Aberrations

definition

All aberrations lead to a blurring of an image formed by an optical system.

                                                          Chromatic Aberrations 

Chromatic aberration occur largely as a result of the form and shape of lens and mirrors as a result and as a result of rays from object points striking spherical surfaces at angle that exceed those set by the paraxial approximation.

  • Chromatic aberrations results because the index of refraction of a material differ for different wavelength.
  • Focal length of a lens is dependent on the index of refraction of lens material.
  • Focal length and image position differ for different wavelength component of the light used in the optical system.
  • Thus, polychromatic light from a point object image not as a point but as a series of point, one for each distinct wavelength.
  • A lens will not focus different color in exactly the same place.
  • The focal length depends on refraction and index of refraction.
  • Short wavelength has higher n and is refracted more than long wavelength.
  • The amount of chromatic aberration depend on the dispersion of glass.

chromatic aberration


Correction of Chromatic Aberrations

  • Chromatic Aberration in lenses can be effectively reducing by using multiple refractive element of opposite power.
  • Of course, image formed in mirror do not suffer from chromatic aberration since the focal length of mirror is independent on wavelength. 
  • It was found by that adding a second lens of different glass composistion this effect could be corrected.
  • Achromatic Doublet  
An achromatic doublet does not completely elliminate chroamtic aberration, but can eliminate it for two colors. Red and blue.