Optimum or under or over exposure-indicating device for use in photography

Claim 1

An optimum or under or over exposure-indicating device for use with a power source in photography, said device comprising: a bridge circuit having four series-connected arms, with two arms thereof each including a variable impedance element, with one arm tHereof including a fixed impedance element and with the remaining arm thereof including a photoconductive member having an impedance related to the incident light thereon, a differential amplifier comprising two amplifying elements and having first and second input terminals and first and second out terminals respectively, said first input terminal being connected to a junction between the arms of said bridge containing said photoconductive member and one of said variable impedance elements, Said second input terminal being connected to a junction between the arms of said bridge, containing said fixed impedance element and the other of said variable impedance elements, the remaining junctions between the arms of said bridge being connectable to said power source, a first amplifier connected to said first output terminal and controlled thereby, a second amplifier connected to said second output terminal and controlled thereby, a first lamp connected in an output circuit of said first amplifier for energization therefrom, and a second lamp connected in an output circuit of said second amplifier for energization therefrom, said first and second lamps both being concurrently energized when said bridge is substantially balanced to indicate an optimum exposure range whereas only one of said first and second lamps is energized to indicate under or over exposure conditions.