The invention claimed is:1. A method for controlling a current of a light-emitting diode to emit a desired luminous flux, comprising:defining, at a predetermined time, the current in dependence on a prior time period during which the light-emitting diode was energized, and in dependence on an operating parameter of the light-emitting diode which existed during the prior time period, andenergizing the light-emitting diode using the current to generate the desired luminous flux, wherein a pulse-width-modulated current signal is used as the current, andwherein defining the current comprises increasing a duty cycle of the pulse-width-modulated current signal, at the predetermined time, proportionally by a value of a time derivative of a luminous flux degradation curve of the light-emitting diode corresponding to aging over the lifetime of the light-emitting diode, the aging due to the energizing of the light-emitting diode, the value of the time derivative corresponding to a slope of the luminous flux degradation curve at the predetermined time.2. The method as claimed in claim 1, wherein the operating parameter represents a temperature of the light-emitting diode during the energizing, in particular the temperature of an active zone of the light-emitting diode.3. The method as claimed in claim 1, wherein a value for the current is further ascertained in dependence on the time of the energizing and in dependence on the operating parameter during the energizing based on at least one formula and/or based on at least one table and/or based on at least one theoretical model.4. The method as claimed in claim 1, wherein the operating parameter represents an amperage or a voltage of the current.5. The method as claimed in claim 1, wherein the operating parameter represents an ambient humidity at the light-emitting diode.6. The method as claimed in claim 1, wherein the operating parameter represents a presence of a predetermined gas, in particular a concentration of the predetermined gas at the light-emitting diode.7. The method as claimed in claim 1, wherein at least one second light-emitting diode is provided, wherein the light-emitting diodes generate electromagnetic radiations having different wavelength ranges, wherein one current value is ascertained for each of the two light-emitting diodes, and wherein the two light-emitting diodes are each supplied using the ascertained current value.8. The method as claimed in claim 1, wherein defining the current for activating the light-emitting diode at the predetermined time is repeatedly ascertained after a periodic predetermined period of time during which the light-emitting diode was energized and subsequently the light-emitting diode is activated using the ascertained current.9. The method as claimed in claim 1, wherein the duty cycle of the pulse-width-modulated current signal is increased in dependence on a temperature of the light-emitting diode.10. The method as claimed in claim 1, wherein the current for activating the light-emitting diode is increased in dependence on a chronological change of the luminous flux degradation of the light-emitting diode.11. The method as claimed in claim 10, wherein the luminous flux degradation of the light-emitting diode is ascertained in dependence on a temperature of the light-emitting diode.12. The method as claimed in claim 10, wherein the luminous flux degradation of the light-emitting diode is ascertained in dependence on the time of the energizing of the light-emitting diode.13. A control unit, which is designed to execute the method as claimed in claim 1.14. The method as claimed in claim 1, wherein the operating parameter further comprises the duty cycle of the pulse-width modulated current signal of the current.15. The method as claimed in claim 1, wherein the operating parameter further comprises a presence of a corrosive gas.16. The method of claim 15, wherein the corrosive gas comprises NOx or H2S.17. The method of claim 1, wherein a value for the current is further defined in dependence on a time step to a next time at which a subsequent current value is to be determined.
