Rydberg atoms are atoms of which the outermost electrons are excited to levels close to the ionization level. The radius of the electron orbits is very large. If an electrical field is present, the orbits are affected, and the levels of the Rydberg atoms are shifted by the Stark effect. This effect can be measured by fluorescence. A ground state atom is excited to an intermediate level by two photons of a dye laser. From that intermediate level, a second laser excites the atom further to a Rydberg level. The fluorescence signal of the intermediate level is recorded. If the wavelength of the second laser is scanned, it passes over a Rydberg level. In the top of the Rydberg level, the density of the intermediate level is decreased because of the optical pumping towards the Rydberg level. This causes a dip in the fluorescence of the intermediate level. By carefully recording the fluorescence of the intermediate level while scanning the second laser over a large wavelength range, several Rydberg states can be mapped out, and the shift of their energy level can be "converted" to absolute values of the electrical field strength.