A non-contact metrology system that provides portable surface profiling, imaging and thickness measurements uses high-precision OptoNCDT 2401 confocal displacement sensors from Micro-Epsilon. Hornet3D, developed by Rhombus VS, is a new metrology system that can measure down to nanometre resolutions. This flexible, modular system can be customised for specific surface-profiling applications. The scalable system can be specified with a range of different sensors and cameras, including Micro-Epsilon's OptoNCDT 2401 confocal displacement measurement sensors.Hornet3D comprises an X-Y scanner and a stage platform with a modular Z-axis fixture and an integrated sensor. If required, the sensor or microscope can be removed and replaced with an alternative unit, providing flexibility for the user in terms of measurement range and resolution. The stage platform can be made larger or smaller depending on customer requirements. The Hornet offers resolutions in the X-Y axis down to 0.1um. Micro-Epsilon's OptoNCDT 2401 confocal sensor provides a Z-axis resolution down to 10nm and a sampling rate of 0.1kHz to 20kHz depending on the scan parameters.
The system also includes Rhombus's integrated Surescan software, which enables the user to define scan regions and resolution scale. The user can create 2D false colour maps, full 3D animated images, cross-section analyses for sample profiling and measurements and basic report generation. Data can also be exported to third-party software packages and reports generated. According to Graeme Elliner, technical director at Rhombus VS, the OptoNCDT 2401 sensors are suitable for the Hornet system because they are designed for harsh industrial environments. The OptoNCDT 2401 confocal chromatic sensor system comprises a light-emitting-diode-based controller and a sensor, connected by a fibre-optic cable, which can be up to 50m in length.
The system has no moving parts and is therefore wear free. There are three choices of sensor designs available: the 2400, the 2401 and the 2403 series, which together cover more than 24 different measurement ranges as standard. A very small confocal sensor, the 2402 series, has a diameter of 4mm and typical lengths of 40-50mm. ATEX-certified versions are also available. The OptoNCDT 2401 is a compact sensor with a large stand-off distance that is able to perform the one-sided thickness measurement of transparent materials such as glass and plastics, as well as multi-layer surfaces including substrates and laminates.
The sensor offers a high spatial resolution for microscopic surface profiling. The spot-size diameter is down to 7um. The resolution on the 3mm measuring range sensor is 0.12um. The confocal measurement principle uses polychromatic white light, which is focused onto the target surface using a multi-lens 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 factory calibration.
Only the wavelength that is exactly focused on the target is used for the measurement. This light reflected from the target surface is passed through a confocal aperture onto a spectrometer, which detects and processes the spectral changes. Extremely high resolutions are said to be possible when using the confocal principle. Resolution in the nanometre range is possible by expanding the colour spectrum. As colour in the focal point is used for distance information, confocal sensors have a very small measuring spot, which also enables measurements on tiny objects.
Even small scratches on a surface or substrate can be detected. The beam path of the sensor is compact and concentric, enabling the system to measure inside drilled holes, which other optical measurement methods (such as laser triangulation) find difficult to achieve due to the formation of shadows. Elliner said: 'Confocal displacement sensors are an excellent fit with our Hornet system as they are modular in their design.
'There is one standard controller and interface; only the sensor head needs to be changed. 'Long cable runs are also possible, so we can solve most application issues. 'We've used the confocal sensor in a wide range of applications to date, including the surface profiling of tracks in printed circuit boards and measuring the thickness of thin-film solar panels, glass substrates, laser micro-machining, inkjet printing and even wear on hip joints,' he added.
