Figure 4. Optical profilometers enable accurate characterization of surface
roughness but operate over only a small field of view.
sary wavefront corrections for the particular asphere under test.
The drawback of this approach is cost: A CGH might cost
$10,000 or more and is useful for testing only one specific
aspheric profile. However, interferometric measurements with a
CGH provide a map of the entire surface, not just along a single
line or lines, and can typically be performed more rapidly than
surface profilometry (once initial interferometer setup is complete). Thus, interferometric testing may be a more economical
alternative for high-volume production.
Interferometric testing is sometimes virtually a necessity with
certain types of parts. For example, high-aspect-ratio parts will
often exhibit some astigmatism after they are removed from the
chuck. While this can be identified by using a sufficient number
of line measurements with surface profilometry, it is far more
readily characterized by interferometric testing with a CGH. Similarly, aspheric profiles that depart substantially from a sphere, or
those that do not have rotational symmetry, are more easily measured with interferometry.
As mentioned previously, surface roughness tends to be a more
significant issue with aspheric surfaces and must be measured in
any quality control/quality assurance protocol for aspheres.
Larger values for surface roughness (100 Å and above) can be
measured with a stylus profilometer, but accurately characterizing
surfaces below this level is generally accomplished using an opti-
cal profilometer. These are microscope-based instruments that de-
liver extremely accurate measurements. However, they capture
only a small field of view at one time, so it’s generally not possi-
ble to measure the entire surface of an optic. Therefore, the pur-
chaser should designate the microscope magnification at which
measurements are to be performed as well as the number and
location of the sampling points. For example, “measure at 10;
at three points (center, middle and edge) on each part.”
Optical profilometers have a limited depth of focus, which can
make it difficult to properly visualize the entire field of view
when looking at particularly steep parts, such as off-axis parabo-
las. In this case, special fixturing might be required to precision-
tilt the part so that proper focus can be obtained across the entire
field of view, and this incurs additional cost.
Meet the authors
Trey Turner is chief technology officer at REO; e-mail: treyt@reoinc.
com. Mark Damery is REO’s vice president and general manager of
worldwide sales; e-mail: firstname.lastname@example.org.