Pulse Energy, PRF and Divergence: Three Specs That Set Range
Three numbers decide how far a laser rangefinder reaches: pulse energy, pulse repetition frequency and beam divergence. Datasheets list them separately, but they only mean something together.
Pulse energy
Pulse energy, in millijoules, is the optical energy in each pulse. More energy means more returning light from a given target, which extends range. The relationship is not linear: because received signal falls as 1/R², doubling energy extends usable range by only about 19% in the ideal case, and less once atmospheric and detector noise floors dominate.
| Model | Pulse energy | Beam divergence | Mass |
|---|---|---|---|
| LDR20K1 | ≥ 20 mJ | ≤ 0.6 mrad | ≤ 290 g |
| LDR25K1 | ≥ 25 mJ | not published | ≤ 380 g |
| LDR40K1 | ≥ 40 mJ | not published | ≤ 460 g |
| LDR60K1 | ≥ 60 mJ | ≤ 0.3 mrad | ≤ 680 g |
| LDR80K2 | not published | ≤ 0.15 mrad | ≤ 850 g |
| LR120K1 | ≥ 120 mJ | ≤ 0.15 mrad | ≤ 2 kg |
Pulse repetition frequency
PRF is how often the laser fires. ERDI 1064 nm modules commonly specify 1 Hz or 5 Hz modes.
PRF sets two things:
- Update rate. At 1 Hz you get one range per second; at 5 Hz, five. For a moving target or a scanning gimbal, 5 Hz matters.
- Unambiguous range. The module must not fire again before the previous echo returns, or a late echo from the first pulse can be attributed to the second. At 5 Hz the interval is 200 ms, corresponding to a round-trip path of about 60,000 km — far beyond any realistic range, so PRF is rarely the limiting factor at these rates. It becomes relevant at kilohertz PRFs used in some scanning systems.
Beam divergence
Divergence sets the spot size at range, and therefore how much of the beam lands on the target:
Spot diameter (m) ≈ divergence (mrad) × range (km)
| Divergence | Spot at 5 km | Spot at 10 km | Spot at 20 km |
|---|---|---|---|
| 0.15 mrad | 0.75 m | 1.5 m | 3.0 m |
| 0.3 mrad | 1.5 m | 3.0 m | 6.0 m |
| 0.6 mrad | 3.0 m | 6.0 m | 12.0 m |
How they interact
Consider two modules at 10 km against a 2 m × 2 m target:
- Module A: 60 mJ, 0.6 mrad. Spot at 10 km is 6 m — much larger than the target, so only about (2/6)² ≈ 11% of the energy hits it.
- Module B: 40 mJ, 0.3 mrad. Spot at 10 km is 3 m, so about (2/3)² ≈ 44% hits it.
Module B delivers roughly 17.6 mJ on target; Module A delivers roughly 6.7 mJ. The lower-energy module wins on a small target — which is why "more mJ" is not the same as "more range".
What to specify
- Pulse energy, with the measurement convention
- Divergence, full-angle or half-angle, with the beam contour definition
- PRF modes available, and whether they can be changed at runtime
- Receiver aperture, since it scales collected energy directly
Frequently asked questions
Is higher PRF better?
It gives more updates per second, which helps with moving targets and scanning. It does not extend range and may reduce energy per pulse in some designs.
Does beam divergence affect accuracy?
Not directly. It affects signal strength and therefore the range at which a valid measurement is possible, not the accuracy of a measurement that succeeds.
Why do some datasheets omit divergence?
Usually because the figure would reveal a wide beam. Ask for it; a rangefinder without a stated divergence cannot be assessed for long-range performance on small targets.
What pulse energy do I need for 10 km?
Roughly 40–80 mJ with a 0.3 mrad beam and a suitable receiver, against a vehicle-sized target in clear air. The exact figure depends on all three parameters together.
Compare modules on the rangefinder collection, or read beam divergence in detail.