Sunday, November 14, 2010

A Broad Overview on Optical (Photogrammetry) Targets

Here comes an esoteric discussion on none other than…optical
targets.  Optical targets help us correlate measurement systems, provide
fixed references directly at the part surface, and act as direct measurements of
hard-fixed points.  They have thickness, they have specularity,
dispersion, edge contrast, absorbtion, spectral shift, and possibly even
translucency.  They come in various sizes, surface types, prices, and
methods of manufacture.  They are important in our achieving of absolute
accuracy, and it is important to quantify their impact on the underlying
metrology.  Yes, these absolute references become a fundamental link in our
error budget.  So let’s get started. 


Optical targets work by creating a reference point, often at the part surface
or at a tooling point location, for a camera to observe as a known location
within the measurement system.  They are often round, but can exist as
other shapes.  The known shape combined with the masked area on the target
(the “dot”) provides a basis for any number of methods to extract the center and
edge parameters.  Books have been written on this extraction, and new
methods are continually being developed.  The ability of these various
algorithms to work in real-world scenarios ultimately depends on the
illumination of the target as seen by the camera, the size of the target
relative to the sensor size of the camera, the quality of the target as
determined by its light scattering or focusing characteristics, the contrast at
the target edge, uniformity of the target’s surface, and degree to which the
target matches the “ideal” shape it is intended to represent.  In general,
a high-quality target should return the same, similar, or at very least a
predictable light/edge response when viewed normal to the target, as at
fairly steep angles to the target.  This single criterion puts many targets
in the category of “unacceptable” for some accuracy budgets, so choose
wisely. 


Once the target’s optical characteristics are determined, they can
be weighted against the precision of the observation, and this overall
uncertainty can be fed into the error budget and calculation of its 3D
position.   After the target’s center point
is determined, an offset correction may be necessary to resolve the 3D
coordinate into a known reference, or a net measured surface. 
  This offset measurement is usually assumed as a fixed value for a
given target type, and is determined via repeated measurement with a micrometer
or similar thickness measurement device.  The offset often takes place
normal to the target’s measured normal vector as determined by the shape of its
edge (such as the plane of the circular “dot”). 


Traditional “dot” targets can be thought of as 1-bit targets, whereas “coded”
targets can range from 1 to n bits in addition to the center bit.  The
additional bit codes provide information such as unique identifiers, and
generally surround the center “dot” of the target with some pattern unique to
the specific decoding mechanism.  They are particularly useful for
disambiguating target locations in uncalibrated or unknown camera states. 
A well-designed coding mechanism will allow for some leeway in the observance of
the code, so that the decoder will not incorrectly identify a code in the real
world.  The same light response, masking, and observation characteristics
that apply to the center “dot” target also apply to the code, so again, choose
wisely.  An important charteristic of the decoder (whether for 1-bit “dot”
or multi-bit targets) is that it successfully handles shadows, partial
obstructions, and other real-world observation and illumination
situations.   


Targets are often considered disposable, and therefore can exist on removable
adhesive.  This adhesive also factors into the error budget, and the
adhesive’s performance on various materials, with various surface finishes, at
various temperatures can affect the target’s usefulness.  On some surfaces,
the adhesive acts as FOD or a corrosive agent, so always make sure the target is
compatible with its application.  Further, when used in “extreme”
temperature, humidity, pressure, electrical, or mechanical ranges, both the
target and any adhesives present must be rated or tested prior to use to verify
stability.  Some targets are manufactured for specialized
environments, so try to find one that matches your needs. 


Long-term application of targets, such as on tooling points or scale bars
should be mechanically coupled to the substrate material.  This is because
in field use, artifacts such as scale bars are subject to human handling, and
often undergo thermal cycles while traveling as checked-in luggage, in hot cars,
snow-covered delivery vehicles, etc.  This stress may loosen or move the
adhesive, and affect the certified scale value.  One method to mechanically
fix the target is to drill a small hole at several locations on the outside edge
of the target and into the substrate material (away from the center “dot” or
code), and use a temperature and material-compatible glue to permanently affix
the target to the bulk material.  Low TCE scale bar targets should be
affixed with Low TCE glue/epoxy, while high TCE scale bar targets should be
affixed with high TCE glue or epoxy. 


Once a target has been characterized, it can serve as a feedback loop into a
metrology system design to optimize camera selection, define performance
specifications, select between lens manufacturers, conduct ambient light or
motion testing, and generally study system repeatability, accuracy, and
robustness. 


Optical targets are often used, sometimes abused, occasionally understood,
yet always in our tool bag.  Whether used for photogrammetric bundling,
single-observation locating, alignment into coordinate systems, correlation in
other devices, or random fitting into a scene, performing the work up-front to
quantify your targets will return both peace of mind and provide that much more
success in your metrology projects.  Once you understand your targets,
they’ll become your go-to “reference.”

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