This post covers some foundations which are often misunderstood. Radiation emitted from a surface (or a gas) can go in all directions and also varies with wavelength, and so we start with a concept called spectral intensity. This value has units of W/m².sr.μm, which in plainer language means Watts (energy per unit time) per square … Continue reading Planck, Stefan-Boltzmann, Kirchhoff and LTE