A highly sensitive absorption experiment for diagnosing glow discharge plasmas is described. This experiment is applicable from the VUV to the IR. A very stable Xe arc lamp is used as a source of continuum radiation. An echelle spectrometer equipped with a gated, image‐intensified, charge‐coupled device detector array is used to disperse and detect the continuum, with absorption features, after it has traversed the glow discharge. Digital subtraction is used to discriminate against the line emission from the glow discharge and detect only the continuum emission from the arc discharge. Estimates of the relative spectral radiances of glow and arc discharges suggests the subtraction technique is broadly applicable to glow discharge studies. A fractional absorption of 10−3is detectable with a signal‐to‐noise ratio limited primarily by shot noise. A detection limit for excited Hg atoms of 7×109cm−2is demonstrated in a 400‐mA Hg–Ar glow discharge. Further improvements in the experiment are proposed.