Confocal sensor systems for displacement, distance, position and thickness
Confocal chromatic sensor systems from Micro-Epsilon Optronic measure small distances and thicknesses according to the principle of chromatic aberration. The sensors measure fine structures because they project a very small spot onto the target. Confocal chromatic measuring systems can be used for distance measurement and thickness measurement of transparent materials. These sensors are being used in the semiconductor industry, glass industry, medical engineering, plastics production etc.
The principle of chromatic aberration
Confocal chromatic measuring devices use the imaging error i.e. »chromatic aberration« (also longitudinal color aberration) of glass lenses for optical distance measurements. A white beam of light is sent from a light source through a special lens arrangement. This is the actual sensor that presents a particularly strong longitudinal color aberration. Therefore this light source does not focus the white light in a single white focal point, but on a rainbow-colored line in the lens axis. Near to the lens is the blue focal point, further away are the green, yellow, red, etc. focal points.
When light hits an object and is reflected, the color of the reflected light can be attributed to the distance between object and sensor, so called color-coded distance measurements.
The reflected light is transmitted along the same path back to the controller where an opto-electrical device, a spectrometer, analyzes its color. This is where the light is being spectrally enhanced and displayed on a line receiver, just like a prism.
The relationship between a color and the distance between measurement object and lens has been configured in the controller for each sensor. This distance is then issued as measurement value.
Thickness measurement with transparent materials
If the object to be measured is transparent, the light is reflected by the surface and by the surface underneath. The spectrometer separately detects the two reflected colors. If the refractive index of the measured object (e.g. glass) is known, the distance between the two surfaces can be calculated and output.