Is a Complete Determination of the Energy Spectrum of Light Possible from Measurements of the Degree of Coherence

It is shown that the analytic properties of the temporal complex degree of coherence γ(τ) in the complex time plane (τ complex) impose a relationship between|γ(τ)| and arg γ(τ) on the real time axis. This relationship involves, in general, the location of the zeros of the degree of coherence in the lower half of the complex τ plane. It is suggested that the analytic continuation of the temporal degree of coherence of many spectral distributions has no zeros at all in this half plane. The spectral profiles of such distributions could be uniquely determined from measurements of|γ(τ)| alone. This possibility is of interest in connection with Michelson's well-known method of visibility curves. It is also of interest in connection with the recently proposed correlation and coincidence techniques (employing square-law detection) for determining narrow spectral profiles, such as those found in the output from an optical maser.