When he detected some "capillary reaction" during Jean-Luc Picard's secondary cardiac implant procedure, the Starbase 515 surgeon ordered that the thoracic polychromatics be made "sharper". His surgical assistant then immediately adjusted a tool that seemed to be emitting some kind of radiation into Picard's chest. (TNG: "Samaritan Snare")

Since the real world study of "polychromatics" focuses on radiation that possesses more than one wavelength, it can be assumed — although the script does not specify it — that the surgeon was merely asking for his technician to adjust the radiation emitter that's in the center of the shot.
"thu-RAS-ik poly-kro-MAT-iks" was the pronunciation of this medical equipment's name from the script pronunciation guide. [1]