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Chapter 12:
Optical Interference Filters
When making almost any measurement, there will be competition between unwanted signals and the desired signal. A photodetector can be sensitive to a wide range of light wavelengths, while the signal of interest is confined to a narrow range of wavelengths. As a result, the detector can be saturated by the unwanted wavelengths of light, and the desired signal can be lost. Even if the detector is not saturated, the unwanted light can easily dominate the detector response and mask the signal. The simplest way avoid unwanted light in the detector is to keep it from entering the detector in first place. This is the role of an optical filter that only allows the desired wavelengths of light to pass. There are many types of optical filters, including colored gel filters and interference filters. Gel filters absorb select wavelengths of light and reduce their intensity on the other side of the filter. Interference filters exploit properties of wave optics such as constructive and destructive interference to remove unwanted wavelengths or to enhance desired wavelengths. These filters, their design, and their use in making composite filters when combined with gel filters will be the major thrust of this chapter. Multilayer dielectric filters can be used to construct bandpass and reject filters as well as low- and high-pass filters. These filters are used in many applications including astronomy, engineering, biology, and commercial color cameras. Such filters are indispensable in our image-laden, optics-driven world. In this chapter, the transfer matrix approach introduced in Chapter 6 will be extended to design and evaluate the properties of optical interference filters, which can be used for many applications. We will discuss how to develop the mathematics for thin film interference filters and design these filters for specific purposes using MATLAB.
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