Variable Filters

Interference filters provide the best wavelength selectivity of any filters available. It is not possible to provide the necessary resolving power required for more complex spectral isolation. The filters therefore are primarily useful for simple systems where passage of a spectral band will meet spectral isolation requirements. 

A circular variable filter is an interference filter constructed so rotation of the filter changes the spectral bandpass of the filter. The filter can be constructed so the wavelength change is a linear function of the rotation angle. 

Circular variable filters can be used in scanning monochromators they are easily calibrated, have constant resolution, are rugged, and require little maintenance. The filters also have application in radiometers, where they have replaced sets of filters of fixed wavelengths. Another application is as order sorter for spectrophotometers. 

FIGURE 3-6. Refraction of light entering a denser medium. 

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Bromba, M. U. A., and Ziegler, H., Variable Filter for Digital Smoothing and Resolution Enhancement of Noisy Spectra, Ana/. Chem. 56,1984, 2052-2058. 

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). 

Model RR/066 351 and 352 pumps models 750/16 variable-wavelength UV monitor detector 750/11 variable filter UV detector, MPD 880S multiwave plasma detector, 750/14 mass detector 750/350/06 electrochemical detector refractive index detector HPLC columns column heaters, autosamplers, pre-columns derivatization systems, solvent degassers, preparative HPLC systems... 

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). 

There has been a flurry of recent activity in the study of the photodissociation dynamics of this molecule (186,187,188). Hermann and Leone used the infrared luminescence technique with a circular variable filter to determine the IR emission as a function of frequency when this molecule was photolyzed with lasers at 248 and 266 nm. From their results, they were able to show that the umbrella bending V2 mode of the CH3 radical was the only mode of CH3 that was excited in the photofragmentation of CH3I. This is in accord with the idea that the photodissociation of this molecule is unusually simple, and involves primarily the scission of the C-I bond with the simultaneous relaxation of the pyramidal structure of the CH3 part of the molecule into its final planar form. The data are used to obtain a vibrational distribution of the CH3 radical that peaks at v" = 2 and extends all the way out to the v" = 10 level. 

Filterbanks. There is still continued research on filter banks for high quality audio coding. Topics include wavelet based filter banks, low delay filter banks [Schuller, 1995] or variable filter banks allowing a higher degree of variability than classic window switching [Princen and Johnston, 1995],... 

Bromba, M.U.A. and Ziegler, H., Variable filter for digital smoothing and resolution enhancement of noisy spectra, Anal. Chem., 56, 2052, 1984. 

Examples of pressure filters include the plate and frame filter press, which gives a variable filter area depending on the number of plates installed on the frame, improving flexibility. Cake washing is also possible with this type of filter. High-pressure contained-plate and frame filters are now available where separations are more difficult or where solvents are being handled. 

Above, we described the solar resource at the top of the atmosphere. The atmosphere acts as a continuously variable filter for the ETR radiation. The atmosphere has stable components of 78% nitrogen, 21% oxygen, and 1% argon and other "noble" gases. There are also variable components in the atmosphere, such as water vapor (0% to 2% of the total composition), and gases dumped into the atmosphere by manmade and natural process, such as carbon dioxide, (0.035%), methane, and nitrous oxides.6... 

These linear variable filters (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. 

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

FIGURE 4 Schematic representation of the steps taken for data scrutiny. (A) Illustration of the batch sequence showing QC samples analyzed in the beginning and then sporadically through the batch. Test samples should be randomized in sets of 5-10 samples. (B) A peak table from the QC data is examined for peak area variability (filter CV<30%). Variables that show CV > 30% are excluded. (C) Variability of features is examined over the RT and ml axis. Red dots represent features with CV > 30% in the QCs, green dots are features that show CV < 30%. This examination could reveal potential analytical pitfalls, for example, in the case where a load of irreproduc-ible features are concentrated in a specific area. (D) Test samples show much higher variability than the QC samples as shown in the corresponding box plots. (E) When the QC data shows... 

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)... 

Sometimes variable filters are applied before the real variable selection is performed (e.g., variables that have no or nearly no variance or variables that are highly intercorrelated with another variable both procedures are fine). On the other hand, the elimination of variables that, taken alone, show no correlation with the biological activity values is a procedure that should not be applied. There is a certain chance that this variable might be able to explain the data set in combination with another variable. A better preselection procedure is the selection from the best of all possible models with three different X variables thousands of such models can be calculated within seconds, using Eq. (25) (rY Yi ... vm)—multiple correlation coefficient rYx vector of rYxt correlation coefficients Rvv matrix of rxi,xj correlation coefficients) [49]. If necessary, highly intercorrelated variables can be eliminated afterward ... 

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)... 

From V. L. Yen, Circular variable filters. Optica Spectra 3, No. 3, 78 (1969). Used by permission of Optica Spectra.]... 

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