1. A method for determining a spatial distribution of plasma optical emission, comprising: igniting a plasma in a plasma processing chamber, the plasma processing chamber having a plasma optical emission measurement system, the plasma optical emission measurement system having a controller for controlling the plasma optical emission measurement system; using the plasma optical emission measurement system, measuring N plasma optical emission spectra integrated along N non-coincident rays across the plasma processing chamber, where N>1, each measured optical emission spectrum comprising M wavelengths, where M≧1; selecting, using the controller, an optical intensity distribution function I(r, θ) comprising a sum of N basis functions F p (r, θ) I ( r , θ ) = ∑ p = 1 N a p F p ( r , θ ) wherein at least one of the N basis functions F p (r, θ) varies with both radial location r and circumferential location θ inside the plasma processing chamber, and wherein each of the N basis functions F p (r, θ) is associated with a fitting parameter a p ; and determining, using the controller, a spatial distribution of plasma optical emission for each of the M wavelengths by fitting the N fitting parameters a p of the selected optical intensity distribution function I(r, θ), to fit the selected optical intensity distribution function I(r, θ) to the N measured plasma optical emission spectra.