Abstract: The paper describes a multi-wavelengthes pyrometer. The pyrometer is based on the planck's law. And it is assumed that the logarithm of emissivity is approximated by its polynomial in terms of wavelength. A representation of four-colour pyrometer is designed. The representing temperature of four-colour wavelength pyrometer come from that of ratiocolour wavelength pyrometers.T_F^-1, reciprocal of four-colour temperature is the linear combination of the T_R1^-1 and T_R2^-1 reciprocal of two ratio temperature. The coefficients of the linear combination are decided by the respective wavelengthes. If one of wavelengthes chosed by a ratio is equal to another one of wavelengthes chosed by the other, the four-colour pyrometer is going to become three-colour one. The four-colour（or threecolour） temperature cuts off the error caused by the influence of the emissivity, with 1n ε assumed as being linear in wavelength.
A optical fiber is used to transmit the radiation emitted by the measured target. It is the use of technique of optical fiber branch that incoming radiation is distributed to the surface of each dectector. A microprocessor is used to calculated the four-colour signal of radiation. The result of the calibration was gived out by the blackbody furnace. The error of four-colour temperature is compared with that of ratio temperature by the experiment on some metal target.
It is said that the technique of multi-wavelength temperature measurement is one. of effective method to decrease the error caused by the emissivity of the measured surface.