Surgical device with: an instrument for acting on a biological tissue by creation of a plasma with a light phenomenon, an appliance for supplying the instrument with electrical HF power, wherein the instrument has at least one HF electrode which is connected to the appliance via a line in order to be supplied with HF power, wherein an HF spark forms between the at least one HF electrode and the biological tissue, with a light-receiving device which is connected to the appliance via a light conductor which extends through the line, an analysis device for determining light features of the light created by the action on the biological tissue, wherein the analysis device has a processing device for determining light features, wherein the light features determined by the processing device are signatures which are calculated from the intensities of several wavelengths or part spectra, and with a discriminator device for assigning light features of the light created during the action to tissue features of the biological tissue subjected to the action, characterized in that the plasma has a plasma temperature which is kept so low that parts of the biological tissue which have vaporized in the plasma are not completely dissociated, that several light-receiving devices (15, 16) are arranged on the instrument, which are each oriented at a partial region of the region containing the light phenomenon and mainly receive light from different portions of the region (15a, 16a, 15b, 16b) containing the light phenomenon, in order to obtain direction information, and that to distinguish between healthy tissue and pathological tissue, each signature is calculated from intensity ratios of selected spectral lines characteristic of vaporized parts of the biological tissue. an instrument for acting on a biological tissue by creation of a plasma with a light phenomenon, an appliance for supplying the instrument with electrical HF power, wherein the instrument has at least one HF electrode which is connected to the appliance via a line in order to be supplied with HF power, wherein an HF spark forms between the at least one HF electrode and the biological tissue, with a light-receiving device which is connected to the appliance via a light conductor which extends through the line, an analysis device for determining light features of the light created by the action on the biological tissue, wherein the analysis device has a processing device for determining light features, wherein the light features determined by the processing device are signatures which are calculated from the intensities of several wavelengths or part spectra, and with a discriminator device for assigning light features of the light created during the action to tissue features of the biological tissue subjected to the action, characterized in that the plasma has a plasma temperature which is kept so low that parts of the biological tissue which have vaporized in the plasma are not completely dissociated, that several light-receiving devices (15, 16) are arranged on the instrument, which are each oriented at a partial region of the region containing the light phenomenon and mainly receive light from different portions of the region (15a, 16a, 15b, 16b) containing the light phenomenon, in order to obtain direction information, and that to distinguish between healthy tissue and pathological tissue, each signature is calculated from intensity ratios of selected spectral lines characteristic of vaporized parts of the biological tissue.