Abstract: A tunable filter in particular for use in the visible or near infrared wavelength region, comprising a nano dispersed composite material contained within a resonant cavity structure comprising dielectric mirrors (15, 16), where the material is polarizable by means of an applied electric field. In particular, the polarizable material may be a nano phase polymer dispersed liquid crystal (PDLC) material where the diameter of liquid crystal droplets (2) within the material (3) is less than 0.1 &mgr;m, and for use in the visible wavelength region is typically between 10 nm and 50 nm. The filter also comprises structure for applying a variable electric field across the faces of the material. The transmission-wavelength characteristic of the filter is such that maxima and minima are shifted in wavelength by varying the electric field.