A few weeks ago a friend showed me a military heliograph he had. If I recall correctly it was part glass and like so much milspec equipment was surprisingly heavy. In my emergency kit I have an item that serves the same purpose. It was made from a polished stainless steel door finger plate. Unbreakable and a fraction of the weight and price of the military item.
“You know how to use the sighting hole?” I asked. I explained that as you aimed the device by looking through the hole it would cast a spot of sunlight onto your face. The heliograph was mirrored on both sides so you could see the reflection of this spot of light in the near side. You then adjusted the mirror till the spot of sunlight moved back to the aiming hole. This gives you the optimum angle for directing sunlight in the aimed direction.
Another technique is simply to hold your hand up between you and your intended target. Some survival heliographs have a little stick you can aim at, but your finger or hand works fine. Angle the mirror until the sunlight shines on your hand, then drop your hand out of the way. This technique can be used with mirrors that lack an aiming hole or items that are only reflective on one side.
One of the disadvantages of heliographs is that you cannot use them to signal in a direction where the sun is behind you. Or can you?
I did some research. One of the interesting things about heliographs is that they only came into widespread use as a communication device in the 19th century. There are some earlier accounts of signalling with light flashes from reflective objects but the true potential of this technique was not exploited. This is interesting since there is no real technological barrier that would have prevented messaging heliographs being invented centuries earlier. The Elizabethans, Tudors or Romans could easily have invented heliograph messaging. Some events in history might have been very different if such a rapid communication system had been available!
Let us look at the setup of the British Mance heliograph. When the sun was before the signaller the signalling mirror was set up facing it. The operator looked into the mirror and moved it until he could see the reflection of his intended message destination. He then erected a sight so that it was in line with the destination and its reflection in the mirror. Alternately the operator stood behind the mirror and looked through the hole in its centre, adjusting the sight until it was in line with the target. The mirror was then adjusted so that it cast its light on the sight. The centre of the Mance mirror was unglazed so it created a dark “shadow spot”. Positioning this shadow spot on the sight gave better alignment.
If the sun was not in the same direction as the message destination the signal mirror was still set up so that it faced the sun. The sight was replaced with a second mirror, termed the “duplex” mirror. In some designs the duplex mirror might be on its own tripod, or might be a second single mirror heliograph.
The duplex mirror was set so that there was a reflection of the intended target. Alternately the hold in the centre could be used to aim it at the target. The signalling mirror was then adjusted so it cast its light onto the duplex. The Mance manual suggests gumming a piece of paper onto the duplex to facilitate correctly placing the shadow spot. Any non-reflective object could be placed before the duplex with the same effect.
By using a pair of reflectors heliograph positions could exchange information in both directions, even if the sun was behind one.
Many survival manuals will tell you about signalling with heliographs. I have never seen on address what to do it the sun is behind you.
There are various ways you can use this technique. “Mirror” in this context means anything that is reflective. A CD, a piece of foil, a mess tin, a polished knife blade, a piece of glass, a watch face, compass face or the screen or polished back of a phone.
If your vehicle has broken down use your windscreen as a duplex reflector and a rear view mirror to direct the sunlight on to it.
If you have a comrade have him hold a reflective object. Point your hand at his reflector and use your heliograph to cast sunlight onto it. Drop your hand out the way. Your comrade using the light cast on his own hand by his reflector to ensure it that it is aimed in the intended direction.
If you are on your own, improvise by mounting the duplex on any natural or self-constructed suitable object.