# Laser encoders glossary

Find definitions of popular technical laser interferometric encoders terminologies.

### Abbé error

Abbé error occurs when the encoder axis is offset from the working axis. If the stage pitches or yaws, the movement of the workpiece is different from the encoder reading. For an offset of d and a stage pitch of θ, the resulting Abbé offset error will be: Abbé error = d × sine θ.

### Accuracy

Accuracy is a qualitative term referring to whether there is agreement between a measurement made on an object and its true (target or reference) value.

### Beam splitter

A beam splitter is an optical device that splits a beam of light into two or more components. It is the key part of most interferometer system.

### Compensation

The process of modifying the position output signal of an encoder to correct for position errors.

### Constructive interference

Constructive interference occurs when two light waves of similar phase and frequency are combined to form a resultant beam of brighter intensity than either of the individual beams. (See also destructive interference).

### Destructive interference

Destructive interference occurs when two light waves of similar frequency but opposing phase combine to form a resultant beam of lower intensity than the brightest individual beam. (See also constructive interference).

### Frequency

Frequency is the measurement of the number of times that a repeated event occurs per unit time. Frequency is calculated by measuring the time between two consecutive occurrences of an event (the period) and then computing the frequency as the reciprocal of this time. The result is measured in hertz (Hz), f = 1/T.

Measuring the frequency of sound, electromagnetic waves (such as radio or light), electrical signals, or other waves, the frequency in hertz is the number of cycles of the repetitive waveform per second.  Frequency has an inverse relationship to the concept of wavelength. The frequency f is equal to the speed v of the wave divided by the wavelength λ (lambda) of the wave: f = v/λ.

### Frequency stability

A measure of the variation in the laser frequency.

### In air

Is a three dimensional space, which is filled with air in the atmosphere.

### Interference

Interference is the superposition of two or more waves resulting in a new wave pattern.  As most commonly used, the term usually refers to the interference of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. (See also constructive interference, destructive interference).

### Interpolation

Interpolation is a method of constructing new data points from a discrete set of known data points. In position control it commonly involves taking an analogue position signal and sub dividing it by a fixed factor to produce a discrete digital signal.

### Interpolator

An interpolator is an electronic device that sub divides an analogue position signal and converts it to a discrete digital count based on the interpolation factor used. For example, a 4096 interpolator used on an analogue signal with a 158 nm period will produce a result with a resolution of 38.6 picometres.

### Lissajous

The lissajous is a method of displaying SIN and COS signals. It combines the signals using triangular algebra into a circular form. With advance signal processing scheme, the mathematical properties of a precise point on the circle represents many characteristics of the interferometers' operation, such as the signal strength and the position at a point in time.

### OEM

Original Equipment Manufacturer is a company that makes a part or subsystem that is used in another company's end product.

### Phase

The phase of a wave relates the position of a feature to that same feature in another part of the waveform (or, which amounts to the same, on another waveform). The phase may be measured as a time, distance, a fraction of the wavelength, or an angle.

A term used to describe a digital serial position signal that is transmitted upon two lines and indicates position on every edge change. These edge changes occur at 90º intervals of phase, so 4 edge transitions are required to travel 360º. As a result, this is called quadrature to indicate 4 points. Often referred to as AQuadB to indicate the use of an A and B line in outputting the signal. This signal is usually RS422 AQuadB and is a standard serial protocol for encoder position feedback. Can also be used to define sine and cosine signals which are said to be in ‘phase quadrature'.

### Refractive index

Refractive index is the ratio of the velocity of light in air to the velocity of light in a refractive medium for a given wavelength. It represents the propagation property of the air, correlated to the conditions such as air temperature, relative humidity and pressure.

### Resolution

The resolution of an encoder is the smallest distance increment it can detect and transmit to a suitable receiver.

### Sub-divisional error (SDE)

This is a non-accumulative error generally caused by non-roundness of the Lissajous.

### Vacuum

A vacuum is a three dimensional space from which air has been partially or completely removed. There are various levels of vacuum:

• Rough or low vacuum - 1 Torr to 1 mTorr
• Medium vacuum - 1 mTorr to 10 μTorr
• High vacuum - 1 μTorr to 10 nTorr
• Ultra high vacuum - less than 1 nTorr

Where: 1 Torr = 133.322 Pascal.  1 atmosphere pressure (pressure at sea level) = 101.325 KPa = 760 Torr

### Wavelength

The wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the Greek letter lambda (λ). In a sine wave, the wavelength is the distance between peaks. Wavelength λ has an inverse relationship to frequency f, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave type divided by the frequency of the wave.