Monday, May 22, 2006

20

Aberrations


Chromatic (color) Aberrations: is the change in light direction in materials with different refractive index due to the different wavelengths of light. A simple plus lens will bend blue light rays more than red rays, leading to the optical aberration known as chromatic aberration. The blue rays come to focus closer to the lens than the red rays. Chromatic aberration occurs strongly in the human eye; with almost 3.00D difference in the focus of the far ends the visible spectrum. This is the basis of the red-green test used for refinement of the sphere power in clinical refraction.

Chromatic dispersion is caused because each wavelength of light has its own index of refraction. Shorter wavelengths (blue) deviate the most in materials with higher index of refraction.


Aberration can be modified by

  • Changing the shape of the lens

  • Changing the refractive index of the lens

  • Changing the aperture size (results in fewer marginal rays)

  • Changing the position of the aperture


In general, it is not possible to eliminate all aberration at once. Minimize one may worsen the other; therefore we need to prioritize and minimize the most irritating aberrations.

  • Aberrations are all object/image distance dependent.


Monochromatic Aberration is caused by non-paraxial rays of light. Monochromatic aberrations include spherical aberration, coma, oblique astigmatism, curvature of field and distortion.

  • Spherical aberration is shape dependent. Spherical aberration normally increases as you move towards the peripheral portion of the lens. This is because the deviating power of the lens increases towards the periphery of the lens (Prentice Rule). To minimize spherical aberration, a biconvex lens is used. Aspheric lenses, lenses where the radius of curvature gets flatter in the periphery (have less power at the edge of the lens) also help minimize aberrations. The cornea is an aspheric surface that gradually flattens towards the periphery.
  • Aperture size: The larger the aperture, the more spherical aberration from marginal rays occurs. Increasing pupil diameter causes greater spherical aberration. This is due to off axis points or extended objects that result in light rays passing through the marginal surface of the lens. This results in the lens not focusing the image at the same point due to para-axial rays. The difference in angles causes the aberration. The pupil of the eye corrects spherical aberration and coma.
  • Coma is an off axis spherical aberration. Peripheral rays produce coma. The image is a series of circles that form a comet shape. This is primarily a problem for large aperture optical systems and can be ignored in spectacles because of the limited affect of the pupil. If we increase the aperture size, we have more coma. Shorter objects have less coma. Objects off axis have more coma. Lens shape will minimize spherical aberration, but not totally eliminate coma. When the aperture is closer to the lens, greater coma occurs.

  • Aplanatic system is free of spherical aberration and coma.

  • Curvature of field is corrected by the curvature of the retina. Curvature of field is an advantageous aberration in the human eye because it produces a curved image on the retina, as opposed to a flat image.

  • Distortion: (Figure 32) is another aberration of thick lenses. It concerns the distortion of straight edges of square objects. There are two types of distortion resulting from lateral magnification of the image that results in a lateral displacement of the image.

  • Barrel distortion - where the rays in the center are more magnified than the rays further off axis. This is due to minification of the corners of a square object, more then the sides, from minus lenses.

  • Pincushion - where the central rays are less magnified than the rays off axis. This is due to magnification of the corners of a square object, more then the sides, from plus lenses.


Click on image to enlarge.

Barrel distortion on the inside – Pin cushion distortion on the outside


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