Profilometer

Profilometer

[proh-fuh-lom-i-ter]

Profilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness. Vertical resolution is usually in the nanometre level, though lateral resolution is usually poorer.

While the historical notion of a profilometer was a device similar to a phonograph that measures a surface as the surface is moved relative to the contact profilometer's stylus, this notion is changing along with the emergence of numerous non-contact profilometery techniques.

Contact profilometers

A diamond stylus is moved vertically in contact with a sample and then moved laterally across the sample for a specified distance and specified contact force. A profilometer can measure small surface variations in vertical stylus displacement as a function of position. A typical profilometer can measure small vertical features ranging in height from 10 nanometres to 1 millimetre. The height position of the diamond stylus generates an analog signal which is converted into a digital signal stored, analyzed and displayed. The radius of diamond stylus ranges from 20 nanometres to 25 μm, and the horizontal resolution is controlled by the scan speed and data signal sampling rate. The stylus tracking force can range from less than 1 to 50 milligrams.

Advantages of contact profilometers:

  • Acceptance: Most of the world's surface finish standards are written for contact profilometers. To follow the prescribed methodology, this type of Profilometer is often required.
  • Surface Independence: Because the stylus is in contact with the surface, this method is not sensitive to surface reflectance or color. Also, contacting the surface is often an advantage in dirty environments where non-contact methods can end up measuring surface contaminants instead of the surface itself.
  • Resolution: The stylus tip radius can be as small as 20 nanometres, significantly better than white-light optical profiling.
  • Direct Technique: No modeling required.

Non-contact profilometers

An optical profilometer is a non-contact method for providing much of the same information as a stylus based profilometer. There are many different techniques which are currently being employed, such as laser triangulation (triangulation sensor), confocal microscopy and digital holography.
Advantages of optical profilometers

  • Speed: Because the non-contact profilometer does not touch the surface the scan speeds are dictated by the light reflected from the surface and the speed of the acquisition electronics.
  • Reliability: optical profilometers do not touch the surface and therefore cannot be damaged by surface wear or careless operators. Many non-contact Profilometers are solid-state which tends to reduce the required maintenance significantly.
  • Spot size: The spot size, or lateral resolution, of optical methods ranges from a few micrometres down to sub micrometre.

Types of profilometry:

Optical Methods:

Contact/Pseudo-Contact Methods:

Road pavement profilometery

One special application is road pavement profilometers (a k a profilographs, as used in the famous 1958-1960 AASTHO Road Test). Modern road profilometers are of non-contact type, most of them use laser triangulation in combination with an odometer and an inertial unit that establishes a large reference plane to which the laser readings are related. The inertial compensation makes the profile data more or less independent of what speed the profilometer vehicle had during the measurements. These high speed instruments are of great use for road management with road safety, environment, economy and ride quality issues in focus. Road profilometers measure longitudinal features, typically unevenness, roughness and/or texture (roads). Road surface texture is divided into megatexture (roads), macrotexture and microtexture. Due to its small scale, microtexture is not profiled despite its importance for dry friction / road safety. Many road profilometers also include a "rut bar", making it possible to also measure cross section features such as rutting, cross slope (crossfall), ridges and more. In addition, many Profilometers measure road alignment, in terms of horizontal curvature, hilliness (longitudinal gradient), drainage gradient et c.

References

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