However, it has been pointed out that the self-heating underestimates the temperature of the particle when many nanoparticles are illuminated. All the NPs in the illuminated area are heated up and collectively increase the temperature of the area and the particles. Collective heating or global heating could raise the temperature (ΔTglobal) of the macroscopic region orders of magnitude higher than that due to self-heating (ΔTnano) even under CW irradiation with moderate laser intensities. Photothermal collective heating originated from heat diffusion or, in another word, temperature field overlap with other NPs surrounding the NP. A nanoparticle j experiences additional heating from the neighboring nanoparticles i heating their environment: