The Diffuse Reflection Problem Of Laser Distance Meter
Usually in order to reduce errors, laser rangefinders will put a reflective surface on the measured end to reduce the errors caused by diffuse reflection. How does the telescope-type laser rangefinder used by snipers overcome this problem? Working principle and Sonar is similar, but will the signal that receives the reflected light be easily interfered by other wavelengths and intensities of light in the environment?
The detection device of laser rangefinder (pulse type) generally uses avalanche photodiodes, which are only sensitive to specific wavelengths of light. If the wavelength is matched, the extremely small light intensity can be detected by it, and the wavelength does not match, even if the light intensity is large. , It cannot be detected either. The laser just has the characteristics of good monochromaticity, and the commonly used wavelength is 905nm. Therefore, the signal that receives the reflected light is not simply interfered by light of other wavelengths and intensities in the environment.
There are two commonly used laser ranging schemes: pulse method and phase method.
The phase method measures the distance by measuring the phase deviation of the returned wave. This required cooperation target is to put a reflecting surface on the measured end. In this case, the transmission power of the rangefinder is small.
Telescope-type laser rangefinders used by snipers generally use the pulse method, that is, announce a pulse, start timing, and stop timing after receiving the reflected pulse to achieve the purpose of ranging. In this case, when there is no cooperative target, the energy loss of diffuse reflected light waves makes it very serious, but generally does not affect the measurement. The reason is as mentioned earlier. Generally, the transmitting power of the rangefinder will be increased to make certain compensation.
Nowadays, there are many types of laser rangefinders and their functions are very different, but from the operating method (perhaps the operating principle), there are only two types, namely: pulsed laser ranging and continuous wave laser ranging. The principle and characteristics of the two rangefinders are explained below:
(1) Pulse laser ranging
The principle of pulsed laser ranging is to use a pulsed laser to send a very narrow light pulse (pulse width less than 50ns) to the policy. After the light reaches the surface of the policy, part of it is reflected. It can be calculated by measuring the time from the light pulse to the receiver. The distance between the rangefinder and the policy. Assuming that the measured distance is h, the reciprocating time of the light pulse is t, and the propagation speed of light in the air is c, then: h=ct/2
Pulsed laser rangefinders can declare very strong lasers. They are talented in range finding. Even for non-cooperative policies, the maximum range can reach more than 30,000m. Its ranging accuracy is generally 5 meters, and the highest can reach 0.15m. Pulsed laser rangefinders can be used not only in the military to measure the range of various non-cooperative policies, but also in the climate to determine the visibility and cloud height. It is also used in the field of precision distance measurement of artificial satellites.
(2) Continuous wave laser ranging
CW laser rangefinders generally use the phase method for ranging. The principle is to first emit a modulated CW laser beam to the policy. After the beam reaches the policy surface, it is reflected, and the modulated laser beam emitted is measured and received by the receiver. The phase difference between the echoes can be derived from the distance between the policy and the rangefinder.
Because of its wide range of uses, the opening of laser rangefinders is unstoppable. Nowadays, there are laser rangefinders in the market: pulsed laser rangefinders and phase laser rangefinders. Although they are all used to measure distances, they have their own interests. The pulsed laser rangefinder has poor measurement planning accuracy, while the phase laser rangefinder has a small measurement plan but high accuracy can reach the millimeter level. During this period, the application fields of pulsed laser ranging have become wider and wider, such as topographic surveying, tactical frontier ranging, missile operation orbit tracking, and measurement of artificial satellites, the distance from the earth to the moon, etc. The pulsed laser ranging method uses the characteristics of a very short duration of laser pulses, relatively convergent energy, and large instantaneous power. With a cooperative policy, pulsed laser ranging can reach extremely long ranges; assuming that only the measured range is used The tiny reflection signal obtained by the diffuse reflection of the pulsed laser can also be used for distance measurement. Therefore, the pulsed laser ranging method is used more frequently.
Now briefly talk about the operating principle of the pulsed laser rangefinder: the laser emitted by the rangefinder is reflected by the object to be measured and then received by the rangefinder. The rangefinder records the reciprocating time of the laser, the product of the speed of light and the reciprocating time. Half of that is the distance between the rangefinder and the object to be measured.
The measurement accuracy of the pulsed laser rangefinder is poor, and the general accuracy can only reach the level of m, so some occasions with high accuracy requirements may require the use of a phased laser rangefinder. However, for long-distance measurement, because of the wide measurement planning, a measurement error of one or two meters is now considered to be very accurate.