Visible emission spectroscopy is used to quantify the carbon impurity concentration in the linear plasma divertor simulator of the Plasma Interaction Surface Component Experimental Station (PISCES-B). A technique has been developed to obtain noninvasively the absolute photometric calibration of a visible spectrometer in order to minimize exposure of equipment in the beryllium safety enclosure of PISCES-B. The principal intrinsic source of carbon appears to be chemical sputtering at the vessel walls, with a typical background concentration similar to 0.2% with deuterium plasmas. Helium plasmas have a lower carbon contamination similar to 0.01%. Methane gas injection is used to increase the carbon contamination in a controlled manner to better simulate tokamak edge plasma conditions. It is found that the spectroscopic method of determining the carbon fraction agrees well with the relative gaseous carbon contamination measured with a residual gas analyzer during plasma operations.