Black Anodizing Reflects Near-Infrared Light: A Hidden Source of Stray Light
Black anodized aluminum is everywhere in optical hardware. Stage plates, lens barrels, mounts and brackets are routinely anodized black because the finish is light, durable and looks matte. It is easy to assume that "matte black" also means "non-reflective" for your sensor.
In the visible range, that assumption mostly holds. In the near-infrared (NIR), it does not. In our measurements, a standard black-anodized aluminum plate reflected about 5% of visible light, but more than 80% across much of the near-infrared range. If your system uses NIR light, anodized parts can become a major source of stray light.
This article shows what we found, and one simple way to fix it.
What is near-infrared light?
Near-infrared light sits just beyond the red end of the visible spectrum, at roughly 750 to 2500 nm. Because it is close to visible light, it behaves in similar ways, and many ordinary cameras can capture part of the NIR band once their IR-cut filter is removed.
NIR is widely used in industry: LiDAR (905 nm and 1550 nm), time-of-flight and face-recognition sensors (850 nm and 940 nm), machine vision, spectroscopy and InGaAs cameras all work in this range.
One useful property of NIR is that two materials that look identical to the eye can look completely different to an NIR sensor. Here is an example with two of our own products.
On the left is VL Flock Sheet, and on the right is IR Flock Sheet. Both are rayon flocked fabrics with almost the same thickness, texture and blackness. In visible light, you cannot tell them apart.
Now look at the same two sheets through a camera sensitive to 750–1000 nm:
The difference is obvious. VL Flock Sheet is designed for visible light and reflects much more in the NIR, while IR Flock Sheet is designed to stay low out to 2500 nm. Materials that look the same to the human eye can absorb and reflect NIR light very differently.
Sony has a good short video showing how NIR sensing can reveal wet areas, separate oil from water, and detect foreign objects:
Black anodizing looks black. Does it absorb NIR?
To check, we bought a standard black-anodized aluminum adapter plate of the kind sold for optical stages, and photographed it next to a sandblasted black PET film.
In visible light the two look about the same, with a total reflectance of roughly 5–6%. But under the NIR camera:
The anodized plate is clearly much brighter than the black PET film. To find out how much, we had the plate measured by an external testing laboratory from 250 to 2500 nm.
Total hemispherical reflectance, angle of incidence 8°, measured by an external testing laboratory from 250 to 2500 nm in 5 nm steps (plotted every 25 nm). Representative values, not a guarantee. FINESHUT SP, the fix described below, is shown for comparison.
The result was striking. Across the visible range, reflectance stayed around 5%, as expected. But just past 700 nm it shoots up, and across most of the near-infrared it stays above 80%, peaking close to 90%.
In other words, to an NIR sensor, a black-anodized part is not black at all: it behaves more like a bright reflector. If anodized aluminum is used inside an optical system that works with NIR light, it can easily become a source of stray light.
The fix: cover the part with FINESHUT SP
Our suggestion is FINESHUT SP, a fine-cell polyurethane foam sheet that contains carbon. It is only 0.2 mm thick (0.4 mm is also available), yet it absorbs light strongly in both the visible and near-infrared ranges.
It does not hydrolyze, it resists abrasion, and because it is a single foam material rather than flocked fibers, it sheds very little dust. That makes it well suited as an anti-reflection lining inside optical equipment.
FINESHUT is supplied laminated with double-sided tape, so it can be applied like a sticker. Here is how we covered the anodized plate.
What you need
- FINESHUT SP 0.2 with thin double-sided tape (DIC #8616UJ), cut a little larger than the plate. The 0.2 mm and 0.4 mm grades have essentially the same optical performance, so either will do. For this method, choose the thin tape.
- A hot knife: a soldering-iron-style tool with a blade tip. These are inexpensive and widely available.
Step 1: Peel the release liner
This is the trickiest part. The adhesive is strong and the liner is a very thin, soft PET film, so it is hard to get started. Gently rub the edge of the FINESHUT sheet with the pad of your finger until the liner lifts.
Step 2: Apply without bubbles
Lay the sheet down gradually from one side so no air gets trapped. Because the anodized surface is matte, this is not difficult.
Step 3: Trim the excess with the hot knife
The hot knife makes this easy. Run the blade along the edge of the plate and the foam cuts cleanly. For holes, hold the blade against the rim and rotate the plate.
Result
To show the difference side by side, we covered only half of the plate:
The edges and holes are trimmed neatly. And under the NIR camera:
The covered half stays dark in the near-infrared as well. The video shows it more clearly:
Care notes
- FINESHUT is very sensitive to organic solvents. Alcohol and similar solvents make the foam swell and degrade it, so do not use them for cleaning.
- If oil gets on FINESHUT, it can be washed with soap or a neutral detergent. Work up a good lather, rinse well, and let it dry.
Summary
- Black anodized aluminum reflects about 5% in the visible range, but more than 80% across much of the near-infrared.
- In systems that use NIR light (LiDAR, ToF, NIR cameras, spectroscopy), anodized parts can be a hidden source of stray light.
- Covering them with a thin NIR-absorbing sheet such as FINESHUT SP is a simple fix.
If stray light from stage plates or housings is a concern in your optical setup, we hope this helps.
Products in this article
Not sure which material fits your wavelength range or geometry? Tell us your application and we will suggest the right grade and size.
Related: The World of Near Infrared Light & Its Applications · What Is Reflectance? The Science Behind True Blackness
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