Measurement methods based on optical quantities are widely used, starting from communication and logistics to precision medicine and materials production.

As a recognition of growing importance of reliable optical measurements, in 2023 a plan was prepared to develop national measurement standard for optical quantities first in the visible range of light (380-780) nm.

The  SI base unit for optical quantities is the candela, cd. Candela is a unit of luminous intensity of light emitted in a particular direction, defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of
frequency 540×10¹² Hz,K_cd, to be 683 when
expressed in the unit lm·W^-1, which is equal to cd·sr·W^-1 or cd·sr·kg^-1·m^-2·s³, where the kilogram,
metre and second are defined in terms of h, c and Δv_Cs, respectively.

One way to realise the candela is via optical power (unit W)  represented by an absolute photodetector  with low measurement uncertainty. For practical considerations, Metrosert is developing the measurement capability of optical quantities based on semiconductor photodetectors (Silicon and InGaAs-based photodiodes). The specially designed photodetectors in conjunction with high-tech electronics make it possible to realise the optical power scale at a level of relative measurement uncertainty of 0.1% at room temperature. The measurement uncertainty is improved by at least one order of magnitude better when cooled. By using our optical power scale, we can provide measurement services for derived optical quantities

• spectral transmission/absorption of optical filters/in materials
• spectral responsivity of photodetectors (also fibre-coupled)
• illuminance on surfaces
• spectral optical power of light sources (also fibre-coupled)

both at ordinary levels of optical power (over 1 microW) and at low photon flux levels (below 1 microW).

Recently, Metrosert acquired a measuring instrumentation for ultra-low photon flux levels. The new equipment will be used to develop calibration and testing services of quantum  sources and quantum detectors, which can be used in further research in quantum technologies and applications.  Presently, our low photon flux measurement system allows operation in a limited visible spectral range. In order to meet growing needs, e.g. in medicine, telecommunication, material science, the measurement capabilities will be gradually expanded into the ultraviolet and infrared parts of the spectrum.

Toomas Kübarsepp
Toomas.kübarsepp@metrosert.ee