The Chroma 3nm OIII filter isolates the 500.7nm primary emission line of doubly ionized oxygen. After H-alpha, this is the next most common emission when imaging nebulae.
All transmission and blocking (OD) data are actual, measured spectra of representative production lots. Spectra varies slightly from lot to lot. Optical density values in excess of 6 may appear noisy because such evaluations push the resolution limit of low light level measurements.