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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 with the emergence of numerous non-contact profilometery techniques.
- Digital holographic microscopy
- Vertical scanning interferometry / White light interferometry
- Phase shifting interferometry
- Differential interference contrast microscopy (Nomarski microscopy)
- Focus detection methods
- Pattern projection methods
- Stylus profilometer (mechanical profilometer)
- Atomic Force Microscopy
- Scanning Tunneling Microscopy
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 50 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: 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. Because the stylus is in contact with the surface, this method is not sensitive to surface reflectance or color.
- Resolution: The stylus tip radius can be as small as 20 nanometres, significantly better than white-light optical profiling. Vertical resolution is typically sub-nanometer as well.
- Direct Technique: No modeling required.
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 (used for profiling of very small objects), low coherence interferometry and digital holography.
Advantages of optical profilometers
- Speed: For small steps and requirements to do 3D scanning, 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. For doing large steps, a 3D scan on an optical profiler can be much slower than a 2D scan on a stylus profiler.
- 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.
Fiber-based optical profilometers
Fiber-based optical profilometers scan surfaces with optical probes which send light interference signals back to the profilometer detector via an optical fiber. Fiber-based probes can be physically located hundreds of meters away from the detector enclosure, without signal degradation. Here are the additional advantages of using fiber-based optical profilometers:
- Flexibility: Thanks to the small diameter of certain probes, surfaces can be scanned even inside hard-to-reach spaces, such as narrow crevices or small-diameter tubes.
- Long profile acquisition: Since these probes generally acquire one point at a time and at very high sample speeds, acquisition of long (continuous) surface profiles is possible.
- Ruggedness: Scanning can take place in hostile environments, including very hot or cryogenic temperatures, or in radioactive chambers, while the detector is located at a distance, in a human-safe environment.
- Ease of incorporating into industrial processes: fiber-based probes are easily installed in-process, such as above moving webs or mounted onto a variety of positioning systems.
Road pavement profilometery
Road pavement profilometers (aka profilographs, as used in the famous 1958-1960 AASHO Road Test) use a distance measuring laser (suspended approximately 30 cm from the pavement) in combination with an odometer and an inertial unit (normally an accelerometer to detect vehicle movement in the vertical plane) that establishes a moving reference plane to which the laser distances are integrated. The inertial compensation makes the profile data more or less independent of what speed the profilometer vehicle had during the measurements, with the assumption that the vehicle does not make large speed variations and the speed is kept above 25 km/h or 15 mph. The profilometer system collects data at normal highway speeds, sampling the surface elevations at intervals of 2–15 cm (1–6 in), and requires a high speed data acquisition system capable of obtaining measurements in the kilohertz range.
The data collected by a profilometer is used to calculate the International Roughness Index (IRI) which is expressed in units of inches/mile or mm/m. IRI values range from 0 (equivalent to driving on a plate of glass) upwards to several hundred in/mi (a very rough road). The IRI value is used for road management to monitor road safety and quality issues.
Many road profilers also measure the pavement's cross slope, curvature, longitudinal gradient and rutting. Some profilers take digital photos or videos while profiling the road. Most profilers also record the position, using GPS technology. Another quite common measurement option is cracks. Some profilometer systems include a ground penetrating radar, used to record asphalt layer thickness.
Another type of profilometer is for measuring the surface texture of a road and how it relates to the coefficient of friction and thus to skid resistance. Pavement texture is divided into three categories: megatexture (roads), macrotexture, and microtexture. Microtexture cannot currently be measured directly, except in a laboratory. Megatexture is measured using a similar profiling method as when obtaining IRI values, while macrotexture is the measurement of the individual variations of the road within a small interval of a few centimeters. For example, a road which has gravel spread on top followed by an asphalt seal coat will have a high macrotexture, and a road built with concrete slabs will have low macrotexture. For this reason, concrete is often grooved or roughed up immediately after it is laid on the road bed to increase the friction between the tire and road.
Equipment to measure macrotexure currently consists of a distance measuring laser with an extremely small spot size (< 1 mm) and data acquisition systems capable of recording elevations spaced at a mm or less apart. The sample rate is generally over 32 kHz. Macrotexture data can be used to calculate the speed-depending part of the friction number between typical car tires and the road surface. The macrotexture also give information on the difference between dry and wet road friction. However, macrotexture cannot be used to calculate a relevant friction number, since also microtexture affects the friction.
Lateral friction and cross slope are the key reaction forces acting to keep a cornering vehicle in steady lateral position, while exposed to exciting forces from speed and curvature. Since friction is strongly dependent on macrotexture and texture, cross slope as well as curvature can be measured with a road profiler, so road profilers are very useful to identify improperly banked curves that may pose a risk to motor vehicles.
- Jean M. Bennett, Lars Mattsson, Introduction to Surface Roughness and Scattering, Optical Society of America, Washington, D.C.
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