The absolute density of singlet oxygen has been quantitatively determined using off-axis integrated cavity output spectroscopy to measure the Q(2)-Q(18) lines of the (0,0) band of the b(1)Sigma(g)(+) <--a(1)Delta(g) Noxon system near 1910 nm. Using derived values for the absorption cross-sections and a fitted rotational temperature of 295 K, a singlet oxygen density of 3 x 10(15) molecules CM-3 (5% conversion efficiency) was obtained in the afterglow of a 13.56 MHz RF discharge. The observed line spectrum for the Q(1 2) transition has a signal-to-noise ratio of 120:1 signifying that singlet oxygen densities as low as 2.5 x 10(13) molecules CM-3 are detectable. (C) 2004 Elsevier B.V. All rights reserved.