Blackbody Line in the Infrared Color-Color Diagram

The blackbodsy line is an important part of the infrared color-color diagram. However, up to now, there is no paper carefully discussing its determination, mathematical property and physical meaning, although these questions are the basical ones. This paper gives the equations of the blackbody line in different infrared color-color diagrams, and also the calculating results. With the equations and results one can easily get the blackbody line, and avoid the complicated deriving and calculating. The mathematical property and the physical meaning of the blackbody line are also discussed in this paper.