Thermally detected optical absorption (TDOA) and photoluminescence experiments were carried out on InGaN/GaN multi-quantum well (MQW) structures grown by molecular beam epitaxy on (0001) sapphire substrates. The absorption coefficient of the QW was extracted by modelling the TDOA lineshape. These experimental findings are confronted with the theoretical optical density evaluated within envelope function calculations of transition energies and oscillator strengths, including the electric field effects. The results show a relative agreement obtained by using a smaller value of the electric field than it has been predicted theoretically. However, other effects must be included in our model in order to achieve a realistic description of the absorption process in this sophisticated system.