Principle of Infrared or Laser Distance Measurement
The ranging principle basically boils down to measuring the round-trip time of light traveling to the target. The distance D is then calculated using the speed of light \(c=299792458\ \text{m/s}\) and the atmospheric refractive index n. Since direct time measurement is rather difficult, the phase of a continuous wave is usually measured instead; such instruments are called phase-shift rangefinders. Of course, pulse rangefinders also exist.
It should be noted that phase measurement does not measure the phase of infrared or laser light itself, but the phase of the signal modulated onto the infrared or laser beam. Handheld laser rangefinders used in the construction industry for house surveying operate on this same principle.
② The plane of the measured object must be perpendicular to the light beam Precision ranging generally requires the use of a total-reflection prism. Rangefinders for building measurement perform measurements by reflecting light directly off smooth wall surfaces, mainly because the distance is short and the reflected signal strength is sufficient. It follows that perpendicular alignment is required; otherwise, the returned signal will be too weak to obtain an accurate distance reading.
③ Diffuse reflection on the measured object plane is permissible This is normally feasible. In practical engineering, thin plastic sheets are used as reflective surfaces to mitigate severe diffuse reflection issues.
④ For pulse laser rangefinders, consumer-grade products offer a display resolution of 1 m with a measurement accuracy of ±1 m. Measurement-grade products have a display resolution of 0.1 m and a measurement accuracy of ±0.15 m.
⑤ Phase-shift laser rangefinders can achieve errors as low as 1 mm, making them suitable for various high-precision measurement applications.
