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5.
MEASUREMENT STRATEGY
MVR systems can be operated with any of 6 telecentric fixed-focus lenses with magnification from
0.30X to 4.0X or with 6.5:1 zoom optics with continuously adjustable magnification from 0.7X to
4.5X. Magnification is the image size at the camera CCD detector place divided by the object size.
Since the CCD size is fixed (namely 9.93 x 8.70 mm for the 2 Mpixel camera), 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 the smaller the field of view.
Note that the centering of the optics can vary by a small amount in the quick-change bayonet
optics mount, so all measurements on the same part need to be taken with the same optics.
5.1
Telecentric Lens Measurement Strategy
Select telecentric optics to perform high accuracy, high throughput field-of-view (FOV) measure-
ments. If all measurements are to be in the FOV, select the highest magnification lens whose
FOV encompasses the entire part. The 0.30X lens accommodates parts up to 51 x 38 mm (2.00
”
x 1.50
”). If the entire part cannot fit into the FOV, no problem. Simply move the stage by up to 200
mm (8
”) for the MVR200 or 300 mm (12") for the MVR300, and the M3 software will seamlessly
integrate FOV measurements with encoder readings from stage motion.
5.2
Zoom Optics Measurement 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,
they do not offer the same low optical distortion that is required for accurate FOV measurements
across the entire FOV. However, they are equally as accurate as telecentric lenses when used at
high magnification in combination with stage motion.
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 zoom magnification setting
accommodates parts up to 11.2 x 9.4 mm (0.44" x 0.37") in the FOV. To measure large parts,
locate the edge of interest at minimum magnification, then take the actual measurement at maxi-
mum magnification using the system's crosshairs. The
zoom optics’ parcentricity feature will
ensure that a feature will remain at the optical center of the video image throughout the
magnification range.
5.3
Illumination Strategy
Once the image has been properly focused and magnification has been set, adjust light levels
as necessary using the slider controls in M3 software. The right lighting is paramount to accurate
measurement with any video-based measurement system. Lighting that is too low will result in a
dark, low-contrast image with indiscernible features. Lighting that is too bright may result in a
washed-out image and blooming, or oversaturated bright regions that distort features.
When adjusting lighting, start with light that is lower than desired, then increase lighting while
viewing the image on the monitor. Maintain constant lighting for consistent results. Always
use the same light level while sampling points for a single feature
– do not to change light levels
during a measurement run.
Depending on the part characteristics and the feature being measured, the right combination
of lighting may aid in bringing out a particular feature. Take time to experiment by balancing the
available light sources.
5.4
Magnification Strategy
In general, higher magnification provides greater resolution and accuracy; however, not all
features should be inspected at the highest available magnification. Too high a magnification
may make it difficult to discern edges by exaggerating edge defects such as burrs or chips.
Try decreasing the magnification until the edge is more clearly identifiable.