an illustrative embodiment of the invention will be described in detail , in conjunction with the accompanying drawings , in which : fig1 is a simplified optical diagram of a binocular operation microscope embodying a dosimeter instrument of the invention , in the context of an eye to be operated upon , the aspect of the diagram being a side elevation ; fig3 is an enlarged view of the front panel of the dosimeter instrument of fig1 ; fig4 is an electrical block diagram schematically indicating certain components of the dosimeter instrument ; fig4 a is a similar schematic diagram for further components of the dosimeter ; fig5 is a simplified and fragmentary optical diagram from the aspect of fig1 in aid of specific discussion ; fig6 is an enlarged , partly broken , longitudinal sectional view of an illumination probe alternatively usable with the dosimeter of the invention ; fig6 a is an enlarged and simplified diagram to illustrate use of the probe of fig6 ; and fig7 is a simplified diagram of a photo - sensor probe optionally usable with the dosimeter of the invention . in fig1 and 2 , the invention is shown in application to a binocular operation microscope wherein a housing 10 is adapted for mounting to a support arm 11 and carries a single main objective 12 serving spaced paths of stereoptical viewing alignments , from convergence at the cornea of an eye 13 , to the respective halves of a binocular tube 14 . also using the same objective 12 is a field - illumination system within a side housing 15 and comprising a light source 16 , a lens system 17 -- 17 &# 39 ;, and a reflecting prism 18 . light thus cast by source 16 is reflected at 19 for traverse of objective 12 at an eccentrically offset location between the respective binocular - viewing paths ; thus - projected light covers the object field at substantially the wide - angle limit of object viewing through the microscope . in the collimated - light region between objective 12 and the ocular portion 14 , the microscope may include one or more of various optical devices , such as a magnification changer 20 , a beam splitter 21 , as to serve a monocular - viewing tube , and a beam splitter 22 to serve a tv or photographic camera . the range of optical accessories optionally usable in this collimated - light region is of course more extensive , and the items mentioned are therefore to be understood as merely illustrative of auxiliary optical means which can have an attenuating or modulating effect upon instantaneous reflected - light flux arising from a given illumination of the object field and reaching the ocular region 14 of the microscope . these attenuating or modulating effects translate to the viewing surgeon as one of the needs to variously control the level of field illumination provided by light source 16 , and a knob control for the purpose is indicated at 30 . in accordance with the invention , the time - integration of a small fraction of the field - illumination flux , whatever the varying setting of control means 30 , is taken as the measure of total dose exposure of the patient &# 39 ; s retina in the course of a given operational procedure . in fig1 this sampling occurs via a transparent flat plate 31 interposed between elements of the projection system and having sufficient internal scattering to permit generation of an electrical - signal output to a dosimeter instrument 32 , the signal being illustratively generated by a photodetector , such as a silicon photodiode , edge - mounted to plate 31 . the plate 31 may illustratively be of plexiglass , and it is shown positioned in a ray - convergence zone of the projection system , between lens element 17 &# 39 ; and reflector 19 . for the projection system shown , the filament of source 16 appears as a first image ( or &# 34 ; secondary &# 34 ; light source , see fig5 ) at the reflector 19 , and light emerging from objective 12 is so projected that a second image can form at the retina of the eye being operated upon . referring now to fig3 and 4 , the front panel 33 of dosimeter 32 is seen to provide four separate decimal dose - related led or liquid - crystal displays , applicable to any one of four optional input signals , as may be selected at 34 ; for the input 1 that is shown to have been selected , the input signal will be in analog form , from the output of the edge - mounted sensor on plate 31 . the display at 35 is labeled &# 34 ; power &# 34 ; and is a four - digit expression of the currently sensed intensity of the projection beam ; the display at 36 is labeled &# 34 ; dose time &# 34 ; and is a four - digit expression of time lapse since commencement of the field - illuminating projected light ; the display at 37 is labeled &# 34 ; dosage &# 34 ; and is the instantaneous value of time - integrated intensity of the projected light ; and the display at 38 is labeled &# 34 ; remaining time &# 34 ; and is the instantaneous value of time remaining to the point of reaching a preset upper limit of dosage , for the currently applicable level of projected illumination . various other controls and indications at panel 33 will appear from description of the circuits of fig4 and 4a , fig4 being relevant to generation of data presented at the displays 35 - 36 - 37 - 38 . in fig4 legend indicates that the output of the light - sampling sensor is first supplied to an amplifier 40 , having panel - mounted means 41 for calibrating adjustment . amplifier output is processed for analog / digital conversion and is directly used for the power display 35 , being symbolized p in fig4 . an independent self - contained generator 42 supplies clock pulses to a pulse counter 43 , whereby to provide a digital output which is a strict measure of time ( t ) since commencement of projected - light exposure to the eye 13 . an integrating circuit 44 accepts the digital output of the clock - pulse generator and of the detected - signal amplifier , effectively continuously multiplying the detected power ( p ) function by each unit of elapsed time ( t ). circuit 44 may be a microprocessor connected and programmed to add an increment of integrated power , for each clock pulse ( or predetermined submultiple thereof ); and for the normal push - button switch connection shown at 45 , the output of circuit 44 establishes the instantaneous integrated dose ( d 1 ) display at 37 ; this display will advance at a faster rate , the greater the detected intensity of projected illumination . to obtain data for the remaining time display at 38 , the dosimeter includes means 46 for accepting the stored preset of a digital expression of maximum acceptable dosage ( d m ). this may , for example , be set by push - button manipulation in the manner of presetting the alarm for a digital watch , the preset button ( not shown ) being preferably inaccessible via the panel 33 , as by having it only accessible on a side or via the bottom of the dosimeter housing ; by depressing the button of switch 45 while actuating the preset means 46 , one has a display ( at 38 ) of the instantaneous value of the preset maximum dose ( d m ). thus , with switch 45 in its normal position , the preset means 46 and the dosage display 37 are sources of the respective maximum ( dm ) and integrated ( d i ) dose values , in digital form . the instantaneous value of their difference is continuously available from a circuit 47 , and the digitally expressed difference is continuously supplied to a circuit 48 , for continuous development of the quotient which results from division by the instantaneous digitally expressed power value p . the remaining time display 38 is established by this quotient . all three of the displays at 36 - 37 - 38 are time - related , and legend indicates that they may all be cleared , for start of the next operational procedure . at panel 33 , however , a single pushbutton actuator 49 will be understood to clear all three of these displays , for zero reset , at the same time . at the integrating circuit 44 , a manually selectable hold / run function is identified at 50 . at panel 33 , this switch 50 is shown in the hold position , meaning that the integrating function of circuit 44 is being temporarily discontinued , as by temporarily ceasing to supply further clock pulses while projected - light illumination is temporarily switched off . in its down or run position , switch 50 will be understood to enable circuit 44 to resume its continuous time - integration of the intensity ( p ) of field illumination . the circuitry of fig4 a is illustrative of means further utilizing the digital dosage expressions d i and d m , for simple colored - lamp indication of the dosage fraction with which a given instantaneous advance of operating exposure exists . a &# 34 ; green &# 34 ; or first - half - period signal ( to illuminate a green lamp 54 ) may be provided by a flip / flop circuit 51 wherein the integrating circuit 44 is accumulating its tally of dosage ( d i ) in an initial phase , say to the halfway value of the setting at 46 of maximum dosage . a first threshold circuit 52 may be set at the divide - by - two expression of the maximum dose ( d m ), to provide a signal ( to illuminate a yellow lamp 55 and to extinguish the green lamp 54 by changing the state of flip / flop 51 ) when the integrated value ( d i ) reaches the d m / 2 threshold . and a second threshold circuit 53 may be set at the maximum level ( d m ) to provide an output signal which illuminates a red lamp 56 ( and extinguishes the yellow lamp 55 ) when the preset maximum - dose level is reached by the integrated dosage ( d i ). this red signal is of course a real warning that further exposure of eye 13 to the projected illumination has now begun to exceed the conservative limit which the surgeon preset ( at 46 ) as his maximum dosage . fig6 and 6a are illustrative of a probe 60 which provides both field illumination and a means of sampling a small fraction of the illumination for operation of the described dosimeter ( fig3 and 4 ). probe 60 is a small - diameter bundle of elongate optical fibers , within a suitable tubular enclosure 61 which has a smaller - diameter branch formation 62 at its axially outer end . most of the optical fibers are straight , being identified 63 with outer ends directly exposed to light from a source 64 , as focused by a lens 65 . a small fraction 66 of the optical fibers extends continuously from within branch 62 to the distal end of the probe , for exposure to light reflected within an eye 13 &# 39 ;, a small surgical aperture in the wall of the eye having been made for entry of the probe into the posterior chamber of the eye . light projected within the eye diverges as suggested by dashed lines , for manipulable illumination of selected areas of the posterior chamber ; and a sampling of reflection of the projected light is available at the branch end for collection and focus by a lens 67 to a photocell 68 , whereby the photocell ( 68 ) may provide a measured - luminance signal at , say , input 2 to the dosimeter 32 . the phantom outline 69 will be understood to indicate suitable miniaturized packaging of the source and sensing components 64 - 65 and 67 - 68 in unit - handling relation with the branched end of probe 60 . it will be understood that in the use situation depicted in fig6 a , manipulated position of and illumination by probe 60 within the posterior chamber of eye 13 &# 39 ; may be monitored by the microscope of fig1 and 2 , as may also the manipulated position and action of an elongate surgical tool 70 , which is shown similarly introduced through the wall of the eye , via the side opposite entry by probe 60 . and the arrangement of fig6 a will be understood to be illustratively applicable either to the suturing of an intraocular implant , or to the performance of vitreous surgery ; the tool 70 may thus be a cutter / aspirator , or an infusor device . further illustratively , the total number of fibers 63 - 66 in probe 60 may be 850 , all contained within a 1 - mm diameter bundle , and of these a small fraction , as for example a bundle of 50 fibers , taken preferably at random from the bundle of 850 fibers exposed at the distal end , may be diverted for reflected - light evaluation via the branch formation 62 . the probe 75 of fig7 is illustrative of a miniature photosensor 76 carried at the distal end of a small - diameter elongate rod 77 in which electrical output leads 78 are embedded , to enable external flexible connection to another of the selectively available inputs to dosimeter 32 . probe 75 may be used for measurement of illumination flux within the eye , as for an operative procedure within the anterior chamber to the eye , and when light from the projector source 16 is sufficient . the described instrumentation will be seen to achieve all stated objects , providing a hitherto unavailable means whereby the surgeon can pace his operative procedure safely with the limiting dosage which he can preset from his own experience . the instrumentation enables the surgeon to establish ( a ) the irradiation level in the wound , ( b ) the duration of the operative intervention , ( c ) the dosage the operative field has received , and ( d ) operation time remaining at the current illumination level . the device can only objectively indicate irradiation levels and total dosage ; it cannot make any conclusive decision about threshold values of dosage which may prove harmful to involved body tissue . for example , in ophthalmic microsurgery , the dosimeter 32 cannot assume the role of safeguard against retinal damage to the patient &# 39 ; s eye , and it is the physician &# 39 ; s responsibility to keep light levels and operation time strictly within safe limits which are based on his professional expertise ; the indications at 35 - 36 - 37 - 38 - 54 - 55 - 56 only aid the physician , based on his own expert preset of a dosage limit at 46 , and he can check this setting at any time , merely by pushing the button 45 and observing the numerical value then displayed at 37 . while the invention has been described in detail for a preferred embodiment , it will be understood that modification may be made without departure from the scope of the invention . for example , dosage measurement finds utility in operations other than when retinal protection is of concern .