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A physicist who measured the fundamental band gap of gallium nitride using photoluminescence excitation. When suitable transmission samples were unavailable, varying the excitation and observing emission provided another route to the absorption edge. Combining those measurements with reflection and luminescence separated exciton transition energies from the underlying band gap.^[ref]

Using $E_{\gamma }=hc/\lambda$ and an ideal band-edge photon energy equal to the quoted gap,At low temperature an electron and hole can form a bound exciton. Its recombination releases approximately $E_g-E_B$, so its wavelength is longer than $hc/E_g$ evaluated using the gap at that same temperature. GaN excitation and emission measurements resolve such excitonic transitions below the fundamental gap.^[ref]