Bandpass filter glass is a kind of glass with narrow bandwidth and obvious cut-off between short wave and long wave, as shown in the figure. Their spectral properties are represented by the maximum (TM) transmission ratio and its wavelength (lambda M) position, short wave cutoff wavelength, long wave cutoff wavelength, and half width. The difference between the wavelength 1 and 2 (1- 1)nm corresponding to half of the maximum transmission ratio of the band-pass glass is called half width. The short wave cutoff wavelength is the wavelength on the peak short wave side with transmission ratio equal to zero. The cut-off length of long wave is the wavelength with transmission ratio equal to zero on the side of peak long wave. In some varieties, if the transmission ratio of glass on the side of long wave cannot reach zero, it is expressed as the transmission ratio of a certain wavelength.
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