There are a large number of situations in modern science and technology requiring the study of very small objects in a volume. These small objects, for example, could be fog, sprays, dust particles, burning coal particles, cavitation and other bubbles in water, fuel droplets in a combustion chamber, etc. In practical situations these objects are small and micrometer order resolution is often required. This resolution itself is not a problem with conventional microscopic techniques. However, a serious problem arises due to the need to study a dynamic volume and not just a plane. An imaging system that can resolve a diameter d has a depth of field of only about d2/λ, where λ is the wavelength of the light used. For d = 10 μm, λ = 0.5 μm, the depth of field is only 0.2 mm! Clearly this is not satisfactory.

Light scattering and diffraction methods depend on models. These models are often based on assumptions of the object shape, pre-knowledge of the refractive index and other physical parameters. These assumptions are good enough when rapid and mean particle size distribution rather than exact shape and other parameters are needed. An example is size distribution measurements in the ceramic industry. These methods being real-time in nature, are very valuable in particular situations. Some of these methods are described in Chapter 9.

These non-imaging methods are not adequate in a large number of situations of practical interest.