Optics 101

What is an Optical Bandpass Filter?

Topics Covered

Introduction
Working Principle
Construction
Applications
References

Introduction

Optical bandpass filters are filters that allow only a certain range of wavelengths of light, and block any light above and below the specified bandwidth. Two important characteristics that are used to describe these filters are the center wavelength and bandwidth. The bandwidth of the filter is also referred to as full width at half of maximum transmission (FWHM).

A bandpass filter has a well defined short wavelength cut-on and a long-wavelength cut off. A filter that rejects a certain wavelength range rather than letting it pass is called a notch filter.

Some bandpass filters are tunable filters, where the central wavelength can be chosen by the user. This datasheet will elaborate on the working, construction and applications of bandpass filters.

Working Principle

A bandpass filter is a combination of a longpass and a shortpass filter. The center wavelength of this filter is deduced as the arithmetic mean of the wavelengths measured at 50% of peak transmission and the value of FWHM is calculated as the bandwidth at 50% of peak transmission.

Construction

Bandpass filters are usually made of colored glass. They are a combination of longpass and shortpass filters.

Applications

Some of the typical applications of bandpass filters are listed below:

  • Astronomy – used in telescopes
  • Excitation filters in fluorescence microscopy.

References

 

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