An apparatus for recognizing tissue during an energy-based tissue-sealing procedure, comprising: an energy-based tissue-sealing instrument (200, 300) for grasping tissue; an optical system for illuminating the grasped tissue or tissue adjacent to the grasped tissue with light; a display; an optical sensor and microprocessor configured to form a spatial distribution of the light transmitted, scattered, or reflected by the tissue into an image of the tissue, and the microprocessor configured to transmit the image to the display to allow a user to recognize the tissue based on the image of the tissue, wherein the tissue includes a vessel and wherein the optical system configured for illuminating the tissue with light includes illuminating the tissue with light having at least a first wavelength and a second wavelength, characterised in that the first wavelength is selected to be about 530 nm so that the difference between absorption or scattering of the light of the first wavelength by the vessel or a marker where introduced into fluid flowing in a vessel of the tissue and absorption or scattering of the light of the first wavelength by tissue surrounding the vessel is sufficient to recognize the vessel based on the light transmitted, scattered, or reflected by the tissue, and the second wavelength is selected to be about 630 nm so that the magnitude of the absorption or scattering of the light of the second wavelength by the vessel or a marker where introduced into fluid flowing in a vessel of the tissue is approximately equal to the magnitude of the absorption or scattering of the light of the second wavelength by the tissue surrounding the vessel. an energy-based tissue-sealing instrument (200, 300) for grasping tissue; an optical system for illuminating the grasped tissue or tissue adjacent to the grasped tissue with light; a display; an optical sensor and microprocessor configured to form a spatial distribution of the light transmitted, scattered, or reflected by the tissue into an image of the tissue, and the microprocessor configured to transmit the image to the display to allow a user to recognize the tissue based on the image of the tissue, wherein the tissue includes a vessel and wherein the optical system configured for illuminating the tissue with light includes illuminating the tissue with light having at least a first wavelength and a second wavelength, characterised in that the first wavelength is selected to be about 530 nm so that the difference between absorption or scattering of the light of the first wavelength by the vessel or a marker where introduced into fluid flowing in a vessel of the tissue and absorption or scattering of the light of the first wavelength by tissue surrounding the vessel is sufficient to recognize the vessel based on the light transmitted, scattered, or reflected by the tissue, and the second wavelength is selected to be about 630 nm so that the magnitude of the absorption or scattering of the light of the second wavelength by the vessel or a marker where introduced into fluid flowing in a vessel of the tissue is approximately equal to the magnitude of the absorption or scattering of the light of the second wavelength by the tissue surrounding the vessel.