  We propose a phenomenological theory working out light illumination effects
on the equilibrium values of any macroscopic crystal tensor (polarization,
magnetization, susceptibilities, strain tensor, elastic coefficients...). it
also encompasses non-equilibrium light-induced quantities such as an electric
current and heat flow. We use a single phenomenological approach based on
Wigner spherical functions for predicting symmetry-related photo-induced
phenomena, including photovoltaic, photoelectric, photomagnetic,
photorefractive, photogalvanic, photoelastic and optic rectification phenomena.
For each crystal magnetic point group and each tensor type, response functions
are calculated vs. the propagation and polarization directions of the incident
electromagnetic beam. Their forms are determined by crystal and light symmetry
group interconnection. We pay special attention to time and space reversal
symmetries, which play a dominant role in the intricate symmetry breakdowns of
multiferroic materials. Photovoltaic and photo-magneto-electric effects in
LiNbO3 illustrate our theory.
