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This page contains answers to common questions handled by our support
staff, along with some tips and tricks that we have found useful and presented here as
questions.
Note: In these answers we will follow a few shorthand conventions for
describing user-interface procedures. Key combinations will be presented like this: Ctrl+Alt+Delete,
which means that you should press and hold down the Control key, the Alt key, and the
Delete key at the same time. Menu selections will be presented like this: File | Open,
which means that you should open the File menu, and then make the Open selection.
- Is Phase MOSAIC a phase shifting data acquisition program?
- How do I register surfaces together?
- Can I register surfaces without using fiducials?
- Graphics appear blocky on my display. Does this mean my results will be
inaccurate?
- How are pixel values chosen for surface frame operations (registration,
addition, subtraction, maximum, minimum,averaging)?
- I am Zernike fitting a surface and the resulting coefficients don't seem
to make any sense. Whats' going on?
- After matching coordinate systems using fiducial groups, the surface now
seems to be skewed. How can I stop this from happening?
At the current time, Phase MOSAIC does not support data acquisition. Instead, it
works with the user's current data acquisition system. This avoids the high cost of
upgrading the acquisition system while adding the benefits of a state of the art analysis
program.
Surfaces are registered or "stitched" together by using coordinate systems.
To start, make sure a proper global coordinate system is defined in each surface to
register. One way to do this is to use fiducials. In each surface file, mark fiducial
locations at known physical locations. Then use the Modify | Coordinate System |
Match | Fiducial Groups menu item. Edit the fiducial values in the group to
match to be the known physical positions. Then fit the coordinate system to this group
using the Match button. Be sure to save a version of the file with the corrected
coordinate system for each surface to be registered. Load the first file and use the Modify
| Pixels | Frame | Register menu item to select the other file. At this time
you can select the aberrations to be adjusted to match the surfaces together.
A short tutorial describing the steps to stitch files together can be found
here.
Yes. Registration uses the coordinate system defined for each file. If each surface to
register uses the same set of pixel locations, the default coordinate system on each
surface can be used to register since it will be the same. One limitation to this is
allowing aberrations higher than focus to be adjusted during registration. Higher
aberrations require a correctly identified center. In this case, the center that is used
for registration needs to be the optical center of the part registered.
Not necessarily. Phase MOSAIC uses the "document-view" model of programming.
This means that the data and the display of the data are separated. Calculations in Phase
MOSAIC use the actual pixel values, not the displayed pixels on the screen. Graphical
display of data can appear blocky if the graphics resolution setting is much lower than
the number of pixels being represented. The graphics resolution of the 2D surface
view can be adjusted by right-clicking in the view and select the Properties
menu item. The render resolution can be changed with the Preferences tab. This adjusts the
desired resolution of the graphical representation of the pixel data. For more
information, see here.
These operations work by choosing a surface height from the surface to be registered,
(or added, subtracted, etc.), and modifying the base surface height based on this value.
Generally, surface height can be chosen by finding the nearest pixel value to the
desired location, or interpolating the value. Interpolation may result in more accurate
results in some situations, but may also result in smoothing of the surface data. For this
reason, Phase MOSAIC chooses the pixel overlay value by finding the nearest pixel value to
the desired location. If interpolation is desired, it is recommended that the surface be
resampled to a square grid using the Modify | Pixels | Resample | Square Pixels |
command. Be sure to check the Use Bilinear Interpolation checkbox. Frame operations can be
executed on the resampled surfaces after saving.
Phase MOSAIC fits Zernike polynomials around the currently defined coordinate system
origin. By default, it then defines a normalization radius to be the distance from the
origin to encompass all of the valid pixel data. If the origin of the coordinate system is
far from the centroid of the valid pixel data, the data will be non-orthogonal and the
resulting fit coefficients will not appear to be correct. In order to evaluate Zernike
polynomial coefficients for an arbitrary surface, translate the origin of the coordinate
system to the centroid of the valid data using the Coordinate
System | Translate | To Centroid menu item.
Phase MOSAIC uses a least-squares optimization to match fiducial groups containing 3 or
more fiducials. This results in all six coefficients of the affine transform matrix being
treated as free parameters. This means translation, x- and y- scaling, rotation, and x-
and y- skew are allowed in the fitted transformation. If only translation, scaling, and
rotation are allowed, match fiducial groups containing only 2 fiducials. The 2 fiducials
can be created from average positions of a larger number of fiducials, if desired. If only
translation is allowed, match fiducial groups containing only 1 fiducial.
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Phase MOSAIC