How many letters does the top answer for “a photographic record of a spectrum” have?

The most common answer is spectrogram (11 letters).

Crossword clue: a photographic record of a spectrum

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

spectrogram 11 letters wn-def

a photographic record of a spectrum

spectrograph 12 letters wn-def

a photographic record of a spectrum

record on photographic film 50% similar → photograph