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 each of N respective 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 α 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 α 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, wherein the step of fitting N fitting parameters α p comprises utilizing an optical collection efficiency w for each one of the N non-coincident rays, each optical collection efficienc being adapted to each ray by performing a simulation or an experiment which determines the efficiency of coupling of light from a given location within each one of the N non-coincident rays to a respective optical fiber connected to a spectrometer.