Densitometers measure ink density and optical density.
Incident light densitometers measure prints or photo prints, transmitted light densitometers measure films for printing plates or screen printing, slide films and negative films.
But compared to a colorimeter or a spectrophotometer, a densitometer can only measure tonal values but no colour tones, they are practically colour-blind.
Important for proofing: Densitometers can indeed measure colour densities, but two identical colour densities do not necessarily have the same colour impression. Densitometrically measured values from a proof cannot be compared with the same values on an offset print, a print cannot be adjusted densitometrically to the values of the proof.
Densitometers from Techkon are very common in pressrooms.