The U.S. Pat. No. 6,018,374 proposes a technology for interfering against such an illegal activity. This technology is based on a difference between the characteristics of human vision and the image capturing characteristics of image capturing cameras and uses an infrared light as a means of interference. Specifically, an infrared radiation apparatus, which is placed at a position close to or far away from an image projector, radiates an infrared light on a screen, so that a reflective light may radiate into an image capturing camera belonging to an individual who is engaged in an illegal activity. In other words, the illegally captured image may record the optical images of the infrared light that is unrelated to the image being presented. As a result, the quality may be lost in the image, that has been illegally captured, and, in some cases, the illegal activity may be identified. Of course, the infrared light is undetectable to a human, and the viewer may not experience any difficulty viewing the image being presented.
Such a use of the infrared light may make it possible to realize an adequate interference effect and prevention effect. However, a diverse interference technology needs to be established in order to protect critical contents.