• Theoretically, the energy resolutions in a low temperature microcalorimeter is primarily limited by thermal phonon fluctuations. In this case, the device resolution is given by

where k is a Boltzman's constant, T is operating temperature and C is the heat capacity of the device. At low temperatures the energy resolution of an X-ray microcalorimeter can be as low as few eV, which makes it an extremely valuable tool for spectroscopy. 

• The TES is a very sensitive thermometer made from a superconducting film typically voltage biased (VBias) on the transition between the superconducting and normal states.

• The TES is thermally coupled to a cold bath whose temperature (TB) is bellow the transition temperature (TC). At bias point, the Joule heating, caused by the current flowing through the TES, is compensated by the heat flowing from the TES to the bath.

• When energy is deposited into the TES its temperature rises causing its resistance to increase. The increase in resistance reduces the Joule heating and current flowing through the TES (under the constant voltage bias). The change in current is typically measured by the SQUID-based current amplifier.

• The energy deposited in the film is removed by a reduction in Joule heating which happens on a much faster time scale than the intrinsic time constant of the device (t=C/G where C is the heat capacity of the TES and G is the thermal conductivity to the bath). Therefore, the energy deposited into the TES is equal to the integral of the change in Joule power which is equal to the bias voltage times the integral of the change in current measured with the SQUID.