Light-Absorbing Spray Coating for Small-Bore Deep-Cavity Lens Barrels

Precision optical systems such as telephoto lenses, endoscopes, LiDAR receivers, and compact camera modules require complete suppression of internal stray light to maintain high contrast and signal-to-noise ratio. A common industry challenge is coating the inner wall of small-diameter, deep-cavity lens barrels – for example, a barrel with inner diameter Φ7.2 mm and length 26.8 mm (aspect ratio ~4:1).

The customer originally used black anodizing on aluminium alloy barrels. However, the hemispherical reflectance of black anodizing was >5% in the visible range. This caused:

· Visible ghost images and flare

· Reduced image contrast

· Unacceptable stray light noise for high-end imaging and sensing applications

Adhesive black films and flocking paper could not be applied because the bore was too narrow and too deep for tooling or manual placement. A new solution was required that could reach deep cavities while providing far lower reflectance than anodizing.

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Customized Solution: Tsingcarbo Sprayable Ultra-Black Coating

Based on its core nanoporous carbon technology, Suzhou Tsingcarbo Nano provided a spray-applied ultra-black coating specifically adapted for small-bore, deep-cavity components. The coating was applied to the full inner wall of the Φ7.2 mm × 26.8 mm aluminium barrel using a fine-nozzle spray process that reaches depths where films cannot enter.

Ultra-low hemispherical reflectance
The coating achieves <1.5% hemispherical reflectance in the visible band (400–800 nm) – a >70% reduction compared to black anodizing (>5%). The coating also maintains low reflectance across UV–VIS–NIR (200–2500 nm), ensuring broadband stray light suppression.

Perfect coverage of small-bore deep cavities
The proprietary spray process uniformly coats bores as small as Φ6 mm with length-to-diameter ratios >5. The entire 26.8 mm depth, including the cylindrical side wall and the barrel bottom, received a defect-free matt black layer with no pinholes or holidays.

High adhesion and cleanliness
The coating exhibits excellent adhesion to aluminium alloy (passes cross-cut tape test) and does not peel or shed particles. It is VOC-free, low-outgassing, and withstands temperature cycling, making it suitable for precision optical assemblies.

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Measurable Performance Improvements

After coating, the customer tested the barrels in their optical module and compared results against black anodized barrels:

Metric	Black Anodizing	Tsingcarbo Spray Coating
Hemispherical reflectance (VIS)	>5%	<1.5%
Stray light / ghosting	Clearly visible	Eliminated
Image contrast	Reduced	Dramatically improved
Applicability to Φ7.2 mm × 26.8 mm bore	Yes (bath process)	Yes – uniform spray
Particle shedding	No	No
Adhesion on aluminium	Good	Excellent (cross-cut 0)

1. Stray light virtually eliminated
Internal reflections that previously caused flare and ghosting are now suppressed below the detection limit of the customer’s imaging sensor.

2. Contrast and SNR improved
The barrel’s deep black interior raises image contrast and signal-to-noise ratio, enabling the optical module to operate reliably under strong ambient or off-axis illumination.

3. Broadband advantage over anodizing
Black anodizing often has higher reflectance in the near-infrared (NIR); Tsingcarbo coating maintains <1.5% from UV through NIR, which is critical for LiDAR and multispectral systems.

4. Enables compact optical designs
With a highly absorbing inner wall, engineers can simplify or remove mechanical baffles, reducing system weight and assembly complexity.

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Why Choose Tsingcarbo for Small-Bore Deep Cavities?

Requirement	Black Anodizing	Black Film	Tsingcarbo Spray Coating
Hemispherical reflectance	>5%	<1% (but inapplicable)	<1.5%
Applicable to Φ7.2 mm × 26.8 mm	Yes	No (cannot reach)	Yes
Uniform coverage of full depth	Yes (bath)	No	Yes (spray)
Broadband absorption (UV–NIR)	Moderate	Limited	Yes
No particle shedding / high cleanliness	Yes	Risk of edge lift	Yes
Prototype turnaround	Standard anodizing line	Sample films available	7-day custom spray