Why Black Materials Reflect Near-Infrared, and 3 Ways to Build a True NIR Absorber
A material that looks perfectly black to the eye can be surprisingly bright to a near-infrared (NIR) sensor. Black anodized aluminum, for example, reflects about 5% of green light at 550 nm, but about 75% at 950 nm.
So how do you make a black that keeps absorbing into the NIR? In our work we combine three approaches: a light-trapping surface structure, dyes that absorb into the NIR, and carbon built into the base material. Dyes cover the common sensor wavelengths of 850 and 940 nm; for the 1500 nm band you need carbon.
This article is an English adaptation of a paper I wrote for the Journal of the Japan Society of Infrared Science and Technology (Vol. 35, No. 2). Because these are products we still sell, some details of our know-how and raw materials are left out.
NIR systems are everywhere, and they need a different kind of black
Technologies that use invisible near-infrared light have spread quickly: LiDAR for autonomous driving, face recognition in smartphones, AR/VR headsets, security cameras and medical devices.
To perform well, these systems need more than good sensors and light sources. They also need control of unwanted light (stray light) inside and around the system, and development teams often need NIR-dark test environments.
Visible and NIR light behave similarly, so lenses and cameras can often be shared between the two. Light-absorbing black materials usually cannot.
NIR wavelengths and what they are used for
"Near-infrared" covers a wide range, and different applications use different wavelengths:
| Wavelength | Typical applications | Characteristics |
|---|---|---|
| 850 nm | IR illumination for security cameras, ToF cameras, eye tracking, short-reach optical links | Close to visible light and detected efficiently by silicon sensors. LED sources can show a faint red glow |
| 905 nm | Automotive and industrial LiDAR | Works with low-cost silicon detectors. It reaches the retina, so output power is limited for eye safety |
| 940 nm | Smartphone face recognition, proximity sensors, ToF sensors | Nearly invisible. Sunlight around 940 nm is weakened by water vapor in the atmosphere, so there is less ambient noise outdoors |
|
1550 nm (1500 nm band) |
Long-range LiDAR, fiber-optic communication, moisture sensing | Absorbed by water in the eye before reaching the retina, allowing higher power ("eye-safe"). Requires InGaAs or similar detectors instead of silicon. Strongly absorbed by water |
Silicon sensors respond only up to about 1100 nm, which is why 850–940 nm is so common in cameras and low-cost sensors, while 1550 nm is used where eye safety and long range matter.
The catch is that the longer the wavelength, the more ordinary black materials reflect. Black anodized aluminum goes from 5.38% at 550 nm to 65.66% at 850 nm and 87.57% at 1550 nm. Always check reflectance at the wavelength you actually use. A full table for our materials is further down.
Why black materials reflect NIR light
Most black materials made for visible light get their color from black dyes. These dyes absorb visible light well, but their absorption drops sharply above about 700 nm. The material still looks black, yet it reflects NIR light.
The table and chart below compare a standard black-anodized part with one of our visible-light absorbers, VL Flock Sheet. Both are low in the visible range, but both rise steeply in the NIR, which makes them unsuitable as NIR absorbers.
| Material | 550 nm | 950 nm | 1500 nm |
|---|---|---|---|
| Black anodizing | 5.4% | 75.4% | 87.2% |
| VL Flock Sheet (visible-light absorber) | 0.3% | 10.0% | 43.7% |
Total hemispherical reflectance, angle of incidence 8°, measured from 250 to 2500 nm in 5 nm steps; 250–1500 nm shown, plotted every 25 nm. Representative values, not a guarantee. The anodized plate is not our product; it was measured under the same conditions for reference.
For more on anodizing, see Black Anodizing Reflects Near-Infrared Light.
Three approaches to absorbing visible and NIR light
We develop sheet materials that work in both the visible and NIR ranges and can be used over large areas. Beyond absorption itself, we need large-area production, reasonable cost and stable properties in real environments. To get there, we combine three approaches:
- A surface structure that traps light
- Dyes that absorb into the NIR
- Carbon built into the base material
Approach 1: trap light with surface structure
How much light a material absorbs depends not only on the material itself but also on the shape of its surface.
On a perfectly smooth surface, light reflects once, like a mirror, so it has only one chance to be absorbed. On a surface with fine, complex texture, light bounces many times between the features and loses a little energy at every bounce. The result is much higher absorption than the material alone could achieve, and this works in both the visible and NIR ranges.
Our materials use three kinds of structure.
Electrostatic flocking: a forest of upright fibers
In electrostatic flocking, short fibers are driven onto an adhesive-coated backing by an electric field so that they stand upright, packed closely together.
Light that enters this structure bounces between the fibers as it travels deeper and back out again. Because it is reflected so many times on the way, the structure absorbs light very efficiently.
How an electrostatically flocked surface absorbs light (schematic)
You can see the same effect with a bundle of sewing needles. Each needle is plated and highly reflective. But look at the bundle from the tips and the inside appears dark, because light bounces between the needles and fades away.
A bundle of sewing needles: from the side (left) and from the tips (right)
The fibers must stand upright. In spray flocking, fibers are sprayed on without an electric field and land in random orientations. Fibers lying flat block the light from going deeper, so it escapes after a shorter path and absorption is noticeably worse than with electrostatic flocking.
Spray flocking (left) and electrostatic flocking (right)
Our VL Flock Sheet and IR Flock Sheet are electrostatically flocked materials.
Foam structure: countless tiny light traps
A foam with open pores on the surface and countless cells inside is also an excellent light absorber. Light that enters a pore bounces around inside the cells and very little of it comes back out.
Surface of a foam-type light absorber (SEM, 200x)
The photo below shows the very same polyurethane sheet twice. On the left is the smooth skin as molded; on the right, the skin has been removed to expose the foam layer. Same raw material, very different blackness, purely because of surface structure.
The same polyurethane sheet: smooth skin (left) and foam surface (right)
Because a foam absorber is made from a single material, it sheds far less particulate than a flocked fabric built from backing, adhesive and fibers. It is also water resistant and easy to cut, which makes it practical for industrial use. Our FINESHUT series is this type.
Porous paint film: a rough stack of droplets
Paint can be improved the same way. Normal spray painting aims for a smooth, wet, even film, which is a poor structure for trapping light.
Our light-absorbing paint is designed so that the spray droplets land half-dry and pile up. The coarse stack leaves many bumps and voids both on and inside the film, where light bounces around and struggles to escape.
Magnified paint film (scale bar: 1000 µm)
This is the film structure of Musou Black Paint.
Approach 2: choose dyes that absorb into the NIR
The dyes used in ordinary black fabrics absorb visible light well, but their absorption falls off sharply beyond 700 nm.
Some black dyes, however, keep absorbing from the visible range up to about 1000 nm. With these dyes we can make materials for 850 nm and 940 nm, the wavelengths most often used by industrial sensors.
Approach 3: build carbon into the material
Longer NIR wavelengths, such as the 1500 nm band, are hard to cover with dyes alone. Here we knead carbon particles into the raw resin, so the material itself absorbs light by a mechanism independent of the dye. This covers wavelengths where dyes stop working.
We use this approach in our foamed polyurethane products and in some of our flocked products.
The chart below shows the total reflectance of two electrostatically flocked fabrics. The one made with conventional black dye (VL Flock Sheet) rises above 700 nm. The one that combines an NIR-absorbing dye with carbon-loaded fibers (IR Flock Sheet) stays below 1% all the way to 2500 nm.
Total hemispherical reflectance, angle of incidence 8°, measured from 250 to 2500 nm in 5 nm steps and plotted every 25 nm. Representative values, not a guarantee.
In the original paper we also compared a third fabric that uses the NIR-absorbing dye alone, with no carbon (our IR950 grade). It stays low up to about 1000 nm and then rises at longer wavelengths. In short: dyes can take you to around 1000 nm; beyond that, you need carbon.
The 1500 nm band matters because eye-safe LiDAR uses it and because water absorbs strongly there, which industry uses for moisture sensing. Absorbers for this band are in high demand.
Reflectance of our materials by wavelength
Total hemispherical reflectance of our materials at 550 nm (visible) and at the four NIR wavelengths above:
| Product | Structure / method | 550 nm | 850 nm | 905 nm | 940 nm | 1550 nm |
|---|---|---|---|---|---|---|
| VL Flock Sheet | Electrostatic flocking | 0.29% | 8.67% | 8.63% | 9.12% | 45.90% |
| IR Flock Sheet | Electrostatic flocking + NIR dye + carbon-loaded fibers | 0.26% | 0.27% | 0.28% | 0.30% | 0.47% |
| FINESHUT SP | Foam structure + carbon | 1.16% | 1.19% | 1.19% | 1.23% | 1.37% |
| FINESHUT KIWAMI | Foam structure + carbon | 0.70% | 0.70% | 0.73% | 0.72% | 0.74% |
| FINESHUT KIWAMI XX | Foam structure + carbon | 0.52% | 0.55% | 0.55% | 0.56% | 0.61% |
| Musou Black Paint (airbrushed) | Porous paint film | 0.56% | 1.30% | 1.68% | 1.90% | 11.71% |
| Black anodizing (reference, not our product) | — | 5.38% | 65.66% | 71.25% | 74.53% | 87.57% |
Representative total hemispherical reflectance (AOI 8°) measured by an external testing laboratory. Not guaranteed values.
As a rough guide:
- Visible light only: VL Flock Sheet or Musou Black Fabric KIWAMI is enough.
- Into the NIR (850, 905, 940 or 1550 nm): choose IR Flock Sheet or FINESHUT.
- For large areas and darkrooms, IR Flock Sheet. Inside equipment where particulate must be kept low, FINESHUT.
Not just materials: cutting and assembly too
Other excellent NIR-absorbing technologies exist. What we focus on is practicality, cost, mass production and a range of grades. None of the three approaches above is new on its own, but combining them lets us make NIR-capable absorbers that are affordable and easy to use.
Beyond selling materials, we also make custom-shaped parts from a single piece (Laser Cut Service) and handle assembly, so stray-light problems can be solved in one place.
Products in this article
Not sure which material fits your wavelength or geometry? Tell us your application and we will suggest the right grade and size.
Source and author
Source
Teppei Kiyofuji, "Technical Approaches for Developing Visible and Near-Infrared (NIR) Absorbing Materials" (可視光・近赤外吸収材開発における技術的アプローチ), Journal of the Japan Society of Infrared Science and Technology, Vol. 35, No. 2 (in Japanese).
Teppei Kiyofuji, Director and General Manager of ANSOKEN, Koyo Orient Japan Co., Ltd.
Joined Koyo Orient Japan in 2009. In 2018 he launched ANSOKEN, a division dedicated to the planning, R&D, converting and sales of light-absorbing materials (the team behind Musou Black). Member of the Japan Society of Infrared Science and Technology.
Related: Black Anodizing Reflects Near-Infrared Light · The World of Near Infrared Light & Its Applications · What Is Reflectance? The Science Behind True Blackness
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