Page 156 - Chalcogenide Glasses for Infrared Optics

132 Cha pte r S i x

Better still would be a Petzval design that used only one material in
the two-element design. Aspheric surfaces and the addition of diffrac-
tive surfaces would improve color correction.
Designers use computer software such as ZEMAX that provides
the ability to iterate their designs to produce maximum performance.
Positions of each lens may be changed one at a time while the thick-
ness of each lens and the radius of curvature, all may be changed
while witnessing the change in performance. Before beginning a lens
design for a camera, the designer needs to know the area of the detector
array and the number of elements in order to calculate the area of
each detector element, the pixel size. The finished lens design must
have enough performance to resolve each detector element. The greater
the number of detector elements, the greater the optical perfor-
mance required. Specified field of view and performance off-axis
requirements must be met as well.
The final product of a lens design is a lens drawing used by the
fabricator to produce this specific part. Besides a drawing of the lens
element, usually drawn to some scale, the material to be used will be
specified along with a callout for the drawing, if available, which
specifies the material requirements. Inner and outer diameters may
be specified with acceptable tolerances. Spherical radii for both
surfaces, if used, will be specified with tolerances. Center thickness is
important. Sag for concave and convex surfaces may be given.
Alignment of the surfaces with the base in terms of wedge may be
specified. Minimum surface figures such as measured with a Zygo
interferometer may be listed. Assembly drawings showing all the
optical elements in place with spacing between each element specified
may be generated by the designer.

6.2 Diamond Turning
The advent of diamond point turning for optics was a major advance-
ment. Optical design is a mathematics-based activity. Computer precise
control of a diamond point applied to a circular lens blank accurately
located on a round rotating mount produced aspheric or diffractive
surfaces (kinoforms) on command. Optical designers use software
that allows them to specify optical elements in size and shape, using
specific optical materials. The refractive index of each material as a
function of wavelength is available to the designer in stored tables.
The spacing between elements is a variable as well as the center thick-
ness of each lens. The program can calculate expected performance in
terms of MTF with spatial frequency on axis and off axis. The vari-
ables may then be adjusted slightly to maximize calculated perfor-
mance for the final design. If the operator could express the surfaces
of each lens he or she required mathematically and load it into the
computer controlling the diamond point, it would be generated one
surface at a time.