Modern warfare often involves poorly defined battle lines accompanied by multilevel fire support systems that can deliver firepower with high precision and devastating lethality from a long distance. The ability to immediately and accurately discriminate through sight between friend and foe is of great importance to military operations for effectively destroying hostile forces while preventing fratricide. This ability is even more crucial for irregular and unconventional warfare such as anti-terrorism operations where US forces often engage enemy combatants well entrenched in urban settings or rugged terrains at night. In such battlefield conditions, using conventional daylight and thermal imagery often exceeds the ability to accurately identify targets as friend or foe, and more reliable battlefield identification methods are needed.

Tagging, Tracking and Locating

Figure 1. Optical taggants can be used effectively in battlefield situations known as “identification friend or foe (IFF)”. Here a hyperspectral imager from the air identifies friendly soldiers in the general area where both sides are present. (Diagram courtesy Brimrose Corporation)
Tagging, tracking and locating represent a valuable technology for both commercial and military applications. For military operations, the tagging systems (taggants and detectors) must satisfy three basic requirements: covertness, (i.e., signals cannot be easily detected by common techniques), quick and accurate identification from a long distance (up to a mile or farther), and both lightweight and ruggedness suitable for field applications. Other important criteria may include two-way communication, the ability to track and identify a large number of subjects and objects, and specific capabilities tailored for unique battlefield conditions.

Materials and devices emitting in the infrared region represent an important class of covert optical taggants. Infrared (IR) light (from 0.75 μm to 1000 μm) is electromagnetic radiation with wavelengths longer than those operating in the visible region. For military applications, the IR wavelength is usually limited to 15 μm. An integrated infrared tagging system consists of three essential parts: infrared emitting (or absorbing) taggants, photodetectors, and an intelligence interface. Certain materials can emit infrared light through chemiluminescence, photoluminescence or electroluminescence.

There are three general groups of infrared emitting materials: organic IR emitting dyes, lanthanide IR emitters, and semiconductor IR emitters. Many pure organic dyes have been developed especially for NIR bimolecular imaging. The use of NIR fluorophores will eliminate background noise caused by the autofluorescence of biosubstrates. Common organic NIR fluorophores include cyanine, oxazine and rhodamine dyes. The fluorescence maxima of these dyes are between 700-850 nm. Organic near IR dyes can be used to make glowstick-type IR light sources through chemiluminescence.

Figure 2. A soldier wearing an optical taggant on his right sleeve. In the left photo, the taggant has not been activated. At the right photo, the optical taggant has been activated. The activated taggant is not available in the visual spectrum, but only when using the hyperspectral imager, scanning at the predetermined wavelength. (Photos courtesy Brimrose Technology Corp.)
Organic dyes with fluorescence maxima extending to far near IR and into short wave IR (SWIR) can be achieved by the formation of metal ion complexes. The most notable group of metals whose ions are capable of narrow band infrared emission is the lanthanide series with atomic numbers 57 to 71 (lanthanum to lutetium). Lanthanide infrared phosphors can also be hosted in inorganic matrices. These inorganic host materials include fluoride and oxyfluoride optical glasses, such as NaYF, SiO2–Al2O3–NaF–YF3, and oxide glass/ceramics including SiO2, ZrO2, Y2O3, and Y3Al5O12 (yttrium aluminum garnet; YAG). These inorganic host materials are generally optically transparent, especially in the IR spectral region.

Infrared emissions of lanthanide are often achieved through photoluminescence. Photoluminescence of lanthanide cations are due to their abundance of 4f–4f and 4f–5d transitions. Well known IR emission wavelengths from lanthanide ions are generally in the 1-3 μm regions, and this has led lanthanide ions to become active centers in laser gain medium materials. It is also known that several trivalent lanthanide ions possess possible emission transitions in the MWIR spectral region (3-5 μm).

Semiconductor Materials

Semiconductors are important optical materials. Unlike organic fluorophores or lanthanide ions whose optical properties are mainly determined by molecular or atomic structures and are usually un-tunable, optical emission and absorption of semiconductors are due to their unique band gap, which often falls into the infrared energy region. Their wavelengths can be further adjusted, either by forming compound semiconductors or reducing the size to cover a broad wavelength range. Semiconductors are the foundation materials for modern infrared detectors as well as infrared light emitting diodes (LEDs) and laser diodes. LEDs and laser diodes are commercially available for infrared light emission at wavelengths from near IR to mid IR (1-5 μm).

LEDs and laser diodes both have their advantages and disadvantages. Compared to LEDs, laser diodes produce much narrower band emissions, but they may require cooling features such as heat sinks, especially when operating at high-energy densities, while for LEDs no cooling is needed. Light emissions from LEDs and especially laser diodes are highly directional. This not only raises concerns about eye safety, but also limits wide-angle visibility. Therefore, light scattering/diffusing media are often integrated with LEDs and laser diodes. Examples of these optical media include side-emitting fibers/strips and light scattering lenses/coatings.