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2D Calibration Overview
The process of calibrating a VIC-2D system is quick and simple. The procedure is software based and can be performed quite easily by clicking the “Calibrate Scale” button under the Calibration option in the VIC-2D program. To calibrate for scale, you must have an image which contains a known distance, either marked on the specimen or on a ruler or target that is in the same plane as the specimen. Here, you simply choose a dimension (mm, inches, etc.) to replace pixels as a form of measurement. If a VIC-3D calibration target is used to define the known distance, subpixel accuracy can be achieved.
3D Calibration Overview
Calibrating a VIC-3D system is straightforward and has been streamlined to a nearly automatic process. The procedure for the 3D system involves moving, imaging, and analyzing a rigid calibration target in front of a stereo camera pair. This precisely calculates the cameras’ intrinsic and extrinsic parameters while triangulating the cameras’ positions and removing lens distortions. This ultimately removes any measurement bias and defines a three-dimensional coordinate system on the specimen’s surface. This entire process typically takes 1-3 minutes.
Calibration target Overview
There are many types of calibration targets available at Correlated Solutions, such as Standard Targets, Double Sided Targets, Glass Targets, Microscope Targets, and IR Targets. Each calibration target is needed for a different type of application and/or field of view. To find out more information on the type of gird you have or need for your application, we have listed and described each of these target’s features, purposes, and corresponding systems below.
The Standard Calibration Target (30mm+)
The Standard Calibration Target set is provided with each VIC-3D™ System and covers fields of view from 30mm and up. Each target features an ultra matte finish, a rigid & durable design, and they are specially coded for automatic spacing detection in VIC-3D v7 and later. These targets can be used for calibrating high-speed and low-speed systems and for both high and low resolution cameras. If exact calibration target measurements are needed, Correlated Solutions can provide NIST traceable certificates which provide detailed measurements on spacing parameters.
Need to replace a target? Users have the ability to create their own standard calibration targets. The user friendly Calibration Target Generator can be found on the Downloads page, or downloaded here: cal_gen.exe (7MB). Self-printed targets must utilize a high-resolution laser printer, and must be adhere to a flat, but more importantly, rigid plate.
Large Scale Calibrations (2m+)
The largest target (36″ x 24″) made by Correlated Solutions calibrates a field of view up to about two meters. If calibrating a larger field of view is required, a single target is no longer practical to use or cost effective to produce. To handle this problem, Correlated Solutions has implemented an intelligent calibration technique called “External Orientation Calibration.” This is a two step technique that first calibrates each camera individually with a small standard calibration target, and then uses a known distance on the specimen’s surface to calibrate for scale. This provides a very accurate, low cost, and practical solution to what may initially seem like a “large” problem. Implementing this technique demonstrates the robustness of VIC-3D products and the flexibility of the calibration procedure.
Double Sided Targets (35mm – 1.5m)
A double sided target is used to simultaneously calibrate two VIC-3D systems facing each other. The target allows DIC data from oppositely mounted systems to be put into a common coordinate system with relative ease. When your application requires the measurement of two sides of the same specimen to analyze thickness or necking, a double sided target simplifies the calibration process.
Double sided targets are manufactured in much the same way as standard calibration targets to ensure an ultra-matte finish and high rigidity & durability for a multitude of testing environments. They are also individually calibrated at CSI’s headquarters to ensure accurate measurements are achieved.
VRO Calibration: Imaging Through Glass or Liquids
The VRO calibration feature corrects for changes in the refractive index between the speckle pattern and a stereo camera pair. This is especially useful when viewing a specimen through one or more panes of glass (e.g., a viewport of a heating chamber) and in bio-medical applications, where the sample is oftentimes submerged in a liquid. While a standard calibration procedure cannot remove the bias that leads to significant errors in shape, deformation and strain data, implementing the VRO calibration model leaves data with no discernible bias.
Glass Target Overview
When calibrating fields of view smaller than 30mm, standard calibration targets are no longer ideal to use. This is due to the fact that printing devices limit the size of the elliptical markers printed with acceptable resolution. Furthermore, imaging standard targets at fields of view smaller than 30mm can also prove difficult with no other hardware to provide assistance during the tilt and rotation process.
For small scale calibrations from 0.7mm to 30mm, Correlated Solutions provides laser etched, blue chrome plated, sodalime glass calibration targets. All glass targets are created through a photo lithography process which creates a plating of blue chrome over the face of the glass plate. With proper backlighting, these targets provide very sharp edges which are clearly seen at high magnifications and are compatible with external hardware, such as target fixtures and gooseneck positioning arms. Since depth of fields decrease at higher magnifications, external hardware is almost always necessary for stabilizing the target for small out-of-plane tilts.
Small Scale Calibrations (8mm – 30mm)
For fields of view from 8mm-30mm on a VIC-3D system with standard optics, one of two glass targets should be used, either the 1″ Target for fields of view 20mm-30mm or the 4-in-1 Target (seen left) for fields of view 8mm-20mm.
Both targets are generally provided together and with the assembly seen on the left, which includes a gooseneck positioning arm, a mini-ball head, and a glass target holder. Using these components together will ensure both stability and flexibility when calibrating small fields of view.
If the sample is 7mm or smaller a standard system can be used, however, a microscope system should be heavily considered in order to take advantage of camera resolution to maximize your test’s potential data points.
Microscope Calibration Target (0.7mm – 7mm)
For fields of view from 0.7mm to 7mm, the VIC-3D Microscope system will be needed. Calibrating the Microscope System is similar to calibrating a standard system, but on an even smaller scale. Both of the targets seen to the left are provided with the microscope system for these fields of view. A back-lit target fixture is also provided on the microscope to ensure the calibration process goes smoothly from the first tilt to the last.
Due to a complex optical path and high magnification, one additional step needs to be taken in the calibration procedure to completely remove optical distortions from the microscope system. This exclusive procedure is essential when performing three-dimension DIC under a microscope. Without it, the measurement bias is on the order of 2,000 microstrain. In this step, a sequence of images is taken of a speckled glass target. By analyzing these image groups with VIC-3D’s Distortion Correction feature, all distortions are then computed and corrected. With the motorized 3-axis precision stage, this additional Distortion Sequence becomes an automated process, saving time and further simplifying the calibration process.
IR Calibration Target s (50mm – 300mm)
If your application requires full-field temperature measurements alongside strain and deformation measurements, the new VIC-3D IR™ System may be of interest to you. If you’d like to add this capability to an existing system, only a couple pieces of hardware need to be added: a camera, lens, and calibration target(s).
To calibrate a thermal camera into the VIC-3D system, a calibration target visible in the thermal camera’s wavelength range and in the DIC cameras’ white light range is needed. The easiest way to achieve this is to use a slightly heated chrome plated glass target or a laser-etched, anodized, aluminum target. The chrome plated glass targets are used to calibrate smaller fields of view and the laser etched aluminum plates are used to calibrate larger fields of view.
Each IR calibration target comes with its own protective case. The case or target displays the target’s calibration parameters, such as pitch (mm), number of dots in x, number of dots in y, offset in x, offset in y, length in x, and length in y.
Correlated Solutions, Inc. can provide calibration targets to VIC-3D system owners as standalone items, complete sets, or as part of a turn-key system for new customers. To include the capabilities these calibration targets can add to your VIC-3D system, please call us today at: +1 (803) 926-7272. If you prefer to communicate with us via e-mail, please direct your request/question to our respective departments:
|Sales@correlatedsolutions.com||Request a quotation, General inquiries, System upgrade information|
|Support@correlatedsolutions.com||Current customers may submit a support ticket here|