Measures of chirality and angular momentum in the electromagnetic field

It is shown that prominent measures of the helicity of chiral electromagnetic radiation have a common basis, in differences between the populations of optical modes associated with angular momenta of opposite sign. The analysis helps to develop the interpretation of chiroptical phenomena in which such metrics have been treated as representing physically distinct “superchiral” phenomena. Constructing theory with due cognizance of photonic character, it emerges only that the spin angular momentum of light is engaged in such observations. In consequence, the term “superchiral” is redundant.