Filter interference

Interference filters are used for selective transmission in a narrow spectral range. Incident radiation of wavelengths outside this transmission range is either reflected or absorbed. One distinguishes between line filters and bandpass filters. 

A higher finesse F caused larger reflectivities of the reflecting films not only decreases the bandwidth but also increases the contrast factor. With R — 0.98 F = 4R/(l — R) = 9.8 x 10, which means that the intensity at the transmission minimum is only about 10 of the peak transmission. 

The bandwidth can be further decreased by using two interference filters in series. However, it is preferable to construct a double filter that consists of three highly-reflecting surfaces, separated by two nonabsorbing layers of the same optical thickness. If the thickness of these two layers is made slightly different, a bandpass filter that has a flat transmission curve but steep slopes to both sides results. Commercial interference filters are currently available with a peak transmission of at least 90% and a bandwidth of less than 2 nm [4.46,4.52], Special narrow-band filters even reach 0.3 nm, however, with reduced peak transmission. 

In the ultraviolet region, where the absorption of most materials used for interference filters becomes large, the selective reflectance of interference filters can be utilized to achieve narrow-band filters with low losses (Fig. 4.58). For more detailed treatment, see [4.45. 52]. 

A line filter is essentially a Fabry-Perot etalon with a very small optical path nd between the two reflecting surfaces. The technical realization uses two highly reflecting coatings (either silver coatings or dielectric multilayer coatings) that are separated by a nonabsorbing layer with a low refractive index (Fig. 4.60). For instance, for nd = 0.5 p,m the transmission maxima for vertical incidence are obtained from (4.62a) at Ai = 1 xm, A2 = 0.5 p,m, A3 = 0.33 p,m, etc. In the 

In low-level fluorescence spectroscopy or Raman spectroscopy, the scattered light of the intense exciting laser often overlaps the fluorescence lines. Here special interference filters are available which have a narrow minimum transmission at the laser wavelength (line-blocking filter) but a high transmission in the other spectral ranges. 

In the ultraviolet region where the absorption of most materials, used for interference filters, becomes large, the selective reflectance of interfer- 

3) The contrast factor, which gives the ratio of maximum to minimum transmission. 

A higher finesse F, due to larger reflectivities of the reflecting films, 

F = 4R/(1 - R) = 1.7x10, which means that the intensity at the transmission minimum is less than l%o of the peak transmission. 

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Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

A simple instrument for measuring absorbance that uses absorption or interference filters to select the wavelength. 

In FT-Raman spectroscopy the radiation emerging from the sample contains not only the Raman scattering but also the extremely intense laser radiation used to produce it. If this were allowed to contribute to the interferogram, before Fourier transformation, the corresponding cosine wave would overwhelm those due to the Raman scattering. To avoid this, a sharp cut-off (interference) filter is inserted after the sample cell to remove 1064 nm (and lower wavelength) radiation. 

Figure 4.25. Experimental configuration for optical pyrometry of shock temperatures induced in transparent minerals. Upon impact of projectile with driver plate, a shock wave is driven into the driver plate and then into the sample. Optical radiation from the sample is detected via six lens/interference filter channels and an array of six photodiodes. Signals from photodiode circuits are recorded on oscilloscopes operating in single sweep model. (After Ahrens et al. (1982).)...

With interference filters two narrow wavelength bands are selected. These are the absorption (or measurement) and reference bands, within which the gas absorption is as high and as low as possible. The filters are mounted on a rotating disk, and the intensities are registered synchronously. The ratio of the intensities is used as the signal related to the gas concentration (Fig, 1... 

FIGURE 13.46 Nondispersive Infrared analyzer based on (d) interference filters and (b) gas correlation cechnictues. M = mirror, D = detector, S source, F = filter disk. WO = motor, FB = baud pass filter. SD = synchronous detection. C = correlation cell. N nitrogen filter. 

Unlike the carbon monoxide measuring instrument discussed above, the Lion Intoximeter 3000 uses an interference filter to produce monochromatic radiation... 

In a simple flame (emission) photometer an interference filter (Section 17.7) can be used. In more sophisticated flame emission spectrophotometers which require better isolation of the emitted frequency, a prism or a grating monochromator is employed. 

Cary 82 filter Interference filters (lower limit)... 

The removal of plasma lines is normally effected by using conventional interference filters. Interference filters, however, have several drawbacks in that they reduce transmission efficiency and do not withstand the intense laser output power over a long period of time. 

The Cary 82 spectrometer employs an optical filtering system which is similar in some respects to the design by Claassen et al. 41). This optical filtering arrangement is shown in Fig. 22. The Cary 82 filter system has higher transmission efficiency than conventional interference filters (Table VII). 

Obviously, the lowering in intensity of the laser beam incident through the beam expander must be compensated for by increasing the source output power. However, it has been found that the intensity loss when using the beam expander is less than that encountered with interference filters. 

High-index films in multilayer interference filters. 

Figure 8. Simultaneous measurement of intracellular Ca and oxidant production in neutrophils. Cells were labeled with Quin-2 and suspended at 2 x lo cells/mL buffer. At time zero, 1 nJf FLPEP was added (upper trace in each panel). In addition, the receptor blocker tBOC was added (3 x 10" M) after 30 s to stop further binding of the stimulus (lower trace in each panel). The excitation wavelength was 3A0 nm. Top panel Quin-2 fluorescence determined on channel B (of Figure 1) using a Corion A90-nm interference filter. The crossover from the superoxide assay has been subtracted. Middle panel Oxidant production (superoxide equivalents) determined by the para-hydroxyphenylacetate assay. Fluorescence was observed at AOO nm (on channel A of Figure 1).

Figure 9. Increase of intracellular Ca stimulated by various HCH isomers. Cells were labeled with lndo-1 and suspended at 2 X 10 cells/mL buffer at 37°C. The HCH isomers were dissolved in DMSO and added to the cell suspensions such that the final HCH concentration was 260 pff and the final DMSO concentation was 0.25% (v/v). The various isomers are indicated in the plot. The control is DMSO alone. The data are plotted as the ratio of fluorescence at 400 nm (measured on channel A) to that at 490 nm (measured through a Corion 490-nm interference filter on channel B).

UV irradiation of this bacterial cells using a broadband mercury-quartz lamp through an interference filter (bandwith 254 nm), provided preferential DNA damage with minimal effects on other subcellular structures. The exposure time ranged from 0 to 180 minutes in increments of 60 minutes, which gave total dose of UV exposure 1.21, 2.43 and 3.64 J/ m. ... 

The light beam passes through a filter system. The filters may be colored glass or special interference filters that allow very narrow spectral cuts. 

The detection of small extrasolar planets (of around the size of the Earth) will be done by registering the infrared light which they emit. Interference filters will blot out the light emitted by the star in question. Because of the huge distance from the Earth, effects due to its atmosphere and its IR radiation will not interfere. Darwin is intended not only to discover planets but to analyse their atmospheres for possible signs of life. 

At a terminal voltage of 4 V and a frequency of 300 Hz the anion Tb(TTFA) shows a very weak eel. It was identified by a broad-band interference filter to appear around 565 nm. The intensity was too low to record the eel spectrum on the emission spectrometer. 

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