Gas in the tube is ionized, current then passes through it. The current excites the ions that emit high energy photons (typically toward the ultra violet end of the spectrum) which impinge on a phosphor that coats the inside of the tube which absorbs the energy of the photon and then re-radiates that energy as heat + light at a different wavelength. (generally lower in the spectrum toward blue) The more "downshifting" the phosphor does the more heat it emits. --Chuck