A Marker Light is essentially an advanced Tau hand-held laser range finder that is used to "paint" targets for other Tau units to fire upon. When activated, a Marker Light projects a simple beam at the target which measures the exact distance between the Marker Light and the user. This and a torrent of other targetting information then is fed into the Cadre tactical network and becomes available on the wider Tau electronic battle network, relaying ranges to the marked target, triangulating optimum firing trajectories and superimposing aiming vectors. All of this information is transmitted to other Tau units to guide their fire and engage the enemy with unerring accuracy. During the Damocles Gulf Crusade, the Marker Light was also dubbed the "Valkyrie's Mark" by Imperial forces, because those marked with it were soon numbered amongst the dead.
Marker Lights allow Tau Fire Warriors and vehicles to:
- Fire more accurately upon their targets.
- Be able to see and target enemies in darkness clearly.
- Call in Seeker Missiles on the target, either from Skyrays or missiles mounted on other Tau vehicles.
- Pin enemy units under fire from Tau troops more easily.
- Allow their aimed shots to strike more accurately at targets who take cover.
The most common and effective use of the Marker Light is to guide indirect or standoff fire which is called in by Pathfinder Teams. Utilising a concealed position, a Pathfinder can direct his Marker Light at an enemy target without the foe being aware of his presence. Seeker Missiles carried by Tau armoured vehicles are then fed the targeting data captured by the Marker Light and fired, often from some distance away or from behind intervening terrain. The missile then “rides the beam,” which the Marker Light user must keep trained on the target until the missile strikes. Because Pathfinders are adept at working their way into flanking positions, they are often able to target the vulnerable sides and rear of their targets, which allows for the guided missiles to destroy them with ruthless efficiency. The only downside to the use of a Marker Light is that the beam, while invisible in normal conditions, becomes visible when it passes through dense airborne gas or vapour (such as smoke or fog) and, in so doing, risks giving away the user's position. Marker Lights must also be relatively stable when painting a target, making them of limited utility when a unit is on the move.
Marker Light technology is commonly employed by Tau Pathfinders, and on Tau vehicles like the Tetra and the Sky Ray. Fire Warriors who achieved the rank of Shas'ui in the Fire Caste can also upgrade their Pulse Rifle or Pulse Carbine with a Marker Light, along with XV15 Stealthsuits and XV25 Stealthsuits who have attained the rank of Shas'vre, and Cadre Fireblades. Sniper Drone Team Firesight Marksmen also commonly make use of Marker Lights, whilst XV84 Crisis Battlesuits mount a spine-mounted Marker Light as part of their main systems. Marker Drones also carry Marker Lights and act as fire support for other Tau units. The Tetra Scout Speeder mounts a High Intensity Marker Light, which emits a far more powerful beam than normal Marker Lights. Drone-controlled gyros keep it steady as a Tetra moves, giving greater accuracy and also allowing multiple strikes to be performed on the one target. A Networked Marker Light is a more advanced version of the standard device and allows the user and his unit to benefit from the Marker Light's use, whereas normal Marker Lights do not aid the user's own fire. The Tau Sky Ray is a notable example of the use of a Networked Marker Light, where its twin-linked Networked Marker Lights allow for its own Seeker Missiles to be launched at the target. Many Tau aircraft Sun Shark bombers, DX-6 Remora Drone Stealth Fighters, Tiger Shark AX-1-0s, and Mantas are armed with a Networked Marker Light as standard issue.
- Codex: Tau (3rd Edition), pp. 19, 25-26, 27
- Codex: Tau Empire (4th Edition), pp. 29, 35-37, 40
- Codex: Tau Empire (6th Edition), pp. 33, 36, 38, 46, 48, 51, 69
- Deathwatch: Mark of the Xenos (RPG), pg. 5