You are a funny guy ian....once again, you prove that you are perfectly willing to be fooled by instrumentation if it supports your beliefs....net radiometers are little more than souped up pyrogeometers..
Net radiometers operate via thermopiles whose warm joints are in thermal contact with the receiver and the upper cool joints are in thermal contact with the lower receiver...via a contrived mathematical model, the temperature difference between the two receivers is supposedly proportional to the net radiation...The temperature difference between hot and cold is converted to voltage by a Seebeck effect.
So congratulations...once again, you have fallen victim to being fooled by instrumentation...net radiometers are not measuring back radiation any more than pyrogeometers are...they are measuring the temperature difference between sensors on a thermopile...again, if you want to actually measure physical radiation (as opposed to radiation contrived via a mathematical model) you must cool the instrument to a temperature lower than that of the atmosphere...and once again, that is not back radiation, it is merely radiation moving from the warmer atmosphere to the cooler instrument.