Photoluminescence excitation (PLE) spectroscopy was carried out to investigate the excitation/emission cycle of MBE grown InGaN quantum structures. Quantum well and Stranski-Krastanov type quantum box samples were chosen that emit from blue to red. The bandgap energy (E-g), determined from the PLE spectrum, was found to decrease concurrently with the detection energy. This indicates that the emission spectrum from the sample is inhomogeneously broadened. A plot of the resultant Stokes shift against detection energy shows a linear trend. Our results agree with those from independent measurements of thermally detected optical absorption (TDOA) in some of the samples. On comparison with absorption and PLE measurements on MOCVD grown InGaN. a difference in the bandgap energies obtained becomes apparent for detection energies below approximate to 2.6 eV.