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Reflected light or backlight?
Why illumination defines what you measure
Why illumination defines what you measure
Following up on our recent blog posts, comments came up that were absolutely valid. One aspect was still missing. Other reflected light scenarios.
At this point, we do not want to provide a classic overview of illumination concepts. There are plenty of those available ... Trainings, papers, comparison charts. All easy to find.
The setup: Same hardware, same reference
For this comparison, we set up and calibrated a system using a 20 MP camera with a 50 mm lens. Using a calibration target with 150 nm accuracy, we were able to verify that the system achieves a measurement capability of ±12.5 µm.
With this setup, we then measured a shim, a part we had already analyzed before. These components are actually great for measurement experiments, even for thickness measurements. Just a small tip on the side :-)
The result: The edge makes the difference
What becomes immediately visible. The shim does not have a perfectly sharp edge. It is slightly rounded.
And this is the critical point.
In reflected light, this rounded edge becomes dark earlier than the silhouette ends in backlight. In other words. In reflected light, the object appears shorter than it actually is.
And this is exactly what the measurement results show.
The numbers: Very clear
The true diameter of the object is 15.475 mm.
Measured with backlight: 15.4761 mm
Measured with reflected light: 15.4271 mm
That is a deviation of 49 µm, measured too small in reflected light.
Not because of software.
Not because of calibration.
But purely due to illumination physics.
