Parameters of Interference Filters

Focal Length:

Generally speaking, interference filters themselves do not correspond to a specific focal length since focal length is usually related to lenses. 

Reflectivity: It is one of the important parameters of interference filters, and there are significant differences in reflectivity among various types of interference filters.

Dielectric interference filters can achieve a high peak transmittance and low reflectivity through coating technology.

For example, certain all - dielectric narrowband interference filters may have very low reflectivity outside the specific wavelength range to ensure high transmittance within the target band [1][3].

Metallic film interference filters usually coat an additional layer of dielectric film on the metal film layer to adjust the reflectivity and other properties [1]. 

Transmittance: It is a key indicator for measuring the performance of interference filters, which determines the ability of the filter to pass light of a specific wavelength.

Narrowband interference filters have a narrow spectral width (usually from 1nm to 40nm), with a high peak transmittance at the center wavelength, but the transmittance will decrease rapidly at other wavelengths [3].

Bandpass filters (a type of interference filter) have relatively high transmittance within a specific bandwidth range, while the transmittance outside this range is lower [5]. 

Length: For interference filters, length is not a fixed and universally meaningful dimension parameter. However, if it is an optical system composed of multiple filter elements or an optical device containing interference filters, its overall length will depend on the specific design and assembly requirements. 

Diameter Tolerance: Similar to general optical elements, the diameter tolerance of interference filters depends on the manufacturing process and precision requirements. High - precision interference filters may have a small diameter tolerance, such as ±0.05mm or less.

While some ordinary - precision interference filters may have a diameter tolerance between ±0.1mm and ±0.2mm. But the specific diameter tolerance values will vary according to different application scenarios and product standards. 

Smoothness: It has a certain impact on the performance of interference filters. Higher smoothness can reduce light scattering and surface reflection loss.

High - quality interference filters usually have a high surface smoothness, and their surface roughness may reach the nanometer level or even smaller. The specific smoothness value will depend on the product quality and application requirements. For example, the Ra (arithmetical mean deviation of surface roughness) value may be between 0.1nm and 1nm. 

Substrate Material: Common substrate materials include glass and quartz. Glass - substrate interference filters have a relatively low cost and are suitable for general optical applications. Quartz substrate has better optical stability and a lower thermal expansion coefficient, so it is often used in occasions that require high accuracy and stability [1]. 

Coating: The function of interference filters is mainly realized through coating. Common coating methods include vacuum coating and chemical coating.

According to different needs, different types and thicknesses of films can be coated, such as all - dielectric films, metallic films or hybrid films, etc. [1].