White light scanner
Encyclopedia
A White Light Scanner is a device for measuring the physical geometrical characteristics of an object using White light interferometry. Frequency domain
analysis (FDA) is an alternate approach that uses all of the information available in the interferogram. This Fourier analysis
method is used to convert intensity data to the spatial frequency
domain, allowing production of an extremely accurate surface map.
White-light interferometry scanning (WLS) systems capture intensity data at a series of positions along the vertical axis
, determining where the surface is located by using the shape of the white-light interferogram, the localized phase of the interferogram, or a combination of both shape and phase. The white light interferogram actually consists of the superposition of fringes generated by multiple wavelengths, obtaining peak fringe contrast as a function of scan position, that is, the red portion of the object beam
interferes with the red portion of the reference beam
, the blue interferes with the blue, and so forth. In other words, a prodigious amount of data is available in white-light interferograms.
In a WLS system, an imaging interferometer is vertically scanned to vary the optical path difference
. During this process, a series of interference patterns are formed at each pixel in the instrument field of view
. This results in an interference function, with interference varying as a function of optical path difference. The data are stored digitally and processed in a variety of ways depending on the system manufacturer, including being Fourier-transformed into frequency space, subject to cross-correlation methods, or analysis in the spatial domain.
If a Fourier transform is used, the original intensity data are expressed in terms of interference phase as a function of wavenumber. Wavenumber k is just a representation of wavelength in the spatial frequency domain, defined by k = 2π/λ. If phase is plotted versus wavenumber, the slope of the function corresponds to the relative change in group-velocity optical path difference DG by Dh = DG/2nG where nG is group-velocity index of refraction
. If this calculation is performed for each pixel, a three-dimensional surface height map emerges from the data.
In the actual measuring process, the optical path difference is steadily increased by scanning the objective vertically using a precision mechanical stage or piezoelectric positioner. Interference data are captured at each step in the scan. In effect, an interferogram is captured as a function of vertical position for each pixel in the detector array. To sift through the large amount of data acquired over long scans, many different techniques can be employed. Most methods allow the instrument to reject raw data
that do not exhibit sufficient signal-to-noise. The intensity data as a function of the optical path difference are processed and converted to height information of the sample.
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
analysis (FDA) is an alternate approach that uses all of the information available in the interferogram. This Fourier analysis
Harmonic analysis
Harmonic analysis is the branch of mathematics that studies the representation of functions or signals as the superposition of basic waves. It investigates and generalizes the notions of Fourier series and Fourier transforms...
method is used to convert intensity data to the spatial frequency
Spatial frequency
In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance. The SI unit of spatial frequency is...
domain, allowing production of an extremely accurate surface map.
Description
White light interferometry, also called coherence scanning interferometry by the International Standards Organization (ISO), is certainly not new. In fact the development of scanning white-light interferometry is in many ways a back-to-basics scenario. As interferometry progressed from using white light to monochromatic light to lasers to computerized fringe analysis to phase shifting techniques and other Structured-light 3D scanners, the path has actually led right back to white light for some applications.White-light interferometry scanning (WLS) systems capture intensity data at a series of positions along the vertical axis
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
, determining where the surface is located by using the shape of the white-light interferogram, the localized phase of the interferogram, or a combination of both shape and phase. The white light interferogram actually consists of the superposition of fringes generated by multiple wavelengths, obtaining peak fringe contrast as a function of scan position, that is, the red portion of the object beam
Signal beam
A signal beam or object beam is one of at least two laser beams used to write holograms. The signal beam is the beam that carries the information to be stored in the hologram. In the case of a holographic picture, this beam is reflected off the object being recorded, into the media...
interferes with the red portion of the reference beam
Reference beam
A reference beam is a laser beam used to read and write holograms. It is one of two laser beams used to create a hologram. In order to read a hologram out, some aspects of the reference beam must be reproduced exactly as when it was used to write the hologram...
, the blue interferes with the blue, and so forth. In other words, a prodigious amount of data is available in white-light interferograms.
In a WLS system, an imaging interferometer is vertically scanned to vary the optical path difference
Optical path length
In optics, optical path length or optical distance is the product of the geometric length of the path light follows through the system, and the index of refraction of the medium through which it propagates. A difference in optical path length between two paths is often called the optical path...
. During this process, a series of interference patterns are formed at each pixel in the instrument field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
. This results in an interference function, with interference varying as a function of optical path difference. The data are stored digitally and processed in a variety of ways depending on the system manufacturer, including being Fourier-transformed into frequency space, subject to cross-correlation methods, or analysis in the spatial domain.
If a Fourier transform is used, the original intensity data are expressed in terms of interference phase as a function of wavenumber. Wavenumber k is just a representation of wavelength in the spatial frequency domain, defined by k = 2π/λ. If phase is plotted versus wavenumber, the slope of the function corresponds to the relative change in group-velocity optical path difference DG by Dh = DG/2nG where nG is group-velocity index of refraction
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
. If this calculation is performed for each pixel, a three-dimensional surface height map emerges from the data.
In the actual measuring process, the optical path difference is steadily increased by scanning the objective vertically using a precision mechanical stage or piezoelectric positioner. Interference data are captured at each step in the scan. In effect, an interferogram is captured as a function of vertical position for each pixel in the detector array. To sift through the large amount of data acquired over long scans, many different techniques can be employed. Most methods allow the instrument to reject raw data
Raw data
'\putang inaIn computing, it may have the following attributes: possibly containing errors, not validated; in sfferent formats; uncoded or unformatted; and suspect, requiring confirmation or citation. For example, a data input sheet might contain dates as raw data in many forms: "31st January...
that do not exhibit sufficient signal-to-noise. The intensity data as a function of the optical path difference are processed and converted to height information of the sample.
Manufacturers
- Phase Vision http://www.phasevision.com
- Veeco Instruments http://www.veeco.com
- Zygo http://www.zygo.com/