Modern gas turbine installations used for generating electrical energy are power-optimized systems, the individual components of which are mostly operated at their material-related roadability limits, such as, in particular, those components which are exposed directly to the hot gas stream occurring within the combustion chamber during the combustion process.  These are, in particular, the guide vanes and moving blades of the gas turbine stage, within which the hot gases emerging from the combustion chamber at maximum temperatures of above 1000° C. perform expansion work which drive the rotor unit which is ultimately connected to a generator for generating electrical energy.  In order to ensure that the gas turbine components exposed to the hot gases do not overheat, care must be taken to ensure that what is known as a maximum permissible operating temperature limit dependent on the respective gas turbine type is not overshot. 
For this purpose, the turbine outlet temperature (TAT) can be measured, and the turbine inlet temperature (TET) can be determined via suitable auxiliary variables.  During normal operation, the latter is kept below a specific limit value by means of appropriate control actions, in order to prevent guide vanes and moving blades from overheating. 
If this limit value is nevertheless overshot on account of a fault, the components which are exposed directly to the hot gas stream are subjected to excessive thermal stress, with the result that the useful life of the overall gas turbine installation may ultimately be reduced considerably. 
In order to avoid an overheating of the gas turbine installation, the hot gas temperature before inlet into the gas turbine stage has been monitored continuously.  In the event of an approach of the hot gas temperature to the maximum limit temperature, measures have been taken in order to avoid a further temperature rise, for example in the form of an emergency shutdown of the gas turbine installation by the supply of fuel being stopped abruptly. 
To measure the turbine outlet temperature, thermocouples are mostly used which, due to the system, are subject to measurement inertia with time constants in the second range.  If the temperature rise of the hot gases occurs sufficiently slowly, thermal sensors can detect in good time an approach to the maximum limit temperature, and therefore appropriate countermeasures can be initiated sufficiently early.  If, however, an overheating of the hot gases takes place abruptly and suddenly, for example within fractions of a second, then there are problems in detecting the overheating event in good time by means of known thermal sensors.  For this reason, it is appropriate to look for alternative protection and monitoring systems, with the aid of which an overheating of gas turbine installations can be ruled out reliably. 
Furthermore, for an exemplary operation of the gas turbine installation, it can be desirable to ensure that the fuel supplied is burnt completely within the combustion chamber.  Modern combustion systems are in this case operated with low flame temperatures very close to the extinction limit in order to minimize the emissions of nitrogen oxides.  In the event of a fault which leads to a lowering of the flame temperature below a critical limit value, the combustion reaction can no longer be maintained, and therefore the flame is extinguished completely or partially.  If fuel continues to be supplied in such a case, this may lead to hazardous situations if the fuel/air mixture ignites downstream of the combustion chamber, for example in a boiler installation coupled to the gas turbine. 
For this reason, the operation of the combustion chamber can be monitored.  This can be carried out with the aid of optical sensors which detect specific flame parameters via a photocell and compare them with defined limit values.  If the parameters are outside the permitted operating window, the emergency shutdown is triggered. 
However, the treatment and evaluation of the measured flame parameters can involve a certain processing time which can be in the region of 1 sec in present-day systems.  Moreover, reliable detection may not always ensured if the flame is not extinguished completely, but only partially.  The latter, however, may likewise lead to potential damage to the installation, for example if fuel metering is increased on account of the power drop of the gas turbine, which occurs in the event of a partial extinction of the flame, and if the flame thereupon reignites completely again. 
There is therefore a desire for supplementary measures for optical flame monitoring which can compensate these deficiencies.