The Spectral analyzer was a device used by the Ghostbusters to analyze photographs and determine whether there was a living presence in them. It worked using ionizing radiation such as x-rays, as Röntgens were a unit on the device.
Ray and Egon used the device to examine photographs taken by Peter of the Vigo painting. Speculating the presence of Multi-Planar Kirlian Emanations, they used a high Röntgen setting on a wide shot. After the analyzer completed its analysis, the photograph now revealed the River of Slime.