The penetration in the terrestrial atmosphere of solar radiation corresponding to the spectral range of the Schumann-Runge bands of molecular oxygen is analyzed between 1750 and 2050 Å. The variation of the absorption cross section with temperature is taken into account and it is shown that average O2 absorption cross sections cannot lead to correct photodissociation coefficients. Reduction factors are defined in order to simplify the computation of the molecular oxygen photodissociation and to permit a simple determination of the photodissociation coefficients of any minor constituent with smoothly varying absorption cross section. Examples are given for O2, H2O, CO2, N2O, HNO3 and H2O2. Numerical approximations are developed for three types of spectral subdivisions: Schumann-Runge band intervals, 500 cm-1 and 10 Å intervals. The approximations are valid from the lower thermosphere down to the stratosphere and they can be applied for a wide range of atmospheric models and solar zenith distances.