The instrument according to the invention for electrosurgically impacting biological tissue comprises an electrode (18) as well as a light guide (21), which is connected to a light inlet window (19), which is formed by means of a fluid body (27). The light guide is connected to a light analysis device (13), so as to absorb the light, which is generated at the electrode (18) in response to the HF surgery and so as to supply it to the light analysis device (13). The light inlet window (19) is arranged at the point of origin of the light, namely immediately at the electrode, that is, at the spark, which is generated. An adulteration of the absorbed light by means of smoke or particle deposition on the light inlet window (19) can virtually be avoided.
