1. Active ranging method

Active ranging needs to emit artificial light to illuminate the object, and determine the distance by analyzing the texture changes of the reflected light of the object or directly measuring the propagation time of light;
Due to the use of the illuminating light source, the concealment is poor and the anti-interference is poor.

1. Laser ranging method

Utilize the advantages of laser (such as monochromaticity, good coherence, strong directionality, etc.), to achieve high-precision measurement and detection.
Pulse laser ranging: Send a laser signal to the target, and it will be reflected back after hitting the target. You only need to record the round-trip time of the laser signal, and multiply the speed of light by half of the round-trip time to obtain the measured distance.
Continuous wave phase laser ranging: The measured target is irradiated with a continuously modulated laser beam, and the distance of the measured target is calculated according to the phase change caused by the round trip. (In order to ensure the accuracy, it is generally necessary to install a laser reflector on the measured target)

脉冲式激光测距一般适用于距离较远对误差要求不高的测距场合，对动态目标的测距比较难实现；连续波相位式激光测距精度可以达到波长级别，缺点在于需要安装专门的激光反射器且要求物体的稳定性高（没有微小的变化）

The structured light ranging method is an active triangulation technique that requires the use of imaging equipment.
Basic principle: The laser projects light spots, light strips or light surfaces onto the imaging object and moves a certain distance $x$ ; the image of the light spot, light bar or light surface is accepted by the imaging device, and the displacement $x^{'}$ . Combining the distance from the intersection of the laser beam and the receiving optical axis to the front main surface of the receiving lens, the distance from the rear main surface of the receiving lens to the center point of the imaging surface, the angle between the optical axis of the laser beam and the optical axis of the receiving lens, and the distance between the detector and the receiving lens The angle between the optical axes can calculate the distance of the measured object.
Advantages: simple calculation, small size, good reliability and easy installation and maintenance;
Disadvantages: The field of view is not large, the system complexity is high and there are blind spots, and the measurement accuracy is inconsistent with the measurement speed.

Measuring principle: The grating pattern is projected onto the surface of the measured object, and the phase and amplitude will be modulated by the height of the object surface to deform the grating image. It is necessary to obtain the phase change containing the height information through demodulation, and finally complete the phase-height relationship according to the triangulation method. conversion between.
Raster production requires high requirements and a large amount of calculation.

The passive ranging method does not need to emit artificial light to illuminate the object, and determines the distance of the object by detecting the natural light radiation of the object and analyzing it. It has good concealment and a high degree of automation in ranging (directly measuring the image distance without human intervention) , good anti-interference, fast ranging speed, small size and light weight.

Basic principle: Use two or more cameras to image the same object from different positions to obtain a stereo pair, match the corresponding image points through various algorithms to calculate the parallax, and then use the method based on triangulation to recover the distance of the object.

The continuous auto-focus camera is used to continuously focus and image the target, and a function for evaluating the degree of focus is established in data processing to evaluate the focus situation; when the system reaches the best focus state, the lens imaging formula can be used to calculate the focus of the object distance.

There are also methods such as triangulation ranging, pixel high frequency vibration ranging and so on.