Far infrared heating tube heating process some red some not how red is what reason?

Electric heating tube even consumed power, that is to say, even if converted into heat energy, due to the different heating pipe internal insulation treatment is different, the effect of heat transfer to the external pipe, heating pipe material, diameter, to adapt to the heating medium is different, and the heating pipe environment different, such as a number of factors, will affect the surface can reach the temperature of the heating tube