How many letters does the top answer for “of or pertaining to photometry, or to a photometer.” have?

The most common answer is photometric (11 letters).

Crossword clue: of or pertaining to photometry, or to a photometer.

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

photometric 11 letters web-def

of or relating to photometry

photometrical 13 letters web-def

Of or pertaining to photometry, or to a photometer.