These neutral density filters are continuously variable over 270° of the filter. Circular variable filters provide linear attenuation of incident light by rotating the filter. Designed to be spectrally flat from 400 to 700nm, they provide excellent attenuation for spectrometers and other optical instruments. The 50mm OD filters are available mounted or unmounted. The durable anodized aluminum mount has an indicator arrow that accurately identifies the angular position of the filter for increased repeatability. The 360° rotating mount comes complete with 4" L, 0.5" Dia. stainless steel post.