Introduction to optical filters
Filters selectively transmit parts of the spectrum while rejecting other parts. Edmund Optics’ optical filters are widely used in microscopy, spectroscopy, chemical analysis and machine vision and are available in a variety of filter types and accuracy levels. This application note details the different techniques used to create EO filters, the definition of some key specifications, and the range of various types of filters we offer.
Key terms for optical filters
In the Key Terms section, we highlight the specifications that filters have in common with other optical components, but also point out many specifications that are unique to filters that need to be understood in order to effectively understand and determine which filter is best for your application
Central wavelength
The center wavelength is the midpoint that defines a bandpass filter and describes the spectral bandwidth transmitted by the filter. Traditional coated filters tend to have maximum transmission near the center wavelength, while hard-coated filters tend to have a fairly flat transmission distribution across the entire spectral bandwidth.
Bandwidth
Is the range of wavelengths used to represent the specific portion of the spectrum in which incident energy passes through a filter.
Filter manufacturing technology
Filters either absorb unwanted light by adding colored glass or dyes, or reflect unwanted light by adding interference coatings. Most EO filters operate on the principle of interference coating, and the coating design and materials are specifically selected to achieve the desired transmission shape and performance. Hard-coated filters feature a single substrate with a dense coating and excellent optical performance.
They are designed to meet strong adhesion, wear resistance, and temperature and humidity requirements, making them ideal for precision requirements and OEM integration. Traditional coated filters are typically a stack of absorptive materials, interference coatings and metallic layers to reduce costs. However, the complexity of the components limits the optical performance and environmental stability of such filters. Nonetheless, traditional coated filters are still ideal for laboratory equipment and analytical instruments. Colored glass filters and other absorbing filters (such as plastic filters and Wratten filters) incorporate elements, compounds, dyes, or other colorants into the substrate to control the spectral properties of the filter. The resulting filter is relatively inexpensive but has less desirable optical properties than similar coated filters. Absorption filters are often integrated into lighting and sensing applications.
Absorptive and Dichroic Filters
Among them, optical filters can be divided into the following two categories: dichroic filters and absorption filters. The filtering methods of the two are different. In the re-absorption filter, the light can be blocked according to the required absorption characteristics of the glass substrate. If the blocked light will not be reflected from the filter, on the contrary it will It is absorbed in, so the absorption filter is well used in applications that do not need to generate noise, and it is not too sensitive to angles.
In contrast, dichroic filters work by reflecting unwanted wavelengths while transmitting the desired part of the spectrum. In some applications, this is a desirable effect because the light can be split by wavelength into two sources. This is accomplished by adding one or more layers of materials with different refractive indexes to take advantage of the interference properties of light waves. In an interference filter, light traveling from a lower refractive index material will be reflected by a higher refractive index material; only light at specific angles and wavelengths will constructively interfere with the incoming beam and pass through the material.
Type of filter
To help understand the similarities and differences among today’s various optical filters, consider the ten most popular types. Please contact us for more information.