Abstract: A device for filtering a selected spectral line from an incident beam of radiation in the form of a cell which contains an atomic vapor of a material, one of whose spectral lines is the selected spectral line, and which has a window which is non-reflective of incident radiation in the region of the selected spectral line when the vapor is absent. The density of the vapor in the cell may be adjusted to an optimum value at which the window-vapor interface will specularly reflect the selected specular line while passing all other wavelengths in the incident beam into the cell.