[en] In this paper the setup and use of a retarding field energy analyser to measure the energy distribution of LMIS will be presented. An introduction in the technology and mechanism of LMIS will be given in chapter 2. The experimental configuration of the retarding field energy analyser, together with an error estimation and a computer program, to control the energy analyser and to calculate the energy distribution, will be discussed in chapter 3. To test the energy analyser an electron source, consisting of a tungsten thermoionic cathode, will be used. Energy distribution measurements of a gallium LMIS will be presented in chapter 4. Finally in chapter 5, the accuracy of the energy analyser and suggestions for improvement will be presented. (orig.)