What is an Actinometer?


An actinometer is a device that is used to measure the intensity of solar radiation. It is a chemical system that determines the amount of photons by measuring the rate of change of photoinduced responses in a chemical system.

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


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

For an efficient actinometer, this quantum performance must be independent of oxygen, trace impurities, temperature and excitation wavelength.

Operating principle

The following are the basic steps used in the principle of operation of chemical actinometers:

*** The gas of interest is filled in a photolysis reactor.
*** The actinometer is exposed to heat radiation.
*** The photochemical speed is measured.

Here, the actinometer gas is exposed to the actinic flow without significantly altering the intensity of the radiation and the spectral composition. For this reason, a transparent quartz cell having a suitable geometric shape is used, and gases with small optical absorption values ​​are used. Depending on these conditions, the frequency of photolysis can be easily evaluated.

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 with a gas valve and covered with an opaque cover to avoid exposure to sunlight.

During the measurement, the actinometer is discovered and exposed to heat radiation during a fixed time interval. After this, the actinometer is closed again and analyzed to determine the change in gas composition.

In the other mode of operation, the actinometer gas constantly passes to the reactor that is exposed to solar radiation. In this case, it is considered that the time interval is the average residence time of the gas in the illuminated reactor.

When the gas passes through the reactor, its composition is analyzed using an in-line gas detector. Photolysis frequencies can be continuously monitored in this mode.


The actinometers are used mainly in meteorology to measure the solar radiation transmitted by the sun, reflected by the earth or dispersed by the atmosphere. They are used in photochemical experiments involving complex irradiation geometry. In addition, the actinometers serve as a first option to calibrate the photochemical detectors used for radiation measurements.

These instruments can be combined with joulemeters to measure the pulse energies of the laser. Actinometers tend to lose sensitivity and accuracy at high laser pulses due to the various photon processes that take place at high densities of photons. However, repeated calibration of the actinometers with controlled precision can be guaranteed by reading the linearity of the joulemeter.

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