Light scalar elds can drive the accelerated expansion of the universe. Hence, they are
obvious dark energy candidates. To make such models compatible with tests of General
Relativity in the solar system and \fth force" searches on Earth, one needs to screen
them. One possibility is the so-called \chameleon" mechanism, which renders an eective
mass depending on the local matter density. If chameleon particles exist, they can be produced
in the sun and detected on Earth exploiting the equivalent of a radiation pressure.
Since their eective mass scales with the local matter density, chameleons can be re
by a dense medium if their eective mass becomes greater than their total energy. Thus,
under appropriate conditions, a
ux of solar chameleons may be sensed by detecting the
total instantaneous momentum transferred to a suitable opto-mechanical force/pressure
sensor. We calculate the solar chameleon spectrum and the reach in the chameleon parameter
space of an experiment using the preliminary results from a force/pressure sensor,
currently under development at INFN Trieste, to be mounted in the focal plane of one of
the X-Ray telescopes of the CAST experiment at CERN. We show, that such an experiment
signies a pioneering eort probing uncharted chameleon parameter space.