1. A photo-therapy method using a display device, the photo-therapy method comprising: receiving a viewing time input and a photo-therapy selection input, wherein the viewing time input represents an expected viewing time of a user and the photo-therapy selection input represents a photo-therapy selected by the user; calculating a period of a photo-therapy image based on the expected viewing time and a suggested treatment time of the selected photo-therapy; and displaying the photo-therapy image with the calculated period while displaying a normal image.