Real-World Results vs “Extended Range”
In excellent conditions our standard LRF model can obtain up to 17km on a NATO-sized vehicle target. Longer ranges could theoretically be achieved with a perfect atmosphere, perfect target, perfect reflection, target larger than the beam, and no stray light. Referred to as “extended range”, this metric is a common rating among LRFs but is a theoretical laboratory calculation for comparison and benchmarking, not a value to be obtained in the real world.
How an LRF Functions
Let’s consider the procedure of ranging a 5km target. First, the photons from the laser beam must travel 5km to the target. Even in ideal conditions, the air through which the laser light travels is not pure, even though it may appear so to the human eye. Air will always contain foreign particles such as dust, pollen, smoke, fog, and humidity. Along the way, any photons that hit particulates or moisture in the atmosphere get absorbed or deflected and won’t reach the target. The ones that do reach the target are spread across a relatively large area, and the majority of those photons are absorbed by the target or reflected in other directions.
This means that only a small amount of photons from that initial beam are reflected back towards the LRF. This diminished beam then travels another 5km back through the atmosphere (again losing photons to particulates and moisture) before hitting the LRF receiver, which has an aperture diameter of 50mm. The receiver will read all of the reflected photons it collects through that aperture, including not just those reflected off the target but also those which may have reflected back from targets at other distances, and will be able to return a reading if it detects a spike of photons at a specific distance, based on the time it took for them to be reflected back to the LRF.
Other Environmental Limitations
Long-range eye safe LRFs operate on the SWIR spectrum of light, typically 1500–1575nm. The sun emits a wide spectrum of light including SWIR, so in daytime the receiver is also reading millions of photons coming either directly or indirectly from the sun. If the reflection of the sun is stronger than the LRF beam, it will provide a false or no reading because this extra, intense light overwhelms and confuses the LRF’s InGaAs sensor. This component is highly sensitive, and InGaAs sensitivity increases with longer-range LRF models. This means that sunny days can result in reduced performance, reduced accuracy, as well as increased occurrences of false or no readings. This effect is especially prevalent in urban environments that have many reflective surfaces.
Cloud cover can help performance, as can ranging from a shadow into a shadow, but only marginally. The very longest ranging can be achieved in overcast weather, at dusk, or at night. These conditions allow the LRF to get within 20% of its rated performance for targets like vehicles (not extended ranges).
Another factor to consider when ranging in coastal regions is that bodies of water can reflect, absorb, and diffuse both the sun and the LRF’s laser. Relative humidity levels over 55% (which are common in coastal environments) can greatly affect performance with long-range distances, as ranging through a high humidity atmosphere is essentially like trying to range through water.
In summary, there are many impediments that the LRF beam faces, and if those impediments are serious enough, they can render the LRF either completely useless or dramatically reduce its range and accuracy.