1) Measuring device 2) Measuring instrument 3) Measuring system 
Actinometers are instruments used to measure the heating power of radiation. They are used in meteorology to measure solar radiation as pyrheliometers. An actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet...
instrument for measuring incident radiation
• (n.) An instrument for measuring the direct heating power of the sun`s rays. • (n.) An instrument for measuring the actinic effect of rays of light.
in chemistry, a substance or a mixture of substances that reacts through the action of light and that, because of the easily determined quantitative ...
Ac`ti·nom'e·ter noun [ Greek ..., ..., ray + -meter ] (a) An instrument for measuring the direct heating power of the sun's rays. (b) An instrument for measuring the actinic effect of rays of light. 
An actinometer is an instrument for measuring heat radiation.
[n] - an instrument for measuring the intensity of electromagnetic radiation (usually by the photochemical effect)
actinometer An instrument for measuring the intensity and penetrating power of actinic rays. Also actinometry.
An actinometer is an instrument for measuring heat radiation. See also: Actinometry, Heat.
noun an instrument for measuring the intensity of electromagnetic radiation (usually by the photochemical effect)
a device for measuring intensity of radiation, esp. that of the sun. Cf. actinograph.