A device (1, 1', 83) for treating skin by means of radiation, which device comprises: - a housing (3, 85), which accommodates a radiation source (9, 91) and which is provided with an exit opening (15, 99) for the radiation; - a detector (43, 63, 103) for detecting the presence of skin directly in front of the exit opening; and - a control unit which activates the radiation source only if the detector detects the presence of skin directly in front of the exit opening; - wherein the housing comprises a skin contact element (21, 101) in which the exit opening is formed and in which said detector is provided near the exit opening; - wherein the detector can suitably be used to measure a scattering coefficient and/or an absorption coefficient of the skin for light of a predetermined wavelength by means of which the skin can be characterized; - wherein the control unit comprises a comparator for comparing a value or condition of the scattering coefficient and/or the absorption coefficient, measured by means of said detector, with a skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient; - wherein the skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient is related to a state wherein the skin contact element (21, 101) is in contact with the skin; - wherein the control unit activates the radiation source (9, 91) only if the value or condition of the scattering coefficient and/or the absorption coefficient, measured by the detector (43, 63, 103), corresponds within predetermined limits to the skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient; and - wherein the detector is provided with a light sensor and a light source for light of said predetermined wavelength, which light source is arranged next to the light sensor and optically separated from said light sensor, the light source and the light sensor being in contact with the skin only if the skin contact element is in contact with the skin, and the detector determining the scattering coefficient and/or absorption coefficient by comparing an amount of light measured by the light sensor with an amount of light generated by the light source; characterized in that the detector is provided with a further light source for light of a further predetermined wavelength different from said predetermined wavelength, which further light source is also arranged next to the light sensor, optically separated from said light sensor and in contact with the skin only if the skin contact element is in contact with the skin, the detector determining the scattering coefficient and/or absorption coefficient for said predetermined wavelength and said further predetermined wavelength by comparing the amounts of light measured by the light sensor with the amounts of light generated by the light source and the further light source. - a housing (3, 85), which accommodates a radiation source (9, 91) and which is provided with an exit opening (15, 99) for the radiation; - a detector (43, 63, 103) for detecting the presence of skin directly in front of the exit opening; and - a control unit which activates the radiation source only if the detector detects the presence of skin directly in front of the exit opening; - wherein the housing comprises a skin contact element (21, 101) in which the exit opening is formed and in which said detector is provided near the exit opening; - wherein the detector can suitably be used to measure a scattering coefficient and/or an absorption coefficient of the skin for light of a predetermined wavelength by means of which the skin can be characterized; - wherein the control unit comprises a comparator for comparing a value or condition of the scattering coefficient and/or the absorption coefficient, measured by means of said detector, with a skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient; - wherein the skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient is related to a state wherein the skin contact element (21, 101) is in contact with the skin; - wherein the control unit activates the radiation source (9, 91) only if the value or condition of the scattering coefficient and/or the absorption coefficient, measured by the detector (43, 63, 103), corresponds within predetermined limits to the skin-characteristic value or condition of the scattering coefficient and/or the absorption coefficient; and - wherein the detector is provided with a light sensor and a light source for light of said predetermined wavelength, which light source is arranged next to the light sensor and optically separated from said light sensor, the light source and the light sensor being in contact with the skin only if the skin contact element is in contact with the skin, and the detector determining the scattering coefficient and/or absorption coefficient by comparing an amount of light measured by the light sensor with an amount of light generated by the light source;