Encoder disc glossary: 30 common terms explained
Common encoder disc and optical encoder terms with concise definitions: incremental, absolute, transmissive, reflective, track, line count, grating pitch, duty cycle, interpolation, Gray code, eccentricity, optical density, reflectance and more.
2026-10-05 · Langfang Jingzheng Optoelectronic Technology Co., Ltd.
The table below covers the 30 terms used most often in encoder disc selection, drawings and inspection, including common alternative English names, for easier communication with customers and readhead manufacturers worldwide.
| Term | English | Definition |
|---|---|---|
| Encoder discs | encoder disc / code wheel | A disc carrying a pattern of light/dark or high/low-reflectance areas; the core angle-measuring element of an optical encoder |
| Optical encoders | optical encoder | A sensor that converts rotation angle into electrical signals using a light source, an encoder disc and a photodetector |
| Incremental | incremental | Outputs pulses or sine/cosine signals as the shaft rotates; must be homed after power-up |
| Absolute | absolute | Each angular position has a unique code, so the position is known at power-up |
| Transmissive | transmissive | Light passes through the disc; the light source and receiver are on opposite sides of the disc |
| Reflective | reflective | Light is reflected from the disc surface; the light source and receiver are on the same side |
| Tracks | track | An annular band around the disc carrying one type of pattern, such as the incremental track, index track or absolute track |
| Index / reference mark | index / reference mark (Z) | A mark that occurs once per revolution, used for homing |
| Line count | line count / lines per revolution | The number of lines around one revolution of the incremental track, e.g. 2500 lines |
| Grating pitch | grating pitch | The distance between two adjacent lines at the reading radius, equal to 2πr ÷ line count |
| Duty cycle | duty cycle | The fraction of one grating pitch occupied by the transparent (or high-reflectance) part; typically 1:1 |
| Reading radius | reading radius | The distance from the center of the readhead's light spot to the center of the disc |
| Edge multiplication | edge multiplication (×4) | Uses the rising and falling edges of the A and B square waves to raise the count to 4 times the line count |
| Interpolation / subdivision | interpolation | Divides one sine/cosine signal period into many parts to increase resolution |
| Sine/cosine signals | sine/cosine (sin/cos) signals | Two analog signals with a 90° phase difference, used for interpolation |
| Gray code | Gray code | A code in which only one bit changes between adjacent positions; common on multi-track absolute discs |
| Pseudo-random code | pseudo-random code (PRBS) | A code common on single-track absolute discs; reading a few consecutive bits is enough to determine the position |
| Resolution | resolution | The smallest angular step that can be distinguished |
| Accuracy | accuracy | The maximum deviation of the reading from the true angle; affected by graduation error, eccentricity and interpolation error |
| Graduation error | graduation error | The difference between the actual and theoretical positions of the lines |
| Eccentricity | eccentricity | Offset between the center of the disc pattern and the center of rotation; causes a once-per-revolution angular error |
| Concentricity | concentricity | Deviation between the pattern center and the bore center; a common incoming inspection item |
| Optical density | optical density (OD) | The ability of the opaque areas of a transmissive disc to block light; OD 3 means a transmittance of 0.1% |
| Reflectance | reflectance | The fraction of light reflected by the high- and low-reflectance areas of a reflective disc |
| Contrast | contrast | The difference between the signals from the light and dark areas; determines signal amplitude and signal-to-noise ratio |
| Photolithography | photolithography | A process that transfers a mask pattern onto the substrate using photoresist, exposure and development |
| Chrome coating | chrome coating (Cr) | A chrome layer deposited on the glass surface as the opaque layer; the graduations are formed by etching it |
| AOI | automated optical inspection | Automatic machine-vision inspection of graduation integrity and surface defects |
| Readhead | read head | The component that contains the light source and receiver and reads the disc pattern |
| Lissajous figure | Lissajous figure | The figure obtained by feeding the sin and cos signals into an oscilloscope in XY mode; used to judge signal quality |
Related articles: How to choose between incremental and absolute encoder discs, The relationship between line count, interpolation and resolution.
This article was compiled by the engineering department of Langfang Jingzheng Optoelectronic Technology Co., Ltd. Jingzheng Optoelectronics manufactures glass encoder discs, reflective metal encoder discs, PC encoder discs and custom encoder discs. Website: www.encoderdisk.cn; phone: 0316-8933331.