An illuminating instrument for diagnostics, therapy or surgery of a body part, comprising: at least one optical fiber for transmitting electromagnetic radiation directly from an electromagnetic radiation source to the body part; and a housing which supports the optical fiber; the optical axis of the light-output end of the optical fiber circumferentially extends in an illumination portion in the housing which is annular shaped and arranged to face the body part; the optical fiber in the illumination portion has one or more diffusing regions for diffusing the electromagnetic radiation to the outside of the optical fiber through the circumferential surface thereof, a coupling device which supports the electromagnetic radiation source for generating the electromagnetic radiation, characterized by further comprising: a conversion means which is supported by the housing without directly contacting the electromagnetic radiation source, and arranged between the diffusing regions and the body part for converting the electromagnetic radiation into visible light for illuminating the body part, and wherein the light-entrance end of the optical fiber is directly optically connected with the electromagnetic radiation source, wherein the conversion means is outside an optical interface between the light-entrance end of the optical fiber and the electromagnetic radiation source. at least one optical fiber for transmitting electromagnetic radiation directly from an electromagnetic radiation source to the body part; and a housing which supports the optical fiber; the optical axis of the light-output end of the optical fiber circumferentially extends in an illumination portion in the housing which is annular shaped and arranged to face the body part; the optical fiber in the illumination portion has one or more diffusing regions for diffusing the electromagnetic radiation to the outside of the optical fiber through the circumferential surface thereof, a coupling device which supports the electromagnetic radiation source for generating the electromagnetic radiation, characterized by further comprising: a conversion means which is supported by the housing without directly contacting the electromagnetic radiation source, and arranged between the diffusing regions and the body part for converting the electromagnetic radiation into visible light for illuminating the body part, and wherein the light-entrance end of the optical fiber is directly optically connected with the electromagnetic radiation source, wherein the conversion means is outside an optical interface between the light-entrance end of the optical fiber and the electromagnetic radiation source. a conversion means which is supported by the housing without directly contacting the electromagnetic radiation source, and arranged between the diffusing regions and the body part for converting the electromagnetic radiation into visible light for illuminating the body part, and wherein the light-entrance end of the optical fiber is directly optically connected with the electromagnetic radiation source, wherein the conversion means is outside an optical interface between the light-entrance end of the optical fiber and the electromagnetic radiation source.