Every form of light is composed of many different wavelengths (a colour spectrum). White light overlaps all visible wavelengths. The visible range for humans starts at 400nm (blue) and ends at 700nm (red). Not all the various wavelengths can be bundled into exactly one point by lenses. This is called the chromatic lens error or the chromatic lens aberration. This can be compared with the depth of field when using microscopes or cameras. It is precisely this effect that is used in confocal measurement technology. The blurring of the focal point of the different colours is extended using special lenses. This means that depending on the distance to the lens, there is precisely one wavelength in the focal point. Only this information is included for measurements.
In order to achieve the specific chromatic aberration, several glass lenses in the sensor are required which split the light according to the measuring range. Using converging lenses, the colour spectrums are bundled along a line before the light is discharged from the sensor, so that an exact focus line is achieved. Without converging lenses, the light from the sensor would scatter and a measurement evaluation would be impossible.
White light reaches the lens through a semi-permeable mirror. The specific chromatic aberration described above occurs there. The wavelengths are reflected from the surface of a target and return to the semi-permeable mirror via the lens. The mirror deflects the wavelengths to a perforated cover, which lets through the best focused wavelengths with the highest intensity.
Blurred spectrums strike the perforated cover as slivers and not as a focused point. The focused wavelength has sufficient intensity to produce a significant peak on the CCD array.
A spectrometer positioned after the perforated cover evaluates the received colour information. This contains an optical grid, which achieves a more or less strong deflection of the wavelength on a CCD array depending on the wavelength. Each position on the CCD array corresponds to a specified distance of the target from the sensor.
30,000 resolved points are obtained via the depth of field (measuring range).
Only the wavelength (λ) is evaluated to obtain the signal. A peak of the amplitude (l) is not taken into account for the signal evaluation. The strength of the intensity does not play any role.
This means that irrespective of how much light is reflected from an object, distance information can invariably be obtained, since each focused reflection results in a more or less high peak, provided that the reflected light is stronger than background emissions. Using the confocal measuring principle, highly reflective materials, including black rubber or transparent materials such as glass or liquids can be measured reliably.
New technology, new possibilities
Extremely high resolutions are possible when using confocal chromatic measurement technology. Resolution in the nanometre range is possible by expanding the colour spectrum. As colour in the focal point is used for the distance information, confocal sensors have an extremely small measuring spot, which also makes measurements on very small objects possible. Therefore, even the finest surface scratches can be detected reliably.
The beam path of the sensor is compact and concentric. This enables the system, for example, to measure inside drilled holes, which other optical methods, such as the triangulation technique, find difficult or even impossible to achieve due to the formation of shadows. For measurements of this nature, Micro-Epsilon’s optoNCDT2402 confocal miniature sensors, which have a sensor diameter of just 4mm, are ideally suited. Another interesting potential application is measuring the thickness of transparent films, boards or layers. In contrast to other methods, the system only requires one sensor for a measurement of this type. The reflection of the focal points on the front and rear surfaces are evaluated. Since the measurement is only performed using white light, no laser safety regulations apply. The sensors can also be used in potentially explosive environments and in systems that are susceptible to EMC. The sensor head itself is completely passive; there are no electronics inside. The controller can also be placed at a safe distance. Distances of up to 50m between controller and sensor are made possible by using a fibre optic cable.
However, it must be noted that no objects or particles are permitted in the optical beam path. These would lead to inaccurate measurements or even make measuring impossible. Due to the optical method, only certain distances between sensor and object are possible.
