According to the theory of infrared radiation and principles of temperature measurement using infrared imager, a universal mathematical model of infrared imager is established. Based on the normal emissivity characteristics of measured surface, the mathematical model is simplified, and the formula of temperature measurement using infrared imager is obtained. Through the relevant experiment, it is proved that the sum of emissivity and reflectivity of objects remained basically unchanged in a certain temperature range. The sum of emissivity and reflectivity of objects is relevant to the object types, surface conditions and the object temperature. The closer an object to Lambertian objects, the greater the sum is and the closer it is to 1. The farther the surface conditions deviate from the Lambertian surface, or the smoother the surface, the smaller the sum is. Experimental results show that if the object is close to Lambertian objects, it could be regarded as Lambertian, without the need for amendments to the actual objects. For non-Lambertian body (especially the smooth surfaces and low-emissivity objects), the amendment is necessary, or the temperature measurement error will increase, or even the obtained temperature is very far away from its true temperature. The study shows that, through the amendment, infrared temperature measurement on non-Lambertian objects is available.