 Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself.To get around these difficulties, the ratio or two-color pyrometer was developed.They rely on the fact that Planck's law, which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck's statement of the intensities at two different wavelengths is divided.