1. Molecular components of the dielectric constant
 = n2 = 1 + el + ion + or
orienting mol. dipoles
ion displacement
from equilibrium structure
electronic displacement
H. Haken & H.C. Wolf “Molecular Physics and Elements of Quantum Chemistry”, Fig. 3.5

2. From current literature:
Laura M. Herz, “How Lattice Dynamics Moderate the Electronic Properties of Metal-Halide Perovskites”, J. Phys. Chem. Lett. 2018, 9, 6853−6863
Figure 1. Schematic representation of the frequency-dependent relative permittivity (real part) of MAPbI3 at room temperature, derived from
averages over a number of literature reports covering various different frequency intervals.4 Features around 4−8 × 1014 Hz are associated
with electronic interband transitions; strong resonances near 1 and 2 THz derive from optical phonon resonances of the lead-iodide lattice. An increase in dielectric response near 100 GHz is associated with collective reorientation of MA cations. The rise in the value of ϵ′ toward the low frequency (≤104 Hz) regime originates from slow migration of ions, in particular, iodide.

3. Definitions of some basic photometric quantities
Radiant energy W (measured in J): total amount of radiant energy in a given context (e.g. as emitted by a light source, transmitted through a surface, collected by a detector)
Radiant power P or radiant flux (measured in W): radiant energy per unit time
Radiant energy density  (measured in J·m-3): radiant energy per unit volume
Consider surface element dA of a light source.
Radiance L (measured in W·m-2·sr-1): power emitted per unit surface into solid angle d around  against surface normal dP = L()·dA·d
Spectral radiant energy W(), power P(), energy density  (): amounts of W, P, , L in frequency interval d around frequency . Pay attention to the units!!!
Irradiance or intensity I (measured in W·m-2): radiant flux per unit area