A novel IR method for measuring the kinetics of N2O photodecomposition has been devised and used to calibrate the flux of Lyman-alpha (10.2 eV) radiation from a H-2/Ar microwave discharge lamp. The photodecomposition of N2O Occurs with a weak pressure dependence due to the operation of a wall effect Consuming some photogenerated active oxygen species. This effect is removed by working at high N2O pressures. The Lyman-alpha flux from the lamp is 1.28 +/- 0.36 x 10(15) photons cm(-2) s(-1).