A safety distance measuring device employing an electromagnetic wave is provided. A laser diode 2 is energized to emit laser beams by a laser diode drive circuit 3 in accordance with a clock signal generated by a control circuit 29. The emitted laser beams are sequentially changed to predetermined scanning directions by a scanner 1 associated with control circuit 29. The scanned laser beams from scanner 1 are reflected by an object 19 to be received by a photodiode 5. An output of the photodiode is applied to control circuit 29 through a light receiving circuit 6 where an elapsed time from emission to receipt is computed to obtain a distance from scanner 1 to object 19. Based on data relating to scanning directions by a scanning position detector 4, a begin-and-end detecting circuit 28 can stop the radiation of electromagnetic waves around a scanning start point and a scanning end point which is danger to human bodies.

A distance measuring apparatus has: light receiving means for receiving light which is emitted from a light emitting element unit and reflected from an obstacle having a reflecting surface, a pair of members of high reflection performance being respectively disposed on portions of the reflecting surface; and a calculating circuit for calculating a distance from the obstacle on the basis of a time period between the emission and the reception of the light. According to the apparatus, in a measurement range of a short distance where light reflected from a reflecting surface positioned between the pair of members of high reflection performance is received, the maximum amount of light received by the light receiving means is set to be smaller than a sensitivity at which a snowfall is detected, and the minimum amount of light is reduced to a level at which an obstacle positioned at the short distance can be detected.

A surveying instrument is provided having a sighting telescope and a distance measuring device. The sighting telescope has a focusing lens group which is moveable to focus an object to be measured. The distance measuring device measures a distance between the object and the surveying instrument. A focusing operation of the sighting telescope is carried out to move the focusing lens group to a focal position of the object in accordance with the distance between the object and the surveying instrument measured by the distance measuring device.

Distance sensors are mounted on both ends of a vehicle and have light transmitting and receiving sections. A distance to an object is independently measured by alternately transmitting "one-side-only enlarged light beams" at a time interval and receiving corresponding reflected beams from the object. In an overlapped area of the "one-side-only enlarged light beams", the distance from the vehicle to the object, as well as the azimuth of the object, is measured by a triangulation distance measurement using an interval between the distance sensors as a baseline. The lengths of the "one-side-only enlarged light beams" in the running direction of the vehicle are about 50m and a width vertical to the running direction of the overlapped area is about 2m slightly greater than a width of the vehicle.

An obstacle detection system for vehicles detects kinetic attributes relative to a vehicle equipped with the system of an object present in a forward path of travel of the vehicle, sets a presumed zone into which the object is expected to have entered at a lapse of a specified period based on the kinetic attributes, and proves an object detected at a lapse of the specified period as the object previously detected if the object of second detection is determined as one having moved into the presumed zone and an obstacle precarious to the vehicle.