Volume of infrared rays radiated from the completely black objects is decided as per the relationship of wavelength of light and the temperature. This is called "Planck's law".
Refer to the below graph. Values of "6000K* ", "3000K" indicate the temperature. Horizontal axis of the graph indicates wavelength, wavelength gets longer as we move to the right side. On the other hand, vertical axis indicates the volume of light (radiation volume). The volume becomes larger as we move to upper side.
From this graph, it can be confirmed that when the temperature is higher, more light is radiated.
When the temperature reaches 1000K, then visible rays are radiated and red light can be seen. Further, when it reaches 2000K, 3000K, then light having short wavelengths is radiated. That is why the flame of matchstick or candle looks more bluish when the temperature rises more and more (visible rays having short wavelength).
From this graph, we can conclude that the temperature of completely black objects could be obtained if the volume of infrared rays radiation is known.
Relationship of wavelength and radiation volume as per the temperature variation
*"K" is read as "Kelvin". When atoms and molecules of certain object completely stop moving, then temperature of that object becomes "Absolute zero". Unit of absolute temperature is "K=Kelvin". Absolute zero could be indicated as "-273.15°C" if described in our familiar expressions.