Arka Das2012TTF2404

CAMOUFLAGE COLOURS IN MILITARY APPLICATIONS

INDIAN INSTITUTE OF TECHNOLOGY, DELHIPRESENTED TO PROF. M. L. GULRAJANI

Military Camouflage

Military camouflage is the use of camouflage by a military force to protect personnel and equipment from visual observation by enemy forces.

In practice, this means applying colour and materials to military equipment of all kinds, including vehicles, ships, aircraft, gun positions and battledress, either to conceal it from visual observation (crypsis), or to make it appear as something else (mimicry).

Sensor Systems in Military Applications

Active. Active sensors emit energy that reflects from targets and is recaptured by the emitting or other nearby unit, indicating the presence of a target. Examples of active sensors are searchlights and radars.

Passive. Passive sensors emit no energy. This type of sensor collects energy, which may indicate the presence of a target. Examples of passive sensors are the human eye, night vision devices, and photographs.

Threats in Military Applications Image Intensifiers Low-Light Television (LLTV). Aerial Reconnaissance, Remote Sensing, and

Imagery Near Infrared (NIR) Sensors IR Sensors UV Sensors Radar

Camouflage Systems

The aim is to make sure that the surface of the soldiers and not form a contrasting shape against the background.

The principles involved in camouflage materials are shape, shine, shadow, silhouette, surface, spacing and movement .

The hues used in the camouflage are green, olive, khaki, brown and black.

Factors influencing Camouflage

Contrast of brightness

Contrasts of colour

Contrasts of texture

The geometry of boundaries between contrasting zones

Distribution of contrasts in the visual field

Previous experience of the subject

Types of Camouflage

UV camouflage Camouflages for visible region NIR camouflage Thermal IR camouflage Anti radar camouflage Camouflages for multiple spectra

UV camouflage

Only in the snow covered environment is UV observation of military importance.

Titanium dioxide pigment which is commonly used as a low-cost widely available treatment for artificial fibres is visually white, but has low reflectance in the UV.

Luckily, other pigments such as barium sulphate are suitable and can be incorporated into textile coatings.

UV camouflage

Camouflages for visible region

In this range we are trying to mimic natural or even artificial backgrounds, not just in terms of colour, but also patterns, gloss and texture.

A tree or bush, for instance, will have a different appearance during different parts of the day as the quality of illumination changes.

In practice, each military nation has adopted its own visual colours and patterns. Colours often include khaki, green, brown and black, with additional colours such as olive, yellow, orange, pink, grey, beige, and sand to extend use to other urban, rural and desert backgrounds.

Infra red rays Infrared (IR) light is electromagnetic radiation with a

wavelength longer than that of visible light.

It is measured from the nominal edge of visible red light at 0.74 micrometres (µm), and extending conventionally to 300 µm.

These wavelengths correspond to a frequency range of approximately 1 to 400 THz, and include most of the thermal radiation emitted by objects near room temperature.

Microscopically, IR light is typically emitted or

absorbed by molecules when they change their rotational vibrational movements .

Types of Infra red rays

The CIE recommended the division of IR into following three bands.

Near Infra red – 700 – 1500 nm

Middle Infra red – 1400 – 3000 nm

Far Infra red – Above 3000 nm.

Near Infrared Camouflage

The development of infrared surveillance technology during the Second World War has had a particularly significant effect on the challenge of supplying military personnel and equipment with effective means of concealment.

It has forced the evolution of a generation of camouflage materials that attempt to minimise the contrast between objects and their environment over the near infrared region of the electromagnetic spectrum in addition to the visible region.

IR reflectance of natural surroundings

NIR range – 700 – 1200 nm. The sand and soil show a slight

rise from visible region to those in the infrared.

As moisture content increase reflectance in visible region decrease in the soil.

Plants tend to have a small increase in reflectance at around 550 nm, which is responsible for the green coloration. Reflectance then decreases slightly in the red region and rises steeply between 680 and 710 run to a plateau at 720 nm. At longer wavelengths the reflectance remains fairly constant until about 1300 nm, where it decreases again.

Black camouflage tends to have low, near-constant reflectance values.

Brown hues have gradually rising IR curves typical of sand and soil.

Green shades must duplicate the 'chlorophyll increase' or 'edge' .

Generally, military authorities require green camouflage to exhibit a steep rise in reflectance between 700 and 720 nm, so that reflectance in the range 700-900 nm exceeds that in the 600-690 nm region .

Contd..

Dyeing of cellulosic fibres

Vat dyes absorb IR are suitable for military uniforms as well as the required fastness to light and washing.

Certain sulphur dyes exhibit low IR reflectance but not used in camouflage due to poor light and wet fastness.

In Table 1, Combination of Cibanone (Ciba) or Indanthren (BASF) ranges of vat dyes are shown.

Suitable vat dyes are generally based on anthraquinonebenzanthrone- acridine polycyclic ring systems.

Vat dye combinations for camouflage for cellulosic

Shade DyesPale Brown

CI Vat Brown 6 ((Cibanone Brown F3B) CI Val Brown 1 (Cibanone Brown FBR) CI Val Orange 15 (Cibanone Golden Orange F3G)

Dark Brown

CI Vat Brown 35 (Cibanone Yellow Brown FG) CI Vat Black 27 (Cibanone Olive F2R) CI Vat Red 24 (Cibanone Red F4B)

Pale green

CI Vat Green 28 (Cibanone Green F6G) CI Vat Black 27 (Cibanone Olive F2R)CI Vat Orange 15 (Cibanone Golden Orange F3G)

Dark green

CI Vat Green 28 (Cibanone Green F6G)

CI Vat Black 27 (Cibanone Olive F2R) Cibanone Brilliant Green F4G

Grey CI Vat Black 30 (Cibanone Grey FOGR) CI Vat Brown 35 or CI Vat Orange 15CI Vat Black 27 (Cibanone Olive F2R)

Olive green shades Suitable vat dyes are generally based on anthraquinonebenzanthrone-acridine polycyclic ring systems

CI Vat Green 3 (X = Y = H), prepared by condensing 3- bromobenzanthrone with I-aminoanthraquinone and fusing the product with alkali

Examples of these olive dyes include: (X = H, Y = NHCOAr), (X = Ph, X = H, Y = (4-halogeno)anthraquinyl-1-amino) and (X=H, Y= SC4H9)

CI Vat Green 3

Grey & Olive shades 2-methyl-1(3)- anthraquinylamino substituent in one of the α-positions of the anthraquinone residue.

Synthesised by condensation of 3-bromobenzanthrone & diaminoanthraquinone, reacted with a bromoanthraquinone.

The subsequent alkali treatment is claimed to cause a double ring closure, i.e. the formation of two acridine residues.

Olive brown shades

Anthraquinone-benzanthrone- acridine dyes are those of general structure (5; R = H, X = NHCOAr).

For example, analogues of CI Vat Black 25 (5; R = X = H). The derivatives (5; R = (cyclo)alkyl, X = H)

Infrared reflectance of about 10-25%.

The dyes (5; R = H, X = OCH3) and (5; R = H, X = SC4H9) are claimed to give grey and brown shades, respectively on cotton

CI Vat Black 25

Olive & Brown shades

Anthraquinone-benzanthrone-acridine derivatives, containing a thiaxanthone residue, prepared 4-aminoanthraquinone thiaxanthone.

The dyes yield olive to brown shades on cotton of low infrared reflectance

They give satisfactory light and wash fastness properties.

Dyes for Non cellulosic fibres for camouflage

Strongly IR absorbing pigments, such as carbon black, can be melt-spun into or printed onto the polymer to raise IR absorption.

PET fabric dyed with the disperse dyes (8; X = NHCH,OH, Y = OH; 0.2% o.m.f.), (8; X = OH, Y = NHCH,OH; 0.2% 0.m.f.) and 9 (0.8% o.m.f.), by a carrier dyeing method.

These doesn’t have satisfactory IR. So, PET spun-dyed with carbon black (0.01

% by weight) and cross-dyed with the same dye combination is claimed to have superior reflectance characteristics which meets the criteria of the Danish Army.

Dyes for other fibres

Camouflage shades can be produced in this fashion from polyamide fibres (disperse, non-metallised acid or metal-complex dyes), cellulose acetate (disperse dyes) and viscose (reactive dyes).

In the case of wool, only a few green and black colorants absorb sufficient IR to be suitable for camouflage purposes.

Using other dyes the required IR can be obtained by forming a blend with suitably dyed or spun viscose or synthetic fibres

If 100% wool materials are required, IR can be lowered by fixing carbon black onto the fibre with a synthetic resin binder.

Pigments used for IR camouflageOrganic Pigments Inorganic pigments

Perylene blackPhthalocyanine bluesGreensVioletCarbazole dioxazine

Chromium oxideRed & yellow oxide of ironFerric oxideCarbon blackLead chromateCarbon blackMetallic pigmentsQuinacridoneIsoindolinone Isoindoline Diketopyrrolopyrrole

The particle size of the pigment should be 0.35 – 0.55 microns For highest reflectivity, the particle size should be more than half the wave length of the light to be reflected.

Camouflage of Cotton Fabrics in Visibleand NIR Region Using Three SelectedVat Dyes

To provide camouflage in near infrared (NIR) region and imitate reflectance profile of greenish leaves, cotton fabrics were dyed with three selected vat dyes, namely C.I. Vat Blue 6, C.I. Vat Yellow 2, and C.I. Vat Red 13.

Reflectance curves of two types of fresh greenish leaves were measured as standard reference.

Experimental Details Cotton fabrics with twill weave (bleached and mercerized)

were purchased from Ardakan textile (Yazd, Iran). C.I. Vat Blue 6, C.I. Vat Yellow 2 and C.I. Vat Red 13 were

used. For dyeing process, 1 g of dye powder was added to 99 mL

distilled water at 60–70°C and stirred for 10 min. 1% owf (on weight of fiber) solution of each dye was

prepared. For reduction of vat dyes, th required amount of dye was

added to dye bath, then 36 mL/L alkali and 36 g/L sodium hydrosulfite for NATO green shade and 51.5 mL/L alkali and 51.5 g/L sodium hydrosulfite for forest green shade were added to dye bath.

Temperature of dye bath was adjusted to 40°C. After this time, temperature raised to 60°C, and the dyeing

process was continued for 40 min.

Dye Structure

As above mentioned, vat dyes which have anthraquinone, benzanthrone, or acridine polycyclic ring structures are suitable for camouflage in NIR region and create suitable spectral reflectance.5 So in term of structure, they are appropriate for camouflage in NIR range.

RESULTS AND DISCUSSION

In reflectance profile of green leaf there is a slight increase in range

of 550 nm. Then in range of about

670–780 nm, the reflection increases steeply.

These amounts for deciduous leaf and coniferous needle are 90 and 50%,

From Fig. 5, it can be concluded that for Vat Yellow 2 there is an absorption peak around 422 nm in visible range.

There is an absorption peak around 520 nm for Vat Red 13.

The absorption peak of Vat Blue 6 occurs at 660 nm.

It can be observed that the reflectance of Vat Yellow 2 increases sharply within the range of 440– 560 nm. After this range, reflectance will be constant in about 67±2%

The reflectance of Vat Red 13 increases rapidly in range of 580–700nm. Then reflectance is 64±6% until 1100 nm. Also for this dye, there is an increase around 420 nm.

The reflectance of Vat Blue 6 increases in range of 700–780 nm and also for this dye there is a pronounced increase around 440 nm. After 780 nm, the reflectance will be in range of 59±2%.

From Figure 9, it can be seen that general shape of the reflectance curves is very similar to reflectance profile of greenish leaves.

Both of them have slight increasing in about 550 nm and also have pronounced increasing in range of 670–780 nm according to reflectance profile of green leaves.

Summary

Three vat dyes were selected (Vat Blue 6, Vat yellow 2, and Vat Red 13) and cotton fabrics were dyed with these dyes to imparting camouflage in NIR region and similarly with the reflectance profile of green leaves.

Compared with the reflection curves of C.I. Vat Yellow 2 and C.I. Vat Red 13 and green leaf, the reflectance profile of Vat Blue 6 is very similar to reflectance curve of greenish leaves in range of 670–780 nm.

For Vat Blue 6, the reflectance curve of dyed fabrics matches with reflectance of leaves at dyeing concentration of 0.85% owf and 1.2% owf, for NATO and forest green shade, respectively.

The dyes have suprerior wash and light fastness and their camouflage characteristics do not change during exposure.

Thermal infrared camouflage waveband

The thermal or far infrared (FIR) wavebands are, militarily, defined as being from 3–5μm, and 8–14μm.

Objects are detected by the heat energy they emit or reflect.

The relationships between energy emitted, emissivities, wavelengths, and temperatures are covered by mathematical relationships derived by Planck,Wien, and Stefan. In simplified terms these are:

λmax*T = a (constant)Where λ is the wavelength and T is absolute temperatureStefan; E = ησT^4η is the emissivity and σ is a constant

Therefore, there are two things that we can do to reduce the thermal signature of targets, reduce the temperature and the emissivity of the target.1) Reduce the temperature of the target: vehicles need

to be designed so that hot exhaust systems are cooled by air or liquids, by insulating the hot components, or by rerouting the hot piping so that it is covered and not visible.

2) Reduce the emissivity of the target: Most surfaces are good emitters, except those which are shiny and metallic. Therefore, we can lower the emissivity of the target by using a shiny reflective cover, although this will obviously interfere with visual camouflage

Thermal infrared camouflage waveband

Radar

MTI Radar. (a)MTI radar is a threat to ground forces near the

battle area. Radar-reflecting metal on dismounted soldiers has been reduced. Kevlar helmets and body armor are now radar-transparent. Plastic canteens are standard issue. Velcro and buttons have replaced metal snaps on most field uniforms. A soldier wearing only the BDU cannot be detected until very close to an MTI radar.

(b)Vehicles are large radar-reflecting objects. Moving by covered routes protects against MTI radar surveillance

Conclusion The development of several surveillance technology has

rendered obsolete textile production techniques that provide camouflage solely in the visible region of the electromagnetic spectrum.

Modern military forces require counter surveillance materials that afford protection against several surveillance technology, camouflage textiles must satisfy for all threats.

The ease of fulfilling such requirements varies with the substrate of the camouflage material. Conventional methods for the coloration of other fibers have been found to be inadequate. Novel dyes and pigments, as well as new techniques, have had to be developed in future.

References 1) Infra red camouflage, S M Burkinshaw, G Hallas & A D

Towns, Rev. Prog. Coloration, 26 (1996) 47.2) HANDBOOK OF TECHNICAL TEXTILES, A R Horrocks and S C

Anand, Woodhead Publishing Limited 3)Camouflage Textiles, D.Saravanan, AATCC Review, 2007.4) Camouflage of Cotton Fabrics in Visible and NIR Region

Using Three Selected Vat Dyes, U. Goudarzi, J. Mokhtari,* M. Nouri, Wiley Periodicals, Inc., 29 June 2012

5) Introduction to camouflage & deception, Dr.J.V.Ramana Rao, DRDO, DESIDOC, 1999.

6) Infrared Reflective Inorganic Pigments, Ashwini KB & Vinod CM, Recent Patents on Chemical Engineering, 2008, 1, 67-79

7) Wikipedia 8) Camouflage Colours, T. y. OVERTON, Stores and Clothing

Research and Development Establishment, Ministry of Dejence, Flagstafl Road, Colchester, Essex

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