Abstract Thermoradiative (TR) power generation in a diode optically coupled to a cold environment is a promising energy conversion mechanism. Here, we develop and validate a simple method to accurately evaluate the device-temperature dependence of TR diode operation using lock-in detection in combination with a reflective optical chopper. A formulation within the framework of detailed-balance theory shows that accurate lock-in detection of the actual short-circuit current is achieved only when the chopper reflectance is sufficiently close to unity if the device and chopper temperatures differ. We experimentally validate this formulation by measuring the generation current from an InGaAs photodiode facing an infrared-emissive surface using chopper blades with different reflectivities. The switching temperature of the device between TR and photovoltaic modes was correctly detected using a highly reflective chopper covered with gold-evaporated film. The proposed method establishes a practical characterization protocol for TR devices, enabling reliable evaluation of their temperature-dependent operation.