Because of the lack of 3-D TV programs, many TV set manufacturers have developed a 3-D simulation technology that can convert 2-D into 3-D on ordinary television sets. But one 3-D TV set salesman says the 3-D simulation technology has some shortcomings. "The simulation technology can help create a depth of field effect, but compared to the genuine 3-D technology, it can't produce a convex effect." Song Linlin, a manager of an electronic appliances shopping mall, suggests that consumers purchase the 3-D TV with a big screen. "Aside from the big screen, the backlight of the 3-D TV set should send light evenly, such as LED. The higher the updated frequency of the pictures is, the better the TV set is." Although 3-D content is enjoyable and lively, experts warn that if viewers watch it for too long, they will experience visual fatigue. They suggest that viewers especially teenagers reduce the time they spend watching 3-D programs.