An energy-based tissue-sealing system, comprising: an energy-based instrument having a mechanism to deform tissue in the form of a vessel and an energy applicator to apply tissue-sealing energy to the tissue; a sealing energy source coupled to the energy-based instrument and configured to generate and deliver tissue-sealing energy to the energy-based instrument; wherein the tissue-sealing energy is light and the sealing energy source is a light source; a tissue monitor configured to monitor at least one optical parameter of the tissue structure during sealing; wherein the tissue monitor includes a light source configured to illuminate the tissue with light, a sensor configured to sense the light modified by the tissue structure, and a processor configured to determine the at least one optical parameter of the tissue structure based on the light modified by the tissue structure; a controller coupled to the sealing energy source and configured to control at least one parameter of the tissue-sealing energy generated by the sealing energy source based on the at least one optical parameter of the tissue structure, wherein: the light source includes a light element and a beam formation system, wherein the light element is configured to generate light, and the beam formation system is configured to form a light beam so that the tissue that is subject to the effects of the sealing energy is exposed to the light beam, wherein an angle θ is the angle of an incident light beam with respect to an axis perpendicular to the vessel, wherein the beam formation system is configured to vary the angle θ of the light beam over a range of angles, and wherein the beam formation system is configured to select the angle θ of the light beam so as to optimize the detection of the at least one optical parameter of the tissue. an energy-based instrument having a mechanism to deform tissue in the form of a vessel and an energy applicator to apply tissue-sealing energy to the tissue; a sealing energy source coupled to the energy-based instrument and configured to generate and deliver tissue-sealing energy to the energy-based instrument; wherein the tissue-sealing energy is light and the sealing energy source is a light source; a tissue monitor configured to monitor at least one optical parameter of the tissue structure during sealing; wherein the tissue monitor includes a light source configured to illuminate the tissue with light, a sensor configured to sense the light modified by the tissue structure, and a processor configured to determine the at least one optical parameter of the tissue structure based on the light modified by the tissue structure; a controller coupled to the sealing energy source and configured to control at least one parameter of the tissue-sealing energy generated by the sealing energy source based on the at least one optical parameter of the tissue structure, wherein: the light source includes a light element and a beam formation system, wherein the light element is configured to generate light, and the beam formation system is configured to form a light beam so that the tissue that is subject to the effects of the sealing energy is exposed to the light beam, wherein an angle θ is the angle of an incident light beam with respect to an axis perpendicular to the vessel, wherein the beam formation system is configured to vary the angle θ of the light beam over a range of angles, and wherein the beam formation system is configured to select the angle θ of the light beam so as to optimize the detection of the at least one optical parameter of the tissue. the light source includes a light element and a beam formation system, wherein the light element is configured to generate light, and the beam formation system is configured to form a light beam so that the tissue that is subject to the effects of the sealing energy is exposed to the light beam, wherein an angle θ is the angle of an incident light beam with respect to an axis perpendicular to the vessel, wherein the beam formation system is configured to vary the angle θ of the light beam over a range of angles, and wherein the beam formation system is configured to select the angle θ of the light beam so as to optimize the detection of the at least one optical parameter of the tissue.