Test & measurement 2. RESOLUTION AND SPOT SIZE
The confocal principle operates using a very small, constant spot size (typically 3-25 micron) through the measuring range, resulting in very stable, nanometre resolution measurements. On polished or highly reflective surfaces and even transparent materials, confocal sensors offer greater stability than on dark, diffuse materials.
3. THERMAL STABILITY
In terms of thermal stability, confocal sensors are more stable than laser triangulation or eddy current sensors. This is due to the design of the sensors, which typically comprises a cylindrical tube with a series of optical lenses. The sensor is considered “passive”, as the controller and electronics are housed separately and so can be located further away from the target object, which normally means they can be mounted in a more controlled temperature environment.
4. TYPE OF MATERIAL OR SURFACE
The confocal principle is considered surface-independent, enabling measurements on any type of surface, both diffuse and specular – from dark, diffuse materials to highly reflective, shiny, mirrored or even transparent surfaces.
With translucent or transparent materials such as glass, a one-sided thickness measurement can be achieved using a single confocal sensor, along with the distance measurement. Also, because the emitter and receiver on a confocal sensor are arranged in one axis (i.e. the sensor measures vertically down and the beam is reflected back vertically up from the target), shadowing is avoided.
If you need to measure inside restricted spaces such as drilled holes, bored holes, cavities or recesses, miniature radial and axial confocal versions are now available. Some confocal sensor suppliers can offer miniature versions with diameters of just 4mm, allowing the sensor to be inserted into very tight or narrow gaps and cavities. Some confocal sensor suppliers offer 90-degree sensor versions that can measure the finest of interior contours.
Continued on page 32... Instrumentation Monthly August 2024 31
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