referring first to fig1 the studio lighting system comprises a flash unit indicated in general at 1 , and a focusing continuous light source or lamp 2 , the illumination from which may be varied by means of a phase control indicated schematically by the rectangle 3 , further details of which will be described below in connection with fig4 . there is also a control circuit comprising the parts within the broken line rectangle 4 , which determines the time for disconnecting the flash light , that is , for terminating the flash from the flash bulb or tube , and according to the invention the time for terminating the flash is adjustable in accordance with the adjusted brilliance of the continuous light from the continuous lamp 2 . the focusing continuous light and the flash unit are connected through a switch 5 to a suitable alternating current power source 6 , such as conventional power mains of 220 volts . the flash unit comprises a flash capacitor or storage capacitor 7 which can be charged through a voltage transformer indicated in general by the broken line rectangle 8 , to a suitable operating voltage of , for instance , 360 volts of direct current . the voltage transformer is developed as an ordinary conventional voltage doubler circuit fed from a 220 volt alternating current power line , and may be transformed into a voltage tripler circuit by closing the switch 9 , so that when this switch is closed , the transformer can be fed from a 120 volt alternating current supply , and yet will be able to produce the desired output voltage of , for example , 360 volts . the storage capacitor 7 can be discharged over a flash tube 10 which , together with a switch thyristor 11 , is connected in parallel to the storage capacitor . the flash tube 10 can be fired by means of its firing electrode , when activated by a firing device indicated in general by the broken line rectangle 12 , which device at the same time fires the switch thyristor 11 , that is , triggers this thyristor so that it becomes conductive . for this purpose , the control electrode of the switch thyristor 11 is connected through a resistor 13 to the collector of a pnp transistor 14 , the emitter of which is connected to a reference voltage source 15 of , for example , 10 volts direct current . the base of this transistor 14 is connected to the same reference voltage 15 through the secondary winding of a pulse transformer 16 , thus being connected through this secondary winding to the emitter of the same transistor . the primary winding of the pulse transformer 16 is connected at one end to the zero or negative terminal of the storage capacitor 7 , and at its other end is connected to a capacitor 18 which forms part of the firing device 12 . this capacitor 18 can be charged from the storage capacitor 7 through a resistor 17 . a thyristor 19 in the firing device 12 is connected in parallel with the series circuit consisting of the capacitor 18 and the primary winding of the pulse transformer 16 . also in parallel with this thyristor 19 there is another series connection consisting of a capacitor 20 and primary winding of another pulse transformer 21 . the capacitor 20 , like the capacitor 18 , can be charged from the main storage capacitor 7 through the resistor 17 . the secondary winding of this second pulse transformer 21 is connected with the firing electrode of the flash tube 10 , and also with the cathode of the flash tube , and the cathode of the flash tube is connected , as already indicated , to the anode of the switch thyristor 11 . the cathode of the above mentioned thyristor 19 in the firing device 12 is connected with the cathode of the switch thyristor 11 , and thus connected also with the negative plate of the storage capacitor 7 . the control electrode of the thyristor 19 is connected with a blocking device 22 for preventing the firing of the flash tube whenever the voltage of the charge on the main storage capacitor 7 is below a given predetermined or desired value . this blocking device is indicated schematically in fig1 by the rectangle 22 , and further details of the blocking device will be described below in connection with fig3 . a resistor 23 is connected in parallel with the anode - cathode path of the switch thyristor 11 . likewise a series connection comprising the quenching capacitor 24 and the anode - cathode path of the quenching thyristor 25 is also in parallel with the anode - cathode path of the switch thyristor 11 . the connection between the quenching capacitor 24 and the anode of the quenching thyristor 25 is connected through a resistor 26 with the positive terminal of the main storage capacitor 7 . the control grid or gate of the quenching thyristor 25 is connected to the control circuit 4 , as will be further described below . to make sure that the storage capacitor 7 is always charged to the desired constant or predetermined voltage when the flash unit is ready for flashing , the voltage transformer 8 is provided with a charge control switch 28 which disconnects the transformer as soon as the predetermined voltage value of the capacitor 7 has been reached , during the charging process . for this purpose , there is provided in the voltage transformer 8 a triac 27 which is conductive as long as the voltage of the storage capacitor lies below the predetermined voltage , and which blocks as soon as the predetermined voltage value of the storage capacitor has been reached . upon the blocking of the triac 27 , the voltage transformer is effectively disconnected ; that is , no further charging current is supplied to the storage capacitor 7 . the triac is controlled by the charge control circuit or switch 28 which compares the voltage of the storage capacitor 7 with a reference voltage and supplies the triac with firing pulses as long as the desired voltage of the storage capacitor has not been reached . this charge control switch 28 is schematically shown as a rectangle in fig1 and further details thereof will be described below in connection with fig2 . the phase control 3 , which may be of a known kind , has an adjusting resistor 29 by means of which the brightness of the focusing light 2 is continuously adjustable . the wiper of the resistor 29 is mechanically connected with the movable tap or wiper of a variable resistor 30 , so that a change in the setting of the resistor 29 causes a corresponding change in the resistance of the resistor 30 . the variable resistor 30 together with a capacitor 31 form a time function element or member , one end of which member is connected to the negative terminal of the storage capacitor 7 . the other end of this member is connected through the emitter - collector path of a transistor 32 to the positive terminal of the source of voltage 15 . the base of the transistor 32 is connected through a resistor 33 and capacitor 34 to the positive terminal of the storage capacitor 7 . parallel to the series connection of the transistor 32 and time function member 30 , 31 , there is connected a unijunction transistor 35 having the base resistors 36 and 37 . the control electrode of the unijunction transistor is connected to the junction point between the resistor 30 and capacitor 31 of the time function member . the base of the unijunction transistor 35 is connected through the base resistor 37 to the negative terminal of the storage capacitor , and is also connected with the gate or control electrode of the quenching thyristor 35 , this connection being through a voltage amplifier circuit comprising the transistors 38 and 39 . it has been mentioned above that the phase control 3 is of known form . however , for the sake of completeness , further details thereof will be described in connection with fig4 . between the input 3a of the phase control and the output 3b thereof , there is connected a triac 40 whose control electrode is connected with the control input 3d through a diac 41 and a resistor 42 . the positive output of the triac 40 is connected with the control input 3c . the adjusting resistor 29 described above is connected to the control inputs 3c and 3d . between the negative output of the triac 40 and the junction point between the diac 41 and resistor 42 , there is connected a capacitor 43 . another capacitor 44 is connected between the negative output of the triac and the control input 3d . fig2 illustrates further details of the charge control circuit 28 in the voltage transformer 8 . an operational amplifier 50 serving as a comparator is connected to a source of voltage indicated schematically at u b . its output forms the output 28b of the charge control circuit 28 which is connected with the gate or control electrode of the triac 27 . the inverted input of the operational amplifier 50 is connected to the intermediate tap of a voltage divider circuit comprising the resistors 51 and 52 in series with each other . this voltage divider circuit is located , as illustrated , between the input 28a of the charge control circuit , and the negative potential of the source of direct current voltage u b . the input 28a of the charge control circuit is connected with the positive terminal of the storage capacitor 7 , as illustrated in fig1 . the non - inverted input of the operational amplifier 50 is connected to a reference voltage which is formed by a zener diode 53 which is connected through a resistor 54 to the operating voltage u b of the operational amplifier 50 . fig3 shows further details of the blocking device 22 , schematically shown merely by a rectangle in fig1 . this blocking device comprises an operational amplifier 60 serving as a comparator whose output is connected through switches 61 and 62 in parallel with each other , and through a voltage divider circuit comprising resistors 63 and 64 , to the zero potential of the source of direct current voltage u b which supplies the operational amplifier . the switch 61 is the conventional synchronous contact switch of the camera with which this studio lighting system is used , and is arranged , in the conventional manner , to be closed in synchronism with the opening of the camera shutter . the switch 62 is a hand or manual switch to enable the flash to be triggered manually . the intermediate tap of the voltage divider 63 , 64 is connected to the output 22b of the blocking device 22 , this output being connected to the control electrode or gate of the thyristor 19 , as illustrated in fig1 . the non - inverting input of the operational amplifier 60 is connected to an intermediate tap of a voltage divider circuit comprising the resistor 65 and the adjustable resistor 66 . this voltage divider circuit lies between the input 22a of the blocking device 22 , and the zero potential of the source of voltage for the operational amplifier . the input 22a is connected with the positive plate of the storage capacitor 7 , as illustrated in fig1 . a zener diode 67 and a resistor 68 are connected between the zero potential and the positive potential of the source u b , and this zener diode supplies the reference voltage to the inverted input of the operational amplifier 60 . the operation of the studio lighting system described above is as follows : when the switch 5 is turned on , the entire studio lighting system becomes ready for operation . first of all , the subject to be photographed is illuminated by means of the continuous focusing light 2 . the photographer can then determine the desired positioning or setting of the subject , and the desired brightness of the continuous light can be adjusted by means of the adjustment resistor 29 . the adjustment of the resistor 29 simultaneously adjusts the resistor 30 , because of the mechanical connection between them , and the adjustment of the resistor 30 serves to adjust the time of the duration of the flash , terminating or quenching the flash at an earlier or later time as the case may be , depending on the adjustment . when the desired time is reached , the circuit gives off a pulse to the quenching thyristor 25 so as to terminate the flash . this period of time is so established that the quantity of light which is given off by the flash tube 10 upon the discharge of the storage capacitor 7 within this period of time , causes an exposure of the photograph which corresponds to the prior illumination of the object or subject being photographed by means of the focusing continuous light . in the condition of readiness for operation , the storage capacitor 7 and the capacitors 18 and 20 are charged to the operating voltage , for instance 360 volts of direct current . this operating voltage is determined exactly by the charge control circuit 28 . when the desired operating voltage of the storage capacitor 7 is reached , the control circuit 28 no longer gives off any firing pulses to the triac 27 , as a result of which the triac blocks and the voltage transformer 8 becomes effectively disconnected . the quantity of light given off by the flash tube 10 can be proportional to the period of time set in the control circuit 4 only if the same predetermined voltage is always present in the capacitor 7 at the time of commencement of the flash , and that is why it is important to make sure that the storage capacitor is always charged to exactly the right voltage when a flash is to be produced ; otherwise the results would not be consistent from one exposure to another exposure . it is for this purpose of insuring that the right charge is carried by the storage capacitor when a flash is to be produced , that the firing device 12 is connected to the blocking device 22 . this blocking device 22 only permits a pulse to arrive at the control electrode of the thyristor 19 upon the closing of the synchronous contact switch 61 or the manual switch 62 , if a positive voltage is present at the output of the operational amplifier 60 . however , this is only possible when the positive plate of the storage capacitor 7 , connected with the non - inverted input of the operational amplifier 60 , has a higher voltage than the inverted input of the operational amplifier . when the predetermined charge voltage of the storage capacitor 7 is reached , then when the synchronous contact 61 or the manual switch 62 is closed , a positive pulse will pass to the control electrode of the thyristor 19 , and this thyristor will thereby be triggered or fired , becoming conductive . the capacitors 18 and 20 now discharge through the thyristor 19 and the primary windings of the respective pulse transformers 16 and 21 . a firing pulse if thus produced by the transformer 21 to the firing electrode of the flash tube 10 , and a negative pulse if transmitted from the secondary winding of the transformer 16 to the base of the transistor 14 , whereby the latter becomes conductive . when the transistor 14 is conductive , the control electrode of the switch thyristor 11 is connected to the source of direct current voltage 15 , so that simultaneously with the firing of the flash tube 10 , the switch thyristor 11 also becomes conductive . the storage capacitor 7 now starts suddenly to discharge through the series connection of flash tube 10 and switch thyristor 11 . the change in voltage at the storage capacitor 7 passes as a negative pulse to the base of the transistor 32 , so that this transistor is driven or becomes conductive , and the control circuit is connected with the start of the flash discharge . the capacitor 31 of the control circuit is charged through the conductive transistor 32 and the resistor 33 , until the voltage at the capacitor 31 exceeds the breakthrough voltage of the unijunction transistor 35 . at this moment the unijunction transistor switches through , and there is a voltage drop in the resistor 37 , which permits the npn transistor 38 to become conductive whereby the pnp transistor 39 is in its turn driven , and a positive pulse passes from the source of voltage 15 to the control electrode of the quenching thyristor 25 . the quenching thyristor 25 thus becomes conductive , and the quenching capacitor 24 discharges through the quenching thyristor 25 and the resistor 23 , whereby a negative voltage occurs at the anode - cathode path of the switch thyristor 11 , so that the latter blocks in known manner . in order to accelerate the disconnecting process of the switch thyristor 11 , its control electrode is connected furthermore , through a resistor 70 , with the negative pole of another source of direct current voltage 71 , for instance , five volts . with the blocking of the switch thyristor 11 , the discharge of the storage capacitor 7 over the flash tube 10 is interrupted and the flash is extinguished or terminated . since no change in voltage occurs anymore at the storage capacitor 7 , there is also no longer any negative control voltage at the base of the transistor 32 , so the latter blocks and the control circuit 4 is disconnected . the taking of the picture has been completed . when the next photograph is taken , the process just described is repeated .