Windows, mirrors, bottle glass, vehicle windscreens and polished metal can flash reflected sunlight for kilometres, so dull, shade or cover every reflective surface that faces open ground and check the site from the real viewpoints in the morning and the afternoon. A flash happens only when the sun, the surface and the observer line up, which is why a site can look safe at noon and be exposed at four o’clock.
Part of Camouflage and Concealment
Reflective surface management is the identification and treatment of glass, metal, water and other shiny materials that bounce sunlight toward an observer and reveal a position.
A Flat Reflector Sends The Sun To One Place At A Time
The parent article warns that windows, mirrors, bottle glass and vehicle windshields can flash reflected sunlight for kilometres, which is how heliographs work, and that unintentional flashes reveal your position. The reason is that a flat, smooth surface returns a narrow beam toward whatever the sun and the surface angle direct it to. That beam sweeps across the landscape as the sun moves. The width of such a beam and why it is so narrow is explained in Mirror and Light Signals.
The practical consequences are: a flash occurs for a short window of the day; it is seen only by observers inside the beam; and it is brightest and most obvious on surfaces facing the open ground that the sun can reach. Rough, dirty or curved surfaces scatter the light and do not flash in the same way.
What You Need
- A list of every reflective item — glass, tin roofing, metal tools, aluminium, jewellery, cooking pots, spectacle and binocular lenses, vehicle glass, a pond or puddle
- Covers — cloth, bark, branches or boards that can hang over an outward-facing window; see Natural Camouflage Materials
- Dulling materials — mud, ash, soot, paint, lime wash, or a thin coat of earth
- A viewing partner at the places observers would stand
Method: Find, Treat, Re-check
Step 1: Walk the site in the morning, midday and late afternoon from the viewpoints that matter, looking for any glint. Check again on a bright day after rain, when wet surfaces shine.
Step 2: Cover glass that faces outward. The parent article suggests covering outward-facing glass and notes that dirty or mud-smeared glass does not flash. Windows covered on the outside with cloth or vegetation serve two purposes, concealment of light at night and removal of reflections by day.
Step 3: Store or cover polished tools when you are not using them. Dulling by dirt, paint or mud removes the shine; the parent article lists dulling spray, paint or mud.
Step 4: Treat large reflectors first. A tin roof, a vehicle windscreen or a greenhouse of glass throws the biggest flash. Paint, cover or move them; park vehicles under a screen, with their glass turned from the viewpoints.
Step 5: Watch your own hands and head. Spectacles, binoculars and a watch face can flash when you turn toward the sun. Shade lenses with a hat brim or a lens hood, and keep them in a pocket when not in use.
Step 6: Re-check after you change anything. Moving a window cover can open a new angle.
Common Mistakes
- Treating the check as a one-off. The sun moves, so a site clear at noon can flash at four.
- Smearing mud on a window you need. You lose your view and your light, and a window you cannot open is also an exit problem; use a cover you can lift.
- Forgetting water. Still water mirrors the sky and, at some angles, the sun.
- Dulling the front and not the sides. A flash can come from any angle that faces the sun.
Safety
Covers, glass and heat
A cover over a window is also a cover over an exit; it must pull away from inside, as the parent article’s blackout rule requires. Do not fix cloth or vegetation over a window with nails or wire. Dry camouflage on a roof or a wall is fuel for fire, so keep it away from chimneys, flues and lamps. Never look at a reflected flash of sun or point one at a person’s face, because the light is bright enough to dazzle and damage eyes; a curved glass or mirror can focus enough heat to start a fire in dry material, so do not leave concave reflectors or lenses lying in sun beside tinder.
What’s Next
- Learn next: Noise Discipline — the next detection channel after sight and light
- Combine with: Glass, Mirrors, Lights and Reflectors — what these materials do when you want a flash