Contrast enhancement filter having first and second transparent glass substrates (16, 17) with first and second surfaces (18, 19; 21, 22) with the first surface (18,21) of each facing the medium in which the filter is disposed. A sheet-like circular polarizer (12) is provided formed of stretched plastic members (13) covered by layers (14) of cellulose acetate butyrate. The circular polarizer is disposed between the first and second glass substrates (16,17) and faces the second surfaces (19,21) of the glass-substrates. A dried plastisol primer layer (26, 27) is disposed on the surface of the layers of cellulose acetate butyrate facing the glass substrates. First and second layers (23, 24) of polyvinyl butyral adhesive secure the layers of cellulose acetate butyrate carried by the circular polarizer with the primer layers thereon to the second surfaces of the glass substrates. An anti-reflection coating (31) is disposed on the first surface of the first substrate.
A method for manufacturing a contrast enhancement fil. ter which comprises the steps of providing a sheet-like circular polarizer (12) formed of stretched plastic members (13) and having outer layers (14) of cellulose acetate butyrate. A thin film (26, 27) of optical clear plastisol primer is applied on the outer surfaces of the cellulose acetate butyrate layers of the circular polarizer. The primer is then dried. A transparent substrate (16) is provided with first and second surfaces (18, 19). A layer (23)) of a polyvinyl butyral adhesive (23) is then applied to the second surface (19) of the first glass substrate. The circular polarizerwith its primer layerthereon is placed on the layer of polyvinyl butyral adhesive. Another layer (24) of polyvinyl butyral adhesive is then placed on the other side of the circular polarizer. A second glass substrate (17) having first and second surfaces (21,22) with the second surface (22) with the second surface (22) facing the polyvinyl butryral adhesive is placed in the sandwich. The various parts in the sandwich are then secured into a temporary unitary assembly. Air is evacuated from between the parts and the layers. Heat and pressure is applied to the assembly to cause the parts to be permanently bonded into a unitary assembly.