Fluorophores are special molecules which are efficient at absorbing excitation photons and at emitting many photons as they relax from the excited state. They are excited by a specific wavelength (or range of wavelengths) of light and emit fluorescence at a higher wavelength of light. This emission wavelength can then be separated from the excitation wavelengths and captured and visualised to image the sample. Proteins of interest can be tagged with fluorophores of different excitation and emission wavelengths in order to determine co-localisation, this is visualised by overlaying the two different images produced by exciting the two different fluorophores separately. Fluorophores can be synthetic, organic molecules, or can be proteins themselves – these are known as fluorescent proteins.