For example I have a glassy sample which is doped with Dy+3 ions. I am trying to find the best excitation wavelength for my photoluminescence (PL) measurements. For this purpose I perform a photoluminescence excitation (PLE) measurement. But I need a fixed wavelength to perform a PLE measurement. I am a little bit confused about choosing the right wavelength for PLE measurement. In the literature they fixed the emission wavelength to perform PLE measurements for Dy+3 doped samples as 575-576 nm. But according to what? A high energy wavelength is enough or does it need to be a specific wavelength? By the way the most intensive emission band of Dy+3 ion with a correct excitation is at 575 nm. But how can I guess this without making a proper PL measurement.