Introduction to Narrow Bandpass Filters
JML’s stock bandpass filters use interference phenomenon to selectively transmit or block specific portions of the UV to NIR spectrum. These devices allow the user to isolate narrow spectral regions. Figure 1 shows the general structure of a single cavity interference filter. The filter consists of alternating layers of high (H) and low (L) index materials with each layer an optical quarter wavelength thick. Multiple iterations of this structure provide increasingly deep out-of-band blocking and more abrupt transition from rejection to transmission.
Figure 1
JML’s stock filters include BP series and DF series parts. The BP series provides industry standard performance. The DF series is a unique group of discriminating filters with extremely rectangular shape, very steep edges and especially deep blocking that exceeds optical density (OD) 6 outside the passband. A common application of the DF series is in fluorescence studies for observing low-level spectral emission while blocking a high-intensity excitation source such as a laser or arc lamp. Figure 2 describes the bandpass profiles for this series of filters.
Figure 2
Figure 3
The blocking strategy employed is determined by the blocking range. Filters that block to the near IR use a combination of absorbing materials and interference coatings. Filters that block to the far IR employ metallic coatings. A composite filter is a structure of several matched elements stacked one on top of the other. Once bonded together with optical epoxy, these filters withstand demanding humidity, temperature and radiation conditions for long periods of time. All mounted filters are mounted in aluminum rings.
The industry standard bandwidth for narrowband filters is 10nm. Most of the narrowband filters JML supplies have a FWHM (Full Width, Half Maximum) of 10nm. The Center Wavelength (CWL) of a bandpass filter is chosen to correlate to a particular laser, analytical or absorption line depending on the application.
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