It is proved that the equality ∆ ln |κ−λ| = 6κ, where κ is the Gaussian curvature of a metric tensor 1 on a 2-dimensional manifold is a sufficient and necessary condition for local realizability of the metric as the Blaschke metric of some affine sphere. Consequently, the set of all improper local affine spheres with nowhere-vanishing Pick invariant can be parametrized by harmonic functions