Instrumentation for determining the absorption coefficient in a plasma in the optical wavelength region has been developed for the study of a hydrogen arc discharge. The arc column is scanned with a beam of its own radiation emitted from the central zone which usually displays the highest brightness. The accuracy of the attenuation measurements near the edges of the plasma column is significantly increased over the accuracy obtainable with the double‐path method. The described instrumentation produces a lateral scan of the plasma in digital form suitable for the computation of the spatial distribution of the absorption coefficient.