Interference fringes, elimination

Mattheyses, A. L., Shaw, K. and Axelrod, D. (2006). Effective elimination of laser interference fringing in fluorescence microscopy by spinning azimuthal incidence angle. Microsc. Res. Tech. 69, 642-7. 

Multiple reflections within the specimen can also reduce the accuracy of quantitative measurements (51,52). The intensity of light transmitted through a plane parallel plate, after the interference fringes have been eliminated, is given by (53) ... 

Thin unsupported solid sheets often show interference fringes that could be mistaken for absorption bands. So do organic coatings On metals when examined by the reflection technique. Interference fringes can be a nuisance, but there are ways of eliminating them. 

In the case of transparent substrates, the sensitivity may be significantly increased with the BML-IRRAS technique (Section 2.3.3) or simply by placing a flat mirror under the substrate while the IRbeam is incident on the other side [16]. The problem that can be met when measuring the IRRAS spectra of films on transparent substrates is interference fringes, which arise due to multiple reflections of the IR beam inside the substrate. To eliminate multiple reflections of the beam and to achieve the... 

If a prepared film exhibits interference fringes in its infrared spectrum, these may sometimes be suppressed in intensity or eliminated by lightly scouring the film surfaces with wire wool or an abrasive paper. 

NIR has been used to measure percent degradation and viscosity of polyester terephthalate films. A method has been developed [86] to eliminate the effect of interference fringes and orientation due to stfetching of the film. A special integrating sphere with PbS detectors at 0 and 90 has been used in combination with a sample cup using frosted quartz windows. 

Several different types of reflection spectroscopy yield more information when the radiation is polarized. For example, very thin films of molecules adsorbed on the surface of metals only absorb radiation polarized parallel to the plane of incidence. The reflection of light from bulk samples depends on the polarization of the light with respect to the plane of the sample. The effective depth into a sample that can be sensed by internal reflection spectroscopy is also different for radiation polarized perpendicular and parallel to the surface. Polarized radiation may even be used to eliminate interference fringes from the spectra of thin polymer films. 

Several ways to eliminate interference fringes have been proposed. One of the more effective ways is to mount a polarizer in the beam and to mount the sample at Brewster s angle, 9, calculated as... 

Interference pattern The spectra of thin unsupported films may exhibit interference fringes. For example, the spectra of thin polymeric films often have a regular interference pattern superimposed on the spectrum. Although possible, it is generally difficult to mistake such a wave pattern for absorption bands. When examined by reflection techniques, coatings on metals may also exhibit an interference pattern. The interference pattern can be a nuisance but can be relatively easily eliminated. The wave pattern observed may be used to determine film thickness (see page 266). 

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