  Near-equilibrium thermal detectors operate as classical calorimeters, with
energy deposition and internal equilibration times short compared to the
thermal time constant of the device. Advances in fabrication techniques,
cryogenics, and electronics have made it practical to measure deposited energy
with unprecedented sensitivity and precision. In this chapter we discuss
performance considerations for these devices, including optimal filtering and
energy resolution calculations. We begin with the basic theory of simple
equilibrium calorimeters with ideal resistive thermometers. This provides a
starting point for a brief discussion of electrothermal feedback, other noise
sources, various non-ideal effects, and nonlinearity. We then describe other
types of thermometers and show how they fit into this theoretical framework and
why they may require different optimizations and figures of merit. Most of this
discussion is applicable also to power detectors, or bolometers, where the
detector time constants may be short compared to variations in the incident
signal power.
