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In the last quarter century, calorimeters have evolved as the particle detectors of choice in experiments at the energy frontier. However, development of the full potential of these detectors, which are based on total absorption of the particles to be detected, is hampered by the fact that electrons and photons generated in the absorption process produce significantly larger signals than equally energetic protons and pions generated in this process. The fact that the energy sharing between these different classes of shower particles (the electromagnetic fraction) varies strongly and in a non-Gaussian manner from one event to the next and is, on average, energy dependent, creates a whole series of awkward problems (non-linearity, non-Gaussian response functions, etc.) that could be avoided in the absence of this effect.

High-quality energy measurements will be an important tool for accelerator-based experiments at the TeV scale. There are no fundamental reasons why the four-vectors of all elementary particles could not be measured with a precision of 1% or better at these energies. However, reaching this goal is far from trivial, especially for the hadronic constituents of matter. Unfortunately, little or no guidance is provided by hadronic Monte Carlo shower simulations in this respect. In the past 30 years, all progress in this domain therefore has been achieved through dedicated R&D project, and this is still the way to go today.

The dual-readout approach is a remarkably successful and elegant way to eliminate the problems mentioned above. In the past several years,
the RD52 collaboration has performed a large number of dedicated tests which have demonstrated that the dual-readout method combines the advantages of compensating calorimetry with a reasonable amount of design flexibility. It holds the promise of high-quality calorimetry for all types of particles, with an instrument that can be calibrated with electrons.