in the actual case , the curve of the measured reflected radiation as a function of time starts to increase after a certain delay time during which the measured reflected radiation is substantially constant . the slope of the curve representing the increase in measured reflected radiation r rapidly increases to its maximum value at the start thereof with subsequent decrease thereof until the curve reaches saturation , i . e ., approximately zero slope . fig1 a shows an approximation of such a curve . as shown in the approximation of fig1 a , for the case in which radiation is supplied over time t , at a constant power po , the measured reflected radiation r = r a follows an approximately horizontal trace over a time δt a . after this delay time δt a , there follows an approximately linear increase of the measured reflected radiation , having the slope s = s *, up to a time equivalent to n times the delay time , and thereafter an increase having a lower slope as *. the factors n and a can be determined experimentally . they are dependent on the size of the area irradiated , but are independent , within wide limits , of the radiated power supplied . finally , the diffuse reflection value r gradually approaches the saturation value r e which for the sake of simplicity is shown in fig1 a as a straight line section . the transition to the value r e takes place with a delay δt e after switching off the radiation po which effects the coagulation process . it has now been found that the value δt a has a relationship fixed with respect to the slopes s * and as *, and also with respect to the saturation value r e , so that it is sufficient to determine δt a in order to be able , thereafter , to predict , substantially , the further course of the curve r = f ( t ). the relationship s *= f ( δt a ) is illustrated in fig2 a curve resembling a hyberbola being the result , thus implying that the product δt a × s * is approximately constant . it is thus sufficient to determine δt a in order to be able , thereafter , to recognize immediately whether the value which is to be expected for r e will be correct , or whether it will be unsuitable , that is to say , too low or too high . it is therefore possible to influence the course of the coagulation process during the initial phases thereof , i . e ., while it is still developing , and , to alter the exposure parameters , and in particular to reduce the degree of coagulation , to switch off completely , and / or to perform repetitive coagulation operations . the following equation then applies for the exposure time t in the case of the approximation represented by fig1 a : ## equ1 ## according to this relationship , the necessary exposure time t is already fixed shortly after the start of the exposure ( at the point in time ## equ2 ## if t & gt ; t max ., and / or e & gt ; e max ., e being the radiation energy supplied and t max . and e max . being the maximum permissible values , the exposure is immediately terminated by automatic means , and only a slight degree of damage occurs in the irradiated area . the desired degree of coagulation is achieved , using the preset parameters , by successively increasing the power and by repeated exposures ( this process can also be effected automatically ). the delay time δt a is determined by means of a conventional device 6 for measuring this delay time ( fig3 ). however , it is also possible to determine the degree of the coagulation from the slope s of the curve r = f ( t ), or from the ratio of an instantaneous diffuse reflection value r to the initial value r a , instead of from δt a , and to control the exposure parameters by reference to s or r / r a . a differentiator 3 serves to measure the slope s , while the above - mentioned ratio of the diffuse reflection values is formed with the aid of a ratio - forming circuit 4 which comprises a memory circuit for retaining the initial value r a with which subsequent values of r are compared . it is also sufficient , under some circumstances , to effect the control of the exposure parameters with the aid of an absolute diffuse reflection value r , this value being measured at a selectable point in time by means of a conventional sample / hold amplifier circuit 5 . the circuit 9 thereby provides a signal representing the difference equivalent to the measured diffuse reflected radiation less the initial measured diffuse reflected radiation ( r - r a ) while the circuit 10 produces a signal representing the absolute value of the measured diffuse reflected radiation and block 8 contains a conventional control circuit for controlling the exposure parameters . not only can each of the circuits 3 , 4 , 5 , 6 , 9 , 10 , illustrated in fig3 be installed independently , but it is also possible to arrange for two or more of these circuits to be connected in parallel . in this case , a logic circuit 7 is needed in order to logically combine the output signals of the individual circuits which are used , for the purpose of generating a control signal which is a function of all the output signals . given , as expressed above , the following equation for exposure time t of the high energy laser in one simplified example : ## equ3 ## and , as also expressed above , the relationship where k is a constant or , when rewritten ## equ4 ## the exposure time t can also be expressed solely as a function of δt a or s as follows : ## equ5 ## where r norm = δr / r a and s norm = s / r a . fig4 and 6 show some of the possible systems for controlling the parameters of a conventional photocoagulation device when utilizing some of circuits of fig3 independently with the processor 7 , instead of being connected all in parallel , as shown in fig3 . in fig4 a conventional time delay detector 6 receives the reflected measured radiation r from the detector 1 in the photocoagulation device , and supplies a delay time δt a to the processor 7 which , in turn , computes the exposure parameters according to a predetermined equation , such as , for instance , equation 5 . the processor 7 then effects control of the exposure control circuit 8 in the photocoagulation device to continue or terminate the exposure of the fundus . in fig5 and 6 , on the other hand , the slope detector 3 and ratio circuit 4 supply corresponding signals to the processor 7 to control the treating laser according to a predetermined equation such as , for example , equation ( 1 ). in contrast to the state of the art , provision may also be made , if appropriate , to construct the device 1 for measuring the reflected intensity to be mobile so that it can be employed not only for picking up the diffuse reflection at different angles , but also for measuring the radiation which is transmitted through the treated tissue in parts of the human body where this is possible . this can be realized , for example , with the aid of flexible light guides placed underneath the tissue to be treated . the need for a second source of radiation is eliminated by an arrangement in which a device 10 for determining the absolute value of the diffuse reflection r and a control element 8 for the exposure parameters are connected in series . since the cirucit used in the various parts of the block diagram of fig3 are of known conventional type , forming no part of the present invention , a detailed description thereof is dispensed with herein for the sake of simplicity . the invention may be embodied in other specific forms without departing from the spirit of essential characteristics thereof . the embodiment is therefore to be considered in all respects as illustrative and not restrictive .
a control system for use with a conventional photocoagulation device for controlling the exposure parameters of irradiation of biological tissue , such as the fundus of an eye . the photocoagulation device includes a source of radiation , a means for initiating and terminating the source of radiation , and sensing means for measuring the extent of coagulation of the biological tissue . exposure parameters of the device include exposure time and the wavelength spot size and power of the irradiating beam . the control system cooperates with the device for determining and regulating the exposure parameters in accordance with the unique mathematical relationship between the extent and progression of coagulation during a photocoagulation operation , thereby to terminate the photocoagulation operation should unsatisfactory results be anticipated .