Meaning
Electromagnetic wave properties describing the correlation between the phases of a light wave at different points transverse to the direction of propagation determine the wave’s ability to produce stable interference patterns. High spatial coherence is achieved when the phase relationship across the wavefront is uniform, which typically requires a point light source or a laser with a well-defined spatial mode. In industrial imaging, this property allows the generation of sharp phase-contrast profiles that outline thin boundaries and internal cracks in low-density materials.
The effective coherence area diminishes as the angular size of the source increases, which limits the use of extended thermal or X-ray sources in phase-sensitive measurements.
Imaging Application
High-contrast interferometry relies on this property to achieve sub-nanometer resolution of surface features on silicon wafers and optical components. The instrument employs spatial filtering, using pinholes or narrow slits, to select a coherent portion of the beam and illuminate the target. This filtration process necessarily reduces the intensity of the light, requiring a trade-off between the contrast of the interference pattern and the exposure time of the detector.
Industrial Limit
Extended X-ray tube sources require long drift paths to establish the required coherence before reaching the sample.
System Measurement
Engineers verify the phase stability by measuring the visibility of diffraction fringes on a calibration grid.
