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Because
many models of Graham Interferometers provide a fixed reference flat as
an integral part of the instrument, it is relatively easy to measure the
sphericity and radius of spherical surfaces using any "good" converging
lens. For example, the operator may select a high quality camera
lens of appropriate aperture and focal length.
This offers a distinct
cost advantage and for many applications is more than sufficient. First
let us consider the measurement of a concave surface.
First let us consider
the measurement of a concave surface. The plane wavefront leaving the reference
surface is converted to a spherical wavefront by the converging lens and
is directed to the focal point after which it diverges  still a spherical
wavefront. Unlike a Transmission sphere, this system does not require
a spherical reference surface. Instead, after striking the test surface,
the beam is reflected back through the lens. If the test surface
is spherical, and positioned at either the Focus of the lens, or with its
center of curvature coincident with the focus of the lens (Confocal Position)
the reflected wave front will be converted back to a plane wave front by
the lens, and then compared to the reference flat of the interferometer.
Thus, a properly aligned
spherical surface will show straight interference fringes. Any deviation
from spherical will result in distortion of the interference fringes.
Any variation in axial position of the test piece will result in closed
(circular) fringes  just the same as when using standard techniques with
a transmission sphere. There is no limit on the radius of curvature that
can be measured on a concave test piece, but the maximum diameter that
can be measured is limited by the F/# of the converging lens  just as
with a transmission sphere.
Just as with a transmission
sphere, the distance between the "Cat's Eye" Position and the Confocal
Position is the radius of curvature.
Now, when measuring a
Convex Surface, the maximum radius of curvature is limited by the focal
length of the converging lens and, of course, the maximum diameter of the
test piece is limited by the F/# of the converging lens. Again this
is no different from the limitation imposed by a transmission sphere.
However, since the converging lens is most likely a quality camera lens,
the relative low cost permits the purchase of a selection of lenses with
a range of focal lengths.
The setups for convex
lenses, illustrated below, are exactly the same as would be employed when
using a Transmission sphere. As when measuring a concave test piece,
the distance from the "Cat's Eye" position to the null fringe position
is the radius of curvature.
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This
page last updated April 23, 2013
