Emissivity factor value can be from 0 to 1.
In case of infrared thermometers which do not allow for adjusting this parameter manually, it is usually stored in internal memory of the device and it has a constant value of 0.95. It is suitable for most commonly examined materials.
A problem occurs in case of materials with a very smooth surface, such as polished metals, emissivity factor value of which is below 0.1.
In order to measure temperature of such objects, it would be good to buy an infrared thermometer which allows for entering emissivity factor manually. If it is not possible, there are some methods that will improve accuracy of temperature measurement:
Both these methods can be used in case of surfaces with unknown emissivity factor.