Computer-generated optical diffraction

Any diffractive optical element may be computer-generated (CGH, computergenerated hologram) or designed directly using a physical interference pattern, either from some object or generated in an interferometer. 

We need to point out that, if the wavelengths of laser radiation are less than the size of typical structures on the optical element, the Fresnel model gives a satisfactory approximation for the diffraction of the wave on a flat optical element If we have to work with super-high resolution e-beam generators when the size of a typical structure on the element is less than the wavelengths, in principle, we need to use the Maxwell equations. Now, the calculation of direct problems of diffraction, using the Maxwell equations, are used only in cases when the element has special symmetry (for example circular symmetry). As a rule, the purpose of this calculation in this case is to define the boundary of the Fresnel model approximation. In common cases, the calculation of the diffraction using the Maxwell equation is an extremely complicated problem, even if we use a super computer. 

Laser diffraction is the most commonly used instrumental method for determining the droplet size distribution of emulsions. The possibility of using laser diffraction for this purpose was realized many years ago (van der Hulst, 1957 Kerker, 1969 Bohren and Huffman, 1983). Nevertheless, it is only the rapid advances in electronic components and computers that have occurred during the past decade or so that has led to the development of commercial analytical instruments that are specifically designed for particle size characterization. These instruments are simple to use, generate precise data, and rapidly provide full particle size distributions. It is for this reason that they have largely replaced the more time-consuming and laborious optical and electron microscopy techniques. 

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