Flash apparatus with flash terminating switch controlled by photosensor

Claim 1

In the electronic flash unit of the type operative for producing flashes the duration of which is automatically controlled in dependence upon the scene brightness during the flash, in combination, a flash element; means for igniting the flash element; terminating means for terminating the flash produced by the flash element and including an electronic flash-terminating switch having a control electrode and operative when activated for causing the terminating means to terminate the flash; an integrating capacitor and means providing a current path for the capacitor; a charging resistor connected in the current path of the integrating capacitor and carrying a component of the integrating capacitor current, the charging resistor being operative for affecting the rate at which the integrating capacitor charges; means connecting the junction between the charging resistor and the integrating capacitor to the control electrode of the electronic flash-terminating switch for controlling the conductivity of the switch in dependence upon the voltage across the integrating capacitor; a light-sensing element connected in the current path of the integrating capacitor operative for making the capacitor current a function of sensed scene light; and a diode connected across the charging resistor with a polarity such as to carry a further component of the integrating capacitor current, the diode presenting to such further current component a resistance which varies in inverse dependence upon variations in the voltage across the charging resistor.