Calbration of an infrared sensor
Your question is worded poorly. I think what you mean is why is it important to also know the temperature of a thermopile itself (as opposed to just the temperature of the target).

This is because thermopiles only measure the temperature DIFFERENCE between themselves and the target. It does not measure absolute temperature. If you want to measure absolute temperature of a target then you have to have another way to measure the thermopile's own temperature and then add that to the temperature difference being measured by the thermopile.


  This seems really logic, would be nice if I can find this back in an equation for example


Now that you know what I just said, I don't see what you expect an equation to do for you here unless the equation you are looking for is just:

\$T_{target} = T_{Thermopile} + \Delta T_{measured} \$