Compared to the Cocconi and Morrison’s proposal to search for radio signals in the nearby stars. Dyson (1960) pointed out it was also useful to look at infrared signals at 10 microns radiated by his hypothesized Dyson sphere. He argued much more advanced civilizations would have built thin shells around their host stars to more efficiently harvest the energy of the host star.
He argued the Malthusian pressure would drive some of the advanced civilizations to build such a sphere to catch up with the exponential population growth. Additionally, he also argued that the such a sphere would have similar orbit size as the Earth and same temperature as well so that we could possibly detect it at the 10 microns which was also transparent to the Earth atmosphere.
I do not really understand how he concluded that the temperature of the sphere would be at the same temperature as the Earth. Would not the energy harvesting efficiency be much higher if the sphere was radiating at a much lower effective temperature. It seems to me that his conclusion about the temperature of the sphere came from our understanding of the Earth and it was 10 microns which was also transparent to the Earth atmosphere made him draw the conclusion that we should observe at 10 microns.
In the later reply letter, he clarified the idea of Dyson sphere to not being a solid sphere which was physically impossible to build but a swarm of objects traveling on independent orbits around their host star. He also admitted whether the alien civilizations obeyed the Malthusian principle or not was a question of taste. Finally, he argued that even if we did not find any aliens, the search for intense infrared signals could help us identify interesting astronomical objects.