The installation of fibre optic systems is recommended if it is not possible to place the flame monitor at an optimal position in order to guarantee best and permanent flame monitoring. A fibre optic system is also useful if the conditions of the environment are harsh to electronic devices. The fibre optic systems D-LL 701 and D-LL 702 have to be used in combination with the flame sensor D-LE 701. Fibre optic systems D-LL 703 and D-LL 704 are suitable for devices of type D-LE 703, D-LX 710 and D-LX 721. The fibre optic systems D-LL 703 and D-LL 704 can be installed directly in the hot zones of the burner. The radiation of the flame is transferred outside by a temperature resistant fibre glass bundle and directed to the flame monitor.