The excitation spectrum refers to the variation in the intensity of a specific spectral line or band of luminescence as a function of the wavelength (or frequency) of the excitation light.
Thus, the excitation spectrum reflects the effect of light excitation of a material at different wavelengths; in other words, it indicates the material's responsiveness to different excitation wavelengths.
This refers to the relative radiant power generated per unit input power at a specific wavelength.