  We examine the recently introduced measure of chirality of a monochromatic
optical field [Y. Tang and A. E. Cohen, Phys. Rev. Lett. 104, 163901 (2010)]
using the momentum (plane-wave) representation and helicity basis. Our analysis
clarifies the physical meaning of the measure of chirality and unveils its
close relation to the polarization helicity, spin angular momentum, energy
density, and Poynting energy flow. We derive the operators of the optical
chirality and of the corresponding chiral momentum, which acquire remarkably
simple forms in the helicity representation.
