These are dark glass filters, nearly black, which transmit the ultraviolet region but absorb the visible region. They transmit slightly in the near-infrared region.
The glass type figure represents the wavelength that provides the maximum transmittance. These filters are chiefly used for single extraction of the ultraviolet rays, and are often utilized for selectively transmitting the 253.7nm and 365nm wavelengths, which are the representative line spectra of mercury lamps.
Color Filter Glass : Ultraviolet Transmitting, Visible Absorbing Filter Data