A cell-space adjustable slit allows the reference beam of a Cary 14R spectrophotometer to be attenuated by a factor of up to 1000. The attenuator transmittance has been measured and the results shown graphically as corrections applicable to observed sample transmittances. The smallest nominal values of recorder full scale deflection obtainable without excessive pen noise are 0·01% in the ultra-violet and visible, and 0·1% in the infra-red ranges of the instrument. The transmittance curves for shade 12 welding filters (luminous transmission factor of about 0·002%) have been measured without thinning the samples over the range 270 nm to 2·6 μm. The technique may also be used for double-beam spectrophotometry of other kinds of dense filters.