Linear variable filters

These LVFs are indicative of the solid-state spectrometers expected in the near future. True process control will be possible only when the cost of process instruments drops from its current level. Simple instruments such as these will allow monitoring throughout the process stream. 

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As for miniaturisation, examples are UV/VIS spectrometers using a linearly variable filter rather than a diffraction grating as the wavelength separation device (matchbox size), and double-focusing mass sensors (postage stamp size). 

Conventional wisdom leans towards a simple measurement system featuring one or two optical elements and with a simple constraction. In practice there are many forms of filter assemblies that can be used and these are not limited to the use of a single filter and implementations featuring multiple filters on a wheel are often implemented in multichannel analyzers. Such assemblies (Figure 6.6C) can be quite small (25mm diameter as illustrated) and can accommodate a relatively large number of filters. Other forms of filter assemblies include clusters or arrays (multiple wavelengths), correlation filters (customized to the analyte), continuously variable filters, such as the circular variable filter (CVF) and the linear variable filter (LVF), and tunable filters (both broad and narrow band). 

As with the linear variable filters, these minimonochromators will likely revolutionize pharmaceutical production. 

Fig. 1. (a) Concept of linear variable filter and wavelength detector, (b) Transmission properties of linear variable filter at a small location. 

Linear variable filter (light transmitted only at specific positions)... 

Noguiera, F.G., Phelps, D., Gutierrez-Osuna, R. Development of an infrared absorption spectroscope based on linear variable filters. IEEE Sensors Journal (in press, 2007)... 

Fig. 16. Configuration for the selective detection method. A special wavelength filter, like a band-pass linear variable filter, is inserted at the reconstructed image plane.

As with the linear variable filters, these mini-monochromators will likely revolutionize pharmaceutical production. Small, rugged, and inexpensive devices are the wave of the future measurements will not only be taken more often, but at many more points along the process stream. 

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