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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.
Bliokh et al. (Wed,) studied this question.