Yes, PHOTOMETER is a Scrabble word!

Letter values in Scrabble US: P 3, H 4, O 1, T 1, O 1, M 3, E 1, T 1, E 1, R 1.

Is it a valid word?

PHOTOMETER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

measuring instrument for measuring the luminous intensity of a source by comparing it (visually or photoelectrically) with a standard source
photographic equipment that measures the intensity of light

Also: light meter, exposure meter.