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Circular Continuously Variable Neutral Density Filter (ND Filter) is a special kind of neutral density (ND) optical filter, unlike a homogeneous neutral density filter that offers even attenuation across the surface of the filter, a circular variable neutral density filter consists of a circular substrate and a coating of which the optical densities changes in continuance across the angular distributions of the circle, allowing the user to adjust the light attenuation as the user rotates the circular variable filter about the center within the coated area of the filter. The optical densities of the circular neutral density filters have a linear relationship with the rotation angles. A circular variable neutral density filter enables precise adjustment of the power of a transmitted beam to desired ratios.

Shanghai North Optics supplies Standard and Custom Circular Continuously Variable Neutral Density Filters. We offer two substrate materials, standard BK7 substrates allowing transmission from 380-2000nm, Fused Silica substrates are available in custom versions, permitting a broad transmission range from 250-25000nm. These continuous variable circular variable neutral density filters offer a convenient adjusting beam attenuation through simple rotation. The outer diameters of our circular variable neutral density filters range from 10-200mm, the coating region covers 0-270°, and there are four optical density range options:  Max. OD1, Max. OD2, Max. OD3 and Max. OD4, the optical densities of the circular variable neutral density filter change from the start of the 0-270° coated areas. As the optical density increases in continuance and in a linear relationship with the rotation angles, through rotating the circular variable filters, one can manipulate the optical densities within the chosen maximum of OD1, OD2, OD3, and OD4.  In addition, by referencing the angular markings, users can easily set the filter to achieve the desired transmitted beam power. The linear density relationship enables motorized rotation, facilitating precise logarithmic sweeps in transmitted beam power.