9-2 Color Temperature Characteristics of Liquid Crystal ND Filters for Color Image Sensing

Abstract

A variable neutral density filter using guest-host liquid crystal devices we investigated for color video image sensing. The liquid Crystal device contains black dichroic dyes whose absorption is controlled by voltage applied to the device. The fabricated filter keeps the spectral balance of transmitted light almost constant in the visible wavelength range, when the light transmittance is varied by the applied voltage. It exhibited a high extincion ratio of 70:1 and response time of 50ms when it was driven by 20V. The color temperature shift of the transmitted light was then suppressed to less than 200K at 5000K. We also demonstrated the automatic light exposure system using it without aperture control, to keep video signal level constant for a great change in the illuminance of objects to be captured.