Solartron Linear Encoders operate on the principle of interference between two diffraction gratings. Solartron uses precision Ronchi type gratings with a 10µm period deposited on a low expansion quartz substrate.
Collimated light from a light emitting diode (LED) is used to illuminate an amplitude diffraction grating thus producing spatially modulated light intensity at the output. At a specific distance behind the modulating grating, a second diffraction grating with similar diffraction properties (the scale) is used to scan this intensity modulation as it is moved. A photodiode positioned behind the two gratings converts the high contrast optical fringes into a current, which is directly related to scale displacement.
The periodic nature of the encoder signals means that the scale displacement can only be determined unambiguously over a short distance corresponding to the scale period. By using a four field scanning grating, the direction of displacement of the scale is determined. Also, the incorporation of a reference mark overcomes the displacement ambiguity after system power down.
The low noise nature of the encoder's output signal allows electronic interpolation to be used to achieve resolutions, typically 0.05 µm or less.
Over relatively long measuring ranges, this technology is extremely accurate though not as rugged as the inductive technology.