Abstract: The invention provides an IR modulator for synchronized interruption of an IR beam path in a thermal imaging device. A layer made of thermorefractive material is arranged in the beam path, and means are provided for changing the temperature of the thermorefractive layer.

Abstract: A method and apparatus are provided for determining the range of vision of the driver of an motor vehicle upon encountering a fog. A transmitter and receiving member is mounted on the front portion of the motor vehicle. A series of beam are transmitted from the range finder to different measured portions of the roadway. The reflections from the roadway are monitored. When the atmospheric conditions change so that a fog or other obstacle appears in the roadway, the reflected signal will take on different characteristics or will not appear at all. The driver of the motor vehicle can have a visual and/or an acoustic warning indication inside the vehicle to note that some dangerous condition exists.