A description is given of a single beam photometer, for measuring ozone concentrations, that requires neither optical filters nor a mechnical chopper. Yellow-orange light-emitting diodes radiate a band whose maximum is shown to match closely the absorption maximum of ozone in the visible region, and the emission band is sufficiently narrow to ensure good Beer-Lambert behavior. The rapid rise and fall times of the diode permit electrical modulation of the light source. Over the emission band, the integrated absorption cross-section for ozone is about 4.2 x 10 ²⁵m², while the peak ozone cross-section is only slightly greater (4.9 x 10 ²⁵m² at lambda