Stray Light Control in Optical Instruments: How to Choose Anti-Reflection Materials

"Black materials" for optics come in many forms: flocked fabrics, sheets, paints. Each has its own strengths. In precision optics and cleanroom environments, low reflectance alone is not enough. Unless you also weigh particle shedding, thickness and how easily the material can be built in, you can end up with a material that does not work on the production line.

This guide is for engineers who design or manufacture optics for cameras, sensors and measuring instruments. It sets out the criteria for choosing an anti-reflection (light-absorbing) material, based on the questions we are asked most often.

What is stray light, and why is it a problem in optical instruments?

Stray light is unwanted light that leaves the intended optical path inside an instrument and reaches the sensor or image plane.

Cameras, microscopes, laser processing machines, spectrometers, LiDAR, semiconductor inspection systems: all of them deliver their designed performance only when light is guided and received exactly as intended. Inside a real housing, however, light reflects slightly off almost every part: the inner wall of the lens barrel, the edges of holders, screw heads, flexible printed circuits (FPCs). When that light reaches the sensor, it lowers image quality and sensing accuracy through reduced contrast, ghosting, flare and a worse signal-to-noise ratio.

"The lens design is perfect, but the shadows are never truly black."
"We painted the part black, and it still causes ghosting."
"Raising the illumination does not reduce the scatter in our measurements."

In many cases, the cause of symptoms like these is not the optical design itself but the anti-reflection treatment inside the housing. Worse, a material chosen only to suppress reflection can introduce a different problem, particles, which land on lenses and sensors and lower yield. Anti-reflection measures always involve a trade-off with particle shedding, thickness and durability.

Where does stray light come from? Three sources

It helps to sort the sources of stray light into three groups.

1. Reflections from structural surfaces
Barrel walls, the edges of apertures and holders, screw heads, the metallic shine of springs and washers. A part may be "black" on the drawing, but in reality the gloss of plastic or a machined surface returns more light than expected.

2. Scattering from surface finishes
Even black paint and black anodizing leave diffuse reflection (scattering), depending on surface roughness. Looking black to the eye and returning no light optically are two different things.

3. Multiple reflections
A reflection weak enough to ignore once can add up when light bounces back and forth, for example between an inner wall and a cover glass, increasing the amount of light that reaches the sensor.

Sources 2 and 3 can be greatly improved simply by lining inner walls with a low-reflectance material. Source 1 is hard to solve without also changing the geometry (light-shielding rings and baffles), so do not rely on materials alone.

Five properties to look for in an anti-reflection material

Light absorption: how black is black enough?

The basic principle of stray light control is to absorb unwanted light instead of reflecting it. The measure for this is reflectance: the share of the light hitting a material that comes back.

For stray light control, what matters is not only the mirror-like specular reflection but the total hemispherical reflectance (diffuse plus specular). The lower this value, the less stray light is scattered in every direction.

First, a few terms that are easily confused when selecting materials:

Specular reflection: the component that bounces off like a mirror, at the same angle as the incident light. It is the main issue with glossy black paint.

Diffuse reflection: the component scattered in many directions by fine surface roughness. It remains even on matte black surfaces and seeds multiple reflections inside the instrument.

Total hemispherical reflectance: the two components together, i.e. the total amount of light that comes back. In practice, this is the value to compare for stray light control.

BRDF (bidirectional reflectance distribution function): a distribution describing how much light arriving from one direction is reflected into each other direction. It is used in rigorous stray light analysis.

Reflection also changes with the angle of incidence. On inner walls and baffles that light strikes at grazing angles, stray light can remain even if the reflectance measured near normal incidence is low. When you compare catalog values, check that the type of reflection, the wavelength and the angle of incidence are the same. Values measured under different definitions cannot be used to choose between materials.

For more on how reflectance works, see What Is Reflectance? Meaning, Formula, Specular vs. Diffuse, and How It Is Measured.

About the reflectance values in this article: They are representative values (not guaranteed) of total hemispherical reflectance, diffuse plus specular, measured with a spectrophotometer and an integrating sphere at an 8° angle of incidence. Values for our products were measured by an independent testing laboratory. Competitor products were purchased by us and measured under the same conditions (reference values).

Low particle shedding: a must in cleanroom environments

Inside an optical instrument, especially around lenses and sensors, even a little dust or a single fiber can degrade performance. Particles on an image sensor block light and show up as black spots or shadows in the image. Particles caught in moving parts cause malfunctions. These problems occur not only during assembly but also after shipping, from vibration and aging. Flocked materials absorb light very well, but fibers can come loose during use or be shed by friction, so they are often avoided in clean environments.

The FINESHUT Series is a sheet, not a flocked fabric, so there are no fibers to come loose, and it sheds few particles. Because of this low particle shedding, it is used by optical and sensor manufacturers inside precision instruments and in cleanrooms. In quantitative particle measurements by a third-party laboratory, FINESHUT KIWAMI XX shed about 1/7 as many particles as an electrostatically flocked material (VL Flock Sheet) (after normalizing for test-piece area).

An often-overlooked point: particles are not only a problem during assembly. Vibration in transit, sliding in moving parts (zoom, focus, shutter) and expansion and contraction from temperature and humidity cycles all generate fibers and particles little by little after shipping. Field complaints of "black spots in the image" can be caused by this kind of gradual shedding. And because the inside of an optical instrument is almost sealed, particles that are generated have nowhere to go. Designing so that particles are not generated in the first place is more effective than adding cleaning steps.

Thickness and ease of processing: working in tight spaces

Parts inside optical instruments are packed tightly. The places most likely to cause reflections, such as the gap between barrel and sensor or the shadows of flex circuits and connectors, are exactly where space is shortest. An anti-reflection material therefore needs to be as thin as possible and easy to apply and build in. A thick material can block the optical path or eat into assembly tolerances, making the design harder.

FINESHUT is very thin, from 0.22 mm, so it fits into narrow spaces. It is available with or without adhesive backing (depending on the grade), and it can be cut to the shape of your parts, so you can design in ease of handling on the production line from the start.

Two requests we hear often are:
"We cut it by hand, so the shape is inconsistent."
"The shape is complex and hard to cut."
If the material is delivered already cut to shape, the variation between workers disappears and application time is reduced.

We offer precision cutting specified in 0.1 mm increments: see our Laser Cut Service. For examples of peel-and-stick parts cut from FINESHUT and IR Flock Sheet, see Custom Infrared-Absorbing Parts.

The absorption you need also depends on the application. Some sensors need absorption not only in the visible range but also in the near-infrared; in other cases, a moderate level of absorption is enough when balanced against cost. Identifying the performance you actually need, rather than simply choosing "the blackest material", is the shortest way to avoid over-specification and extra cost.

Is black paint or black anodizing enough? Three common mistakes

When anti-reflection measures fail, the same patterns tend to appear.

Mistake 1: "We painted it black, so it's fine."
Black paint and black anodizing may look black in visible light, yet ordinary black paint typically still reflects about 3–10%, and in the infrared reflectance can rise even further. Check with measured values, not with your eyes. (See Black Anodizing Reflects Near-Infrared Light.)

Mistake 2: Choosing by visible-light data only
In instruments that work in the infrared, such as LiDAR and IR cameras, stray light can remain even if you pick a material with low visible reflectance. Decide based on the value at the wavelength you use.

Mistake 3: Solving reflection and creating a new problem
Choosing a flocked material for absorption alone and then getting errors from particles. Applying a thick material and squeezing the assembly tolerances. Light absorption, particle shedding, thickness and durability need to be considered together.

Comparing performance at the prototype and evaluation stage helps you avoid going back to the drawing board.

Why engineers choose the FINESHUT Series

The FINESHUT Series is a sheet-type light-absorbing material that combines low reflectance, low particle shedding, thinness and ease of processing. Here are the three features that customers value most when selecting it.

Three light-absorption grades for different applications

The FINESHUT Series comes in three grades with different reflectance.

FINESHUT SP
Total hemispherical reflectance 1.2–1.4% (550–1500 nm). For general optical instruments and industrial cameras, balancing sufficient absorption with cost.

FINESHUT KIWAMI
Total hemispherical reflectance 0.7% (550–1500 nm). One step lower than SP, and nearly flat from visible to near-infrared (0.70–0.74% measured at 550, 950 and 1500 nm). KIWAMI has a grooved surface, and these values apply when the grooves run perpendicular to the incoming light (the recommended orientation).

FINESHUT KIWAMI XX
Total hemispherical reflectance 0.5–0.6% (550–1500 nm). The lowest reflectance in the series, for high-precision sensors and other applications that demand the strictest stray light control.

As a rough guide to choosing a grade by application:

The final choice depends on your wavelength and the level of stray light you can accept. If you are unsure, ask us. For how astronomers use FINESHUT inside telescopes, see Stray Light Control in Telescopes.

Performance into the infrared: built for sensing applications

More and more sensing applications use not only visible light but also near-infrared (NIR) and short-wave infrared (SWIR). Many materials that look black in visible light become more reflective in the infrared. The FINESHUT Series keeps its reflectance low into the infrared, which makes it well suited to internal parts of LiDAR sensors, IR cameras and NIR spectrometers.

For example, LiDAR uses the 905 nm and 1550 nm bands, and spectrometers for biological and food applications may work at 1000–1700 nm. These wavelengths are invisible to the human eye, so you cannot judge by how black a material looks. Whether a catalog lists near-infrared values, or whether the supplier can provide data at your wavelength, is what really separates one material from another. We publish FINESHUT values at three wavelengths, 550, 950 and 1500 nm, and can discuss measurements at other wavelengths such as 1550 nm.

The chart below compares the reflectance of the three FINESHUT grades from visible to near-infrared (250–1500 nm).
The yellow line is VL Flock Sheet, a flocked fabric designed for visible light. Its reflectance is very low in the visible range, 0.3–0.5%, but it rises sharply above 700 nm and reaches about 58% at 1250 nm.
The purple line is a commercially available black anodized aluminum plate. It reflects 5–6% of visible light, but it too rises sharply above 700 nm, reaching about 75% at 950 nm and about 87% at 1500 nm. A material that is black in visible light is not necessarily black in the near-infrared.

Besides the FINESHUT Series, we also offer flocked fabrics for the near-infrared. To compare near-infrared absorbing materials for sensors, LiDAR and dark rooms, see NIR Absorbing Materials for Sensors & LiDAR. For background on why black materials reflect NIR, see Why Black Materials Reflect Near-Infrared.

Durability: contact resistance and water resistance

During assembly and maintenance, materials in optical instruments touch tools, fingers and other parts, and get wiped during cleaning. With flocked materials, fibers that have been flattened may not recover, reducing performance. FINESHUT SP has a surface that resists damage and has the contact resistance to withstand light wiping. On FINESHUT KIWAMI and FINESHUT KIWAMI XX, dust can get into the fine grooves and textures of the surface, and pressing hard can leave marks, so we recommend SP where parts are touched or cleaned often.

FINESHUT SP 0.4 and FINESHUT KIWAMI XX are also water resistant, so they can be used where condensation or high humidity occurs. Dirt can be washed off with water and a foamed neutral detergent. Do not wipe them with ethanol or other organic solvents for cleaning or degreasing: solvents make the material swell and deform.

Thanks to this contact and water resistance, we also make laser-processed markers from FINESHUT. See Increase Recognition Accuracy with Sticker-Type AR Markers.

FINESHUT vs. flocked fabric and black paint: which to use where?

Here is how the FINESHUT Series compares with our IR Flock Sheet, a flocked fabric widely used as an anti-reflection material that also absorbs near-infrared light. Rather than one being better than the other, it is more practical to think that the best choice depends on the environment and the performance you need.

On reflectance alone, IR Flock Sheet is the lowest. However, inside optical instruments where particles cannot be tolerated, and where thinness and contact resistance are required, the FINESHUT Series is the better fit. With FINESHUT KIWAMI XX, you can keep reflectance down to 0.5–0.6% while keeping particle shedding low. For large backdrops, or general environments where cleanliness matters less, IR Flock Sheet is also a strong option.

Our paint, Musou Black Paint, is often compared too. Paint is easy to apply to complex shapes and large parts, but the coating is weak against contact, film thickness can vary, and fixtures need masking.

Musou Black Paint is designed for visible light. Its reflectance is a low 0.6% at 550 nm, but it rises in the near-infrared, to 2.0% at 950 nm and 10.7% at 1500 nm. For parts used at near-infrared wavelengths, compare it with the sheet materials above.

Values for a sample applied with an airbrush.

FAQ

Q. How do I choose between the three FINESHUT grades?
A. Balance how strictly you need to suppress stray light against cost. For high-precision sensors and astronomical telescopes, where reflection must be kept as low as possible, FINESHUT KIWAMI XX is the best fit. For general industrial cameras and measuring instruments, FINESHUT SP or FINESHUT KIWAMI often performs well enough at a lower cost. If space is tight, note that FINESHUT KIWAMI XX is 0.71 mm thick including its tape. Tell us about your application and we will help you choose.

Q. Should I use a flocked fabric or the FINESHUT Series?
A. Where particles cannot be tolerated, such as cleanrooms and the inside of optical instruments, the FINESHUT Series is the better fit. For backdrops or large-area stray light control where particles are less of a concern, flocked fabrics are also an option. Thickness, near-infrared performance and contact resistance are useful criteria too.

Q. How is FINESHUT different from black paint or black anodizing?
A. Black paint and black anodizing look black, but they can still reflect several percent of visible light: ordinary black paint typically reflects about 3–10%, and the black anodized aluminum we measured reflected about 5% at 550 nm. In the infrared, reflectance can rise much further (about 75% at 950 nm for that anodized plate). The FINESHUT Series reflects 0.5–1.4% at 550 nm, 950 nm and 1500 nm.

Q. Can FINESHUT be used in 905 nm or 1550 nm LiDAR?
A. We publish total hemispherical reflectance at 950 nm and 1500 nm, close to the 905 nm and 1550 nm LiDAR bands: 0.6–1.4% depending on the grade. For 1550 nm or other wavelengths not in our published data, contact us about measurement.

Q. Can I check outgassing, heat resistance and the operating temperature range?
A. The standard operating temperature range is −20 to +80 °C. Outgassing, vacuum and chemical compatibility depend on the environment, so tell us about your application and we will advise you individually. Please use our contact form.

Q. Can you cut sheets to the shape of my part, with or without adhesive?
A. Yes. FINESHUT SP is available with or without adhesive backing; FINESHUT KIWAMI and FINESHUT KIWAMI XX come with double-sided tape as standard (adhesive-free versions on request). Our precision laser cutting service, specified in 0.1 mm increments, cuts sheets to the shape of your part, removing the variation of hand cutting and reducing assembly time.

Q. Can I try small quantities? Do you offer samples?
A. We do not offer free samples, but you can buy small standard sizes, such as 245 × 200 mm (SP 0.4), 190 × 53 mm (KIWAMI) and 136 × 112 mm (KIWAMI XX), for prototyping and evaluation.

Summary: protecting the performance of your optical instruments

When choosing an anti-reflection material for optical instruments, five things matter: light absorption, low particle shedding, thinness, ease of processing and durability. If you also define the performance you need at your operating wavelength (visible or infrared), you can reduce stray light effectively without over-specifying. The FINESHUT Series is an anti-reflection material for precision optics and sensor surroundings that is designed to meet all of these needs, with the right grade for each application.

If stray light is a problem in your design, start by comparing the grades of the FINESHUT Series.

Not sure which to choose? Need to work at a special wavelength or in a special environment?

Related articles

• Black Anodizing Reflects Near-Infrared Light: A Hidden Source of Stray Light
• Real-World Test: Reducing Mount Adapter Internal Reflections with FINESHUT KIWAMI
• Stray Light Control in Telescopes | Space Research Pt 2

Originally published in Japanese on October 21, 2026. Translated into English in October 2026.