question: What are the purpose(s) of coloured lenses in military flashlights?
comment: Greetings from /r/flashlight. We get people asking for colored light often, most commonly red. It's usually one of two reasons: * Red light has less impact on dark adaptation than white light at the same level. A chemical in the human eye (rhodopsin) that increases sensitivity to light gets destroyed by exposure to bright light, and the effect is not as strong with red. For most uses, hobbyists have found that very low levels of white light work better, because much less white is required to see clearly. * Military doctrine usually says that red light is less likely to attract the attention of the enemy. I'm not sure whether that's true because **any** point source is pretty easy to notice in an area with minimal light pollution. People smoking cigarettes get spotted easily, for example, and that's a very dim red/orange light. When there isn't an external mandate to use red like there would be in the military, we usually try to talk people out of this. I'm not sure about purple. These are usually incandescent lights that don't produce enough UV to be useful, so I don't think it's that.
passages: Since tactical lights are intended for use in situations where lethal force is likely to be used, reliability is important.
  
  Section::::Lighting features.:Battery.
   The second version, with blue-green filters over the projectors, was trialled on board the corvette HMCS "Chambly" in May 1941. This gave better results as the filters removed the reddish bias to the lamps when at low intensity (lower colour temperature). The supports too were retractable, so the delicate projectors could be stowed away for protection when not in use. This second version reduced "Chambly's" visibility by 50% in most conditions, and sometimes by as much as 75%. This was enough to justify development of a more robust version.
   Certain accessories for a flashlight allow the color of the light to be altered or allow light to be dispersed differently. Translucent colored plastic cones slipped over the lens of a flashlight increase the visibility when looking at the side of the light. Such marshalling wands are frequently used for directing automobiles or aircraft at night. Colored lenses placed over the end of the flashlight are used for signalling, for example, in railway yards. Colored light is occasionally useful for hunters tracking wounded game after dusk, or for forensic examination of an area. A red filter helps preserve night vision after the flashlight is turned off, and can be useful to observe animals (such as nesting Loggerhead sea turtles) without disturbing them. 
   BULLET::::- Historically, white was used in combination with red bulb lenses in the Federal signal company Beacon Ray, introduced in the late 1940s and the later 1960s and 1970s model P A lights equipped with Clear domes on through the 1980s, when revolving bulb and reflector type emergency lighting including the later 1970s Twinsonic and aerodynamic light bars were supplanted with newer strobe types. The clear dome extended Hi base 4-Bulb Beacon Ray with colored lenses over the bulbs were used extensively in the 1960s by the NYPD with two red bulbs alternated with a white bulb and an amber bulb canted upwards at a 45-degree angle so the amber flash would reflect off upper-floor building windows for added traffic attention/visibility. This clear dome/colored bulb lens combination also found extensive use across the US as fire apparatus and ambulance lighting.
   Section::::Handheld lights.
   Section::::Lighting features.:Illumination types.
   BULLET::::- Submarines: Oftentimes submarines are “rigged for red,” meaning that the boat is going to be surfacing or coming to periscope depth at night. During such times illumination within certain compartments is switched to red light to allow the eyes of the lookouts and officers to adjust to the darkness prior to looking outside of the boat. Additionally, compartments on a submarine may be illuminated with red light in order to simulate night conditions for the crew.
  
  Section::::Accelerating dark adaptation.:Vitamin A.
   Colour may also communicate information about the nature of the tell-tale, for example red may signify that the vehicle cannot continue driving (e.g. oil pressure).) Many older vehicles used schemes which were specific to the manufacturer, e.g. some British Fords of the 1960s used green for oil pressure warning, while many other makes of the same era used amber for oil pressure. 
  
  Section::::Types.:Oil pressure.
   Counter-illumination has not so far come into widespread military use, but during the Second World War it was trialled in ships in the Canadian Diffused lighting camouflage project, and in aircraft in the American Yehudi lights project.
  
  Section::::In marine animals.
  
  Section::::In marine animals.:Mechanism.
  
  Section::::In marine animals.:Mechanism.:Counter-illumination and countershading.
   Handheld lights are generally restricted to use with handguns, as long guns such as rifles and shotguns require two hands to operate. While just about any handheld flashlight can serve in the role of tactical light with the proper technique, some features are more readily adapted to the role. Because the flashlight needs to be reliable and rugged, specialized, more expensive tactical flashlights are generally used by law enforcement officers and soldiers, rather than inexpensive consumer-grade units. Some manufacturers sell lights specifically designed for use as handheld tactical lights. Police training programs in shooting while holding a flashlight date back to at least the 1930s.
   Modern armies use a variety of equipment and discharge devices to create artificial light. If natural light is not present searchlights, whether using visible light or infrared, and flares can be used. As light can be detected electronically, modern warfare has accordingly seen increased use of night vision through the use of infrared cameras and image intensifiers.
  
  Section::::Theory.
   Tactical lights can be fitted with lenses to produce certain colors, Colored lights or filters provide flexibility for different purposes. Red lights are best for preserving night vision for the law enforcement officer, due to their minimal impact on the rod cells in the eye, while blue light provides high contrast for detecting blood. Light in the infrared spectrum is only visible through night vision devices, allowing the operator to see clearly while reducing the visibility to those not equipped with night vision equipment. Tactical lights are sometimes combined with a laser to form a multifunction unit, able to provide low-light targeting, illumination, or both.
   Aboard naval ships, battle lanterns may be used as emergency portable lighting. Installed in major compartments of the ship, a battle lantern can be detached from its mounting and used as portable lighting in the event primary lighting is out of service. Battle lanterns may use either incandescent or LED lamps, and may have either disposable primary or rechargeable batteries.
   This same study compared different light colors for glare and detection time under both daylight and night conditions. While red and blue both compared favorably with amber for glare under various conditions, some contradictory findings were observed for detection time. When all colors were held at equal intensity, amber had the poorest detection time both daytime and night. However, when the light source was held at constant intensity, the amber filters, which generally let the most light through, had the best detection time.
  
  Section::::Scientific research.:Potential hazards.
   Section::::Military prototypes.:For aircraft.
   Section::::Military use.:First World War.
  
  Searchlights were first used in the First World War to create "artificial moonlight" to enhance opportunities for night attacks by reflecting searchlight beams off the bottoms of clouds, a practice which continued in the Second World War. The term "artificial moonlight" was used to distinguish illumination provided by searchlights from that provided by natural moonlight, which was referred to as "movement light" in night-time manoeuvers. Searchlights were also heavily used in the defense of the UK against German nighttime bombing raids using Zeppelins.
  
  Section::::Military use.:Second World War.
   In 1940, Edmund Godfrey Burr, a Canadian professor at McGill University, serendipitously stumbled on the principle of counter-illumination, or as he called it "diffused-lighting camouflage". Burr had been tasked by Canada's National Research Council (NRC) to evaluate night observation instruments. With these, he found that aircraft flying without navigation lights remained readily visible as silhouettes against the night sky, which was never completely black. Burr wondered if he could camouflage planes by somehow reducing this difference in brightness. One night in December 1940, Burr saw a plane coming in to land over snow suddenly vanish: light reflected from the snow had illuminated the underside of the plane just enough to cancel out the difference in brightness, camouflaging the plane perfectly. Burr informed the NRC, who told the RCN. They realized that the technique could help to hide ships from German submarines in the Battle of the Atlantic. Before the introduction of centimetre radar, submarines with their small profile could see convoy ships before they were themselves seen. Diffused lighting camouflage might, the RCN believed, redress the balance.
   Section::::Prototyping.
  
  Section::::Prototyping.:Royal Canadian Navy.
  
  Burr was quickly called to Canada's Naval Services Headquarters to discuss how to apply diffused lighting camouflage. Simple tests in the laboratory served as proof of concept. In January 1941, sea trials began on the new corvette HMCS "Cobalt". She was fitted with ordinary light projectors—neither designed for robustness, nor waterproofed—on temporary supports on one side of the hull; brightness was controlled manually. The trial was sufficiently promising for a better prototype to be developed.
   BULLET::::- Flashlight, a red filter saves night vision, but reduces sight distance and signalling; carry a spare bulb.
   The Admiralty report on the "Trillium" trials stated that "Under certain weather conditions, the Canadian trials, in spite of the crude equipment used, gave highly satisfactory results. The experience gained during the present trials indicated that in various other types of weather this same equipment gave a much less conclusive indication of its value", and described the technical difficulties that any future version would face.
  
  The Admiralty informed the prime minister, Winston Churchill, at the end of that month, stating that the "results seem quite promising".
   The use of headlight covers and the modification of light produced by vehicles has continued into the present day in both the military's development of special blackout head and tail light technologies and in the civilian sector as well. In addition to headlight covers' use in light reduction they have also been employed to protect headlamps from damage in both civilian and combat environments. In fact, 3M now makes a wide variety of aerospace and military films solely for this purpose. 
   Another type of illumination device was the tripflare. This consisted of a pyrotechnic instrument activated by a tripwire, which was planted near trails to provide early warning of enemy movement. By 1944, the Wehrmacht was making extensive use of tripflares in Italy, after their perimeters had been repeatedly infiltrated at night by U.S. Army Rangers. Tripflares helped foil an attack launched by the Fifth Army on January 20 at the Rapido Rivers.
  
  Section::::History.:Post-World War II.
   For Police S.W.A.T. use, long-range weapon lights with exceptional range have always been a problem. Tactical weapon lighting companies such as ExtremeBeam for example have created small light duty lights which S.W.A.T. officers are able to mount to their modern weapons giving them an extended range out to beyond 300 meters (325+yards). This has allowed officer to blind suspects and protect officers closer at hand from being detected and thereby allowing for a better resolution of the situation with less lethal force.
  
  Section::::Lighting features.
   Churchill replied the next day, suggesting that "Surely all this business should be pressed forward on a broader front than the one ship?"
  
  Accordingly, in April 1941 the Admiralty ordered further development work for "full scale trials". The British General Electric Company developed a manually operated diffused lighting system, which was trialled on the ocean boarding vessel HMS "Largs" and the light cruiser HMS "Penelope". 
   First-generation passive devices, introduced during the Vietnam War and patented by the US Army, were an adaptation of earlier active GEN 0 technology and relied on ambient light instead of an extra infrared light source. Using an S-20 photocathode, their image intensifiers produced a light amplification of around 1000, but they were quite bulky and required moonlight to function properly.
  
  Examples:
  
  BULLET::::- AN/PVS-1 Starlight scope
  
  BULLET::::- AN/PVS-2 Starlight scope
  
  BULLET::::- PNV-57E tanker goggles
  
  BULLET::::- PAS 6 Varo Metascope
  
  Section::::History.:United States.:Generation 2 (GEN II).
  