light reflected from the eye returns through the same objective assembly 30 along a path 8b which strikes the inside face 61b of mirror 60 and is reflected to the inside face 71b of mirror 70 , thereby undergoing inverse x - and y - transformations to produce an output beam along an output path 7b which remains stationary as the steering mirrors move . the inner faces 61b , 71b of the x - and y - steering mirrors are separated by a relatively small distance , which for a one centimeter square galvanometer - actuated mirror may , for example , be under one - half centimeter , so that steering related distortion of the steered beam is minimized without requiring additional precision relay optics to place each inside steering mirror face separately at a conjugate position . when the steering system of fig2 is employed , for example , in a fundus imaging system as shown in fig1 the conjugate pupil plane may be located between the mirrors . in that case , a single artificial pupil 72 positioned between faces 61b , 71b results in spatial filtering of the collected light to obtain both high image quality and high signal collection . in particular , the problem of decentering of the pupil position and the mirror rotation axes is minimized , greatly reducing focal shifts and optical aberrations in the collected image . in fig2 the input pupil 36 , or region of the iris of the eye through which the input light path 8a is directed , and the output or observation pupil 37 through which return light is collected , are each slightly offset from the optical axis of the objective assembly 30 , and are conjugate , via assembly 30 , with the central pivot axes of the respective mirror faces 71a , 61b . thus only the second mirror in each path may introduce mechanical scan distortion . this distortion is minimized , in the case of the return imaging path 8b , by the close placement of the mirror inner faces 61b , 71b and the absence of diffractive elements between the faces . fig3 shows the system of fig2 with the optical elements 63 , 64 replaced by separately mounted lenses 107 , 110 and with the input light path relocated to the right side for clarity of illustration . laser 101 , optics 102 , 103 and turning mirror 104 direct a laser beam at the outer face of one steering mirror 105 , and the mirrors 106 , 108 , 109 and lenses 107 , 110 relay the beam to the other steering mirror 110 . objective optics 30 are shown as separate lens assemblies 12 , 14 with the front lens 14 forming an image of the eye fundus at plane 13 , and the rear lens 12 relaying the image to the mirrors , such that the pupillary plane of the eye lies in the plane of an artificial pupil 118a located between the mirrors . the turning axes of both inside mirror faces , as well as the artificial pupil 118a , thus lie in the plane in which the pupil of the eye is imaged . the lenses 107 , 110 are adjusted to place the outer faces of the steering mirrors 105 , 111 in conjugate relation , so that mechanical scan distortion and image decentration are largely avoided . the image light on path 7b passes by imaging optics 119 , 120 to an image - receiving unit 122 which , as indicated previously , may be a photodetector , ccd array , camera , reflectometer , doppler analyzer or other imaging or light - analyzing apparatus . a confocal diaphragm 115 conjugate to the fundus plane screens out light from other regions of the eye . a second diaphragm 118 which forms virtual pupil 118a is conjugate , via the optics , to the virtual pupil and to the iris of the eye . fig4 shows another embodiment structurally identical to fig3 except that mirror 105 is replaced by a polygon 105a . the input light beam direction is oriented to strike one face &# 34 ; a &# 34 ; of the polygon , and is reflected to the inner face of mirror 111 , while the return light strikes a co - rotating face &# 34 ; b &# 34 ; of the polygon which is conjugate , via mirrors 108 , 109 and lenses 107 , 110 , with the outer face of mirror 111 . such a polygon introduces mechanical scan distortion due to the large translational component of motion , and thus introduces special design constraints and may be suitable only for small angular fields . however , it permits high periodic scanning speeds . in this embodiment , the positions of image analyzer 122 and laser 101 may be interchanged . advantageously , the essential symmetry of the input and output paths , and their fixed separation are exploited in one preferred instrument by providing an optical trigger or alternating shutter such as a rotation beamsplitter in a position that is effective to alternately interchange the paths followed by the illumination and the imaging beams . fig5 shows such a system , with the various elements numbered identically to corresponding elements of fig3 . the laser and imaging instrument paths 7a , 7b are oriented such that a rotating beamsplitter 123 either passes illumination along 7a straight through lens 103 whence it is deflected to the outer face of mirror 105 , or reflects the illumination from path 7a through lens 119 to the inner face of mirror 111 . as the beamsplitter rotates , the inner and outer faces are used alternately to steer the illumination beam from path 7a , while the return beam is steered , by the set of mirror faces not in the illumination path , to the imaging unit 122 . one set of directional arrows is drawn in to illustrate the light paths when a reflective sector of rotation beamsplitter is positioned at the junction of paths 7a , 7b . by reversing each arrow except the one nearest to the laser and the instrument , the same drawing illustrates the light paths followed when a transmissive sector of rotation beamsplitter 123 is positioned at the path junction . fig5 a illustrates a typical rotation beamsplitter 123 . a thin disc 123 formed of a transparent material is mounted for rotation about a central axis c . one sector 123a of the disc is silvered or otherwise rendered reflective at the wavelengths employed , and another sector 123b is not reflective . two or more sectors may be provided , depending on the desired sampling or frame rate . the disc is positioned at an angle to the two optical paths 7a , 7b so that as it rotates both paths either proceed through the beamsplitter , or both paths are reflected off the beamsplitter and are redirected . in order to further reduce the level of noise in a opthalmic instrument utilizing a steering system in accordance with the invention , the means for translating one steering mirror face to a position conjugate to the other mirror face are realized in a preferred embodiment with a pair of focusing mirror elements rather than the separate planar mirrors 106 , 108 , 109 and lenses 107 , 110 of fig3 and 5 . fig6 a illustrates a steering system 300 according to this aspect of the invention . two curved mirrors 306 , 309 place the central turning regions of the outer faces of mirrors 105 , 111 in conjugate relation , thus replacing the three mirrors 106 , 108 , 109 and two lenses 107 , 110 . the number of optical surfaces in the conjugation assembly is thus reduced from seven to two , thereby reducing scattering . the use of two reflective elements in this fashion to place the outer faces of the steering mirrors in conjugate positions results in a low noise optical path which is especially suitable for such low light applications as tracking microscopic features at the back of the eye . in other respects , the embodiment of fig6 a is similar to that of fig5 . for completeness , the rotation beamsplitter 123 in fig6 a is illustrated in a position which passes the light from laser 101 without reflection , so that the illumination and observation light paths are reversed from those of fig5 for all paths between beamsplitter 123 and the eye . in particular the illumination and observation windows in the eye pupil are interchanged , and the relevant faces of each steering mirror are switched . fig6 b shows an instrument similar to that of fig3 but having curved mirrors 109a , 108a which operate , together with plane mirror 106 to place the central turning axis at the outer faces of the steering mirrors in a conjugate relation . the pupillary conjugate plane containing the turning axis of both steering mirrors 105 , 111 , is indicated by p . aperture 116a also lies in plane p conjugate to the eye pupil . the relay mirrors 108a , 109a have a curvature to provide unity magnification of the translated beam between mirror 106 and mirror 111 , and thus maintain an identity between the input illumination angle and the light collection angle . conjugate confocal diaphragm 115 filters out light not originating at the retinal plane , so that a ccd , reflectometer or electronic tracking or analysis device may be positioned at any position downstream of diaphragm 115 . alternatively , if diaphragm 115 is omitted , such ccd or the like may be positioned at , or conjugate to , the indicated plane of 115 . the eye objective lenses 12 , 14 provide a magnification of approximately 2 ×, and the steering mirrors 106 , 111 effectively act as &# 34 ; virtual &# 34 ; mirrors or apertures positioned in the eye pupil . in order to avoid placing a diaphragm in the small space between these two mirrors , virtual aperture 118a is preferably formed by a physical aperture 118 downstream of and conjugate to 118a . one useful property of this construction is that the reflective elements lying between lenses 119 and 12 in the figure may be rotated as a unit around the axis of those lenses without interfering with their function . thus , when used with an imaging instrument of the type shown in fig1 which has an image rotator to set up and align the field of view , the image rotator 50 need not occupy any space ahead of the steering optics but may constitute or be mounted within the housing of these optical elements . fig7 shows an instrument 700 for stabilizing a diagnostic beam on the fundus . instrument 700 operates by tracking the position of two microscopic tissue features on the eye fundus , through an objective steering system which also aims the diagnostic / imaging elements . in this respect , the instrument operates on principals similar to those illustrated in the aforesaid u . s . pat . no . 4 , 856 , 891 , wherein separately aligned tracking beams are directed through a common steering system with the instrument &# 39 ; s principal beams . the precise structure of the instrument of fig7 however , employs three dual path steering mirror systems together with a separated prism tracking beam element to assure precise alignment and low noise of the many optical paths . broadly , instrument 700 employs an eye objective steering assembly 750 through which a diagnostic laser beam is introduced from first laser 701 , turning mirror 702 and lens 703 via a dichroic mirror 704 which is reflective at the wavelength of laser 701 . a visible return image returns through assembly 750 and is viewed through optics 716 , 717 via a second dichroic mirror 715 . a tracking assembly 760 injects a pair of tracking beams along paths 771 , 772 which pass through the dichroic mirrors 715 , 704 respectively , so that the two tracking beams are also steered by the steering assembly 750 . in the tracking assembly 760 , a second laser 725 sends a beam of tracking illumination at a different wavelength along a path defined by mirrors 726 , 728 . a fifty percent reflective mirror 727 directs half the light to a first steering assembly 721 , and the remainder of the light passes to a second steering assembly 722 . each assembly 721 , 722 is identical , except for size , to the assembly 300 of fig6 a , and includes a pair of two - sided steering mirror elements and a pair of curved relay mirrors placing the outside faces of the steering mirror elements conjugate to each other . each of the two steering assemblies 721 , 722 is used to steer an input tracking beam which serves as a horizontal tracking beam a h or a vertical tracking beam a v , respectively , to illuminate a fundus region . each steering assembly also receives a corresponding return image beam b h ( respectively b v ) from the region of the fundus illuminated by its tracking beam . the image beams go to respective detectors 723 , 724 , which may be high resolution ccd arrays as described in the aforesaid u . s . patent . the four tracking system beams a h , b h , a v , b v all pass through a prism 720 having offset faces 731 , 732 which fold the illumination beams into and unfold the imaging beams from a bundle oriented axially with the optics 718 , 719 and steering system 750 , while preserving four separate pupils for the four beams , above and below , and left and right of the principal axis . the prism is a single optical element which serves to define the fixed spacing between the different beams . as further described in the aforesaid u . s . patent , which is incorporated by reference for this purpose , each tracking beam is initially aligned , by manipulating joysticks that control its steering assembly , to illuminate and image a target such as one of a pair of orthogonally oriented microscopic vessels on the fundus . thereafter , the detector outputs are processed to develop control signals which move the two principal steering mirrors 705 , 709 of assembly 750 to maintain the two selected target images stationary on detectors 723 , 724 . this automatically also maintains the beam of the diagnostic laser 701 , which may , for example be an ablating or coagulating beam , aimed at a fixed spot on the retina . thus the instrument of fig7 employs separate x - and y - tracking assemblies , each directed at a separate vessel or other target on the fundus , to provide steering control signals to x - and y - deflection mirrors 705 , 709 . the diagnostic or imaging light is spectrally separated from the tracking light , and the input illumination paths are separated from the much lower intensity observation paths , thus eliminating a major source of image noise . the prism 720 precisely maintains all four tracking beam paths separate from each other , and allows the paths of laser 701 and observation optics 716 , 717 to pass through other separate regions of the eye pupil , further enhancing signal clarity . in a variation of this structure , the fifty percent reflective mirror 727 may be replaced by a rotation beamsplitter , arranged so that the x - and y - tracking corrections are performed successively , rather than concurrently . the beamsplitter is preferably rotated at a rate to provide several hundred or more steering corrections per second . for example , an imaging rate of one thousand frames per second may be required to maintain a five - micron beam stably positioned for laser surgery . this completes a description of a bidirectional steering assembly and illustrative instruments employing such assembly for opthalmic imaging in accordance with the present invention . the teachings of the invention being thus disclosed , different variations and modifications will occur to those skilled in the art , and all such variations and modifications are included in the scope of the invention , as defined by the claims appended hereto .
an optical steering assembly includes first and second steering elements , such as orthogonally - oriented glavanometer mirrors , scanners or acousto - optical cells , for redirecting light in both forward and return optical paths without crosstalk or mixing . input light traveling in the first direction is directed from the first side of the first element to the first side of the second element , undergoing two orthogonal steering deflections to a redirected output path . return light along the same or a closely adjacent path is directed at the second side of one element , where an optical relay system tranlates it to the second side of the other element . the return light thus strikes the opposite sides of the same elements as the input light , and undergoes corresponding steering corrections while maintaining complete beam separation . the assembly is especially useful in instruments where low light levels , scan distortion , or crosstalk would otherwise limit performance . a two dimensional fundus tracker employs one such assembly as an objective steering system for all illumination , and two further assemblies for positioning tracking beams directed into the objective steering system .