The present invention relates to an electronic flash apparatus and, more particularly to an exposure control system for a series control type electronic flash apparatus in which a flash discharge lamp and a semiconductor switching element are connected in series.
In FIG. 1, there is shown a conventional series control type flash apparatus. As shown, a main capacitor 2 is connected in parallel with a power source 1, and a series circuit of a silicon controlled rectifier element (thyristor) 4 and a flash discharge lamp 3 is connected in parallel with the main capacitor 2. To the trigger electrode of the flash discharge lamp 3 is connected a flash discharge lamp trigger circuit 5, and to the gate electrode of the thyristor 4 is connected a thyristor trigger circuit 6.
In operation, when a trigger switch 7 (synchro contact), interlocking with a shutter release button of an associated camera, is closed, a trigger signal generaing circuit 8 produces a flash initiation trigger signal and applies it to the trigger circuits 5 and 6. Then, the flash discharge tube 3 is triggered and the thyristor 4 is rendered conductive, so that the main capacitor 2 discharges through the series circuit of the lamp 3 and the thyristor 4 to cause the lamp 3 to produce flash light.
The flash light from the lamp 3 illuminates an object to be photographed and the reflected light from the object is sensed by a photosensitive element 9 where the light is converted into current. The current is further converted into voltage by a current-to-voltage converting circuit 10. The voltage is amplified by an amplifier 11. The output voltage from the amplifier 11 is applied to an integrator 13 through a film sensitivity selection circuit 12. The circuit 12 is used for setting an integration time constant of the integrator 13 to a value corresponding to the sensitivity of a film used. The integrator circuit 13 is so designed as to initiate the integration from the initial value in response to the trigger signal.
The output voltage from the integration circuit 13 is applied to one of the input terminals of comparator 14 where it is compared with a reference voltage Vref applied to the other input terminal of the comparator. When the output voltage from the integrator circuit 13 exceeds the reference voltage Vref, that is, when the amount of the reflected light from the object reaches a given level, the comparator 14 produces a flash terminating signal. The flash terminating signal is applied to a quenching tube trigger circuit 17 connected to the trigger electrode of a quenching tube 16 which is connected through a commutating capacitor 15a to connection point between the flash discharge tube 3 and the thyristor 4. Resistors 18 and 19 connect the positive and negative electrodes of the commutating capacitor 15 to the positive terminal and the negative terminal of the power source 1, respectively, whereby a charging path for the commutating capacitor 15 is formed.
The quenching tube 16 is rendered conductive by the flash terminating signal, so that the thyristor 4 is reverse biased by the voltage across the commutating capacitor 15 to be rendered nonconductive.
The flash apparatus as above mentioned presents the following problems. At the time when the thyristor 4 is rendered nonconductive, the impedance of the discharge tube 3 is very low and a relatively large amount of charge is left in the main capacitor 2 and the capacitor 15. Accordingly, discharge current flows through a closed loop formed by the main capacitor 2, the discharge tube 3, the commutating capacitor 15 and the quenching tube 16. As a result, the flash discharge lamp 3 is caused to produce flash light even after the thyristor has been rendered nonconductive, that is, even at the time in which the lamp 3 should not produce flash light. For this reason, it is very difficult to obtain a proper exposure. FIG. 2 illustrates this state. As seen from FIG. 2, a relatively large amount of flash light is produced, as indicated by the hatched portion, even after the flash terminate signal is produced and thus the thyristor 4 is rendered nonconductive at time t.sub.1.
The amount of light of the hatched portion causes an excessive exposure. The earlier the time t.sub.1, that is, the closer the photographing distance, the more distinguished becomes the degree of the excessive exposure. The reason why a peak value of the intensity of the flash light is present immediately after the time t.sub.1 is that the sum of charges of the main capacitor 2 and the commutating capacitor 3 is applied to the lamp 3 after the thyristor 4 is rendered nonconductive.
In Japanese published patent specification Nos. 49408/'76 and 49410/'76, to prevent the exposure error caused by the flash light after the thyristor is rendered nonconductive, a by-pass capacitor is connected in parallel with a flash lamp to by-pass discharge current after the thyristor is rendered nonconductive. This approach, however, uselessly consumes the charge stored in the main capacitor. This makes it difficult to conduct successive and quick flash photographing. Further, the voltage of a battery used as a power source is remarkably reduced.
Accordingly, an object of the invention is to provide an improved electronic flash apparatus of series control type which removes the abovementioned drawbacks of the conventional flash apparatus and includes means for obtaining a desired flash exposure.