What to Know about Optical Filters
Optical filters transmit a part of the optical spectrum while rejecting other parts. They are commonly used in microscopy, chemical analysis, spectroscopy, and machine vision. They come in various filter types of precision levels. Although filters share a lot of the same specifications with other optical components, some specifications are unique to filters that must be understood to effectively determine the filter that suits an application of optics.
Important Terminology to Understand
When understanding optical filters, it is necessary to understand the following terminology:
- Central wavelength. This is used to define bandpass filters. It describes a spectral bandwidth’s midpoint over which the filter transmits. Conventional coated optical filters can achieve a maximum transmission near the center wavelength.
- Bandwidth. This wavelength range is used for denoting a certain part of the spectrum that passes incident energy through the filter. It is also called Full Width-Half Maximum (FWHM).
- Blocking range. This wavelength interval is used for denoting a spectral region of energy attenuated by the filter. Typically, the degree of its blocking is specified in terms of optical density.
- Optical density. This describes the amount of energy that a filter blocks or rejects. A high optical density value means low transmission. An optical density of at least 6 is utilized for extreme blocking needs like fluorescence microscopy.
- Dichroic filter. This kind of filter is used for transmitting or reflecting light. It is commonly used for longpass and shortpass applications.
Types of Optical Filters
Generally, filters absorb unwanted light by adding colored glasses or dyes or reflect unwanted light by adding interference coatings. The majority of filters operate on the principle of interference coatings. Their coating designs and materials are chosen to achieve the desired transmission performance and shape. Below are the different kinds of optical filters.
- Hard coated optical filters. This features a single substrate with dense coatings and exceptional optical performance. It is designed to meet the adhesion, temperature, abrasion, and humidity requirement.
- Traditional coated optical filters. Typically, these optical filters are a stack of absorbing materials, metallic layers, and interference coatings, laminated together to make a low-cost, efficient filter. But, because the assembly is quite complicated, the filters’ optical performance and environmental stability are limited. But, they are still ideal for laboratory equipment and analytical instrumentation.
- Colored glass filters. These filters introduce elements, dyes, and compounds to a base substrate to manipulate the spectral properties of the filter. These inexpensive filters have less desirable optical properties than similar coated filters.