225 ILCS 80/5
Every person to whom a valid existing license as an optometrist has been issued under this Act, shall be designated professionally as an "optometrist" and not otherwise, and any such licensed optometrist may, in connection with the practice of his or her profession, use the title or designation of "optometrist", and, if entitled by degree from a college or university recognized by the Department of Financial and Professional Regulation, may use the title of "Doctor of Optometry", or the abbreviation "O.D.". When the name of such licensed optometrist is used professionally in oral, written, or printed announcements, prescriptions, professional cards, or publications for the information of the public, and is preceded by the title "Doctor" or the abbreviation "Dr.", the explanatory designation of "optometrist", "optometry", or "Doctor of Optometry" shall be added immediately following such title and name. When such announcement, prescription, professional care or publication is in writing or in print, such explanatory addition shall be in writing, type, or print not less than one-half the size of that used in said name and title. No person other than the holder of a valid existing license under this Act shall use the title and designation of "Doctor of Optometry", "O.D.", or "optometrist", either directly or indirectly in connection with his or her profession or business.
225 ILCS 80/5