Actinometer

a device for measuring intensity of radiation, especially that of the sun.
Historical Examples

It is a good plan to employ an actinometer to judge the correct exposure.
Photographic Amusements, Ninth Edition Walter E. Woodbury and Frank R. Fraprie

A Johnson’s actinometer is very useful to time the exposure.
Photogravure Henry R. Blaney

You may sometimes find that the actinometer indicates a very different exposure from what the eye would lead you to expect.
Scientific American Supplement, No. 492, June 6, 1885 Various

The actinometer indicated an exposure of thirty seconds where in good light one would be right.
Scientific American Supplement, No. 492, June 6, 1885 Various

We can, however, by chemical means arrive at an exact estimate of the active power, and for this purpose an actinometer is used.
Scientific American Supplement, No. 492, June 6, 1885 Various

For judging long exposures, the use of an actinometer (issued in many inexpensive forms) is helpful.
How to Observe in Archaeology Various

A very suitable instrument for timing the exposure of carbon tissue is Sawyer’s actinometer.
Photogravure Henry R. Blaney

We then shift the paper in the actinometer so as to get a fresh portion under the tissue strips, or we substitute a new piece.
The Barnet Book of Photography Various

Now refer to the actinometer and see what has taken place on the silver paper which we put into it.
The Barnet Book of Photography Various

noun
an instrument for measuring the intensity of radiation, esp of the sun’s rays
actinometer
(āk’tə-nŏm’ĭ-tər)
Any of several instruments used to measure radiation, such as a pyrheliometer.