These ND filters provide linearly varying attenuation or beamsplitting in the visible and near infrared region. The optical density varies linearly across the length. Attenuation is achieved by a thin film of aluminum protected with an overcoat. Variable beamsplitting is an added benefit due to the inherent reflectivity of aluminum. The rear surface is coated with a broadband antireflection coating for the visible region. The precision polished BK 7 or UV grade fused silica substrates have a slight wedge between the faces to eliminate unwanted fringe patterns and cavity feedback.

Linear variable filters can be used individually or in matched pairs. A matched pair will eliminate the density gradient over larger apertures when moved in opposing directions. Our Model 925Bcompensated attenuator utilizes a matched pair of these filters and the mechanics to move them precisly.