Described herein are technologies to facilitate computed tomographic techniques to help identifying chemical species during plasma processing of a substrate (e.g., semiconductor wafer) using optical emission spectroscopy (OES). More particularly, the technology described herein uses topographic techniques to spatially resolves emissions and absorptions in at least two-dimension space above the substrate during the plasma processing (e.g., etching) of the substrate. With some implementations utilize optical detectors positioned along multiple axes (e.g., two or more) to receive incident incoming optical spectra from the plasma chamber during the plasma processing (e.g., etching) of the substrate. Because of the multi-axes arrangement, the incident incoming optical spectra form an intersecting grid.