Module system for a radiometric measuring device, comprising: - a basic module comprising - a sensor arrangement which is designed to generate a measurement signal on the basis of radiation which strikes the sensor arrangement, - a signal evaluation unit which is electrically coupled to the sensor arrangement and is designed to determine a measurement variable on the basis of the measurement signal, - a control device interface, the basic module being able to be coupled to at least one control device by the control device interface for the purpose of interchanging data, the basic module being designed, in a basic operating condition, to be supplied with electrical energy solely via its control device interface, and - an expansion module interface, - an expansion module which is separate from the basic module and comprises - a basic module interface, the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging data and/or interchanging measurement signals, and the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging energy, - a number of functional groups, and - an energy supply interface which is able to be coupled to an energy supply unit separate from the basic module and the expansion module, wherein energy provided via the energy supply interface is used to supply energy to the expansion module, and - a housing, the basic module and the expansion module being able to be arranged within the housing. - a basic module comprising - a sensor arrangement which is designed to generate a measurement signal on the basis of radiation which strikes the sensor arrangement, - a signal evaluation unit which is electrically coupled to the sensor arrangement and is designed to determine a measurement variable on the basis of the measurement signal, - a control device interface, the basic module being able to be coupled to at least one control device by the control device interface for the purpose of interchanging data, the basic module being designed, in a basic operating condition, to be supplied with electrical energy solely via its control device interface, and - an expansion module interface, - a sensor arrangement which is designed to generate a measurement signal on the basis of radiation which strikes the sensor arrangement, - a signal evaluation unit which is electrically coupled to the sensor arrangement and is designed to determine a measurement variable on the basis of the measurement signal, - a control device interface, the basic module being able to be coupled to at least one control device by the control device interface for the purpose of interchanging data, the basic module being designed, in a basic operating condition, to be supplied with electrical energy solely via its control device interface, and - an expansion module interface, - an expansion module which is separate from the basic module and comprises - a basic module interface, the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging data and/or interchanging measurement signals, and the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging energy, - a number of functional groups, and - an energy supply interface which is able to be coupled to an energy supply unit separate from the basic module and the expansion module, wherein energy provided via the energy supply interface is used to supply energy to the expansion module, and - a basic module interface, the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging data and/or interchanging measurement signals, and the basic module interface being able to be coupled to the expansion module interface for the purpose of interchanging energy, - a number of functional groups, and - an energy supply interface which is able to be coupled to an energy supply unit separate from the basic module and the expansion module, wherein energy provided via the energy supply interface is used to supply energy to the expansion module, and - a housing, the basic module and the expansion module being able to be arranged within the housing.