Wavelength-selective filters

Band-pass filters are characterized by the following parameters  

Band-pass filters can be produced to transmit only a very narrow range of wavelengths. For example, off-the-shelf interference filters (see Section 11.6 for details) are available with bandwidths of typically 10 nm. Bandwidths of 1 nm are also available for use with a range of common visible laser lines. However, the latter often suffer from relatively low peak transmission (usually less than 50 per cent). 

Blocking filters may be viewed as being the inverse of band-pass filters they only block one particular line or wavelength interval and are transparent for aU other 

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Figure 8.1 Schematic representation of NIR chemical imaging instrument operating in diffuse reflectance mode. Radiation from the illumination source interacts with the sample. Light reflected off of the sample is focused onto a NIR sensitive 2D detector after passing through a solid-state tunable wavelength selection filter.

Spectral condensation Efficient spectral narrowing that occurs when a wavelength-selective filter is added to a dye laser cavity the oscillating linewidth may be reduced maity orders of magnitude, while the output power or energy is reduced typically by less than half... 

Wavelength selection. Filters or monochromators may be used for wavelength selection. In commercial instrumentation the choice is usually reflected in the cost and sophistication of the instrument and may be a combination of the two types. The choice then is again dependent upon the application. 

The maxima in the modulation index correspond to the use of an unrealistically narrowband optical filter. Figure 9 is a 3D plot, showing modulation index as a function of optical selection filter bandwidth and centre wavelength. Optical filters with centre wavelengths between 1.9 pm and 2.1 pm, and bandwidths between 0.01 nm and 80 nm were considered. 

This method is used to determine sodium and potassium in food, water and blood serum. The flame can be hydrogen/oxygen, methane/ oxygen or methane/air fueled. Wavelength selection can be by filter, prism Fig. 9.2 or grating and by either one or two detectors. 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

Fig. 10.9 A MEMS IR gas spectrometer, which measures the IR absorption of a gas at different wavelengths selected by a tunable interference filter. After [Lai 1 ].

The imaging detectors, whether for point mapping, line scanning, or array detection, can be coupled with different types of spectrometers. Instrument types are classified by wavelength selection modality into imaging Fourier transform (FT) and tunable filter (TF) spectrometers, both of which are presented below, and dispersive spectrometers. FT imaging systems are classical laboratory instruments while TF spectrometers are compact and robust systems for chemical imaging. 

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