Laser rangefinder

The optical distance measurement, also called laser distance measurement, is an electronic distance measurement based on delay measurement, the phase angle measurement or laser triangulation of light, usually laser.

More active and passive optical distance measurement techniques are, inter alia, the light sectioning method and triangulation in the field of geodesy and surveying.

The laser triangulation and the laser interferometer are preferably suitable for short distances (a few microns to 100 meters), the transit time method, however, rather large distances (one meter to 1011 meters).

Transit time measurement

In the transit time measurement, a time the light pulse is emitted. The pulse duration is the time it takes the light beam to be reflected to the source. By measuring the travel time, to determine the distance between the source and object of the speed of light. The factor 0.5 reflects the fact that the light must pass through the distance to the object and back. The speed of light is reduced by the surrounding medium with the refractive power / refractive index.

The advantage of this method is the low reaction time. The method has measurement ranges from one meter to several 10 km. The disadvantage is the required measurement a very short time ( nano-to pico-seconds ), it is therefore difficult to achieve a resolution higher than a few centimeters.

To reduce the requirement on the timing, processes are used in which the laser beam is itself frequency modulated or is modulated with a high frequency signal.

This method uses the lidar, the satellite distance measurement, the TOF camera, PMD sensor.

Measurement of the phase angle

The phase shift of the reflected laser beam or its modulation compared to the emitted beam is dependent on distance. This phase shift can be measured and used to determine the distance traveled. If the laser frequency itself is used to overlay, the unit operates as a laser interferometer. Laser interferometers do not measure absolute path lengths but only the relative change in displacement of the target and a reference mirror. When moving the mirror, the sum of ausgesandtem and reflected beam is periodically modulated ( interference). It passes through exactly one period in the shift by a half wavelength of light. Counting the passages and multiply it by the light wavelength, one obtains the desired distance. With a more precise evaluation of the signal to reach accuracies of about wavelength, which are in visible light a few nanometers. The wavelength of light is, however, dependent on the refractive index of air and varies with temperature, pressure and humidity. For accurate measurements, the count of the wavelengths of light must be corrected ( air density correction ) with these properties of air.

For longer distances you are working with a high-frequency modulation of the laser amplitude and does not evaluate the laser wavelength, but the phase position of these modulated high frequency signals. If one assumes that the transmitted beam with a frequency f = 1 / T was modulated, we obtain the following graph:

The phase difference is obtained by the equation:

The distance can be

Be calculated.

The advantage of these methods is compared with runtime process higher resolution, which can be realized with less metrological effort. The measured distance is - due to a necessarily continuously operating at low power laser - lower.

Another problem is the lack of uniqueness of the signals at distances of a multiple of half the laser wavelength or modulation.

Reaching an absolute measurement

Interferometric method, the ambiguity can be circumvented by using a frequency modulation of the laser or the high-frequency modulation signal. It thereby leads to the phase measurement a runtime component a. Due to a lower frequency ( longer period ), we obtain a greater distance a unique result, but a lower resolution. To see even in principle FMCW radar. Procedure with RF modulated laser achieve maximum measuring distance of about 200 meters.

Two methods to obtain an absolute distance measurement by measuring the phase of:

Continuous frequency modulation ( FM function as a radar); Comparing now the original with the reflected signal is a frequency difference between the two. This difference is proportional to the distance.

Taking account of the phase difference:

However, it is not possible to control the wavelength of the laser accurately. Therefore, its wavelength has to be used as a reference.

In direct frequency modulation of the laser to achieve resolutions of 1 micron. However, we obtain with conventional lasers a maximum measuring distance of 1 meter.

In order to eliminate the uncertainty of the relative interferometric measurement in two or more discrete frequencies, the phase position is measured. The frequencies can turn the laser frequency itself ( different lasers, with the smallest distance) or modulation frequencies of one and the same laser be (frequencies must match distances and measuring range).

Laser triangulation

In laser triangulation, a laser beam ( with low requirements, the radiation of a light-emitting diode ) onto the object to be measured and observed with a camera, a position-sensitive photodiode or a CCD line sensor located in the next. Changing the distance of the object from the sensor, also the angle at which the light spot is observed, and thus the position of its image on the photoreceptor. Changes From the position change, the distance of the object is calculated from the laser projector using the trigonometric functions.

The photodetector is a light-sensitive element, which determines the position of the light spot in the image. From this screen position, the distance between the sensor and the object is calculated.

One advantage of triangulation is the fact that it is purely trigonometric relationships. The measurement can therefore be continuous and is thus ideally suited for distance measurement on moving objects. In order to reduce the stray light sensitivity and the influence of inhomogeneous reflecting surfaces, the measuring point must be as small and light. Often, these sensors operate in the pulse mode.

The method is suitable only for short distances, since its sensitivity to the fourth power ( Zweiwegdämpfung ) decreases with the distance between transmitter and receiver. Laser and photodetector are generally placed together in a housing.

The above scheme illustrates the relations between the different distances. With the aid of trigonometry, it is possible to determine the distance from the measured distance:

Summary

Meters

In geodesy, the distance measuring devices are called the work according to the principle of transit time measurement or phase modulation tachymeter or distancer.