Image Credit: E Diego/Shutterstock.com
One way to measure the distance to a target is to use a measuring tape. An alternative way is to shoot a laser pulse at the target, detect its reflection, and calculate the distance based on the time between the emission and detection of the pulse. This is the “time of flight” principle and is the basis on which a laser distance meter functions.
The laser source within the laser distance meter emits a pulse in the direction of measurement. This pulse is reflected off the target surface and travels back to the distance meter where it is detected and the time difference is recorded. Since the speed at which the laser pulse travels through the atmosphere is known, the distance meter uses an onboard computer to calculate the distance between itself and the target using the formula:
Distance = Speed of light x Time taken / 2
The speed at which the laser pulse travels is fairly constant. This means that the accuracy of a simple laser distance meters can be in the order of a few millimeters to centimeters across distances of several feet to a few miles. With specialized equipment like telescopes, the time-of-flight method of calculation can be used to measure distances as large as that between the earth and the moon with high accuracies of around a few centimeters.
The laser source is a crucial component for operations that require such high accuracies. The beam quality of the laser and its pulse duration has a direct impact on the maximum distance a laser distance meter can measure. In order to have spatial accuracies of about 15 cm, the laser source must have a temporal accuracy of 1 ns. Typical pulse durations are of the order of hundreds of picoseconds to a few tens of nanoseconds.
Another requirement for long-range measurements is a small beam divergence. This is usually achieved by using large telescopes to increase the beam diameter thereby increasing its Rayleigh length, which consequently results in small beam divergence.
Moderate distance measurements require a laser pulse with a duration of about 1 ns with a pulse energy of 10 uJ, which can be achieved with eye-safe lasers. However, long-range measurements may require very high pulse energies that may cause laser safety issues. Additionally, the wavelength of the laser used in such applications must be in the eye-safe range.
Though measurements with laser are inherently better in every way compared to other methods of measurement, the laser pulse is just as prone to distortions in undesirable atmospheric conditions like deserts or excessive greenery as white light. In certain cases, the target surface could be made of a material that scatters a large portion of the incident laser light. These conditions could result in inaccurate readings with time-of-flight measurements. Here, the phase-shift method, which uses an intensity-modulated laser beam, is more effective.
The sensitivity and performance of a laser distance meter can be increased by incorporating optical filters to cut out unwanted background noise and undesirable spectral ranges.
A LiDAR is a more complex device that uses lasers pulses to map its surroundings. It operates on the same principle of time-of-flight like the laser distance meter.
Sources and Further Reading