Metrology Science of measurement

The comparability of measurement results

The term metrology comes from ancient Greek μετρεῖν (metreín – measure) and stands for the science of measurement. It is concerned with many aspects of measurement – in theory and practice:

  • Quantification of physical sizes and properties
  • Definition and realization of central units of measurement
  • Calibration of measuring instruments
  • Determination of measurement uncertainties

The aim of metrology is to guarantee the reliability and comparability of measurement results. This is only possible when their requirements apply to all areas: from science to technology to modern life. It is applied in industry, commerce, health care and environmental protection – and, of course, in light measurement technology.

Every measuring method has its own measurement uncertainty

An important finding in metrology is that every measurement has a measurement uncertainty that cannot be avoided. The measured value therefore provides only an estimate of the value in question and the measurement uncertainty is a measure of the reliability of the reading. Besides the numerical reading with unit, in metrology a complete result always includes the measurement uncertainty determined. This depends on the measurement method used.

Light intensity as internationally recognized metric unit

The comparability of measurands can only be ensured with internationally recognized, clearly defined measurement units to which the measurements refer. For this purpose, the Metric Convention Treaty was signed in 1875, creating an internationally valid metric system for units of measurement, known as the SI system (Système International d’Unités). Since the revision of 2019 the SI system comprises nine SI units based on seven natural constants, e.g. speed of light c. This ensures that the selected definitions of SI units are independent of a concrete implementation. Photometric measurands are all related to light intensity (candela, cd). Their definition is via the photometric radiation equivalent Kcd that ensures the link to the historic definition of candela.


SI unit


Natural constant












Δ ν

Electric current








Amount of substance




Luminous intensity




Traceability to national standards

National metrology institutes are tasked with the primary realization and preservation of SI units. In Germany this is the German National Metrology Institute (PTB). Its duties include representing the units and disseminating them with the smallest possible uncertainties. For this purpose, the PTB develops national standards that apply to the entire field of metrology in the Federal Republic of Germany. A constant comparison with metrology institutes of other countries (e.g. NIST) ensures that the units are the same all over the whole world. The relationship to the SI unit is conveyed to end users in the form of “calibration standards” (more about Calibration). If there is a clear relationship between readings and the primary realization of the SI unit of a national institute, we speak of metrological traceability of the result (Figure: right side).

Chart showing the metrological traceability of measurands in light measurement within the framework of the SI system.

Internationally uniform implementation of traceability

A complex network of organizations, contracts and standards ensures the internationally uniform implementation of metrological traceability to the SI unit system (Figure: left part). Stringent requirements are imposed in calibration and test labs by means of standards, e.g. ISO/IEC 17025, in order to guarantee reliable processes within the scope of traceability.

Accredited service providers such as Instrument Systems undergo regular independent competence tests (in Germany by DAkkS) and document the results of their tests in calibration or test certificates.