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actinometer| Instrument devised by English astronomer John Herschel in 1825 to measure the amount of heat received from the Sun by a given surface in a given time. |
| The term is now sometimes used to describe any instrument having this purpose, for example a pyrometer. |
| ‘Actinic’ is sometimes used to describe the chemical action of radiant energy, the most common being that on a photographic plate. Thus actinometer is an alternative name for what is now usually described as a photographic exposure meter. |
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