| 摘要 |
<p>PURPOSE:To locally prevent dewing by providing a heating means in the inside of an infrared ray transmitting part so as to heat a part of the infrared ray transmitting part. CONSTITUTION:When outside air temperature becomes below prescribed set temperature, an outside air temperature thermo-switch 22 operates, a heater 21 is energized, and the length and the layout of the heater 21 are set so that the infrared ray transmitting part 2 is heated and kept at the temperature higher than the outside air temperature at an area through which infrared ray beam quantity 1/100-times as small as at fine and dry weather, at least, can be incident to a light receiving element 7. Thus, since the dewing is never caused within a corresponding heated range, erroneous information is never generated even if the dewing occurs in other part of the infrared ray transmitting part 2.</p> |
