This paper presents a multipass optical absorption cell that is based on a spherical cavity. Ozone concentrations have been detected in the visible region at 603 nm, in the Chappuis band. A 2-inch diameter (50.8 mm) integrating sphere has been modified for use an optical absorption cell. A method of calculating the effective optical path length of the integrating sphere is also presented. It is reported that the effective optical length of the sphere is reduced as the ozone concentration is increased as predicted by the effective path length formula. © 2007 IOP Publishing Ltd.