Non-contact methods of measuring very minute distances and gaps has proven itself indispensable instead of contact methods. While other non-contact methods exist, such as electric and magnetic types, optical techniques (particularly those using laser beams) have made it possible to use the properties of light to achieve high precision measurement. However, measuring the absolute value of a distance or gap to the regular surface of the object measured is problematic with optical measurement (apart from cases where this is known to be within a half wavelength of the light), and this is used only for high precision measurement of fluctuation (displacement) over time of the absolute distance. However, because of optical measurement's high vertical resolution (that is, resolution in the direction of the measurement beam), high horizontal resolution (in the direction perpendicular to the measuring beam), and its lack of susceptibility to the effects of electromagnetic induction, in addition to the advantage of non-contact, such techniques have proven themselves to be the best in recent years for sub-micron high mounting and application property evaluation of parts and systems that have undergone high density lightening and miniaturization.
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