Digital Optical Comparator™ (DOC™)
Most conventional optical comparators are difficult to use and do not yield accurate results unless meticulous care is exercised. MPM has developed a digital optical comparator measurement system that is user friendly and yields both accurate and precise results. The system can be used to measure the dimensions of any part or piece of equipment that can be imaged with the optics. The camera and lens can be mounted on the supplied table top rack and pinion stand, or they can be used as a portable measurement unit. This system can also be conveniently used for percent shear determination and for Brinell hardness measurement.
System Features
The MPM digital imaging system consists of a camera, a telecentric lens, an opaque lighting source, an infrared back lighting source, a rack and pinion stand, data acquisition software, and image analysis software. The 7200 system can be provided with fixed zoom detents (that is, pre-defined magnification points) or can be upgraded to the 7300 system which incorporates a motorized system for continuous zoom adjustment. It is desirable to use the detents so that the calibration at each magnification can be used. These calibrations only need to be updated annually. For the 7300 system, the software returns to precisely the magnification specified by the user.
In a typical measurement, the system will automatically capture the image and use the system tools to make the user-specified measurements. The user can also make various measurements interactively using, for example, length measurement tools, radius tools, angle tools, pattern tools, polygon tools, and geometry tools.
Figure 1 - Digital Optical Comparator System
Calibration
The software has the capability to calibrate distances and areas in real-world coordinates. The calibration can be performed in one dimension (1D) or in two dimensions (2D). The 1D calibration is achieved using a precision reticle as shown in Figure 2.
While the telecentric lens greatly reduces parallax distortion and other optics problems such as lens axis perpendicularity, a recently developed two-dimensional calibration technology eliminates residual errors in the optics. With this technology, it is possible to re-map the pixel grid to yield a nearly undistorted image. An example 2D calibration grid is shown in Figure3. The 2D calibration approach is the most accurate and is recommended by MPM.
Figure 2 - Example 1D Calibration using a Precision Reticle
Figure 3 - Example 2D Dot-Matrix Distance Calibration using Precision Grid
Example Measurements
An example measurement would be the verification that a series of machined notches meet the specified tolerances on notch radius, notch depth, and notch angle. As shown in Figure 4, the software can be configured to automatically acquire the image, make the measurements, and store the data in a file. The user can also interact and use the measurement tools manually. Other potential applications are shown in Figures 5 through 7.
Figure 4 - All Three Desired Measurements are Made Automatically and Stored for Reference
Figure 5a - Auto Threshold and Rotate Image 180 Unchecked
Figure 5b - Auto Threshold and Rotate Image 180 Checked
Figure 6 - Diameter Measurement Tool
Figure 7 - Radius Measurement Tool
Example System Verification Data
Precision reticles traceable to NIST can be used to verify the system accuracy precision. Example verification results for both a 1D and a 2D calibration are shown in Tables 1 and 2. MPM has determined that the small errors are primarily due to inaccuracies in the operator's trace of the length or area to be measured.
Table 1 - One-Dimensional System Verification Data
Table 2 - Two-Dimensional System Verification Data
Other Measurements
Please refer to the Percent Shear, Lateral Expansion, Notch Verification and the Brinell Hardness Measurements pages for a description of other features that are included with these systems.