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Contact and Non-contact profilometers

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Contact and Non-contact profilometers

The diamond stylus moves vertically into contact with the sample and then moves laterally across the sample for a specified distance and a specified contact force.A profiler can measure small surface changes in vertical stylus displacement as a function of position.A typical profiler can measure small vertical features between 10nm and 1mm in height.The height position of the diamond stylus produces an analog signal which is converted into a digital signal, stored, analyzed and displayed.The radius of the diamond stylus ranges from 20 nm to 50 µm, and the horizontal resolution is controlled by the scanning speed and the sampling rate of the data signal.Stylus tracking forces can range from less than 1 to 50 milligrams.

Advantages of contact profilometers include acceptance, surface independence, resolution, and it is a straightforward technique without modeling.Most of the world's surface finish standards are written for contact profilers.A profiler of this type is often required in order to follow a prescribed method.Touching the surface is often an advantage in dirty environments, where non-contact methods may end up measuring surface contamination rather than the surface itself.Because the stylus is in contact with the surface, this method is not sensitive to surface reflectance or color.The stylus tip radius can be as small as 20nm, significantly better than white light optical profiles. Vertical resolution is also typically sub-nanometer.

Non-contact profilometersprofilometers

Optical profilers are a non-contact method that provide much of the same information as stylus-based profilers.Many different techniques are currently being used, such as laser triangulation (triangulation sensors),confocal microscopy (for profiling of very small objects), coherent scanning interferometry and digital holography.The advantages of optical profilers are speed, reliability and spot size.For the small steps and requirements of 3D scanning, since the non-contact profiler does not touch the surface, the scanning speed is determined by the light reflected from the surface and the speed of the acquisition electronics.For large steps, a 3D scan on an optical profiler can be much slower than a 2D scan on a stylus profiler. Optical profilers do not touch the surface and therefore cannot be damaged by surface abrasion or by a careless operator.Many non-contact profilers are solid state, which tends to significantly reduce the maintenance required. Optical methods with spot sizes or lateral resolutions ranging from a few microns to sub-microns.

Time-resolved profilometers:

Non-scanning technologies such as digital holographic microscopy enable real-time 3D topography measurements.The 3D terrain is measured with a single camera acquisition, so the acquisition rate is only limited by the camera acquisition rate, some systems measure the terrain at a frame rate of 1000 fps.Time-resolved systems can measure topographical changes, such as the healing of smart materials or the measurement of moving specimens.A time-resolved profiler can be used in conjunction with a stroboscope to measure MEMS vibrations in the MHz range.The strobe unit provides an excitation signal for the MEMS and a trigger signal for the light source and the camera.

The advantage of time-resolved profilers is that they are resistant to vibrations.Unlike scanning methods, time-resolved profilometer acquisition times are in the millisecond range.No Vertical Calibration Required:Vertical measurement does not depend on the scanning mechanism, digital holographic microscope vertical measurement has an inherent vertical calibration based on the wavelength of the laser source.The sample is not static, and the sample topography responds to external stimuli.With flight measurements, the topography of a moving sample can be acquired with short exposure times. MEMS vibration measurements can be done when the system is combined with a strobe unit.

Fiber-based optical profilometers:

Fiber-optic-based optical profilers scan a surface using an optical probe that sends an optical interference signal back to the profiler detector through an optical fiber.Fiber-based probes can be physically located hundreds of meters away from the detector housing without degrading the signal.Other advantages of using a fiber-based optical profiler include flexibility, long profile acquisition, robustness, and ease of integration into industrial processes.Due to the small diameter of some probes, surfaces can be scanned even in hard-to-reach spaces, such as narrow cracks or small-diameter pipes.Since these probes typically acquire one point at a time and at high sampling rates, long (continuous) surface profiles can be acquired.Scanning can be performed in harsh environments, including very hot or cryogenic temperatures or in radioactive chambers while detectors are located remotely, in a human-safe environment.Fiber-optic-based probes are easily installed in-process,e.g. above a moving net or on various positioning systems.

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