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If your thermal camera has an adjustable emissivity setting, you should calibrate the camera to the emissivity of the surface you are measuring. Emissivity can change with temperature so, make the adjustment at operating temps if you can.
An easy way to do this is to place black electrical tape, which has an emissivity of about 0.97, on the surface you want to measure. Set the emissivity on the camera to 0.97 and measure the temperature on the tape. Then, look at the temperature adjacent to the tape and adjust the emissivity the value on the camera until the temperature reading matches what you saw on the tape. That will be the emissivity of the surface you want to measure.
In the case of surfaces that are to hot or cold curvy for the tape, substitute black tempura paint for the tape.
This procedure will help ensure you are seeing accurate temps with the camera.
Here is a link to emissivity values for common materials.