Introduction

This module briefly introduces a few optical methods that are based on the scattering phenomenon. In optical techniques related to interferometry, schlieren, and shadowgraph, the medium under question was taken to be transparent. In contrast, scattering is concerned with the interaction of light (in general, radiation) with matter. Methods such as infrared thermography rely on thermally stimulated emissions from the surface and do not need a separate radiation source. Methods of interest to the present discussion employ an external light source such as a laser and track changes in the optical properties of light after scattering; for a review see Tropea (2011) (included in this module as review article on light scattering, 2011).

Consider a light beam of wavelength falling on a particle of size , its characteristic dimension. The scattered energy will show changes with respect to intensity, directionality, wavelength, phase, and other properties of the wave. The property that shows the most pronounced change depends on the ratio of the wavelength and the particle diameter . Broadly speaking, we have the following limits:

Ray optics :

Wave optics:

Quantum optics:

Ray optics (also called geometric optics) is the applicable limit when the particle size is much greater than the wavelength of the incident radiation. The particle may be opaque and simply block the passage of light, casting a dark shadow. An example already studied in the context of velocity measurement (module 3) is particle image velocimetry where the particle is transparent and glows when illuminated by a sheet of light. The wave nature of light is revealed when the particle size matches that of wavelength. An example would be laser Doppler velocimetry where the frequency of light is altered by the speed of the particle. A second example is liquid crystal thermography where the color of radiation is selectively enhanced depending on the spacing between atomic layers in a liquid crystal material (module 6). Quantum optics refers to wave-particle interactions that alter the electronic states in the material, thus inducing emissions of its own. Such trends are obtained when the wavelength of light is large when compared to the particle size. It is to be understood that all three effects are jointly present in any application. For example, very small particles in PIV may show light intensity variations as a function of angle with respect to the incident, mainly because of interference effects. The phenomenon that is highlighted in a given measurement, thus, depends of the wavelength-particle size ratio. A second aspect of scattering techniques is the reduction in signal strength as one progresses from geometric to wave and finally to quantum optics. Accordingly, one can expect a considerable increase in the cost of the instrumentation in this sequence that demands extremely high laser power for quantum measurements to capture mild emission signals. The present chapter is intended to give a short introduction to wave and quantum optics from a measurement perspective.