PRECITEC CHRocodile C, Руководство по эксплуатации

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2.3 Measuring principle
2.3.1 Optical principle
For most industrial applications the chromatically coded distance detection method turned out to be
very well suited. CHRocodile C is based on this method and more precisely on the Confocal
Chromatic principle.
This principle combines the properties of confocality and axial chromatism.
Axial Chromatism:
That method takes advantage from a lens optical error commonly known as axial chromatic aberration:
the axial position of the focal point depends on the wavelength (color) of the light to be focused. For
example, in the visible spectral range, the focal distance for blue light is shorter than for red light. The
focal points of intermediate wavelengths are located in between according to a continuous axial
position variation. Thus, considering white light passing through an optical objective provided with
axial chromatic aberration, a continuum of color along the optical axis is generated, as an axial
rainbow.
Confocality:
That method also takes advantage from confocal opto-mechanical configuration. A confocal optical
system uses illumination point source and a pinhole in an optically conjugate plane in front of the
detecting system to eliminate out-of-focus signal. As only in focus light can be detected, the image's
optical lateral and axial resolution is improved. Consequently the pinhole act as a spatial filter which
block light which is out of focus or light which come from an external light source.
Confocal Chromatic Imaging:
Considering both confocality and axial chromatism properties, a white light illumination point is imaged
through the chromatic objective on a target object. Depending on the distance of the target from the
focusing chromatic objective, light of just a very narrow wavelength bandwidth is perfectly focused on
the target’s surface. All other spectral components of the light source are out of focus. In the back
path, from the target’s surface to the detector, the reflected light passes through the chromatic
objective, the optically conjugate pinhole which is in front of the spectrometer. The pinhole filters all
wavelengths except the narrow bandwidth which is in focus. The spectrometer analyses the spectrum
of the light reflected back by the target’s surface, and only a chromatic peak is observed
corresponding to the narrow wavelength bandwidth perfectly in focus. The analysis and the barycenter
calculation of this chromatic peak allow to determine the distance of the target surface from the
chromatic objective. (
Cf. Fig. 2.2 )