In a screen light computation device, a first polarizing filter (93) blocks P waves reflected by a reflective surface, and screen light; the reflected light sensor (95) receives S waves; a second polarizing filter (83) allows the screen light to pass while blocking the S waves; and a screen light sensor (85), being set at Brewster's angle, where the P waves is not received, still receives some leakage P waves. A screen light extraction computation (LUT; not shown) stores a correlation between the detection value by the reflected light sensor 95 and the detection value by the screen light sensor 85. The LUT is queried, a value of only the screen light that does not include reflected light is computed, and the screen light that does not include ambient light (reflected light) reflected in a display screen of the monitor is detected without turning off a backlight of a monitor.