What is the difference between absorption and excitation spectra?

While an excitation spectrum shows the wavelengths of light that a sample will absorb to be able to emit at a specified wavelength, an absorption spectrum shows all of the wavelengths at which light is absorbed by the sample. Generally, both the absorption and excitation spectra of a sample will peak at the same wavelength, although there are exceptions. In addition, absorption spectra is measured using a UV-Vis spectrophotometer, while excitation spectra is measured using a fluorescence spectrophotometer.