A method for the practical determination of the absolute concentration of single (a 1 D g ) oxygen is discussed. The method is based on sensitive off-axis integrated-cavity-output spectroscopy (ICOS). Off-axis ICOS allows narrowband, continuous-wave lasers to be used in conjunction with optical cavities to record sensitive absorption measurements. The details of the method as well as spectroscopic data confirming the first observation of the ͑1, 0͒ band of the b Oxygen has systems in the near-infrared (a 1 D g X 3 S g 2 ) and the visible (b 1 S g 1 X 3 S g 2 ) regions that are atypically weak but are important to the radiative properties of the atmosphere. As a result, these transitions are well characterized spectroscopically and are the focus of much recent work.