Chromatic-confocal distance measurement
The chromatic confocal distance measurement uses the dispersion of white light into a focus lens by the distance of a reflecting surface to determine the sensor. So it makes use of the chromatic aberration .
principle
A white point light source, realized by a very small pinhole aperture of only a few micrometers in diameter, is focused on the object with a non-color-corrected - i.e. dispersive - lens. Due to the dispersion, the blue light components are focused closer to the lens and the red light further away. The reflected light is imaged again via the same lens and decoupled from the illuminating beam path with a beam splitter and imaged onto a structurally identical pinhole as on the illumination side. In accordance with the same principle as with the confocal microscope , the light that does not come from the object plane is blocked out. A spectrometer behind the pinhole then determines the dominant wavelength of the reflected light. From the knowledge of the focal lengths of the individual wavelengths, the object distance can be determined directly from the dominant wavelength.
Features of the technical implementation
The advantage of the method over confocal microscopy is the lack of moving components. In contrast to confocal microscopy, the depth measuring range can also be set largely independently of the working distance.
The use of broadband white light of high intensity and uniform spectral distribution is disadvantageous. So far (2010) this has excluded the use of inexpensive, energy-saving and small LEDs as a light source. Because only light of one wavelength reaches the sensor, the signal intensity is very low compared to a confocal microscope . The signal must therefore be evaluated with correspondingly complex technology.