To provide an illuminance sensor that does not respond to changes in an external light due to an illumination light, but will immediately respond to the change in a natural light.
This illuminance sensor uses a solar battery SB to detect the lightness of external light and makes a variable impedance element VI conductive, when the external light is a prescribed value or over. A liquid crystal panel LC, whose transmittivity is adjusted by an external signal, is placed to an incident path of the external light to the solar battery SB. The transmittivity of the liquid crystal panel is controlled smaller, when the variable impedance element VI is conductive such that when the variable impedance element VI is not conductive. Furthermore, the transmittivity of the liquid crystal panel LC differs between the visible light region and infrared ray region. Thus, a hysteresis characteristic with respect to the illumination light and that with respect to natural light are made different. Then the illuminance sensor has the hysteresis characteristic, that does not respond to the illumination light or light of head light at night, even when the light is received as the external light but that immediately responds to a sun light at daybreak.
MURATA YUKIHIRO
AKINARI YOSHINORI
