Wavefront
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- :For the notion of wave front in functional analysis, see wave front set.
Simple wavefronts and propagation
Strictly speaking, all optical systems can be described with Maxwell's Equations. However, given the above simplifications, Huygens' principle provides a quick method to predict the propagation of a wavefront through, for example, free space. The construction is as follows: Let every point on the wavefront be considered a new point source. By calculating the total effect from every point source, the resulting field at new points can be computed. Sophisticated computational algorithms are often based on this approach. Specific cases for simple wavefronts can be computed directly. For example, a spherical wavefront will remain spherical as the energy of the wave is carried away equally in all directions. Such directions of energy flow, which are always perpendicular to the wavefront, are called rays.
The simplest form of a wavefront is the plane wave, which when propagating can be seen to give rise to new plane wavefronts, as the corresponding rays are parallel to each other. Technically, this is referred to as a collimated beam of light. Such a plane wavefront is a good model for a section of a very large spherical wavefront; for instance, sunlight strikes the earth in wavefronts of radius 93 million miles. For all practical purposes, such a wavefront is considered planar.
Wavefront aberrations
Methods utilizing wavefront measurements or predictions can be considered an advanced approach to lens optics, where a single focal distance may not exist due to lens thickness or imperfections. Note also that for manufacturing reasons, a perfect lens has a spherical (or toroidal) surface shape though, theoretically, the ideal surface would be aspheric. Shortcomings such as these in an optical system cause what are called optical aberrations. The best-known aberrations include spherical aberration and coma.
However there may be more complex sources of aberrations such as in a large telescope due to spatial variations in the index of refraction of the atmosphere. The deviation of a wavefront in an optical system from a desired perfect planar wavefront is called the wavefront aberration. Wavefront aberrations are usually described as either a sampled image or a collection of two-dimensional polynomial terms. Minimization of these aberrations is obviously very desirable in optical systems.
Wavefront sensor
A wavefront sensor is basically a device which measures the wavefront aberration in a coherent signal to describe the optical quality or lack thereof in an optical system. A very common method is to use a Shack-Hartman lenslet array. Probably the most common application of such systems is in measurement of the eye itself. In one approach, a weak laser source is directed into the eye and the reflection off the retina is sampled and processed.
See also
External links
- [LightPipes] free Unix wavefront propagation software
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