Fig. 3From: Laboratory investigation of earthquake lightning due to landslidea Photoemission spectrum upon impact for granite. b Planck’s black-body radiation energy spectrum in the wavelength range from 600 to 800 nm, cI(740 nm)/I(620 nm) vs. T (K), black circle: I(740 nm)/I(620 nm) determined from Planck’s law for temperatures curves of 1000–3000 K; white triangle: I(740 nm)/I(620 nm) = 3.6, determined from the spectrum shown in aBack to article page