n.
Pronunciation: ˌ ak-t ə - ' nä-m ə -t ə r
Function: noun
Date: 1833
: any of various instruments for measuring the intensity of incident radiation especially : one in which the intensity of radiation is measured by the speed of a photochemical reaction
– ac · ti · no · me · tric \ -n ō - ' me-trik \ adjective
– ac · ti · nom · e · try \ - ' nä-m ə -tr ē \ noun