Sensors & Transducers


Con-focal chromatic displacement sensors

March 2009 Sensors & Transducers

Thickness measurement and cavity inspection with optoNCDT 240x series.

The measurement principle

Polychromatic white light is focused onto the target surface by a multilens optical system. The lenses are arranged so that the white light is dispersed into a monochromatic light by controlled chromatic aberration. A specific distance to the target is assigned to each wavelength by a factory calibration. Only the wavelength which is exactly focused on the target is used for the measurement. This light reflected from the target surface is passed through a con-focal aperture onto a spectrometer which detects and processes the spectral changes.

System configuration

The system consists of an LED-based controller a fibre-optic cable and one of the sensor heads of the series 2400/2401, or the world first miniature sensor series – 2402; the fibre-optic cable, which can be of almost any length, connects the two components. This system has no moving parts and is therefore wear free and suitable for use in ATEX/EX environments.

Performance and special features

This unique measuring principle enables displacements and distances to be measured with high precision and extreme spatial resolution. With transparent materials a one-sided thickness measurement can be accomplished along with the distance measurement. Since the emitter and receiver are arranged in one axis, shadowing is avoided. In contrast to conventional triangulation sensors the optoNCDT 2401 system is able to measure in narrow apertures, small gaps and cavities.

The measuring range for these sensors extends from 120 μm to 24 mm with a measuring spot size starting at just 7 μm. The miniaturised version in this range have a sensor diameter of only 4 mm and are ideal for inspecting small cavities and inner wall grooves, or checking the fill level in medical trays.



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