Apparatus for regulating the light energy decay characteristic of a pulsed flash device

Claim 2

In an apparatus for rapidly interrupting the output of a high-speed flash tube, coupled to a flash capacitor, when a predetermined quantity of light energy has been emitted therefrom, wherein a portion of the emitted light energy is reflected onto a photosensitive film to expose the film to yield a film optical density distribution that is normally proportional to the predetermined quantity of light energy but that is normally less than proportional to the predetermined quantity when the exposure time of the film falls below a prescribed value, an arrangement for compensating for the deviation from proportionality at such short exposure times, which comprises: means rendered effective at the start of the flash for generating a control signal that varies in proportion to the light energy emitted from the flash tube; at least one normally disabled thyristor; resistive means for connecting the transconductive path of the thyristor across the flash tube to establish a shunting path for the flash tube to interrupt the emitted light energy when the control electrode of the thyristor is excited, the resistance of the resistive means being sufficiently low so that the resistance of the shunting path is lower than that of the flash tube but sufficiently high relative to the thyristor to set the resulting time constant of the shunting path and the flash capacitor at a value which delays the extinction of the emitted light energy after such interruption for an interval sufficient to make the film optical density more nearly proportional to the predetermined quantity of light energy, and means for coupling the output of the generating means to the control electrode of the thyristor.

Claim 3

In an apparatus for rapidly interrupting the output of a high-speed flash tube, coupled to a flash capacitor, when a predetermined quantity of light energy has been emitted therefrom, wherein a portion of the emitted light energy is reflected onto a photosensitive film to expose the film to yield a film optical density distribution that is normally proportional to the predetermined quantity of light energy but that is normally less than proportional to the predetermined quantity when the exposure time of the film falls below a prescribed value, an arrangement for compensating for the deviation from proportionality at such short exposure times, which comprises: first means rendered effective at the start of the flash for generating a control signal that varies in proportion to the light energy emitted from the flash tube; first and second normally disabled thyristors; first and second resistive means for individually connecting the transconductive paths of each of the thyristors across the flash tube to respectively establish first and second shunting paths for the flash tube when their associated control electrodes are excited, the resistance of each of the first and second resistive means being sufficiently low so that the resistance of the associated shunting path is lower than that of the flash tube but sufficiently high relative to that of the associated thyristor to set the resulting time constant of each of the shunting paths and the flash capacitor at a value which delays the extinction of the emitted light energy after such interruption for an interval sufficient to make the film optical density more nearly proportional to the predetermined quantity of light energy; first means for coupling the output of the first generating means to the control electrode of the first thyristor for triggering the latter into conduction when the control signal has reached a predetermined value; second means responsive to current flow through the first thyristor during conduction thereof for generating a proportional voltage Of a magnitude sufficient to trigger the second thyristor into conduction; and second means for coupling the output of the second generating means to the control electrode of the second thyristor.

Claim 4

In an apparatus for rapidly interrupting the output of a high-speed flash tube, coupled to a flash capacitor, when a predetermined quantity of light energy has been emitted therefrom, wherein a portion of the emitted light energy is reflected onto a photosensitive film to expose the film to yield a film optical density distribution that is normally proportional to the predetermined quantity of light energy but that is normally less than proportional to the predetermined quantity when the exposure time of the film falls below a prescribed value, an arrangement for compensating for the deviation from proportionality at such short exposure times, which comprises: first means rendered effective at the start of the flash for generating a control signal that varies in proportion to the light energy emitted from the flash tube; first and second normally disabled thyristors; first and second resistive means for individually connecting the transconductive paths of each of the thyristors across the flash tube to respectively establish first and second shunting paths for the flash tube when their associated control electrodes are excited, the resistance of each of the first and second resistive means being sufficiently low so that the resistance of the associated shunting path is lower than that of the flash tube but sufficiently high relative to that of the associated thyristor to set the resulting time constant of each of the shunting paths and the flash capacitor at a value which delays the extinction of the emitted light energy after such interruption for an interval sufficient to make the film optical density more nearly proportional to the predetermined quantity of light energy; first means for coupling the output of the first generating means to the control electrode of the first thyristor for triggering the latter into conduction when the control signal has reached a predetermined value; second means responsive to current flow through the first thyristor during conduction thereof for generating a proportional voltage Of a magnitude sufficient to trigger the second thyristor into conduction; and second means for coupling the output of the second generating means to the control electrode of the second thyristor.