The output is independent of the partial coherence of the
source;
There are fewer focusing and alignment problems;
Because there is no sampling slit, the overall system
transfer function is improved;
The measurement of singly diffuse density makes interlabora-
tory comparison easier.
5.9.2.2 Interferometric Methods:
Interferometric testing essentially measures the pupil
function from which the optical transfer function, the spread function,
and aberration coefficients may be computed. The pupil function
must be used to evaluate cascaded optical systems and systems employing
coherent or partially coherent illumination.
Most quantitative interferometric testing systems are expensive,
time-consuming, and require operational expertise. The application
of the Wavefront Shearing Interferometer (WSI) developed at the
U. S. Bureau of Standards?2 eliminates some of these undesirable
features, employing an interferometric cube into which the adjustments
are fixed. The WSI into which the cube is installed is portable
and easily set up on an optical bench.
The resulting inteferograms, however, cannot be interpreted
by simple inspection in terms of wavefront aberrations. Hence the
major cost of testing goes into data reduction and analysis.
The cube interferometer operates by amplitude dividing the
incoming wavefront into two equal parts and introducing a small
angular shear into one of the beams. For two 45? -90? -45? prisms
contacted along the diagonals to form a cube, the pivot point of
rotation lies in the plane of the back face of the cube. A small
shear is then introduced.
As the cube is not used in parallel light, corrections
are made for the known aberrations which are introduced by the
optical path differences.
Working with photographic transparencies of the interfero-
grams, the test data is supplied in fringe-peak locations. The
technique of autoconvolving the fringe density profile in the vicinity
of a maximum or minimum was used to locate the fringe-peaks. (Peak
of the autoconvolution function corresponds to the fringe-peak.)
This method has the advantage of averaging grain noise effects during
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