Magfrenl.len - A Fresnel diffractive magnifier

Magfrenl.len - A Fresnel diffractive magnifier

The Fresnel diffractive magnifier is a plastic element with an aspheric Fresnel surface on the front and a diffractive surface on the back. It is an interesting system for both its tutorial value and its optical performance. The optical performance is much better than that of a simple lens, as shown by the ray analysis below.

A Fresnel surface in OSLO is one that has the power of a curved surface, but is actually placed on a substrate having a different curvature (usually flat). This is accomplished by dividing the surface into prismatic zones, so that the surface normal approximates that of the curved refracting surface at any point. As the number of zones increases, the approximation becomes better. Fresnel surfaces were originally used in searchlights, where a substantial reduction in weight was accomplished. More recently, Fresnel surfaces have been used in a variety of consumer optics products made from embossed plastic.

A diffractive surface is outwardly similar to a Fresnel surface, but there is an important difference between the two: With a Fresnel surface, light from different zones combines incoherently, while with a diffractive surface, light from different zones combines coherently. The former follows the laws of refraction, while the latter follows the laws of diffraction. Curiously, the chromatic aberration of the two has opposite sign, which is used in the magfrenl.len magnifier. The lens data is shown below.

*LENS DATA
Fresnel diffractive magnifier 5X
SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS SPE   NOTE
  0        --        1.0000e+20    4.3661e+18             AIR

  1        --        250.000000      5.000000 A           AIR

  2     32.626000      2.000000     20.000000           CARBO C *
  3        --         48.700000     20.000000             AIR   *

  4        --            --          5.000000

*CONIC AND POLYNOMIAL ASPHERIC DATA
SRF        CC          AD          AE          AF          AG
  2      -0.969340 -1.5465e-07      --          --          --

*DIFFRACTIVE SURFACE DATA
  3   DOE DFR   2 - SYMMETRIC DIFFRACTIVE SRF       DOR   1 DWV      0.587560
      DF0        --     DF1     -0.001041

*SURFACE TAG DATA
  2     FRN   1   FCV       --       FCC       --

Note that surface 2, the Fresnel surface, is made aspheric to correct higher order aberrations. Surface 3, the diffractive surface, contains only a quadratic phase term to add some focusing power and correct the chromatic aberration. The ray analysis on the previous page shows that the system is well-corrected for primary but not secondary color.

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