Correlated Solutions Vic-3D 8 User manual

SAFETY AND RISK MANAGEMENT
SAFETY
•Some mechanical assemblies contain pinch points - use proper care and tools when assembling to avoid injury.
•Incandescent lights can get very hot. Use caution when moving or working around these lights, and beware of
excessive heating of the specimen, cameras, lenses, or other surfaces. Leave lights off when possible.
•When mounting cameras, it may be possible to create an unstable or unbalanced situation. Be careful not to
cantilever cameras excessively, especially heavier or high-speed cameras.
•Personnel should be in a safe location whenever a dangerous or destructive test is being performed. Various
solutions are available for extending cabling/controls to allow the system to be operated in a safe, remote location
– please contact Correlated Solutions for more details.
•Various paints, chemicals, and solvents may be used in preparing specimens for testing. Always observe label
precautions.
•If any cables become frayed or damaged, replace immediately to avoid risk of shock.
•Do not attempt to repair or disassemble the computer or any electronic parts. Risk of shock or damage may result.
RISK MANAGEMENT
•Always fully support cameras when moving, assembling, or adjusting, until all fasteners are completely tight. For
larger high-speed cameras, two people may be required. Cameras can suffer severe damage if dropped.
•Always support lenses when installing or removing. In addition, for certain lenses, during focusing operation, the
front must be supported to prevent drops.
•While most power and data connections are keyed, it may be possible to incorrectly plug in or force a USB, 1394a,
or 1394b connection. Always check orientation and compatibility before making any connection.
•For any test where excessive heat, shock, or flying debris may be present, take steps to protect the cameras and
equipment. Various shielding solutions are available – please contact Correlated Solutions for more details.

INTRODUCTION
Completing a test with Vic-3D is fairly straightforward but a few pointers can help to get the best results in the shortest
period of time. This document explains the basics of a test from start to finish. A typical sequence will be:
•Preparing the specimen
•Setting up, pointing, and focusing the cameras on the specimen
•Calibrating the camera system
•Running the test and acquiring images
•Image correlation
•Viewing and reducing data
PREPARING THE SPECIMEN
Begin by preparing the region of interest on your specimen with a speckle pattern. For more information on techniques and
guidelines, please see the application note AN1701, Speckle Pattern Fundamentals. In this example, our specimen is
prepared with a laser-printed speckle pattern on adhesive paper.

SETTING UP THE CAMERAS
POINTING THE CAMERAS
To begin, set your prepared specimen in its testing location. Be aware of the orientation and potential camera placement;
for example, for a dog-bone specimen in a test frame, the prepared face of the specimen should face outwards from the
test frame rather than facing the frame’s columns.
Our test specimen for this example is a small demonstration fixture which is designed to load an aluminum panel in
bending. An air bladder behind the panel can be inflated to provide the load.
To assist in setting up the cameras, start Vic-Snap to view a live image set.
A window is shown for each camera in the system. The red areas in each image indicate overdrive/saturation, where the
pixel is driven to its highest possible brightness value.
The images can be zoomed by placing the mouse over an image and using the scroll wheel. This is useful for checking focus
or examining details but affects only the display and not the saved image.
To adjust the exposure time, use the large slider at the bottom of each window. The default range is 0-50ms – to select a
higher maximum, or a smaller range (to allow finer adjustments), you can right-click on the slider. Select a preset, or
“Custom” to specify.
After starting Vic-Snap, position the camera rig. The distance between the camera system and the specimen will be
determined by your available lenses; when multiple lenses are available, you should use the shortest one that works for
your setup. In some cases, test or room setup may require you to place the cameras farther away and compensate with
longer lenses; here, there are no such restrictions, so we use 8mm lenses. The short lenses are generally easier to work with
and can give somewhat better results.

The distance is set so that the specimen roughly fills the field of view. If the specimen is larger than the field of view, we
lose data at the edges; if the specimen is much smaller, our spatial resolution suffers. Note that the entire area of interest
must be visible in both cameras – generally, the specimen should be made just a bit smaller than the field of view, so that
pixel-perfect alignment isn’t necessary.
Also note that the object must remain in the field of view for the entire test in order to collect data, so if large motions of
the sample are predicted, the field of view should be adjusted accordingly. For instance, if a rubber specimen will be
straining downwards 100%, it should only be filling the top half of the field of view.
Too small – loss of spatial resolution Too large – lost information at edges A good size
Keep in mind that the entire specimen might not always be of interest – for example, if we were very interested in details
near the two holes, we could zoom in on that area for better spatial resolution.

Once a lens and approximate distance is selected, the cameras can be pointed.
The cameras should be positioned somewhat symmetrically about the specimen;
this will keep the magnification level consistent. The exact angle included
between the cameras is not critical but selecting a correct stereo angle will give
best results: the angle should be at least 25º for short lenses (8mm, 12mm), at
least 20º for medium lenses (35mm), and at least 15º for longer lenses (70mm).
The angle should be kept below approximately 60º.
ADJUSTING FOCUS
When the cameras have been positioned, the next step will be to set focus. Use
the focus control on your lenses to achieve a sharp focus on the entire specimen.
Usually, it will be necessary to zoom in on the image to check fine focus; slight
defocus will not be visible with the image zoomed out to fit on screen. While
zoomed in, look closely at both the far and near edge of the specimen to ensure
that the entire surface is in focus, before proceeding.
Tip: to aid in focusing, open the lens’s aperture all the way. This will reduce the
depth of field and make any focus issues very obvious. Then return the aperture
to the appropriate setting for the test. (You will need to temporarily reduce the
exposure time to compensate – see following section.)
Focusing with large aperture – small DOF After closing aperture – focal plane is well centered
Depth-of-field
Depth-of-field
Specimen
Focal plane
Camera

APERTURE AND EXPOSURE TIME
As you make the image sharp through the focus adjustment, it will also be necessary to adjust the brightness of the image.
There are two controls available for this: the aperture/iris setting on the lens, and the exposure time setting of the camera.
•Aperture: opening the aperture allows more light to fall on the sensor. The aperture setting is also called the f-
number; f-numbers are usually indicated on the lens’s aperture ring and typically go from an open setting of F/1.4
or F/2.8 to a closed setting of F/22 or F/32. Using a bigger aperture (lower f-number) will make the image brighter.
However, it will also decrease the depth of field – the range over which the focus is sharp. Even for a flat specimen,
some depth of field is necessary because each camera is oblique to the plane of the specimen; also, poor depth of
field may make it difficult to achieve a wide range of calibration target positions (see following section).
At higher magnifications and with higher resolution cameras, care should be taken when using very small
apertures (high F-numbers). In some cases, this can limit resolution due to diffraction; for example, with a 5
megapixel camera and a 75mm lens, using apertures of F/16 or higher will result in very blurry images. In these
cases, it will be necessary to find the best balance of depth of field (requiring high F-numbers) and resolution
(requiring low F-numbers).
Note that the aperture may not be changed after the system is calibrated.
•Exposure time: this is the amount of time the camera sensor gathers light before reading out a new image. Longer
exposure times make the image brighter but can also create blur if significant motion happens during the exposure
times. For many tests, blur is not a concern for the specimen itself, but can be an issue when acquiring images of a
hand-held calibration grid. A rule of thumb is to keep the exposure time below 1/f, where f is the focal length of
the lens (in mm). So, for a 50mm lens, this would mean a limit of approximately 20ms.
In contrast to aperture, exposure time may be adjusted after the system is calibrated if lighting conditions change
or the specimen becomes brighter/darker.
Controls for focus and aperture differ by lens; two common C-mount styles are shown below.
This lens has a focus and aperture ring, each with
locking knob. The aperture ring is normally closest
to the camera. Loosen the locking knob (if
present); make any adjustments; and tighten the
lock before calibrating.

This lens has an aperture ring with a locking knob.
To focus, loosen the collar (for this particular lens,
a 2mm hex driver is used), and rotate the entire
body of the lens. Loosen the lens body
(counterclockwise) to focus closer; tighten
(clockwise) to focus farther. Tighten the collar
when complete.
Caution – the lens body is not captive and will fall
if screwed all the way out.
When exposure and aperture settings are complete, the image should be bright and there should be no overdriven (red)
pixels in the area of interest. In many cases, ambient light will not be enough to achieve this without using unacceptably
large exposure times or aperture settings; for these cases, supplemental light will be required.
Too bright – overdriven Too dim Correct brightness
For many moderately sized specimens, a simple task lamp of 50-100 watts will give excellent light levels while providing
diffuse, even illumination. For very small specimens at high magnification, a fiber optic illuminator can be used. For very
large specimens, a 300-500 watt halogen light or a specialized machine vision lighting solution may be required. If the
specimen is reflective or highly curved, specialized solutions or careful positioning may be required to avoid
reflections/highlights on the surface.
WARNING: IF THE LIGHT IS POSITIONED IN FRONT OF AND BELOW THE CAMERAS, OR IF THE SURFACE BEGINS TO HEAT UP
FROM THE LIGHTING, REFRACTIVE HEAT WAVES CAN APPEAR IN YOUR IMAGE. THESE WAVES MAY NOT BE VISIBLE IN A
ZOOMED OUT IMAGE BUT WHEN ZOOMING IN THEY CAN BE SEEN AS A SWIMMING/SHIMMERING EFFECT WHICH CAN
BOTH DEFOCUS AND DISTORT THE IMAGE.
IF HEAT WAVES ARE PRESENT IN YOUR IMAGE THEY CAN EASILY INTRODUCE FALSE STRAINS OF SEVERAL THOUSAND
MICROSTRAIN SO IT IS CRITICAL THAT THEY BE AVOIDED.
HEAT WAVES CAN BE MINIMIZED BY EITHER REPOSITIONING THE LIGHT SOURCE, OR, IF NECESSARY, A SMALL DESK OR
STAND FAN CAN BE SET UP TO BLOW ACROSS THE SCENE. THIS WILL GENERALLY MIX THE AIR WELL ENOUGH TO
COMPLETELY ELIMINATE THE HEAT WAVES.

TO CONCLUSIVELY CHECK FOR HEAT WAVES, YOU CAN SIMPLY TAKE SEVERAL IMAGES OF THE SAME SCENE (NO
LOAD/MOTION) AND RUN THEM. IF HEAT WAVES ARE PRESENT, YOU WILL SEE LARGE DISPLACEMENT FIELDS THAT
CHANGE RANDOMLY FROM IMAGE TO IMAGE.
Once the camera positions have been set and the focus and aperture adjusted, calibration can begin. After this point,
changing any aspect of the camera system will invalidate the calibration; so, all adjustments should be carefully fixed at this
point.
•Tighten the tilt and rotation adjustments on the camera mounts.
•Lock the focus and aperture adjustments on the lenses, if possible.
•Tighten the tripod column.
•Tie the camera cables firmly to the tripod or crossbar; this will prevent external force on the cables from applying
leverage to the cameras.

ANALOG DATA
For systems equipped with data acquisition hardware, several channels of analog data may be acquired along with the
image data. To view the analog data, click the Analog Data button in the toolbar. (If this button is not present, analog
acquisition is not installed. If it is present but grayed out, the acquisition is installed but not active.) A dialog will appear
showing the voltage for each channel present in the device (typically, 4 or 8 channels).
You can double-click on a channel heading to remove it from the display (double-click again to return it). You can also scale
and rename these channels; right-click and select Edit channels. You can enter a title, range, multiplication factor, and
offset for each channel. Selecting the appropriate range will give higher accuracy. To view scaled data, right-click and select
Show scaled.
This data is saved in the CSV log file associated with the project. This file will have the same name as the project prefix and
for each image set, contains the image count, the filenames, the exact times, the unscaled analog data, and the scaled
analog data.

CALIBRATION
SELECTING A GRID
To begin, select a grid that approximately fills the field of view.
Too large Correct Too small
If the grid is too large, it will be difficult to keep it fully in the field of view in both cameras while taking images. However,
calibration images will be useful as long as all three of the hollow marker dots are visible. If any of the three marker dots
cannot be seen, then the image cannot be used.
If the grid is too small, it may be difficult for Vic-3D to automatically extract points; additionally, more total images will be
required to cover the field of view, including the corners. However, if no ideally sized grid is available, smaller grids can
work quite well.
Occasionally, lighting configuration can affect grid selection. Some grids may be slightly reflective; under intense or
directional light sources, these reflections can wash out the grid image. For these cases, especially matte grids should be
used, or care taken to avoid reflections.
POSITIONING THE GRID
The calibration procedure calculates variables about the camera geometry and imaging; it is not specific to a plane or
volume in space. Therefore, it’s not necessary to position the calibration grid in the exact same location as the intended
specimen. Still, it will be most convenient to place the grid in roughly the intended plane. This will ensure that the cameras
point at it correctly and that the grid is in good focus.
Optimally, the specimen can be moved and replaced with the grid during the calibration. If this is not practical, it’s often
possible to calibrate directly in front of the specimen; this method does require some extra depth of field because the grid
will be in front of the focal plane rather than directly in it.
If the specimen cannot be moved and there is insufficient depth of field to calibrate in front of the specimen, it may be
necessary to move or rotate the stereo rig for calibration. In this case, it is very important not to disturb the camera
position. The best way to achieve this may be to smoothly rotate the entire mount; this involves a minimum of vibration
and shock compared to dragging the tripod away. Then, calibrate to the left or right of the specimen position before
returning the rig to center.
If the tripod must be moved, it may be helpful to mark the original location of each leg; otherwise, finding the original
rotation and position may be difficult.

ACQUIRING GRID IMAGES
Before acquiring calibration images in Vic-Snap, select a name for the images by clicking Edit Project in the menu or toolbar.
A consistent suffix such as “cal” will make future reference easier. In this case, we’ll be keeping both the calibration and
test images together in a folder called “bending-test”.
Click Ok to accept. Then, for each grid position, capture an image in Vic-Snap by using the space bar, or the remote.
At least four calibration images must be acquired. More calibration images will give a more accurate result; in addition,
acquiring redundant images leaves more room to discard poor images (images that contain highlights, defocus,
obstructions, or other issues that makes them unsuitable for use). For a typical setup, 15-20 images will be a good number.
•To acquire calibration images, capture several images of the grid in various poses. Include significant rotations
about all 3 axes.
•To accurately estimate perspective information, the grid should be tilted off-axis and/or moved closer/farther from
the cameras for some images.
•To estimate aspect ratio accurately, the grid should be also rotated in-plane in some images.
•To estimate distortion accurately, grid points should cover the corners of the image field in some images. If a small
grid is used, this will require specifically moving the grid to each corner and acquiring images. If the grid nearly fills
the field, it will naturally fill in the corners.
•For each image pair, the grid should be visible in both images. If calibration is performed in roughly the same plane
as the specimen, this will happen naturally.
•Calibration images should be fairly well focused across the width and height of the target.
•When using a grid which doesn’t fill the field of view, take more images of the grid in all regions of the field, as well
as moving it closer and farther from the camera. (For a small grid, tilt alone will not provide the necessary
perspective information).
•If a grid dot is partially off the edge of the image, it will be discarded. However, if it is partially blocked by, i.e., a
thumb, Vic-3D may estimate the center incorrectly. This should be visible as a high error for that particular image.
•Some overdrive/saturation is acceptable, as long as black grid dots don’t appear shiny and white.

CALIBRATION IN VIC-3D
To calibrate using the acquired grid images, start Vic-3D. Select Calibration images from the start page, or click Project…
Calibration images. Navigate to the correct folder and select all your calibration images; click Open.
The selected images will appear under the Calibration Images section in the Images tab. To begin calibration, click the
Calibrate button in the toolbar, or select Calibration… Calibrate stereo system.
If using a coded target, Vic-3D will detect the target parameters and spacing and begin extraction.
For a non-coded target, select the target you used using the drop-down box at the top left of the dialog. If your grid is not
listed, click the “+” next to the dropdown. Vic-3D will attempt to identify the geometry of the grid, but the spacing must be
entered. Then, click Analyze to extract the target points.
At this point, you will see the images in sequence, with the identified target points highlighted.

For each image, a number of points extracted is displayed. When the extraction is complete, the calibration will be
computed; a score will be displayed for each image; and a final score displayed in the lower right.
IMAGING SYSTEM AND DISTORTION ORDER
To edit the characteristics of the imaging system model, click Edit by the imaging system drop-down menu. For most
setups, a standard system with first order radial distortion is appropriate. Some short focal length lenses or very large
camera sensors may require an order of 2 or, rarely, 3.
If you are unsure of the distortion order of your lens, try the calibration at each radial distortion order – 1, 2, and 3. If the
calibration score becomes significantly better when you increase the order from 1 to 2, or 2 to 3, then the lens does have
higher-order distortions. You can also add parameters to describe the prismatic and tangential distortions if necessary. You
will only need to do this process once for each new lens you work with.

For lenses with higher distortion orders, more images may be required, and it becomes even more important to take grid
images where points are present in the corners of the field of view. As many as 30 images may be required to accurately
estimate 3rd order distortions.
Complex distortion setups involving cameras imaging through refractive interfaces (such as through glass and water) can be
modelled using the Variable Ray Origin (VRO) option. Using VRO 4th order is recommended unless the setup involves
multiple refractive interfaces.
HIGH-MAGNIFICATION CALIBRATION
For macro lenses and small fields of view, it will become very difficult to get significant off-axis tilt in the target without
severe defocus. Because of this, the “center” values may be poorly estimated. The best remedy is to attempt to get as much
tilt as possible to get a better estimate, but failing this, the High magnification option will force the center values to be the
numerical center of the lens.
INTERPRETING CALIBRATION RESULTS
After the calculation is complete, you will be presented with a report of calibration results and error scores. The errors will
be displayed per image, as well as an overall error score:
The overall error (Standard deviation of residuals for all views)should be displayed in green. If you have a good set of
calibration images with good tilt and coverage of the image field, and the score is green, then the calibration is good and
you can click Accept to finish.
If the score is displayed in red, you may need to remove some images or recalibrate. Vic-3D will automatically remove very
poor images, but you can remove additional images by right-clicking in the table of scores and selecting Remove row.
If the result is uniformly high and not due to just a few outliers, or if you have several high scores, there may be a problem
with the setup. Check that:
•The grid images are in focus.
•The exposure times are short enough to eliminate motion blur.
•The cameras are secure on the stereo rig.

•The grid is rigid.
•The grid is evenly lit. For a backlit glass grid, this is particularly important.
•If using a glass grid, confirm the correct face is towards the camera.
•The cameras are synchronized.
•Correct any potential problems and recalibrate.
Below the calibration scores, the calibration results are listed. Each result is listed with a confidence interval; if the interval
is very high, it may indicate a poor image sequence, even if the error score is low.
For each camera, the following values can be displayed:
•Center (x,y): the position on the sensor where the lens is centered. It should be roughly in the physical center of
the sensor.
The confidence intervals for center (x) and (y) will generally be higher for long-focal-length lenses. At very small
fields of view and high magnification lenses (70mm and up), the interval may be higher in magnitude than the
value itself – if the centers themselves are reasonable then it’s okay to proceed. If a reasonable center estimate
cannot be obtained, you may need to check the High magnification option (see above).
•Focal length (x,y): the focal length of the lens, in pixels. Multiplying this number by the known pixel size of the
camera will give a number roughly equal to the specified focal length of the lens.
•Skew: indicates the out-of-square of the sensor grid.
•Kappa (1, 2, 3): the radial distortion coefficients of the lens.
•p (1, 2, …): the prismatic distortion coefficients of the lens (if applicable)
•t (1, 2, …): the tangential distortion coefficients of the lens (if applicable)
For the rig as a whole, the following values are given:
•Angles: the three angles between each camera. In general, two angles will be small and one (the stereo angle) will
be larger.
•Distances: the distance between camera 1 and camera 2, measured from camera 1.
When the error score and confidence intervals are acceptable, click Accept to finish. The calibration data will be displayed
in the Calibration tab at left.
To save the new calibration, click the Save icon in the toolbar, or select File… Save, and select a project file name.

RUNNING THE TEST
Once calibration is complete, you may run the test. (Note that you may proceed directly from acquiring calibration images
to acquiring test images. However, you may not uncover any problems with the setup until it is too late. For important
tests, you should check the calibration before testing.)
Double-check the position of the specimen and stereo rig; confirm that the lighting is correct and that the entire specimen
is in sharp focus. Remember that the aperture, focus, and position of the cameras must not be changed without
recalibrating.
In Vic-Snap, click Edit Project in the menu or toolbar to give your images a name.
For a shape measurement only, you can simply press the space bar to acquire a single image. Otherwise, for a typical quasi-
static loading test, the Timed Capture option is a convenient way to acquire a sequence. To begin, show the Timed Capture
menu by right-clicking the toolbar and selecting Timed Capture.
Select an interval that is appropriate to the test length and number of images required. In general, it’s better to acquire
more images rather than fewer; images can always be discarded or ignored later. Here, we’ll plan to acquire around 30
images; the test should take around 30 seconds, so we’ll select an interval of 1 second. We leave the Stop after box
unchecked; we will stop the test manually.
When the test is ready to begin, start the acquisition, then the test (any excess images at the beginning can always be
discarded). The image counter in the toolbar will begin counting up. You can monitor the live images to be sure that the
specimen remains in focus, and that there are no lighting changes due to motion. If necessary, adjust the exposure to keep
the specimen bright but not overdriven.
When the test is complete, click Stop in the timed capture dialog, then Close. Return to Vic-3D to analyze the new images.

Note: even for timed capture tests, you may want to acquire and analyze a single static image of the specimen before
running the test. This will verify that the test is ready to proceed and there are no problems with the lighting, calibration,
focus, or speckle pattern. This is a good safeguard before running any type of expensive or destructive test and only takes a
few moments. Simply acquire the image; analyze according to the instructions in the following section; and confirm that the
error is low and that the shape looks correct.

CORRELATION IN VIC-3D
SPECKLE IMAGES
If you closed Vic-3D before acquiring test images, re-open it; then, select the saved project file with the calibration from the
start page, or use File… Open. Next, add the speckle images. Select the speckle image tool or click Project… Speckle Images.
Select the desired images.
The selected images appear under Speckle Images in the Images tab at the left. The first image in the list has a red arrow
next to it; this indicates that this is the reference image.
This image is the beginning state of the specimen; all displacements and strains will be relative to this reference image. For
most tests, the first image will be the correct reference image; to select a difference reference, right-click on a different
image and choose Set reference image.

DEFINING THE AOI
Before running the correlation, we have to define an area of interest (AOI). This is the portion of the image that contains
the speckle pattern and which will be analyzed for shape and displacement. To begin, double-click on the reference image
to open the AOI Editor. The reference image will be displayed; here, we select the Polygon tool from the set of AOI tools.
Then, click a series of points to define the boundaries of the AOI; double-click to finish.
Here, we want to exclude the two holes in the image from analysis. We’ll select the Cut circle tool from the AOI tools and
click three points on the edge of each circle. The mouse wheel can be used to zoom in for detail work.