The Kodak 3 x 3" #102 Yellow Green Wratten Gel Filter converts the response characteristics of a barrier-layer photocell (as in a densitometer) to the luminosity response of the eye.
Most Kodak gelatin filters are available in 3 x 3" (75 x 75mm), 4 x 4" (100 x 100mm), 6 x 6" (150 x 150mm), 14 x 18" (350 x 450mm) and 4 x 12" (10.2 x 30.5cm).