A spectral scan was a type of sensor scanning technique.
In 2371, the USS Defiant ran a spectral scan upon entering the Trialus system for the first time. The scan noted that the system's sun gave off modulated gamma bursts which indicated an anomaly in the sun's fusion cycle, prior to the appearance of the planet Meridian. (DS9: "Meridian")
When the USS Voyager encountered the planet Tarakis in 2376, Captain Kathryn Janeway ordered a full spectral scan be completed of the planet to, "look for anything unusual." Lieutenant Tuvok reported that there was nothing on geometric sensors but Ensign Harry Kim did pick up power signal emanating from the northern-most continent. (VOY: "Memorial")