While most optical detection is based on light absorbance, the FLD makes use of the fluorescence of substances for detection. A xenon flash lamp sends white light of very high intensity to a substance which is excited in a wavelength range and emits fluorescence radiation in a different wavelength range. The detector can collect both excitation and emission spectra and continuous signals at up to four different excitation and emission wavelengths. While not all substances can be detected by a FLD, the sensitivity and selectivity for suitable substances is excellent, e.g. for anthracene the limit of detection is 10 fg (10-14g).