In researching the history of small inflatable rafts I accumulated some knowledge of the history of the US Military signal mirrors that I thought might be of interest.
History of the Emergency Signal Mirror in the US Military
There is a long history of using signal mirrors for communication (“optical telegraphy”) by the military in the form of heliographs (from the Greek “sun” and “write”). Professor Carl Friedrich Gauss introduced the first reliably documented heliographic device in 1821. This device was first used in geographic surveys for indicating position, especially in British Great India Survey. In ~1869 Sir Henry Christopher Mance added a tilting mechanism to allow flashes of light to be used for communication; thus began “heliographic optical telegraphy”[1]. The heliograph was important in defeating the Indian chief Geronimo in the American Southwest in the 1880s [2], by the British in the Boer War and in the African desert during WWII (British Mark V heliograph [3]).
Emergency signal mirrors were developed mainly for use at sea where ease of sighting is an important consideration. The signaling platform may be moving violently so preferably the sighting and signaling may be done with one hand.
In the 1930s the navy Bureau of Aeronautics (BuAer) issued a round double-sided reflective chromium plated metal signal mirror. The US Coast Guard issued a rectangular metal mirror. The signal mirror has a sighting hole in the center.
Signaling is done by looking at the image of the spot formed by sunlight shining through the hole on the face, hand, clothing, etc. as reflected in the mirrored backside of the mirror and moving that image to overlay the sighting hole. The object to be signaled is then viewed through the sighting hole keeping the reflected image overlaying the sighting hole (“reflected image signaling”). The hole also allows the easy use of the “finger sighting method” where the reflection shines on a finger (thumb) (or in the “V” formed by two fingers) in-line with the object to be signaled.
The metal mirrors suffered from the fact that they would corrode, scratch, warp, and bend. This was the type of emergency mirror was used at the beginning of WWII and some stayed in use in the military until the end of the war.
In Sept. 1942 the BuAer began studies to find a more suitable signal mirror. This culminated in the acceptance of a two-sided mirror of tempered glass as patented by Larry L. Young [4]. The mirror side facing the object to be signaled is a “2nd surface” mirror while the mirror side facing the signaler (backside) is a “front surface” mirror. The mirror had a sighting hole through the reflecting surfaces in the shape of a cross. The backside of the mirror is coated with a black paint except for large diameter circular mirror area around the sighting hole, which is coated with a clear lacquer. Instructions for the use of the signal mirror are printed on the blackened part of the back of the mirror.
The Mil Specs were standardized in June 1943 as M-580 (Mil Spec 40653) and the unit became designated as the ESM/1 and was adopted by the Navy (4X5”) and the Army Air Force (AAF) (ESM/2 - 3X5”). Signaling was done by the “reflected image signaling” method described above where instead of a spot you have a cross shaped image. It is interesting to note that in Aug. 1943 a training film ”The Signaling Mirror: WWII US Government training film” (11:04 minutes – you tube) was issued in Aug. 1943 though the priority date on Young’s patent is Dec. 7, 1943. Apparently GE was the only contractor that made the ESM/1 and production reached 80,000 per month.
Instructions for using the M-580 (ESM/1 & ESM/2)
- Holding this side of the mirror a few inches from your eye. Face it half way between the sun and the rescuer.
- If sun shining through cross does not fall on your person, intercept it with hand or other surface. Tilt mirror so as to see in this round reflector the image of the bright cross thus formed.
- While sighting rescuer through center of cross tilt mirror to direct bright image back through cross. Signal is now aimed directly at rescuer.
In 1942 Charles M. Learned presented a signaling mirror using a retro-reflecting surface position at 30 degrees to the plane of the sight-hole [5-7]. Initially this design was not accepted, probably due to the difficulty in manufacturing. Later a version of the design was put into production with the angled red-colored retro-reflecting surface in the form of a “pop-up” reflector (~80 red retro-reflecting surfaces) on a flap over a square viewing window. The flap is attached to one end of a leaf spring and “pops-up” when the metal lid was removed from the container. To sight you look under the flap after it is in the “pop-up” position to see the near reflected image and red-spot as well as the object to be signaled. This signal mirror was designated the M-580A by the BuAer and was placed into service in 1944. The M-580A was contained in a metal container that included instructions and a warning “not to bend the flap”. The M-580A also had 8 orange reflectors on the back. Some (all?) of the M-580A signal mirrors were made by Signal Service Corporation in Irvington, New Jersey. This signal mirror is obviously much more complicated to manufacture than others of the time. It also more prone to damage in use.
Instruction for using the M-580A mirror are printed on sheets (2) of paper in the metal container box. The Instructions read:
TO AIM THE SIGNAL FLASHES:
- Hold the mirror by edges with your eye at the crack of the raised button and the square window (see diagram). Don’t shade mirror with fingers, head, or hair.
- Reflect sunlight from the mirror onto some nearby surface where you can see the reflected light.
- Look through the window at this reflected light and you should see a red spot.
- By twisting the mirror, move this red spot to your target.
And on the back
DON’T BEND FLAP
It Automatically takes correct position
Keep mirror dry and clean
In the latter part of WWII the AAF adopted a retro-reflective type mirror designated the B-1 (3X5” - Mil Spec. 41063). The B-1 uses an annular solid red-colored filter in front of a retro-reflective material [8] around the sighting hole [9]*. The backside is protected by a circular glass plate glued over the sighting-hole area. This gives the mirror its characteristic “bump” (mesa) on the back of the mirror.
Instructions for use of the B-1
- Holding mirror black side toward you and close to one eye shine it on a nearby visible surface. Sight through the hole at the reflection and note bright red sunspot which appears in line with reflection. This indicates aim of beam.
- Keeping this red spot always in view, tilt mirror to shine toward desired target. When red spot is on target or rescuer mirror is properly aimed.
By the Vietnam War the Signal Mirror Mark 3 (Mil-M-18371E) Type I 2X3”] or Type II [3X5”] retro-reflective signal mirror was the standard. The mirror is two laminated tempered glass plates and uses a wire mesh design for the retro-reflective surface to provide a wider field of view [10]. This design is more easily sighted than earlier designs. The retro-reflective surface is produced by gluing tiny spherical glass beads to an open wire mesh (~30 openings per inch; ~90% transmission). This is still the design used for the most effective signal mirrors**. Hunter’s patent [10] also covers a low-transmissivity surface, such as retro-reflective tape, around the sight-hole though this reduces the field of view.
Signaling is done by holding the mirror close to your eye, locating the bright spot (virtual image of the sun – “fireball”) in the sighting hole on a nearby object and then moving the mirror so that the bright spot is on the object being signaled as seen through the mesh-free “sight hole” (“bright spot signaling”). “Bright spot signaling” is much easier than the “reflected cross signaling” or the “finger sighting” method especially if the raft is bouncing around.
There are several plastic (polycarbonate - Lexan™) retro-reflective signal mirrors on the market [11,12]. The plastic mirror is a less effective reflector than the glass-type mirror but weighs less. Some versions of the plastic mirrors are made to float if dropped in the water. Some signal mirrors (e.g. Coghlan’s “Sight-grid” signal mirror) do not have a “sight hole” but you sight through the mesh – this makes the “fireball” more visible but makes it harder to aim, particularly on a moving target.
Many other mirror configurations may be used for signaling in emergency situations. One of the most widely lauded is the ubiquitous CD. Signaling is done by sighting through the center hole and using the “finger signaling method.” Signaling using the “finger sighting method” may be done by sighting over the top of any shiny surface.
*The annular retro-reflective wire mesh (or tape) surface is placed around the sight-hole and is sandwiched between glass plates. The retro-reflecting surface reflects most of the incoming sunlight that strikes it back toward the sun (the same effect that makes a cat’s eye “shine” in the dark) and a small portion of that light is reflected from the air-glass interface on the front surface of the mirror. That virtual image enters the signaler’s eye from a direction along the path of the main reflected rays but in the reverse direction. The signaler sees a virtual image of the sun in the direction of the main reflected sunlight. By superimposing the virtual image of the sun on the object being signaled as seen through the “sight hole” the main reflected beam is directed toward that object.
**In commercial products beware of signal mirrors with a screen-printed mesh with no glass beads – it is not retro-reflective and has a very weak bright spot compared to a mesh or solid retro-reflective surface mirror (http://www.equipped.com/phony signal_mirrors.htm). If you can, use a Mil Spec. Type 3, (or e.g. Coghlan’s 9902 made by Vector 1 in Japan) signal mirror to understand what a “bright spot” (sometimes called a “fireball”) should look like before you purchase what may be an important outdoor tool. (The non-retro-reflective signal mirror with a mesh that I have has a relatively high transmission >50%, the mesh appears flat and it has a silver color. The instructions printed on the back suggest that it is a retro-reflective mirror but it doesn’t say so directly – the “fireball” is very weak).
References:
- Recent Improvements in the Art of Signaling: For Military and Commercial Purposes. The Finley Helio-telegraph and its Modifications, 2nd edition, J.P. Finley, The American Helio-telegraph and Signal Light Company (1888)
- Charles F. Parker, “Signals in the Sun,” Arizona Highways 43(6) 32-39 (June 1967)
- http://royal-signals.org.uk/Datasheets/Heliograph.php
- “Light-directing Device,” Larry L. Young, USP 2,395,605 (priority/filing date Dec. 7, 1943; publ. date Feb. 6, 1946)
- “Mirror Signaling Device,” Charles Horace Learned, USP 2,589,618 (priority/filing date May11, 1944, publ. date Mar. 18, 1952)
- “Heliographic signaling device” Cass Simpson, USP 2,467,165 (priority/filing date May 11, 1944; publ. date April 12, 1949)
- “Signaling mirror,” Richard S. Hunter, USP 2,504,982 (priority /filing date July 14, 1944; publ. April 25, 1950)
- ”Flexible lenticular Optical Sheet,” Philip V. Palmquist, USP 2,354,048 (priority/filing date Aug. 3, 1940; publ. date July 18, 1944)
- “Signaling Mirror”, Richard S. Hunter, USP 2,412,616 (priority/filing date April 20, 1945; publ. date Dec. 17, 1946)
- “Signaling mirror,” Richard S. Hunter, USP 2,557,108 (priority/filing date Dec. 4, 1946; publ. date June 19, 1951)
- “Signal mirrors,” Malcom G. Murray, USP 3,335693 (priority/filing date Aug. 31, 1965; publ. date Aug. 15, 1967)
- “Reinforced signal mirror,” Malcom G. Murray, USP 5,777,810 (priority
date, May 31, 1995; filing date, Nov. 12, 1996; publ. date July 7, 1998)