Because of the lack of rapid, accurate shop floor measurement devices, inspectors often lack confidence in
their assessments. The overly cautious estimations of
feature sizes that result means that rejection rates are
typically much higher than necessary. To improve yield
and process control, repeatable, accurate high-resolution
surface metrology must be accessible on the shop floor.
A gage based on PSL
A new technique, polarized structured light (PSL),
has been developed and productized for handheld, three-dimensional measurement of surface defects and features, with micrometer resolution.
The PSL technique is based on a combination of two
technologies: fringe projection (structured light) and
In fringe projection, a known pattern is optically projected against the test surface and imaged at an angle onto
a detector. Based on the distortion of the projected pattern, the surface shape may be reconstructed. Typically
with fringe projection, multiple patterns, wavelengths
or angles are necessary to create an accurate three-dimensional representation of the test piece. Because the
resulting acquisition time is relatively long, vibration
and air turbulence will degrade or destroy measurement
results, making handheld operation of the measurement
The second technology — dynamic interferometry —
enables measurement despite vibration. In a dynamic
interferometer, polarization is used to encode the mea-
surement light beam. A specialized detector with dif-
ferent polarizers in front of each camera pixel decodes
the signals to allow the simultaneous acquisition of
all information for three-dimensional reconstruction.
Acquisition takes only tens of microseconds, enabling
the instrument to measure accurately in noisy environ-
ments — even when handheld.
The combination of structured light and dynamic interferometry creates a new method of vibration-immune,
high-resolution 3D metrology. This new PSL technology
has been packaged in a handheld device to allow for rapid
and accurate metrology that can be employed on a factory
floor, replacing less accurate and more time-consuming
methods of part assessment.
PSL technology allows features from 2. 5 to 2500 microns deep or tall to be accurately quantified. The immunity to vibration opens the way for PSL use in handheld
systems for much improved flexibility over profilometers. Further, with no moving parts necessary for measurement and novel mechanical designs made possible
through additive manufacturing, system size can be
greatly reduced. This results in a final device roughly the
size of a flashlight.
The micrometer resolution achievable with PSL is sufficient for imaging defects, part marking and fine-scale
geometries, even for the most rigorously controlled machined components. Measurement results are instantaneous, reducing the measurement cycle from minutes or
hours to several seconds.
A handheld PSL-based gage can inspect both simple and complex part geometries, such as measuring
scratches and nicks on an aircraft engine shaft and fine-scale voids on a composite nose cone (Figure 1). Handheld operation and portability make it possible to measure such components in situ, despite noise and vibration.
The highly portable instrument is extremely useful for
repair/refurbishing facilities that work on large and complex components. It is also well-suited to new-make facilities that need to quantify surface imperfections prior
to assembly or to shipment. The device employs efficient
LEDs and CMOS imaging to reduce power consumption.
A battery pack on a mobile cart can run the instrument
and its computer all day, moving wherever needed on the
factory floor. A touchscreen interface makes quantified
results as simple and fast for an operator as visual inspec-
tion, but with confidence in the accuracy of the result.
The handheld PSL-based gage can read fine-scale
features on any shaped component, from relatively flat
turbine blades to corrosion in corners or nicks on blade
edges. With a 2.5-mm measurement range and 35-mm
standoff, the device can measure most geometries.
A PSL gage can also be used to assess pitting on the
corner of an engine cowling, as well as damage to the
underside of a flange (Figure 2). A plastic tip extending
from the otherwise noncontact device sets the instrument
at the correct distance from the surface for measurement
and helps the operator quickly align the gage to the corner region. The tip also can be replaced with a fold mirror
to access areas without line-of-sight access, as in the example on the right. This ability, combined with the small
size of the instrument, even enables measurement of the
sidewalls of pipes and shafts far into their interiors.
In addition to defects, the PSL gage can measure intentional surface features to ensure acceptability. A
PSL gage, for instance, can quantify the depth of laser
markings or dot peen strikes to ensure compliance with
specification. With a dot-peen marked part, each feature
can be individually analyzed for depth, volume, area and
location (Figure 3). The measurement analysis takes less
than two seconds, effectively providing instantaneous
feedback to the inspector. Other features that can benefit
from rapid quantification are rivet depth and shape, small
part radii, cylindricity of shafts, and edge break and feature recession.
An analysis of PSL data can highlight, for example,
the individual pits on a corroded sample (Figure 4). The
analysis software automatically removes the overall
shape of the surface, enabling accurate measurement of
the pits despite any part curvature. The operator can set
thresholds such that only pits below a certain depth (or
peaks above a given height) are analyzed, as shown at
The handheld polarized structured
light (PSL)-based gage can read
fine scale features on any shaped
component, from relatively flat
turbine blades to corrosion in corners
or nicks on blade edges.
Figure 2. Handheld devices assess damage on an aircraft engine fan disk (left) and on the corner of an engine
cowling (right). The black tip is an alignment aid that sets the gage at the measurement distance and helps the
operator to align the instrument.
Figure 3. Analysis of a part marked by dot peen
determines depth, volume, area and location for