Blocking is measured in Optical Density. For UVPASS applications, you generally need (99.99% blocking) in the visible spectrum (400nm-700nm). High-end scientific FT-UVPASS filters achieve OD > 6 for laser-induced fluorescence (LIF) applications where stray visible light would overwhelm the UV signal.
The "bandwidth" is critical. A narrow FWHM (10nm – 40nm) provides high spectral purity but lower total light throughput. A wide FWHM (50nm+) allows more light but risks contamination from deep violet light. FT-UVPASS filters typically specify FWHM explicitly, e.g., "FT-UVPASS-365-40" indicates a 40nm wide band around 365nm. ft-uvpass