Quasi-monochromatic systems

In much of the interference theory of the acoustic microscope, and especially the theory of how the contrast varies with defocus, it is assumed that the microscope is perfectly monochromatic. Of course, the monochromatic theory could be summed over a frequency spectrum actually used in a given 

For quantitative applications the quasi-monochromatic circuit of Fig. 5.2(b) is better. The basic principles of this circuit are similar to those of Fig. 5.2(a), but there are some important differences. The r.f. oscillator and the local oscillator are two frequency synthesizers that are phase-locked to one another to give a difference frequency of precisely defined phase. This difference frequency is very much lower than in the simple heterodyne circuit, 

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Fig. 5.2. Schematic r.f. systems, (a) Simple heterodyne circuit, SI determines the pulse length, S2 switches the lens from transmit to receive, and A1 amplifies the reflected signal (b) quasi-monochromatic circuit the two oscillators and the pulse repetition frequency are phase-locked, and the final signal is lock-in detected (courtesy of John...

For the determination of the spectral sensitivity complicated and expensive laboratory equipment are required in conjunction with experienced personnel. Such a facility should be capable in providing calibrated monochromatic radiation of sufficient power to be sensed by the broadband detector in question. A typical system should be comprised by a double monochromator coupled with a stable, high-power source (e.g. a 1000-Watt Xenon lamp). A spectrally calibrated detector should also be available to measure the quasi-monochromatic radiation emitted by this system. Such facilities are rarely available and only a few laboratories worldwide are able to cany out such characterizations. 

Neutrons with a wavelength of 5 to 10 A are often used. A narrow range of wavelengths is usually selected to give a quasi-monochromatic beam. This can be achieved by a crystal monochromator or, more usually, by a velocity-chopper system of rotating discs which select neutrons of a given velocity. 

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