A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller. The encoder can be either incremental or absolute. In an incremental system, position is determined by motion over time; in contrast, in an absolute system, motion is determined by position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current. Optical technologies include shadow, self imaging and interferometric. Linear encoders are used in metrology instruments, motion systems, inkjet printers and high precision machining tools ranging from digital calipers and coordinate measuring machines to stages, CNC mills, manufacturing gantry tables and semiconductor steppers. Physical principle Linear encoders are transducers that exploit many different physical properties in order to encode position: Scale/reference based Optical Optical linear encoders dominate the high resolution market and may employ shuttering/moiré, diffraction or holographic principles. Optical encoders are the most accurate of the standard styles of encoders, and the most commonly used in industrial automation applications. When specifying an optical encoder, it's important that the encoder have extra protection built in to prevent contamination from dust, vibration and other conditions common to industrial environments. Typical incremental scale periods vary from hundreds of micrometers down to sub-micrometer. Interpolation can provide resolutions as fine as a nanometer. Light sources used include infrared LEDs, visible LEDs, miniature light-bulbs and laser diodes. Magnetic Magnetic linear encoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils, Hall effect or magnetoresistive readheads. With coarser scale periods than optical encoders (typically a few hundred micrometers to several millimeters) resolutions in the order of a micrometer are the norm. Capacitive Capacitive linear encoders work by sensing the capacitance between a reader and scale. Typical applications are digital calipers. One of the disadvantages is the sensitivity to uneven dirt, which can locally change the relative permittivity. Inductive Inductive technology is robust to contaminants, allowing calipers and other measurement tools that are coolant-proof. A well-known application of the inductive measuring principle is the Inductosyn. Eddy current US Patent 3820110, "Eddy current type digital encoder and position reference", gives an example of this type of encoder, which uses a scale coded with high and low permeability, non-magnetic materials, which