The minimum average optical signal power, Pmin., in optical communications is limited by the photodetector quantum efficiency and by noise. In this paper, the effect of thermal photons irradiated by all materials at absolute temperatures T>0 on optical information detection in communication lines is quantitatively considered. Usually, only the thermal current fluctuations in the photodetector are taken into account. Basing on the negentropy principle of information, assuming the Planck's blackbody radiation spectral distribution of photons, and describing the optical communication channel as non-symmetric noisy binary channel we have calculated the minimum energy required for the detection of one bit of information, ε= 6.5kT/bit, k =1.38×10-23 J/K being the Boltzmann constant. This ε value corresponds to the large error probability q = 0.20. At T = 20°C ε=4.05×10-21 J/bit and for the bit rate of R = 1010 bits/s one finds Pmin = Rε2.63×10-7 mW. In the case of more realistic value of q=10-9 ε=26kT/bit=1.05×10-19 J/bit, Pmin = 1.05×10-6 mW. This is only about 10 times lower than the quantum photodetection limit of conventional photodetectors. For more sensitive photodetectors the thermal photon noise can become important. It is shown that the minimum signal energy estimate ε≈10-19 J/bit is applicable also in a wider error probability range of q=10-3-10-15.