Bolometer n. (Physics) An instrument for measuring minute quantities of radiant heat, especially in different parts of the spectrum; called also actinic balance, thermic balance.
... the last-named investigator has extended our knowledge of it to a point much beyond the lowest visible ray, there yet remains a still remoter region, more extensive than the whole visible spectrum, the study of which has been entered on at Alleghany, by means of the linear bolometer. ... — Scientific American Supplement, No. 363, December 16, 1882 • Various