A new Time Delayed Source (TDS) has been designed to work with a transmitted wavefront
interferometer. The TDS splits a short-coherence wavelength source into two halves, introduces a time
delay between the two halves and then recombines the two halves. The recombined wavefronts are fed into
a transmitted wavefront interferometer to measure the optical path difference (OPD) between the two
surfaces of a part. The time delay is adjusted to match the separation between the two surfaces of the part
allowing phase shifting techniques to be used to measure the OPD. The measurement approach can be
adapted to measure nominally constant thickness windows, spherical domes and tangent ogives through the
design of suitable wavefront matching optics. Producing the wavefront to measure a tangent ogive is
accomplished by a series of sub-apertures, each measuring a section of a tangent ogive. The tangent ogive
is placed on a rotary stage, and as it rotates, sub-aperture measurements are taken and later stitched together
to generate a complete measurement of the part. Alignment of each sub-aperture instrument is essential to
obtain an accurate measurement. The setup and alignment of a tangent ogive transmitted wavefront
interferometer is also described.
A time-delayed source interferometer manipulates the output of a short-coherence length source so that light
reflected from the two surfaces of a nominally constant-thickness optical component interfere. The interference pattern is
a measure of optical thickness variation and can be phase-shifted. The approach is well suited to optical components that
are nominally constant thickness over some portion of the surface. Interferometers suited to the measurement of
windows, hemispherical domes and tangent ogives have been built. Data acquisition, calibration tooling and processing
methods are described for the stitching of phase data.
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