Light discipline is the deliberate control of personal light sources and revealing reflections. A person isolated during military operations limits when, where and how brightly they use a lamp, a display or a fire, and conceals conspicuous illuminated surfaces and shiny equipment. This allows necessary tasks to be carried out without needlessly exposing their position. During signalling, the same relationship is deliberately reversed: light is used to make the position visible to the intended observer.
Light source, illuminated surface and line of sight
An exposed torch or headlamp, a bright display, an indicator light, a flame or a glowing cigarette can stand out as a point of light. The area around the source matters just as much: a concealed lamp can illuminate hands, face, clothing, branches or the inside of a shelter. The observer need not see the lamp itself. Shiny items also require no power supply. A watch glass, polished tool or uncovered signal mirror can reflect sunlight or another light source and draw attention to an otherwise inconspicuous person.
What matters is the path between the source or illuminated surface and the observer. Terrain or a sufficiently closed screen can interrupt that path in one direction while leaving gaps to the sides or an open view from above. Light and contrast against the background help determine what an eye or camera can recognise. A bright patch against a dark background can attract attention even while the person beside it remains concealed. Choosing a work position therefore requires considering possible viewing directions as well as finding a dark surface.
Using necessary light with limited means
Before switching on, identify the actual task: what must be seen, and can it be done in a more sheltered position? Assessing a wound, operating important equipment correctly or identifying an immediate obstacle may require light. Lay out the necessary items within reach first, so the lamp does not sweep through pockets and the surroundings while searching. Its controls and switch position must be understood; an accidentally selected bright mode or a waking display emits light before the task begins.
Keep the light source and work surface behind the available concealment, direct the beam only onto the required area and choose the lowest brightness that still allows the task to be completed reliably. Available clothing can screen the sides of the work area if it also prevents light on the face and hands from escaping outwards. Do not wrap hot lamps or flames in fabric. If weak light hides a critical detail, a short check under adequate light in a better screened position is preferable to a wrong decision. After the task, switch off the lamp and display or close their existing covers. A continuously lit headlamp changes the illuminated area with every movement of the head.
Personal checks begin with visible faults: light on hands outside the screen, a bright patch on the ground, an uncovered indicator or a reflection as equipment turns. Correct the direction, brightness or position accordingly. Recheck escaping light after changing posture. A person alone cannot see every outside surface of their work position at once. Stay concealed while checking and account for the directions that cannot be inspected; a view from inside cannot replace a test with the opposing observer's equipment. Stow or cover shiny items when they are not in use, without making necessary optics, displays or other essential functions unusable.
Red light and personal dark adaptation
The eyes adapt to low illumination. Bright working light can disrupt that adaptation, making faint outlines and movement harder to recognise at first after it is switched off. Dim red light can help preserve dark adaptation for suitable tasks, but remains visible light to an observer. It also changes colour perception: coloured markings can look similar or disappear into darkness. If distinguishing a colour correctly is necessary for the task, use adequately dimmed white light and screen it. The lamp's colour replaces neither control of its brightness nor checks of the surfaces it illuminates.
Night vision, infrared and thermal imaging
An image intensifier makes weak available light usable. Depending on the device, it detects near-infrared as well as visible light. Active infrared illumination instead emits radiation that a suitable sensor receives after it reflects from the surroundings. Near-infrared lies outside human visible perception, but observation equipment can detect it. An IR lamp or an IR filter on a signalling device can therefore be inconspicuous to the unaided eye while producing a clear signal for an appropriately equipped opponent.
Thermal imagers use a different process: they form images from differences in thermal radiation from bodies and surfaces. Switching off a visible lamp removes neither a person's thermal radiation nor that of a fire and the area it has heated. A red or near-infrared lighting mode is also no measure against thermal observation.
Fire between survival needs and detection
A fire can provide heat, cooking and working light. It also produces flames or embers and illuminates its surroundings. Smoke, smell and thermal radiation add further signs. Blocking a direct view of the flame leaves those other indications capable of revealing the position.
Before lighting it, assess the actual need: do available clothing and a sheltered position provide enough protection to retain warmth, can food be used without heating, or is heat needed to remain capable of acting? If available means meet the requirement, the additional fire source can be avoided. When fire is needed, consider its purpose, position and necessary duration together, including the view of the illuminated surroundings and possible smoke. A particular time of day, bad weather or a screened flame does not remove those considerations.
Using light deliberately as a signal
Recovery may require becoming visible. A lamp, strobe or aimed mirror reflection helps only when a suitable observer is actually present, can perceive the signal and has a usable line of sight. An electrical signalling device also requires a working power supply and an appropriate mode. Follow known signal arrangements; use the available device deliberately towards confirmed friendly forces. An aircraft sound or a visible light alone does not establish that the other party belongs to the recovery effort. Without reliable contact, assess the threat and the actual rescue opportunity when deciding whether becoming visible justifies using the signal.
An IR signal requires compatible equipment on the receiving side and can be seen by an opponent with equivalent capabilities. The direction of emission also depends on the particular device: on the military MS-2000(M2) strobe, white and infrared modes radiate in all directions, while the blue filter directs the emission. When using the IR mode, its filter cover must be seated correctly to prevent unwanted visible leakage. Switch off the electrical device after the required signal and check its actual state, preserving power for another opportunity. Cover a signal mirror when it is not in use. Locating a person and confirming their identity remain separate tasks: a point of light indicates a place, but does not by itself establish who is there.