In our previous exploration, we delved into the complexities of the LiDAR equation, focusing on factors that limit the range of LiDAR systems. Now, let's turn our attention to a critical component: the power output of the emitter, denoted as Po in the LiDAR equation. While it might seem intuitive to boost LiDAR range by simply increasing laser power, the reality is constrained by laser eye safety regulations and the inherent properties of laser diodes. Let's unpack why.
The Illusion of Unlimited Power
At first glance, enhancing LiDAR range appears straightforward—just install a more powerful laser. However, this approach hits a legal and ethical wall due to laser eye safety standards. Human eyes are sensitive to certain laser wavelengths, and overexposure can cause serious harm. The Maximum Permissible Exposure (MPE) defines the highest level of laser radiation to which a person may be exposed without hazardous effects, considering factors like wavelength, energy, and exposure duration.
Understanding Wavelength Sensitivity
The human eye's sensitivity varies across different wavelengths. In the graph below, the sensitivity of the cornea and the lens is plotted against wavelength:
(Image Credit: Söderberg, P., et al. "Does infrared or ultraviolet light damage the lens?" Eye, 30(2), 241–246, 2016.)
Three common LiDAR wavelengths are marked on the graph: 850 nm, 905 nm, and 1550 nm. Notably, the eye absorbs higher wavelengths more effectively, reducing the risk to the retina. Lower wavelengths, closer to the visible spectrum (approximately 400–700 nm), are less absorbed, meaning more laser energy reaches the sensitive back of the eye.
For instance, an 850 nm LiDAR system operates near the visible spectrum. You might even notice a faint red glow if you look directly at such a sensor in the dark (though it's strongly advised not to do so). This proximity to the visible spectrum necessitates stricter power limits to remain eye-safe.
Balancing Power and Pulse Duration
The MPE isn't just about wavelength; it's also a function of the total energy involved and the duration of exposure. A common red laser pointer, classified as Class 1, must have an output of less than 1 milliwatt (mW) of continuous power. LiDAR systems, however, emit energy in short pulses rather than a continuous beam.
These pulses are incredibly brief, typically measured in nanoseconds (one billionth of a second) or even picoseconds (one trillionth of a second). The brevity of the pulse is crucial for two reasons:
- Range Measurement Accuracy: LiDAR determines distance by measuring the time it takes for a pulse to travel to an object and back. Shorter pulses allow for more precise timing and better resolution between objects that are close together.
- Eye Safety Compliance: Short pulses mean that even if the peak power is high, the total energy exposure remains within safe limits because the duration is so brief.
The Importance of Pulse Length
Let's consider two scenarios to illustrate the effect of pulse length on detection:
- Graph A: A longer pulse length (~15 nanoseconds) is emitted toward two objects five meters apart. The extended duration of the pulse means that it overlaps both objects simultaneously. The return signal is a blend, making it difficult to distinguish between the two objects or determine their exact positions.
- Graph B: A shorter pulse length (~1 nanosecond) is used. The pulse reflects off the first object before reaching the second, resulting in two distinct return signals. This clarity allows the LiDAR system to accurately detect and differentiate between the two objects.
Short pulses not only enhance resolution but also allow for higher peak power within the MPE limits. This is because the eye's exposure is averaged over time; a brief, intense pulse delivers the necessary energy for detection without exceeding safety thresholds.
Maximizing Range Within Safety Limits
By optimizing pulse duration, LiDAR systems can increase peak power safely, thus extending the effective range. For example, a LiDAR unit operating at 1550 nm can emit pulses with peak powers exceeding 100 watts while remaining eye-safe due to the absorption characteristics of the eye at that wavelength and the ultra-short pulse duration.
This approach contrasts sharply with continuous-wave lasers like standard laser pointers, which are limited to much lower power outputs to prevent eye damage.
The Reality of 'More Powerful' Lasers
When discussing power output, it's essential to recognize that most LiDAR manufacturers use similar laser diodes, often sourced from established companies like Osram. These diodes are designed to comply with eye safety standards while delivering optimal performance. Therefore, the 'Po' in the LiDAR equation is relatively consistent across different systems operating at the same wavelength.
If a LiDAR salesperson claims their system has a significantly more powerful laser than competitors (without changing the wavelength), they might be:
- A) Misrepresenting the facts: Exaggerating capabilities to make a sale.
- B) Ignoring safety regulations: Offering a product that exceeds legal MPE limits, which is both dangerous and illegal.
Conclusion
The power output of a LiDAR emitter is a delicate balance between maximizing range and adhering to stringent eye safety standards. By utilizing ultra-short pulses at appropriate wavelengths, LiDAR systems achieve the necessary power levels for effective range detection without compromising safety.
Understanding these principles clarifies why simply increasing laser power isn't a viable solution and highlights the sophistication involved in LiDAR technology design. It's not about brute force but about intelligent engineering within the boundaries of safety and component capabilities.
