This kind of glass only transmits the light within a narrow wavelength range as shown in figure. Its spectral characteristic is composed of maximum transmissivity (TM) wavelength of the peak (lM), front-cut-off wavelength, behind-cut-off wavelength, and half-peak width.
The wavelength corresponding to the half of the maximum transmissivity are l2 and l1. And the half-peak width is determined by l2 - l1. Front (behind)-cut-off wavelength is corresponding to the wavelength of "zero" transmissivity. But there are a few types which transmissivities of the behind-cut-off wavelengthes can't approach to zero, then they are determined by the transmissivity of the certain wavelength. 
Band-pass filter glass is named DTB, and wavelength of the peak is used as its serial number.

Technical Indexes