  We present an electromagnetic chirality measure for 2D arrays of
subwavelength periodicities under normal illumination. The calculation of the
measure uses only the complex reflection and transmission coefficients from the
array. The measure allows the ordering of arrays according to their
electromagnetic chirality, which further allows a quantitative comparison of
different design strategies. The measure is upper bounded, and the extreme
properties of objects with high values of electromagnetic chirality make them
useful in both near- and far-field applications. We analyze the consequences
that different possible symmetries of the array have on its electromagnetic
chirality. We use the measure to study four different arrays. The results
indicate the suitability of helices for building arrays of high electromagnetic
chirality, and the low effectiveness of a substrate for breaking the transverse
mirror symmetry.
