  We propose a legitimate and easily computable nonclassicality indicator for
the states of electromagnetic fields based on the standard deviation in the
measurement of the homodyne rotated quadrature operator. The proposed
nonclassicality indicator is the nonclassical area projected by the optical
tomogram of the quantum state of light on the optical tomographic plane. If the
nonclassical area projected by the optical tomogram of a quantum state is
greater than zero, the state is nonclassical, and the area is zero for the pure
classical state. It is also noted that the nonclassical area of a quantum state
increases with an increase in the strength of nonclassicality inducing
operations on the state such as squeezing, photon addition, etc. We have tested
the validity of the nonclassical area measure by calculating the same for
certain well-known nonclassical states and found that essential features of the
nonclassicality shown by the states are captured in the nonclassical area. We
have also shown that the nonclassical area is robust against
environment-induced decoherence of the states. Nonclassical area projected by
the optical tomogram of a quantum state of light is experimentally tractable
using the balanced homodyne detection of the quadrature operator of the field,
avoiding the reconstruction of the density matrix or the quasiprobability
distribution of the state.
