  We present an analysis of the optical response of lumped-element
kinetic-inductance detector arrays, based on the NIKA2 1mm array. This array
has a dual-polarization sensitive Hilbert inductor for directly absorbing
incident photons. We present the optical response calculated from a
transmission line model, simulated with HFSS and measured using a Fourier
transform spectrometer. We have estimated the energy absorbed by individual
component of a pixel, such as the inductor. The difference between the
absorption efficiencies is expected to be 20% from the simulations. The
Fourier-transform spectroscopy measurement, performed on the actual NIKA2
arrays, validates our simulations. We discuss several possible ways to increase
the absorption efficiency. This analysis can be used for optimization of the
focal plane layout and can be extended to other kinetic inductance detector
array designs in millimeter, sub-millimeter and terahertz frequency bands.
