Analog visibility computer

Claim 1

Apparatus for providing an indication of visual range comprising: means for generating a voltage which varies with time from a predetermined level, said varying voltage generating means being resettable to said predetermined level; first circuit means connected to said voltage generating means and responsive to an input field sensor measured signal commensurate with the instantaneous atmospheric transmittance and to said varying voltage for computing visual range as a function of a constant existing illuminance threshold and a constant luminous intensity of a preselected object, said first circuit means providing an output signal when the magnitude of said varying voltage reaches a level commensurate with the computed visual range; second circuit means responsive to said input signal commensurate with the instantaneous atmospheric transmittance and to said varying voltage for computing visual range as a function of a constant contrast of the preselected object and a constant observed contrast, said second circuit means providing an output signal when the magnitude of said varying voltage reaches a level commensurate with the computed visual range; logic circuit means connected to said voltage generating means and to said first and second circuit means, said logic circuit means being responsive to the output signals provided by said first and second circuit means for terminating variation of the magnitude of the output voltage of said voltage generating means when both of said first and second circuit means provide output signals; and means for sampling the output of said voltage generating means when the variation in magnitude thereof is terminated whereby said sampling means will measure a voltage commensurate with the greater of the visual ranges as computed by said first and second circuit means.