FRET donor quantum yield ($Q_D$)

The FRET donor quantum yield is the emitted fluorescence photon fraction per absorbed excitation photon, measured without a FRET acceptor. In the constant-rate model used for FRET efficiency, $Q_D=k_{\mathrm{r}}/(k_{\mathrm{r}}+k_{\mathrm{nr}})$. Changing the FRET donor quantum yield changes the Förster distance, even if label positions remain fixed.