Circular variable filters

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. 

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

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. 

The circular variable filter (CVF)-IR-ernission spectra of the aforementioned pyrolants based on MgH2, MgB2, Mg3N2 and Mg2Si do not exhibit any features related to the included elements, in particular, no FBO or SiO are observed (Figure 9.35) [19]. This may be due to the general lower resolution of CVF spectra compared to FTIR spectra. However, the spectra display differences with respect to overall intensity as is discussed in section radiometric properties in Table 9.5. 

The same interference principle may be applied to make a circularly-variable filter. The cavity thickness, and hence the centre wavelength, varies systematically around the circumference of the filter. Rotating the disk in a position between the source... 

The infrared probe, which resembles a specific ion electrode, contains a sensitive element that is dipped into the sample. To operate the probe, (1) the user selects the proper wavelength by rotating a calibrated, circular variable filter (2) then adjusts the gain and slits to bring the meter to 100% (3) next, the probe is lowered into the sample. The meter indicates the absorbance. This value can be converted into concentration by reference to a previously prepared calibration curve. To detect the presence or absence of a particular fimctional group, the user scans through the portion of the spectrum where absorption bands characteristic of that group appear. 

Wyatt, C. L. (1975). Infrared spectrometer liquid-helium-cooled rocketbom circular-variable filter. Applied Optics, 14, 3086-91. 

The observed variation in Gr) with bias is consistent with an IR active layer that is completely depleted even at relatively low bias fields. Gt) values as high as 3.8 were observed at 11.5/um at effective biases near 4V. 17/jd measurements show results near 20%. The rough bias-independence of r)IP indicates that at these fiux levels backgroimd limited performance obtains for effective bias values greater than 0.7V. Responsivity was found to vary only weakly with temperature between 5K and 8K. Variation in Gr) with wavelength was measured using a circular variable filter and was found to increase from 26% at 4.5jum to 52% at 11.5 at constant bias and temperature. 

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