A rangefinder sends one laser flash to the target and measures how long the light takes to come back: that gives the distance. A designator shines a coded laser spot on the target; a bomb or missile with a laser seeker steers towards that spot.
In practiceA tank's fire-control system lases the target to know the range before firing. A JTAC on the ground or a targeting pod under an aircraft "paints" a target for a laser-guided bomb such as the Paveway; the laser code makes sure the bomb follows the right spot.
The two workhorses of the military laser are found in almost every modern sensor chain — from sights and tank fire control to targeting pods and ship directors. They look alike (both fire pulsed laser light at a target) but their function differs fundamentally: the rangefinder measures, the designator marks.
Laser rangefinder
The laser rangefinder (LRF) measures the distance to a target from the travel time of a laser pulse — identical to the radar principle of Distance from a target to the radar:
The difference from radar lies in the wavelength: laser light (≈ 1 µm) is ~30,000 × shorter than X-band radar (3 cm). As a result:
- Almost no beam divergence: you measure the distance to exactly that one vehicle, not to "something in that direction". No clutter problem.
- No large antenna needed: the whole rangefinder fits into binoculars or a sight.
- But: line of sight is required; smoke, fog and rain attenuate it strongly; and the target can notice with a laser warning receiver that it is being "lased".
Operationally, the LRF is the link that completes passive sensors (Laser): EO/IR provides the direction, the LRF provides the range at the decisive moment — and with that the fire control solution is complete (Executing the fire control sequence using the EMS). In tank fire control systems the LRF feeds the ballistic computer directly; in artillery observation it provides the target coordinates together with a compass and GPS.
Because a single laser pulse can already trigger a warning, you lase a target as briefly and as late as possible: track passively, and only range when the measurement is really needed.
Laser designator
The laser designator "paints" a target with a series of coded laser pulses. The target reflects those pulses diffusely in all directions; a semi-active laser (SAL) seeker in a bomb or missile sees that reflected "splash" and steers towards it (compare semi-active radar guidance, Semi-Active Radar Homing — the same principle, a different part of the spectrum).
The pulse code (an agreed PRF pattern, set as a 3- or 4-digit code) is essential:
- The weapon follows only pulses with the right code → several designators can work in the same area at the same time without hijacking each other's weapons.
- Codes make deception by the enemy (false laser spots) much harder.
The designator can stand on the ground (JTAC/observer), hang under the aircraft in a targeting pod, or sit on a drone. Buddy lasing is crucial: the party illuminating the target does not have to be the party releasing the weapon — a drone can mark what a fighter hits from kilometres away.
The same physics as any laser: line of sight is needed right to the end (the "spot" must stay on the target until impact), it is sensitive to smoke/dust/rain, and modern targets carry laser warning receivers that can report direction and code type — followed by laying smoke, manoeuvring or firing back at the source. In addition, a poorly aimed spot can cause "spillover" (the beam partly misses and illuminates something behind the target), on which the weapon then homes.