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Optical methods for topography, also known as optical profilometry technique, use light to determine morphology, roughness and other details of a surface.
Achieved a number of different ways, optical profilometry methods are based on directing light at a surface in such a way that it can provide information about that surface.
Optical profilometry techniques are seen as an alternative to stylus profilometry methods, which use a physical probe to determine surface topography information. Optical methods offer several advantages over stylus-based tests. First, there is the possibility the stylus can deform the sample surface during testing, corrupting results. Second, large, dull stylus tips used to avoid deformation do not provide the most precise imagery. Third, optical measurement techniques are generally faster than stylus-based methods. Consider the following optical methods for topography.
Shape from Focus
Shape from focus is a technique based on a microscope-like system. The technique involves white light being reflected off a surface and back into a microscope objective and an optical sensor array. Using an extremely narrow depth of focus, the system varies the focal position vertically to generate a series of images with different areas of the surface are in focus. The topography of the surface can then be determined based on the shape of the surface at different focal positions.
Each location on the sample surface has an image where it is in focus and because the focal positions are known, the system can then calculate the height of each location.
Shape from focus is the first choice in man applications because it provides a color image of the surface. This image is laid over a 3-D surface topography map to provide a convenient analysis.
Chromatic Sensor
Chromatic sensor tests are based on the phenomenon known as a chromatic aberration: For any particular lens, the focal position is based on the wavelength of light.
In this method, the testing device sends light through fiber optics to the sample surface. The light is reflected back from the surface, gathered by the device and passed back into the optical fiber. The reflected light is then assessed with a spectrometer.
The focal position of the various wavelength components differs in z-directional position. This means the power of the reflected light is strongest at the wavelength where the surface is at a precise focus. The distance between the sample topography and the sensor head can then be established.
Structured-Light Imaging
This analysis involves projecting a series of several known light patterns on the test surface and recording the resulting altered reflection pattern. With the known geometry of the system, the topography of the surface can then be determined.
While this technique is only able to capture a single plane at a time, a 3-D topographical map of an object can be created by revolving the target through 360 degrees and capturing maps of its complete surface.
Laser Confocal Microscopy
This investigation uses optical light and a laser light source to scan a surface and gather high-resolution image data. The test instrument assesses the intensity of reflected laser light in relation to the z-position of the laser to find out nanometre-level topography of the surface.
Photometric Stereo
A photometric stereo test involves the capture of two or more camera images taken directly above of the sample. For each photo, the sample is illuminated from a different angle around the sample, but at the same radius and height above the sample, referred to as the slant angle. The slant angle is selected based on the roughness of the sample, with “flatter” slant angles featuring more of the surface topography, but possibly creating undesirable shadows that disrupt the test.
The inference of the surface gradients from the pictures is derived from the fact that the intensity in a point is founded on the gradient at that point, considering imaging geometry and surface reflection principles.
As soon as the surface gradients have been determined, the surface topography can be calculated.
Sources and Further Reading
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