What do the numbers on my filter mean?

Accepted practice in the optics industry is to define bandpass filters by Center Wavelength (CWL) and Half Bandwidth (HBW) of Full Width at Half Maximum (FWHM). FWHM is defined as the bandwidth at 50% of the maximum transmission of the filter.  So, to use example from the Omega Optical site




For the example shown on the left, the center wavelength is 555 nm, and the FWHM is 30 nm.  That means that the filter transmits light only between 540 nm and 570 nm, that is, 555 nm +/- 15 nm, half the FWHM.  Larger FWHM filters are better for low signal because they allow a wider range of (and thus often more) light through.  Smaller FWHM are better for multiplex studies because their small transmission range allows one to image one fluorophore while excluding the fluorescence from others.