# Counter-illumination

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Active camouflage using light matched to the background

Principle of the counter-illumination camouflage of the firefly squid, *[Watasenia scintillans](/source/Watasenia_scintillans)*. When seen from below by a predator, the animal's light helps to match its brightness and colour to the sea surface above.

**Counter-illumination** is a method of [active camouflage](/source/Active_camouflage) seen in [marine animals](/source/Marine_animal) such as [firefly squid](/source/Firefly_squid) and [midshipman fish](/source/Midshipman_fish), and in military prototypes, producing light to match their backgrounds in both brightness and wavelength.

Marine animals of the [mesopelagic](/source/Mesopelagic) (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, often [from predators](/source/Anti-predator_adaptation) but also from their prey, by producing light with [bioluminescent](/source/Bioluminescence) [photophores](/source/Photophore) on their downward-facing surfaces, reducing the contrast of their [silhouettes](/source/Silhouette) against the background. The light may be produced by the animals themselves, or by [symbiotic](/source/Mutualism_(biology)) [bacteria](/source/Bacteria), often *[Aliivibrio fischeri](/source/Aliivibrio_fischeri)*.

Counter-illumination differs from [countershading](/source/Countershading), which uses only pigments such as [melanin](/source/Melanin) to reduce the appearance of shadows. It is one of the dominant types of [aquatic camouflage](/source/Aquatic_camouflage), along with transparency and [silvering](/source/Silvering_(camouflage)). All three methods make animals in open water resemble their environment.

Counter-illumination has not come into widespread [military use](/source/Military_camouflage), but during the [Second World War](/source/Second_World_War) it was trialled in [ships](/source/Ship_camouflage) in the Canadian [diffused lighting camouflage](/source/Diffused_lighting_camouflage) project, and in [aircraft](/source/Aircraft_camouflage) in the American [Yehudi lights](/source/Yehudi_lights) project.

## In marine animals

Further information: [Underwater camouflage](/source/Underwater_camouflage) and [List of camouflage methods](/source/List_of_camouflage_methods)

### Mechanism

#### Counter-illumination and countershading

Further information: [Underwater camouflage](/source/Underwater_camouflage) and [Countershading](/source/Countershading)

Counter-illuminating photophores illuminating the underside of the hatchetfish *[Argyropelecus olfersii](/source/Argyropelecus_olfersii)*

In the sea, counter-illumination is one of three dominant methods of [underwater camouflage](/source/Underwater_camouflage), the other two being transparency and silvering.[1] Among marine animals, especially [crustaceans](/source/Crustacean), [cephalopods](/source/Cephalopod), and [fish](/source/Fish), counter-illumination [camouflage](/source/Camouflage) occurs where [bioluminescent](/source/Bioluminescent) light from [photophores](/source/Photophore) on an [organism](/source/Organism)'s ventral surface is matched to the light radiating from the environment.[2] The [bioluminescence](/source/Bioluminescence) is used to obscure the organism's silhouette produced by the down-welling light. Counter-illumination differs from [countershading](/source/Countershading), also used by many marine animals, which uses pigments to darken the upper side of the body while the underside is as light as possible with pigment, namely white. Countershading fails when the light falling on the animal's underside is too weak to make it appear roughly as bright as the background. This commonly occurs when the background is the relatively bright ocean surface, and the animal is swimming in the [mesopelagic](/source/Mesopelagic) depths of the sea. Counter-illumination goes further than countershading, actually brightening the underside of the body.[3][4]

#### Photophores

Main article: [Photophore](/source/Photophore)

[Photophores](/source/Photophore) on a [lanternfish](/source/Lanternfish), the most common deep sea fish worldwide

Counter-illumination relies on organs that produce light, photophores. These are roughly spherical structures that appear as [luminous](/source/Luminescence) spots on many marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye, equipped with lenses, shutters, colour filters and reflectors.[5]

[Sagittal section](/source/Sagittal_section) of the large eye-like light-producing organ of Hawaiian bobtail squid, *[Euprymna scolopes](/source/Euprymna_scolopes)*. The organ houses symbiotic *[Aliivibrio fischeri](/source/Aliivibrio_fischeri)* bacteria.

In the [Hawaiian bobtail squid](/source/Hawaiian_bobtail_squid) (*Euprymna scolopes*) light is produced in a large and complex two-lobed light organ inside the squid's [mantle cavity](/source/Mantle_cavity). At the top of the organ (dorsal side) is a reflector, directing the light downwards. Below this are containers (crypts) lined with [epithelium](/source/Epithelium) containing light-producing symbiotic bacteria. Below those is a kind of [iris](/source/Iris_(eye)), consisting of branches (diverticula) of its [ink sac](/source/Ink_sac); and below that is a lens. Both the reflector and the lens are derived from [mesoderm](/source/Mesoderm). Light escapes from the organ downwards, some of it travelling directly, some coming off the reflector. Some 95% of the light-producing bacteria are voided at dawn every morning; the population in the light organ then builds up slowly during the day to a maximum of some 1012 bacteria by nightfall: this species hides in sand away from predators during the day, and does not attempt counter-illumination during daylight, which would in any case require much brighter light than its light organ output. The emitted light shines through the skin of the squid's underside. To reduce light production, the squid can change the shape of its iris; it can also adjust the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. The light production is correlated with the intensity of down-welling light but about one third as bright; the squid is able to track repeated changes in brightness.[6]

#### Matching light intensity and wavelength

At night, nocturnal organisms match both the [wavelength](/source/Wavelength) and the [light intensity](/source/Brightness) of their bioluminescence to that of the down-welling moonlight and direct it downward as they swim, to help them remain unnoticed by any observers below.[6][7]

Spectrum of [visible light](/source/Visible_light) showing colours at different [wavelengths](/source/Wavelength), in [nanometres](/source/Nanometre)

In the eyeflash squid (*[Abralia veranyi](/source/Abralia_veranyi)*) a species which [daily migrates between the surface and deep waters](/source/Diel_vertical_migration), a study showed that the light produced is bluer in cold waters and greener in warmer waters, temperature serving as a guide to the required [emission spectrum](/source/Emission_spectrum). The animal has more than 550 photophores on its underside, consisting of rows of four to six large photophores running across the body, and many smaller photophores scattered over the surface. In cold water at 11 Celsius, the squid's photophores produced a simple (unimodal) spectrum with its peak at 490 nanometres (blue-green). In warmer water at 24 Celsius, the squid added a weaker emission (forming a shoulder on the side of the main peak) at around 440 nanometres (blue), from the same group of photophores. Other groups remained unilluminated: other species, and perhaps *A. veranyi* from its other groups of photophores, can produce a third spectral component when needed. Another squid, *[Abralia trigonura](/source/Abralia_trigonura)*, is able to produce three spectral components: at 440 and at 536 nanometres (green), appearing at 25 Celsius, apparently from the same photophores; and at 470–480 nanometres (blue-green), easily the strongest component at 6 Celsius, apparently from a different group of photophores. Many species can in addition vary the light they emit by passing it through a choice of colour filters.[8]

Counterillumination camouflage halved predation among individuals employing it compared to those not employing it in the [midshipman fish](/source/Midshipman_fish) *[Porichthys notatus](/source/Porichthys_notatus)*.[6][9]

Diagram of a small type of [photophore](/source/Photophore) in the skin of a [cephalopod](/source/Cephalopod), *[Abralia trigonura](/source/Abralia_trigonura)*, in vertical section

#### Autogenic or bacteriogenic bioluminescence

Further information: [Bioluminescence](/source/Bioluminescence)

The bioluminescence used for counter-illumination can be either [autogenic](https://en.wiktionary.org/wiki/autogenic) (produced by the animal itself, as in [pelagic](/source/Pelagic) cephalopods such as *[Vampyroteuthis](/source/Vampyroteuthis)*, *[Stauroteuthis](/source/Stauroteuthis)*, and pelagic octopuses in the [Bolitaenidae](/source/Bolitaenidae)[10]) or bacteriogenic (produced by [bacterial](/source/Bacteria) [symbionts](/source/Symbiosis)). The luminescent bacterium is often *[Aliivibrio fischeri](/source/Aliivibrio_fischeri)*, as for example in the Hawaiian bobtail squid.[6]

### Purpose

Photophores on a nocturnal [midshipman fish](/source/Midshipman_fish), whose bioluminescence halves its rate of predation[6]

#### Hiding from predators

Reducing the silhouette is primarily an [anti-predator defence](/source/Antipredator_adaptation) for mesopelagic (mid-water) organisms. The reduction of the silhouette from highly directional down-welling light is important, since there is no refuge in the open water, and [predation](/source/Predation) occurs from below.[3][11][12] Many mesopelagic cephalopods such as the [firefly squid](/source/Firefly_squid) (*Watasenia scintillans*), [decapod](/source/Decapoda) crustaceans, and deep ocean fishes use counter-illumination; it works best for them when ambient light levels are low, leaving the diffuse down-welling light from above as the only light source.[6][3] Some deep water sharks, including *[Dalatias licha](/source/Dalatias_licha)*, *[Etmopterus lucifer](/source/Etmopterus_lucifer)*, and *[Etmopterus granulosus](/source/Etmopterus_granulosus)*, are bioluminescent, most likely for camouflage from predators that attack from beneath.[13]

#### Hiding from prey

Besides its effectiveness as a predator avoidance mechanism, counter-illumination also serves as an essential tool to predators themselves. Some shark species, such as the deepwater [velvet belly lanternshark](/source/Velvet_belly_lanternshark) (*Etmopterus spinax*), use counter-illumination to remain hidden from their prey.[14] Other well-studied examples include the [cookiecutter shark](/source/Cookiecutter_shark) (*Isistius brasiliensis*), the [marine hatchetfish](/source/Marine_hatchetfish), and the Hawaiian bobtail squid.[6] More than 10% of shark species may be bioluminescent, though some such as [lantern sharks](/source/Lantern_shark) may use the light for [signalling](/source/Signalling_theory) as well as for camouflage.[15]

### Defeating counter-illumination camouflage

An animal camouflaged by counter-illumination is not completely invisible. A predator could resolve individual photophores on a camouflaged prey's underside, given sufficiently acute vision, or it could detect the remaining difference in brightness between the prey and the background. Predators with a visual acuity of 0.11 degrees (of arc) would be able to detect individual photophores of the Madeira lanternfish *[Ceratoscopelus maderensis](https://en.wikipedia.org/w/index.php?title=Ceratoscopelus_maderensis&action=edit&redlink=1)* at up to 2 metres (2.2 yd), and they would be able to see the general layout of the photophore clusters with poorer visual acuity. Much the same applies also to *Abralia veranyi*, but it was largely given away by its unlit fins and tentacles, which appear dark against the background from as far away as 8 metres (8.7 yd). All the same, the counter-illumination camouflage of these species is extremely effective, radically reducing their detectability.[2][a]

## Military prototypes

Main article: [Active camouflage](/source/Active_camouflage)

[Active camouflage](/source/Active_camouflage) in the form of counter-illumination has rarely been used for military purposes, but it has been prototyped in [ship](/source/Ship_camouflage) and [aircraft camouflage](/source/Aircraft_camouflage) from the Second World War onwards.[16][17][18]

### For ships

[Diffused lighting camouflage](/source/Diffused_lighting_camouflage) prototype, not quite complete and set to maximum brightness, installed on [HMS *Largs*](/source/HMS_Largs) in 1942

Main article: [Diffused lighting camouflage](/source/Diffused_lighting_camouflage)

[Diffused lighting camouflage](/source/Diffused_lighting_camouflage), in which [visible light](/source/Visible_light) is projected on to the sides of ships to match the faint glow of the night sky, was trialled by [Canada's National Research Council](/source/National_Research_Council_(Canada)) from 1941 onwards, and then by the [Royal Navy](/source/Royal_Navy), during the Second World War. Some 60 light projectors were mounted all around the hull and on the ships' superstructure such as the bridge and funnels. On average, the system reduced the distance at which a ship could be seen from a surfaced submarine by 25% using binoculars, or by 33% using the naked eye. The camouflage worked best on clear moonless nights: on such a night in January 1942, [HMS *Largs*](/source/HMS_Largs) was not seen until it closed to 2,250 yards (2,060 m) when counter-illuminated, but was visible at 5,250 yards (4,800 m) unlighted, a 57% reduction in range.[16][19]

### For aircraft

[Mary Taylor Brush](/source/Mary_Taylor_Brush)'s 1917 patent application for camouflaging a [Morane-Borel monoplane](/source/Morane-Borel_monoplane) using light bulbs

Main article: [Yehudi lights](/source/Yehudi_lights)

In 1916 the American artist [Mary Taylor Brush](/source/Mary_Taylor_Brush) experimented with camouflage on a [Morane-Borel monoplane](/source/Morane-Borel_monoplane) using light bulbs around the aircraft, and filed a 1917 patent that claimed she was "able to produce a machine which is practically invisible when in the air". The concept was not developed further during the [First World War](/source/First_World_War).[20]

Forward-pointing [Yehudi lights](/source/Yehudi_lights) on [Grumman TBM Avenger](/source/Grumman_TBM_Avenger) raised the average brightness of the plane from a dark shape to the same as the sky.[b]

The Canadian ship concept was trialled in American aircraft including [B-24 Liberators](/source/Consolidated_B-24_Liberator) and [TBM Avengers](/source/Grumman_TBM_Avenger) in the [Yehudi lights](/source/Yehudi_lights) project, starting in 1943, using forward-pointing lamps automatically adjusted to match the brightness of the sky. The goal was to enable a radar-equipped, sea-search aircraft to approach a surfaced [submarine](/source/Submarine) to within 30 seconds from arrival before being seen, to enable the aircraft to drop its [depth charges](/source/Depth_charge) before the submarine could dive. There was insufficient electrical power available to illuminate the entire surface of the aircraft, and outboard lamps in the manner of diffused lighting camouflage would have interfered with the airflow over the aircraft's surface, so a system of forward-pointing lamps was chosen. These had a beam with a radius of 3 degrees, so pilots had to fly with the aircraft's nose pointed directly at the enemy. In a [crosswind](/source/Crosswind), this required a curving approach path, rather than a straight-line path with the nose pointed upwind. In trials in 1945, a counter-illuminated Avenger was not seen until 3,000 yards (2.7 km) from its target, compared to 12 miles (19 km) for an uncamouflaged aircraft.[17]

The idea was revisited in 1973 when an [F-4 Phantom](/source/F-4_Phantom) was fitted with camouflaging lights in the "Compass Ghost" project.[18]

## Notes

1. **[^](#cite_ref-16)** The pattern of photophores may, in addition to matching background brightness, also serve to break up the animals' silhouettes, just as spots and stripes of coloured paint do in [disruptive coloration](/source/Disruptive_coloration), but in the absence of experimental evidence it is uncertain how useful this is: it would only help when the sea surface background was uneven.[2]

1. **[^](#cite_ref-22)** The effect may be seen by standing back a little from the image and half-closing the eyes. The upper image becomes indistinct where the lower image remains as a dark shape.

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1. **[^](#cite_ref-14)** Claes, Julien M.; Aksnes, Dag L.; Mallefet, Jérôme (2010). ["Phantom hunter of the fjords: camouflage by counterillumination in a shark (*Etmopterus spinax*)"](https://web.archive.org/web/20110927154130/http://www.bio.uib.no/modelling/papers/Claes_2010_Phantom_hunter.pdf) (PDF). *Journal of Experimental Marine Biology and Ecology*. **388** (1–2): 28–32. [doi](/source/Doi_(identifier)):[10.1016/j.jembe.2010.03.009](https://doi.org/10.1016%2Fj.jembe.2010.03.009). Archived from [the original](http://www.bio.uib.no/modelling/papers/Claes_2010_Phantom_hunter.pdf) (PDF) on 2011-09-27. Retrieved 2010-11-14.

1. **[^](#cite_ref-15)** Davies, Ella (26 April 2012). ["Tiny sharks provide glowing clue"](https://web.archive.org/web/20121122091054/http://www.bbc.co.uk/nature/17812363). BBC. Archived from [the original](https://www.bbc.co.uk/nature/17812363) on 22 November 2012. Retrieved 12 February 2013.

1. ^ [***a***](#cite_ref-NavalMuseumQuebec_17-0) [***b***](#cite_ref-NavalMuseumQuebec_17-1) ["Diffused Lighting and its use in the Chaleur Bay"](https://web.archive.org/web/20130522231113/http://www.navy.forces.gc.ca/navres/NMQ_MNQ/researches_recherches/diffusedLighting_camouflageLumineux/index-eng.asp). *Naval Museum of Quebec*. Royal Canadian Navy. Archived from [the original](http://www.navy.forces.gc.ca/navres/NMQ_MNQ/researches_recherches/diffusedLighting_camouflageLumineux/index-eng.asp) on 22 May 2013. Retrieved 3 February 2013.

1. ^ [***a***](#cite_ref-NDRC_18-0) [***b***](#cite_ref-NDRC_18-1) Bush, Vannevar; Conant, James; et al. (1946). ["Camouflage of Sea-Search Aircraft"](https://web.archive.org/web/20131023061821/http://www.dtic.mil/dtic/tr/fulltext/u2/221102.pdf) (PDF). *Visibility Studies and Some Applications in the Field of Camouflage*. Office of Scientific Research and Development, National Defence Research Committee. pp. 225–240. Archived from [the original](http://www.dtic.mil/dtic/tr/fulltext/u2/221102.pdf) (PDF) on October 23, 2013. Retrieved February 12, 2013.

1. ^ [***a***](#cite_ref-Dann2011_19-0) [***b***](#cite_ref-Dann2011_19-1) Dann, Rich (2011). ["Yehudi Lights"](https://web.archive.org/web/20111007163146/http://www.public.navy.mil/airfor/centennial/Documents/vol3iss3.pdf) (PDF). *Centennial of Naval Aviation*. **3** (3): 15. Archived from [the original](http://www.public.navy.mil/airfor/centennial/Documents/vol3iss3.pdf) (PDF) on 2011-10-07. Retrieved 2017-02-19. the prototype Grumman XFF-1 .. was fitted with lights as an active camouflage method .. Counter-illumination was tested again in 1973, using a U.S. Air Force F-4C Phantom II with lights, under the name COMPASS GHOST

1. **[^](#cite_ref-20)** *Trial Report D.L. 126: DL Trials on HMS*Largs*in Clyde Approaches*. [The National Archives, Kew](/source/The_National_Archives%2C_Kew): [British Admiralty](/source/British_Admiralty). 1942. ADM/116/5026 Diffused Lighting.

1. **[^](#cite_ref-ASMag_21-0)** D'Alto, Nick (2016). ["Inventing the Invisible Airplane: When camouflage was fine art"](https://www.airspacemag.com/military-aviation/art-camouflage-180959768/). Air & Space Magazine. Retrieved 9 March 2020.

## External links

- [Scientific American: 10 Bioluminescent Creatures](http://www.scientificamerican.com/slideshow.cfm?id=bioluminescent-avatar)

- [Science Magazine: Bioluminescence in Mesopelagic Squid](https://www.science.org/doi/abs/10.1126/science.208.4449.1286)

- [Nova: Science Now: Glowing in the Dark](https://archive.today/20130416025419/http://www.pbs.org/wgbh/nova/sciencenow/0305/04-glow-07.html) (Squid *Abralia veranyi* belly lights)

v t e Camouflage Methods Camouflage Countershading Active camouflage Counter-illumination Disruptive coloration Coincident disruptive coloration Disruptive eye mask Distractive markings Motion camouflage Multi-scale camouflage Multi-spectral camouflage Self-decoration Snow camouflage Urban camouflage In nature As evidence for natural selection Crypsis Decorator crab Flower mantis Mimicry Batesian Müllerian Aggressive Underwater camouflage People Early Edward Bagnall Poulton The Colours of Animals Abbott Handerson Thayer Concealing-Coloration in the Animal Kingdom Camoufleurs Mary Taylor Brush Lucien-Victor Guirand de Scévola John Graham Kerr Norman Wilkinson Everett Warner Leon Underwood Johann Georg Otto Schick Hugh Cott Adaptive Coloration in Animals Geoffrey Barkas Timothy O'Neill Researchers Roy Behrens Tim Caro Innes Cuthill Thomas N. Sherratt Martin Stevens Military Topics Military camouflage Aircraft camouflage Camouflage clothing in Trinidad and Tobago Dazzle camouflage List of countries that prohibit camouflage clothing Middle East Command Camouflage Directorate Ship camouflage USN WWII camouflage measures Patterns Up to WWII German WWII Splittertarnmuster (1931) Platanenmuster (1937) Rauchtarnmuster (1939) Palmenmuster (c 1941) Sumpfmuster (1943) Erbsenmuster (1944) Leibermuster (1945) Other Camouflage tree (1915) Lozenge (1917 aircraft) Telo mimetico (1929 tent) Denison smock (1941) Frog skin (1942) Ghillie suit Post-war Lizard (1947) Strichtarn (1960) KLMK (1968) Late 20th century Jigsaw (1958) Tiger stripe (1962) Rhodesian Brushstroke (1965) ERDL (1967) Disruptive Pattern Material (1969) wz. 68 Moro (1969) Six-Color Desert Pattern (Chocolate Chip) (1981) U.S. "M81" Woodland (1981) Australian Disruptive Pattern (1982) TAZ 83 (1983) M84 (1984) Type 87 (China) (1987) wz. 89 Puma (1989) Camouflage Daguet (1989) M90 (1990) Desert Night Camouflage (1990) Flecktarn (1990) Tropentarn (1990) Desert Camouflage Pattern (1990) Camouflage Central-Europe (1991) Soldier 2000 (1993) TAZ 90 (1993) wz. 93 Pantera (1993) CADPAT (1997) Dubok (1997) M98 (1998) Flora (1998) 21st century MARPAT (2001) (Marine Corps Combat Utility Uniform (2002)) MultiCam (2002) Tactical Assault Camouflage (2004) Universal Camouflage Pattern (2004) ESTDCU (2006) M05 (2007) Airman Battle Uniform (2007) Type 07 (2007) EMR (2008) Multi-Terrain Pattern (2010) Australian Multicam (2014) MM-14 (2014) Varan (camouflage) (2015) HunCam (2015) Operational Camouflage Pattern (2015) Multitarn (2016) Netherlands Fractal Pattern (2019) Xingkong (2019) K20 (2020) Technology Deployed Berberys-R Nakidka Prototypes Diffused lighting camouflage (1941) Yehudi lights (1943) Adaptiv (2011) Related Dazzled and Deceived Stealth technology Cloaking device Invisibility

v t e Vision in animals Vision Birds Chameleons Dinosaurs Fish Toads Mammals horses dogs cats Eyes Arthropod eye Compound eye Eagle eye Eye shine Simple eye in invertebrates Mammalian eye human Mollusc eye cephalopod gastropod Holochroal eye Parietal eye Schizochroal eye Evolution Evolution of the eye Evolution of color vision Evolution of color vision in primates Coloration Albinism Animal coloration Aposematism Camouflage Chromatophore Counter-illumination Countershading Crypsis Deimatic behaviour Disruptive coloration coincident Eyespot (mimicry) Mimicry Structural coloration Underwater camouflage Related topics Animal senses Blindness in animals Eyespot apparatus Feature detection Infrared sensing in snakes Monocular deprivation Ommatidium Palpebral (bone) Pseudopupil Rhopalium Underwater vision Visual perception

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Adapted from the Wikipedia article [Counter-illumination](https://en.wikipedia.org/wiki/Counter-illumination) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Counter-illumination?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.