Video-sensing effects with liquid crystal neutral-density filter

A variable neutral-density filter using guest-host liquid crystal devices was fabricated for color video image sensing. The liquid crystal devices contain black dichroic dyes whose absorption is controlled by voltage applied to the device. The filter keeps the spectral balance of transmitted light almost constant for three primary color wavelength regions, even when its transmittance is changed. The fabricated filter exhibited a high extinction ratio of 70:1 for an applied voltage of 20 V_rms, and the color temperature shift of the transmitted light was less than 200 K. We then developed an automatic light exposure system in which the variable filter is driven so that the video signal level is kept constant. By applying aperture control of a zoom lens to this automatic exposure system, we were able to achieve a new video effect where the focal depth of a captured image could be independently controlled while keeping the video level constant.