A signal mirror is a hand-held optical signalling device that directs reflected sunlight towards an observer. A person isolated during military operations can use it to draw the attention of search or recovery forces to their position. Producing this reflection requires no battery; the person must align the sunlight, mirror surface and direction to the observer.
Reflective surface, aiming device and heliograph
A smooth mirror surface reflects incoming light in a particular direction. Tilting and turning the mirror changes that direction. Simply facing the mirror towards an aircraft is therefore insufficient: the aircraft may lie outside the reflected light even though the operator can see it clearly. An aiming device lets the operator identify the reflection's direction. Some signal mirrors provide a bright aim indicator in a see-through sight; others use a separate small foresight held in front of the mirror. A plain hole initially shows only the direction of sight.
Heliograph also appears in British product names and older Australian aviation equipment as a name for these hand-held survival mirrors. A historical heliograph used for mirror telegraphy has a mirror aligned on a tripod and transmits Morse messages through flashes of light. The actual design of a carried “heliograph” therefore needs to be checked first: a hand mirror with a foresight is operated differently from a mirror with a built-in aim indicator.
Aim the mirror actually available
With a signal mirror that has a built-in bright aim indicator, first reflect sunlight onto your hand or another nearby object. This establishes a position in which the mirror produces a visible reflection. Bring it to eye level, look through the sight with one eye and locate the intense bright spot. Hold the mirror close to the eye and slowly turn it until this indicator lies over the intended observer. The spot shows the outgoing reflection's direction; it does not confirm that the other person has already seen the flash.
With a mirror that has a separate foresight and a shadow-spot aiming arrangement, hold the foresight at arm's length. The reflective front receives sunlight and the back faces the operator. Align the openings in the mirror and foresight with the observer. Then adjust the mirror until its sunlight reflection falls on the foresight and the shadow spot appears in the foresight's opening. This second alignment directs the reflection along the line of sight. Looking through both holes alone can leave the observer in view while the flash misses them. The procedure using a bright indicator in a built-in sight cannot be transferred to this design.
If only an ordinary mirror or a smooth piece of reflective metal remains, the free hand can provide a rough aiming reference. Hold it forward, keep the observer just beyond its edge or between spread fingers and bring the reflected patch of light onto the hand. Carefully move the mirror so that the reflection crosses that sightline towards the observer. The hand provides only a nearby directional reference. Its distance from the eye, finger movement and an uneven reflective surface introduce aiming errors; adding a hole does not produce the bright sight indicator that shows the reflection's direction in a purpose-built signal mirror.
Sunlight, line of sight and physical limits
A strong flash requires enough sunlight to reach the mirror and a clear optical path to the observer. A canopy can block the sun while the person sees an aircraft through a gap. Conversely, the mirror can be in full sunlight while a slope or dense vegetation blocks its reflection. Cloud and haze weaken the available light or its visibility. The operator must adjust as the sun's direction changes or an aircraft moves. Keep the body from shading the mirror; if the current position produces no usable reflection, reposition the body and mirror.
The operator's own hand shows whether a usable patch of light is produced at all. Next check that the correct aim indicator has been found and that the direction to the observer remains clear. Shaking, exhaustion, impaired vision or an injured hand can prevent stable alignment. Supporting the arms and making short attempts with rest pauses can make operation easier where the terrain permits it. None of these aiming methods requires looking into the sun. Mirror size alone cannot determine a fixed range: light, atmospheric conditions, aiming accuracy and the actual observer's attention also matter.
Directed contact and detection risk
Under hostile threat, the same flash can reveal the position to an enemy observer. A directed reflection therefore provides no covert communications channel. Cover the mirror while taking it out, stowing it and between attempts. In US combat use, it is employed as directed by recovery forces; without radio contact, the signal is directed towards confirmed friendly forces. Sweeping the horizon with mirror flashes is a conspicuous distress method for a non-combat situation. Doing so where enemy observation is possible increases the number of potential recipients and the risk of detection. An acute threat to survival may require such exposure; that risk is then weighed against the actual opportunity for the signal to be noticed.
A detected light signal helps locate the person. Prepared procedures govern recognition of friendly forces. Personal authentication uses the agreed questions, replies or signals over a contact channel that is actually available. Recovery forces check the reply against previously provided person-specific information or the authentication method prescribed for that contact. A random mirror flash carries no such identity information; a change of course or a single returned flash does not replace this check. If no reaction follows, first check the light patch, aiming device and line of sight; the observer may have looked elsewhere or been outside the reflection.