How does a Black Body IR Emitter work?
This device is intended to be used with the heater current modulated, generally by a square wave. This allows the system to analyze the received radiation as an AC signal and eliminate DC effects such as drift. More traditional systems use a chopper wheel to do the same thing.

In order to avoid overheating the resistive element (the membrane), the note implies that during the on-portion of the excitation, you will monitor both applied voltage and current. From this you can calculate the resistance, and from the resistance you can infer the temperature. So it's up to you to do these things.