7 Understanding the Measurement Strategy
HDV systems are available with the following optics:
• 6.5:1 zoom optics that enable magnification to be continuously adjusted from 0.7X to 4.5X. 12:1 zoom optics with adjustable magnification from 0.4X to 4.7X.
• Telecentric, fixed-focus lenses with magnification options of 0.24X, 0.16X, 0.5X, and 0.11X.
Magnification is the image size at the camera charge coupled device (CCD) detector place divided by the object size. Since the CCD size is fixed, each magnification has a corresponding field of view (FOV), which is the CCD size divided by magnification. The higher the magnification, the higher the resolution, but also the smaller the field of view.
The following information discusses the measurement strategy for the available optics.
7.1 Telecentric Lens Magnification Strategy
Telecentric optics allow high accuracy, high throughput measurements within their field of view. For highest throughput, select a telecentric lens with a field of view that encompasses the entire part to be measured. The 0.11X lens can accommodate parts up to 76 x 64 millimeter (3.0 x 2.5 inch) in size. If the entire part cannot fit into the field of view, no problem. Simply move the stage by up to 500 millimeters (20 inches), and the M3 software will seamlessly combine FOV measurements with encoder readings from stage motion. Note that stage motion will increase the measurement time.
7.2 Zoom Optics Magnification Strategy
The 6.5:1 zoom optics allows magnification to be continuously adjusted from 0.7X to 4.5X. While the zoom optics can provide the same FOV as the four higher magnification telecentric lenses, these lenses do not offer the same low optical distortion that is required for accurate FOV measurements. Select zoom optics to measure large parts which would not fit into a single FOV, also for smaller parts where extremely high magnification is required. The lowest magnification range accommodates parts up to 0.44 x 0.37 inches (11.2 x 9.4 millimeters) in the FOV. To measure large parts, locate the edge of interest at minimum magnification, then take the actual measurement at maximum magnification using the crosshairs of the system. The zoom optics’ parcentricity feature ensures that a feature remains at the optical center of the video image throughout the magnification range.
7.3 Focus Strategy
Always work at best focus. Focus is determined by adjusting the horizontal Z distance from the lens to the workpiece. To set focus, move the stage from front to back until you find the two points on both sides of the point of best focus where the image just starts to defocus. The point of best focus will be approximately midway between these two points.
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