  After a brief introduction to the field of light-responsive materials, this
paper provides a general theory for modeling the photomechanical response of a
material, applies it to the two best-known mechanisms of photothermal heating
and photo-isomerization, and then describes an experimental apparatus for
quantitative measurements of the stress response. Several different materials
are characterized to illustrate how the experiments and theory can be used to
isolate the contributing mechanisms both through photomechanical measurements
and auxiliary measurements of laser heating and thermal expansion. The
efficiency and figure of merit of the photomechanical response is defined on
several scales form the molecule to the bulk, and the photomorphon -- the basic
material element that determines the bulk response -- is introduced. The
photomorphon provides a conceptual model that can be expressed in terms of
viscoelastic elements such as springs in series and parallel with the
photoactive molecule. The photomechanical response, figure of merit, and the
deduced microscopic photomechanical properties are tabulated and proposals for
new materials classes are made.
