Thursday, June 19, 2008

Optical Aberration

Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system.

It occurs when light from one point of an object after transmission through the system does not converge into (or does not diverge from) a single point. Instrument-makers need to correct optical systems to compensate for aberration.

The articles on reflection, refraction and caustics discuss the general features of reflected and refracted rays.

Aberrations fall into two classes:
  • Monochromatic aberrations (Gr. monos, one) produced without dispersion. These include the aberrations at reflecting surfaces of any colored light, and at refracting surfaces of monochromatic light of single wavelength. These include:
    • Piston
    • Tilt
    • Defocus
    • Spherical
    • Coma
    • Astigmatism
    • Curvature of field
    • Image distortion

  • Chromatic aberrations (Gr. croma, color), where a system disperses the various wavelengths of light
    • Axial, or longitudinal, chromatic aberration
    • Lateral, or transverse, chromatic aberration

Piston and tilt are not actually true optical aberrations, as they do not represent or model curvature in the wavefront. If an otherwise perfect wavefront is "aberrated" by piston and tilt, it will still form a perfect, aberration-free image, only shifted to a different position. Defocus is the lowest order true optical aberration.

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