Light reflection method

Fundamentals. Reflection of light (monochromatic or white) at an electrode surface will depend basically on two influences Absorption of light by colored surface layers and scattering in case of rough or otherwise not ideally reflecting surfaces. Both influences may depend on electrode potential. 

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For a series of stress amplitudes the average number of cycles to craze initiation, as determined by the light reflection method, and the average number of cycles to... 

A quite different means for the experimental determination of surface excess quantities is ellipsometry. The technique is discussed in Section IV-3D, and it is sufficient to note here that the method allows the calculation of the thickness of an adsorbed film from the ellipticity produced in light reflected from the film covered surface. If this thickness, t, is known, F may be calculated from the relationship F = t/V, where V is the molecular volume. This last may be estimated either from molecular models or from the bulk liquid density. 

Interferometry is based on the fact that light reflected from the front and back interfaces of a film travels different distances, producing interference effects. The method has been applied to Langmuir-Blodgett films (Section XV-7) and to soap films (Section XrV-8) [147-149]. 

For higher accuracy, a method involving ampHtude modulation of a continuous laser beam is used. Again, a detector receives light reflected from the object where the distance is to be measured. The phases of the modulation in the outgoing beam and in the reflected return are compared. For a total phase shift A( ) between the two signals, the range R is... 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

PIV has become the most popular technique to measure velocity and turbulent properties (Figure 15.1). The movement of seed particles in a millimeter-thick laser sheet is measured by correlating two photos taken a few milliseconds apart. With two cameras, it is also possible to obtain a 3D vector of the velocity in that plane. The method gives, in general, very good resolution of the flow, but it requires optical access. Also, measurement close to walls can be problematic due to light reflections that disturb the measurements. One extension of PIV is the micro-PIV that uses fluorescent tracer particles, which allows all direct light, for example, reflections at the walls, to be filtered out [1]. 

Rather than measure the light transmitted through a sample, as in all of the IR sampling methods discussed thus far, some methods measure the light reflected from a sample. There are various methods employed to measure this reflectance, depending on the properties of the sample. 

The diffuse reflectance method is a method for solids in which the powdered solid, held in a cup, is irradiated with the IR beam. The scattered reflected light is captured by the detector and the spectrum displayed. 

Selection of visible light reflectance as a candidate nondestructive test method was based on results from probing experiments. It was observed that as cotton was mechanically cleaned, its visible light reflectance increased. Conversely, addition of trace amounts of particulate (trash and dust) to extensively cleaned cotton resulted in a decrease in visible light reflectance. Finally, it was noted that off-colored cotton was rendered whiter with repetitive mechanical cleaning. 

ELLIPSOMETRY The structure of liquid surfaces with monomolecular films can be studied by measuring the light reflected from the surface. The range of thickness that one generally considers to be measured varies from 100 to 1000 A (10-100 nm). However, in monolayers in which the molecules are oriented and the thickness involved is 5-50 A, the methods have been not easily pursued. In a differential method in which two beams of light from the same incandescent lamp were directed... 

The thickness of the compression zone is about 10" cm at atmospheric pressure thus the time for compression is about lO" sec. Since the wave length of visible light is of this order of magnitude, the optical reflectivity method was developed. Experiments were carried out in which the angle of incidence on the shock front was kept constant, and the angle of observation varied. In the case of simple air shock... 

The quality of the relief thus obtained can be determined directly in the course of etching because the light reflected from the etched grating produces a diffraction pattern that characterizes the depth and shape of the grating profile at any given moment. To this end, one has just to measure the intensity of the reflected light. The method possesses a high sensitivity the occurrence of the relief can be detected when its depth is only of the order of 0.01 /an. 

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