Infrared radiation, commonly known as IR, is a type of electromagnetic radiation with wavelengths longer than visible light, measuring from 700 nanometers to 1 millimeter. IR is emitted or absorbed by molecules and atoms as they change their rotational-vibrational movements. This occurs due to changes in the nuclear-spin states of the molecules.
IR has a wide range of applications, including in remote temperature sensing, spectroscopy, night vision devices, astronomy, cooking, and more. A common question that arises is whether IR can pass through fabrics and cloths. This article will analyze the factors that determine if IR is able to pass through cloth materials.
Properties of Infrared Radiation
IR lies between the visible light and microwave portions of the electromagnetic spectrum. It can be divided into multiple ranges, which are distinguished based on the wavelength and photon energy levels:
| Range | Wavelength | Photon Energy |
|---|---|---|
| Near IR (NIR) | 0.75 – 1.4 μm | 1.65 – 1.24 eV |
| Short-wavelength IR (SWIR) | 1.4 – 3 μm | 0.89 – 0.41 eV |
| Mid-wavelength IR (MWIR) | 3 – 8 μm | 0.41 – 0.124 eV |
| Long-wavelength IR (LWIR) | 8 – 15 μm | 0.124 – 0.082 eV |
| Far IR (FIR) | 15 – 1000 μm | 0.082 – 0.0012 eV |
The shorter wavelength IR ranges (NIR, SWIR) can mostly pass through many non-metallic and non-polar mediums like glass, cloth, and plastic. The longer wavelengths (MWIR, LWIR, FIR) have more difficulty traveling through these materials and are more likely to be absorbed or reflected instead.
Factors That Determine IR Transmission Through Fabrics
There are several factors that influence the ability of IR radiation to transmit through fabric and cloth materials:
Fiber Composition
The chemical composition of the fibers in a cloth material affects its interactions with IR radiation. Materials made of polar molecules like water can absorb specific IR wavelengths due to molecular vibrations or rotations in the bonds. Non-polar hydrocarbons found in many synthetic fabrics and plastics transmit IR more readily.
Some common cloth fiber types and their IR transmission properties:
– Cotton – Natural fiber made of cellulose. Moderate IR transmission.
– Wool – Natural protein fiber. Poor transmission of longer IR wavelengths.
– Silk – Protein fiber. Behaves similarly to wool.
– Linen – Plant fiber made from flax. Transmits IR wavelengths well.
– Polyester – Synthetic polymer fiber. Good general IR transmission.
– Nylon – Synthetic polymer. Transparent to some IR bands.
– Spandex – Synthetic elastomer fiber. Good IR transmission.
Fabric Density and Weave
The thickness and density of a fabric impacts IR penetration. As cloth density increases, less IR is able to pass through the material. IR has an easier time traveling through thinner, less dense, or more sheer fabrics.
Additionally, the weaving pattern can influence IR transmission. A tight, compact weave blocks more IR compared to a looser, more open weave. Densely woven cloths like canvas tend to be more opaque to IR compared to fabrics like chiffon or voile.
Dyes and Treatments
Additional chemicals present in fabric dyes, coatings, or treatments can interact with IR radiation. Darker colored dyes, for example, are more likely to absorb certain IR wavelengths. Some chemical additives are specifically designed to block IR transmission through clothes.
Common examples include:
– Metal-complex dyes – Contain metals that absorb IR radiation.
– Sun-protective coatings – Use chemicals like zinc oxide to reflect IR.
– Flame retardants – Compounds that inhibit IR-induced combustion.
Presence of Metallic Components
The incorporation of metallic wires, threads, sequins, or jewelry on a fabric can significantly hinder the passage of IR. This is because metals strongly reflect IR radiation. Even small metallic details in clothing embellishments or accessories may create IR hotspots.
Thickness of Material
Generally, IR penetration decreases as cloth thickness increases. Thinner fabrics provide less obstruction for IR waves to pass through. Materials like fine silk or chiffon scarves can transmit more IR compared to a thick wool blanket or canvas tarp.
IR Transmission Through Common Cloth Materials
Based on the factors above, here is how various cloth types typically interact with IR radiation:
Cotton
Cotton, one of the most ubiquitous cloth materials, allows some IR transmission across the near, short-wave, and mid-wave bands. Its moderate density and thickness blocks the majority of far IR. Bleached white cotton is more transmissive overall than dyed cotton fabrics.
Polyester
IR easily passes through synthetic polyester fibers due to their non-polar hydrocarbon composition. Uncoated polyester fabrics have very high IR transparency from near to far wavelengths.
Wool
The protein structure of wool makes it essentially opaque to mid and far IR radiation while letting smaller amounts of near IR through. Wool fiber density and weave style impacts overall transmission.
Silk
Silk demonstrates good IR transmission properties, although not as strong as synthetics like polyester. Its thin structure and low density allow more IR to pass versus other natural fibers.
Linen
Linen made from flax fibers exhibits perhaps the best IR transparency of common plant-derived fabrics. It outperforms cotton in allowing IR wavelengths to penetrate material layers.
Nylon
Similar to polyester, nylon’s synthetic polymer composition makes it highly conductive to the majority of IR bands. Coatings applied to nylon can reduce IR passage depending on their chemistry.
Leather
Various leather and suede materials reflect or strongly absorb IR radiation, making them relatively impermeable. IR interactions depend on leather thickness, density, and chemical processing.
Denim
The twill weave, dye chemistry, and cotton composition of denim make it moderately resistant to IR penetration. Passage of some near and short-wave IR occurs through denim but decreases with darker indigo dyes.
Select IR Transmission Examples
To illustrate the real-world IR opacity of fabrics, here are transmission percentages of sample IR wavelengths through different cloth materials:
| Fabric Type | 1 μm IR | 3 μm IR | 5 μm IR | 8 μm IR |
|---|---|---|---|---|
| White cotton | 85% | 78% | 63% | 45% |
| Polyester chiffon | 95% | 92% | 89% | 83% |
| Wool suiting | 75% | 51% | 34% | 19% |
| Silk charmeuse | 90% | 82% | 74% | 61% |
| Blue denim | 79% | 66% | 53% | 42% |
| Canvas tarp | 62% | 43% | 29% | 17% |
This table illustrates how synthetic fabrics generally outperform natural ones for IR transmission. It also shows the steep drop-off in IR penetration for thicker, more dense cloths like canvas compared to delicate weaves like silk.
Using IR Imaging to See Through Fabrics
IR imaging cameras use sensors capable of detecting different IR wavelengths emitted from objects. This allows IR radiation to pass through certain fabrics and still create discernible images, depending on the camera sensor technology.
Some uses of IR imaging to see through textile obstructions:
– Security and surveillance – Identify objects concealed under clothing.
– Search and rescue – Detect body heat of humans under debris or fabric structures.
– Medical diagnostics – IR thermography through bandages to analyze injuries.
– Materials testing – Evaluate IR transmission qualitatively or quantitatively.
– Manufacturing – IR scans to inspect alignment of materials or product layers.
– Art documentation – Reveal underdrawings and underpaintings under surface layers.
– Archaeology – Discover artifacts within textile wrappings or coverings.
Near-IR vs Longer Wavelength IR Imaging
Near-IR wavelengths (0.7 – 1.4 μm) transmit through more fabrics than longer IR bands. This allows better visualization of objects underneath clothing or other coverings. But near-IR is more limited in penetration depth compared to mid-wave (3-8 μm) or long-wave (8-15 μm) IR.
Longer IR wavelengths reveal deeper details because they are absorbed less by atmospheric moisture. This makes them ideal for some applications, like analyzing injuries through gauze. But clothing blocks more of these longer IR bands.
Multispectral IR cameras combining both near and long-wave IR capability provide the most flexibility for seeing through fabrics. Short-wave IR supplements the material penetration of near-IR, while mid and long-wave IR provide greater imaging depth.
Cloth Materials That Block IR
While many fabrics allow some IR transmission, there are certain cloth types engineered to purposefully block IR radiation:
IR-Opaque Fabrics
Specialty textiles can be made opaque to IR using thick, dense constructions combined with IR-blocking dyes or additives. These have applications in clothing, blankets, and paint booth curtains.
Metal-Coated Fabrics
Fabrics incorporating metal foils, metal-coated fibers like stainless steel yarn, or metallized films reflect IR radiation. These are used in Protective suits, curtains, and military camouflage netting.
IR-Reflective Safety Wear
Workwear meant for protection from IR exposure integrates materials like aluminized cloth, silver coatings, or metal filaments to reflect IR from sources like molten glass or metal.
Space Blankets
Extremely thin, lightweight blankets with a metallic coating reflect IR radiation back towards the body. This conserves radiant body heat that would otherwise escape.
Conclusion
The ability of IR to transmit through clothing and textiles depends on many variables: fiber chemistry, density, thickness, dyes, and more. While no single fabric is completely transparent to all IR wavelengths, those with low density made of non-polar materials generally allow the most passage of IR radiation. Synthetics transmit more IR compared to natural fabrics.
With the right IR camera system, even relatively opaque cloths still permit some visualization of objects underneath. But specialty fabrics incorporating thick constructions, metals, and IR-blocking dyes can more effectively obstruct the passage of unwanted IR radiation. Understanding the IR transmission properties of textiles is crucial for imaging applications and IR protection.