What are the optical properties of fluorescence bandpass filters?
Center Wavelength (CWL): the wavelength at which the filter has the highest transmittance, which needs to match the emission peak of the fluorescent dye (e.g. FITC: ~525 nm, DAPI: ~460 nm).
Full Width at Half Maximum (FWHM): the wavelength range in which the transmittance is ≥50%, commonly narrowband (10~40 nm) or broadband (50~100 nm).
Peak Transmittance (Tpeak): usually >90% (up to 95% or more for high quality filters).
Blocking Range: optical density (OD) in the non-passband region (e.g., excitation band) should be ≥ OD4 (i.e., transmittance < 0.01%).

Parameter	Typical Value	Influence
CWL	Selection based on fluorescent dye (e.g. Cy5: ~670 nm)	It must match the fluorescence emission peak or the signal is lost.
FWHM	10~40nm (narrow band)	The narrower the bandwidth, the higher the signal-to-noise ratio, but the signal strength may be reduced.
Cut-off Range	OD4~OD6 (excitation light band)	Ensure complete blocking of excitation light (e.g., 488 nm laser) to avoid background interference.
AOI	0° (vertical incidence optimal)	Tilted incidence may result in a CWL offset (requires selection of an angle-compensated design).