Editorial Feature

What is an Actinometer?



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Article updated on 09/03/20 by Ben Pilkington


An actinometer is a device that is used to measure the intensity of solar radiation. It involves a chemistry technique that determines the number of photons by measuring the rate of change of photo-induced responses in a chemical system.


The actinometer was invented by John Herschel in 1825. It works based on the principle that the rate of photolytic conversion of molecules within an actinometer cell is equal to the rate of absorption of photons in the actinometer.


A physical device such as a bolometer, photodiode, and photomultiplier can be employed to convert the incident photon energy into a quantifiable electrical signal. However, a chemical actinometer is the most widely used device in which the quantum yield of a reference substance undergoing a photochemical reaction is determined and calibrated.


For an efficient actinometer, this quantum yield has to be independent of oxygen, trace impurities, temperature, and excitation wavelength.

Working Principle

There are three steps to follow to operate a chemical actinometer:

  • First, the gas of interest is filled into a photolysis reactor.
  • Then, the actinometer is exposed to heat radiation.
  • Finally, the photochemical rate is measured.

In this process, the actinometer gas is exposed to actinic flux without significantly altering the radiation intensity or spectral composition. For this reason, a transparent quartz cell with a suitable geometrical shape is employed, and gases with small optical absorbance values are used. Based on these conditions, the photolysis frequency can be easily evaluated.

Modes of Operation

There are two basic modes of actinometric operation: static batch mode and flowing gas mode.


In the static batch mode, the photolysis reactor is filled with actinometer gas, sealed off by a gas valve, and covered with an opaque hood to prevent exposure to sunlight. During measurement, the actinometer is uncovered and exposed to heat radiation for a fixed interval of time. Following this, the actinometer is closed again and analyzed for changes in the gas composition.


In the flowing gas mode, the actinometer gas is constantly passed into the reactor that is exposed to solar radiation. In this case, the time interval is considered to be the mean residence time of the gas in the illuminated reactor. When the gas passes through the reactor, its composition is analyzed using an online gas detector. Photolysis frequencies can be continuously monitored in this mode.


Actinometers are chiefly used in meteorology to measure solar radiation emitted by the sun, reflected by the earth or scattered by the atmosphere. They are used in photochemical experiments that involve complex irradiation geometry. Besides, they serve as a first choice for calibrating photochemical detectors used for radiation measurements.


These instruments can be combined with a joulemeter to measure laser pulse energies. The actinometers tend to lose sensitivity and precision at high laser pulses owing to various photon processes taking place at high photon densities. However, repeated calibration of actinometers with controlled accuracy can be ensured upon the realization of the linearity of the joulemeter readout.

References and Further Reading

Kuhn, H.J., Braslavsky S.E., Schmidt, R., (2004). Chemical Actinometry (IUPAC Technical Report), 76 (12), pp.2105-2146. [Online] International Union of Pure and Applied Chemistry. Available at: http://publications.iupac.org/pac/2004/pdf/7612x2105.pdf (Accessed on 6 March 2020).


Heard, D.E., ed. (2006). Analytical Techniques for Atmospheric Measurement. [Online] University of Leeds. Available at: https://books.google.co.in/books?id=OO8Z78CJep8C&pg=PA421&lpg=PA421&dq=actinometer+working#v=onepage&q=actinometer%20working&f=false (Accessed on 6 March 2020).

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