A low temperature radiometer includes a main body, a main cavity, an exit cavity, a suspended thermometer, and an attached thermometer. The main cavity is disposed within the main body and is defined through an off-axis parabolic concentrating cone formed of the inner walls of the main body. The exit cavity is disposed within the main body and is defined through a cylindrical inner surface of the main body. The suspended thermometer is suspended within the exit cavity and is disposed to be in communication with radiation entering the main cavity and being diverted to the exit cavity. The attached thermometer is attached to the outer surface of the main body and is in thermal communication and contact with the main body.