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Sensors

A sensor is the part of a measuring system that is directly affected by what is being measured — heat, light, pressure, motion — and turns that effect into a signal, usually a voltage or current, that electronics can read and digitise. Every sensor has limits: its resolution, its measurement uncertainty and the calibration that links its readings to real values. A camera's CCD, which won a share of the 2009 Nobel Prize in Physics, is a grid of millions of light sensors.

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Metrological vocabulary (VIM)

  • Sensor (VIM 3.8): element directly affected by the phenomenon carrying the quantity to be measured.
  • Measuring transducer (VIM 3.7): device whose output has a specified relation to its input.
  • Calibration (VIM 2.39): relate standards' values (with uncertainties) to indications, then use that relation to obtain results.
  • Resolution (VIM 4.14): smallest change in the quantity causing a perceptible change in the indication; may depend on noise or friction.
  • Measurement uncertainty (VIM 2.26): non-negative parameter characterising the dispersion of attributed values.
y=f(x)  ⇒  x^=f−1(y)y = f(x) \;\Rightarrow\; \hat{x} = f^{-1}(y)

Calibration estimates the instrument's response f, then inverts it to recover the measured quantity from the indication y.

Systems combine many sensors. GPS has space, control and user segments: satellites broadcast their position and time, the control segment adjusts satellite clocks and orbits, and the receiver calculates a three-dimensional position and the time.

Common misconception: 'Self-calibration' is usually adjustment, not calibration: VIM notes that calibration should not be confused with adjusting a measuring system.
Full explanation — the complete reference version every reading depth is based on

What a sensor does

The International Vocabulary of Metrology defines a sensor as the element of a measuring system that is directly affected by the phenomenon carrying the quantity to be measured — for example the sensing coil of a platinum resistance thermometer, the rotor of a turbine flow meter or the float of a level gauge. In engineering, a sensor typically produces a voltage or current that represents the property measured, such as speed, temperature or flow.

From the world to numbers

  1. Sensing element: a physical effect changes something measurable (resistance, charge, voltage).
  2. Transducer: produces an output with a specified relation to the input (thermocouples, strain gauges, pH electrodes are examples).
  3. Conversion: the electrical signal is turned into digital numbers a computer can process.
  4. Calibration: comparison with measurement standards turns raw indications into trustworthy values.

Light sensors: the CCD and CMOS

A charge-coupled device (CCD) is a silicon chip holding a grid of photocells. Through the photoelectric effect, light knocks electrons free, and more light frees more electrons. The charges are read out row by row and translated into digital ones and zeros, one value per pixel. Willard Boyle and George Smith outlined the idea in 1969 and shared half of the 2009 Nobel Prize in Physics for it. CMOS sensors use the same photoelectric effect but read each photocell out where it sits.

pixels=width×height\text{pixels} = \text{width} \times \text{height}

A sensor's image capacity; 1280 × 1024 = 1 310 720 pixels, about 1.3 megapixels.

Sensing position: GPS

A GPS receiver works out where it is from signals sent by satellites. The satellites fly about 20 200 km up, circle the Earth twice a day and broadcast one-way signals giving their position and the time. The constellation is arranged so that at least four satellites can be seen from virtually anywhere on Earth, and the receiver uses their signals to calculate its three-dimensional position and the time.

Good measurements

  • Resolution: the smallest change in the measured quantity that causes a perceptible change in the reading.
  • Uncertainty: a non-negative parameter describing the spread of values reasonably attributed to the measured quantity, including systematic effects.
  • Calibration: relating readings to standards with known uncertainties, so that indications become measurement results.

Worked example (our calculation): a phone camera sensor 4000 pixels wide and 3000 tall has 4000 × 3000 = 12 000 000 pixels, i.e. 12 megapixels. More pixels raise the number of samples, but each pixel's accuracy still depends on noise and calibration.

How we know

The words sensor, transducer, calibration, resolution and uncertainty have agreed international meanings, published in the International Vocabulary of Metrology (VIM) by the bodies behind the SI. The CCD account comes from the Nobel committee, and the GPS facts from GPS.gov, the US government's official GPS information site.

Assumptions and limits

A sensor's reading is only as good as its calibration, and resolution can be limited by noise or friction. A camera's pixel values depend on noise and calibration as well as on light, and a GPS receiver depends on receiving signals from enough satellites — which is why the constellation is arranged so that at least four are in view from virtually anywhere.

Common misconception: Misconception: 'a reading with more digits is more accurate'. Extra digits only show resolution. A sensor can display 0.01 °C steps and still be wrong by a whole degree if it is badly calibrated.
Info: Connections: sensors give Robotics its perception; many are built from Semiconductors (like the CCD) and turn Light into electrical signals; their images feed Computer vision; CCDs fill the cameras of Telescopes; and sensors are the input stage of Computers.

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Content status: published 1 October 2026.

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