An optical assembly, for use in a skin treatment device, the optical assembly comprising: a light source; and a first prism including a first surface, a second surface inclined with the first surface and a third surface adjoining the first and the second surfaces; characterized in that the optical assembly further comprises: a first guiding element comprising enclosed reflective faces (121a, 121b) disposed facing each other, and arranged to guide the light transmitted from the light source, through the first surface of the first prism; and a second guiding element comprising enclosed reflective faces (131a, 131b) disposed facing each other, and arranged to receive, through the second surface of the first prism, the light reflected from the third surface of the first prism and output the received light for illuminating the skin, wherein the first surface and the second surface of the first prism are separated from the first guiding element and the second guiding element, respectively, by a refractive index interface and, during use, each act as a total internal reflection surface by being disposed in contact with a medium having a refractive index less than a refractive index (n p) of the first prism. a light source; and a first prism including a first surface, a second surface inclined with the first surface and a third surface adjoining the first and the second surfaces; characterized in that the optical assembly further comprises: a first guiding element comprising enclosed reflective faces (121a, 121b) disposed facing each other, and arranged to guide the light transmitted from the light source, through the first surface of the first prism; and a second guiding element comprising enclosed reflective faces (131a, 131b) disposed facing each other, and arranged to receive, through the second surface of the first prism, the light reflected from the third surface of the first prism and output the received light for illuminating the skin, wherein the first surface and the second surface of the first prism are separated from the first guiding element and the second guiding element, respectively, by a refractive index interface and, during use, each act as a total internal reflection surface by being disposed in contact with a medium having a refractive index less than a refractive index (n p) of the first prism. a first guiding element comprising enclosed reflective faces (121a, 121b) disposed facing each other, and arranged to guide the light transmitted from the light source, through the first surface of the first prism; and a second guiding element comprising enclosed reflective faces (131a, 131b) disposed facing each other, and arranged to receive, through the second surface of the first prism, the light reflected from the third surface of the first prism and output the received light for illuminating the skin, wherein the first surface and the second surface of the first prism are separated from the first guiding element and the second guiding element, respectively, by a refractive index interface and, during use, each act as a total internal reflection surface by being disposed in contact with a medium having a refractive index less than a refractive index (n p) of the first prism.