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Measurement Principle

WSI/PSI Measurement Principle

Even though WSI and PSI are different measurement principle, they use the same optic and measurement system, except using multi-wavelength or solid color wavelength.

The common characteristic of two measurement method is using interference signal. Interference is an optical device that divides a beam of light exiting a single source (like a laser) into two beams and then recombines them to create bright and dark parts. It is called interference signal.

For example, if you throw two stones into a quit lake, there will be two waves in the same pond, waves from one interfere with waves from another. Where the crests of one meet the troughs of another, the waves disappear. They disappear completely if the two disturbances are of equal intensity and they stay in the same place if the waves both have the same wavelength. In this case, interference pattern will be generated. (See the Figure 1.)

With the principle mentioned above, WSI/PSI can be applied as measurement methods (See the Figure 2.)

Interference is circular shape on circle and linear shape on plane surface. Interference lines represent same height position like contour lines.

Please refer below pictures and explanaion of WSI/PSI to understand the detail measurement principles.

WSI (White-Light Scanning Interferometry) Principle

Characteristic of WSI

– Scanning continuously and measuring the scan range.
– Possible to measure height in scan range.

PSI (Phase Shifting Interferometry) Principle

Characteristic of PSI

– measuring 4 times (step movement) as ¼ of Centered wave (about 600nm)
– Regardless Scan range, it possible to measure about under 250nm.


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