Reducing the noise in the ratio of the signals for the two beams in a double beam spectrometer that uses a pulsed laser as a light source depends upon the ability of each beam to respond equally to all parts of the cross section of the laser beam. Optical components that are free of bulk and surface scattering sites are important. An opal glass filter placed near each detection system, in a position to fill its aperture, helps to improve the shot-to-shot reproducibility of the ratio. The best results are obtained when the spatial mode structure, or the cross section intensity pattern, of the laser is relatively reproducible.
