Long Range Target Location Using a Laser Sensor

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I began testing my Mindstorms NXT Laser Target Sensor with 40mm P.V.C. Pipe Tubes 50mm Long wrapped with 3M Scotchlite 8910 Retroreflective Silver Fabric. The distance between the Laser Sensor and targets varied from between 1 to 2 Metres in this Test. The Sensor uses a 650nm 5mW (Red) laser module. The Sensor can however reliably detect objects at 2.4m (8′), dependant on ambient light conditions.

When the Laser Beam is pulsed at 50ms on and 10ms off, it gives the best detection rates. The biggest issue is avoiding having a bright light sources hitting at 90 degrees to the targets which causes the sensor to be flooded by the reflection. Overhead and lights inclined 45 degrees and above cause no issues. This response to light sources is due to the shape of the retroreflector crystals on the tape.

Long Range Target Location Using a Laser Sensor

A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave’s source. The device or surface’s angle of incidence is greater than zero. This is unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence.

Corner Reflector
Working principle of a corner reflector

Retroreflectors are devices that operate by returning light back to the light source along the same light direction. The coefficient of luminous intensity, RI, is the measure of a reflector performance, which is defined as the ratio of the strength of the reflected light (luminous intensity) to the amount of light that falls on the reflector (normal illuminance). A reflector will appear brighter as its RI value increases.

The RI value of the reflector is a function of the color, size, and condition of the reflector. Clear or white reflectors are the most efficient, and appear brighter than other colours. The surface area of the reflector is proportional to the RI value and increases as the reflective surface increases.

The entrance angle is a function of the orientation of the reflector to the light source. For example, the entrance angle between an automobile approaching a bicycle at an intersection 90 degrees apart is larger than the entrance angle for a bicycle directly in front of an automobile on a straight road. The reflector appears brightest to the observer when it is directly in line with the light source.

The brightness of a reflector is also a function of the distance between the light source and the reflector. At a given observation angle, as the distance between the light source and the reflector decreases, the light that falls on the reflector increases. This increases the amount of light returned to the observer and the reflector appears brighter.

Lego Mindstorms NXT Laser Target Sensor
Lego Mindstorms NXT Laser Target Sensor

Too improve the sensitivity of the Laser Target Sensor, a 650nm NBP narrow-bandpass optical filter placed in front of the detector would improve the reliability of detections in varying light conditions, and would also increase the detectable distance of a target as well. This said, for most applications it works well as is.

Listed below is the Downloadable NXC Code, used to Test the Laser Sensor:

NXC Source Code:


A Gold Corner Cube Retroreflector

A Gold Corner Cube Retroreflectors like the one above are used in commercial and scientific Laser range finding. The distance measurement to the target is acquired by measuring the optical delay of the reflected beam. Astronauts on the Apollo 11, 14, and 15 missions left retroreflectors on the Moon as part of the Lunar Laser Ranging Experiment. Even under good viewing conditions, only a single reflected photon is received every few seconds from the reflectors on the moon. This makes the job of filtering laser-generated photons from naturally-occurring photons somewhat challenging. Visit the Apollo 15 Laser Ranging Retroreflector Experiment for more details on the only remaining Apollo project still operating to this day.

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