Emergency illumination panel that displays different pictures depending on room brightness

We fabricated an emergency illumination panel by stacking two translucent pictures, one of which was painted with ink and the other with phosphors. When the panel was illuminated by backlight or room light, only the ink-printed picture was visible to the human eye and only the phosphorescent picture was visible in darkness. A two-way mirror was inserted between the two pictures both to prevent super-imposition of the two images and to enhance phosphorescence. This phosphorescent emergency sign acted as an illuminator independently of electric power sources and provided sufficient brightness for reading newspaper headlines.