Abstract: A variable fluid media radiation filter apparatus comprised of a pair of spaced, opposing panes transmissive to radiation, sealed at their opposing edges so as to form a cavity therebetween in communication with a pair of sealed, fluid-retaining, upper and lower bellows forming, with the cavity, a closed volume containing a plurality of immiscible fluids of differing specific gravities and radiative transmissivities, whereby the fluid within the cavity may be changed by selectively compressing or expanding the bellows, thereby changing the radiation transmission characteristics of the filter.