photographometer

from The Century Dictionary.

• noun In photography, an instrument for determining the sensibility of a film employed in photographic processes, relatively to the amount of radiation, luminous and chemical.
• noun A rotating photographic apparatus for recording automatically the angular position of objects around a given point.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Photog.) An instrument for determining the sensibility of the plates employed in photographic processes to luminous rays.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun dated An instrument for determining the sensibility of the plates employed in photographic processes to luminous rays.