Related Parameters of Neutral Density Filter

Focal Length: Neutral density filters themselves do not have a specific focal length, as the focal length is mainly related to lenses. However, they can be applied in different optical systems and used in conjunction with lenses and other elements. 

Reflectivity: The reflectivity varies among different types of neutral density filters. For example, precision anti - reflection (AR) coated absorptive neutral density filters have a coating specification with an average reflectivity Ravg ≤1.0% (450 - 700nm, at an incidence angle of 0°). 

Transmittance: Transmittance refers to the ratio of the intensity of light allowed to pass through the filter to the intensity of incident light, expressed as a percentage. The attenuation strength of neutral density filters is usually represented by decibels (dB) or transmittance (T), and there is a mathematical relationship between them: D = -10 * log₁₀(T), where D is the dB value and T is the transmittance.

The transmittance range of different products varies significantly. For example, some products have an optical density between 0.1 and 4.0, from which the corresponding transmittance can be calculated. 

Length: In common materials, the parameter of “length” is rarely mentioned. Instead, the diameter and thickness of the filters are more commonly involved. The thickness of filters has different specifications. For example, the thickness of absorbance - stable neutral density filters is less than 2.0mm, while that of reflectance - stable and reflectance - step - variable neutral density filters is 2.0mm. 

Diameter Tolerance: There is no universal information found about the diameter tolerance of neutral density filters. The diameter tolerance may vary depending on different manufacturers and specifications of products. Common filter diameter specifications include Ø25.4mm, Ø50mm, Ø100mm, etc. 

Smoothness: Generally, surface smoothness is expressed by scratches/pits. For example, the surface smoothness of absorbance - stable and reflectance - stable neutral density filters is 40/20, that of reflectance - step - variable neutral density filters is 60/40, and the surface quality of precision AR - coated absorptive neutral density filters is 80 - 50. 

Substrate Material: The substrate materials of neutral density filters are diverse. For example, the base material of absorbance - stable neutral density filters is NG11; that of reflectance - stable neutral density filters is N - BK7; that of reflectance - step - variable neutral density filters is UVFS; and precision absorptive neutral density filters use SCHOTT NG colored glass substrate. 

Coating: Different types of neutral density filters have different coating conditions. For example, the coatings of reflectance - stable and reflectance - step - variable neutral density filters are chrome - nickel - iron alloy (on one side), and the radial angle of the coating for reflectance - step - variable type is 297.5°±5°; precision AR - coated absorptive neutral density filters adopt anti - reflection coating (BBAR) for the range of 450 - 700nm.