black coatings (original) (raw)

Definition: coatings with strong light absorption

Alternative terms: light-absorbing coatings, high-absorptance coatings

Categories: general optics, optical materials

• absorbing coatings
  • black coatings

Related: reflectancebeam dumpsanti-reflection coatingsthermal radiation

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DOI: 10.61835/e8d Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Black Coatings?

There are various types of coatings that are strongly absorbing light so that they appear black. Some of them also exhibit strong absorption in the infrared or in the ultraviolet. Their main application is to suppress stray light in optical systems.

Strong absorption, i.e. a high absorptance, is associated with strong thermal emission, i.e., a high emissivity. Therefore, black coatings may also be needed where a maximum of thermal radiation from some parts is required. Here, a low reflectance in the infrared is usually of highest importance. However, there are also coatings which are black for visible light while exhibiting a low emissivity (thus also low absorption) in the infrared.

Examples of Applications

Black coatings can be applied to a variety of substances, such as metals, polymers, glasses, semiconductors and ceramics.

In a monochromator, e.g. of Czerny–Turner type, different wavelength components are spatially separated with a diffraction grating, and only a narrow range of wavelength components are transmitted through the exit slit. It must be made sure that all other wavelength components, which should be suppressed, do not get through the exit slit in other ways by scattering within the monochromator's housing. Therefore, the housing must be equipped with light-absorbing materials — ideally, not only the inner side of the box, but also light shields at well chosen positions. A very low degree of reflectance is desirable. For that purpose, optimized black coatings can be used. This is particularly important for applications where faint spectral components need to be detected in the presence of strong light at other wavelengths — for example, in Raman spectroscopy.

Similarly, in imaging applications it is desirable to suppress stray light as much as possible. For example, a photographic objective may be hit by sunlight at an angle for which there is no nominal transmission of light to the image sensor. However, light may be scattered within the device, and such scattering should be suppressed with a suitable black coating to avoid a substantial degradation of image quality.

Displays, e.g. used by the automotive industry, often need to contain black parts having black coatings. A low reflectance can help to suppress disturbing influences of sunlight.

In a thermal power meter, incident light must be fully absorbed and converted to heat. This may be accomplished with a black coating on a thin metal part connected to a heat sensor. Similarly, black coatings are needed for pyroelectric detectors and for beam dumps.

Thermal solar panels need to efficiently absorb solar radiation and transfer it to water flowing through connected pipes. Strong absorption in the infrared and visible ranges is important, also a long lifetime and the durability at elevated temperatures.

In some space applications, the high emissivity of black coatings is utilized, for example to provide effective cooling by thermal radiation into space, in infrared heaters, or in reference standards for thermal radiation.

Performance Figures

The performance of a black coating cannot be fully characterized by a single number, for example with the hemispherical reflectance at a particular wavelength for normal incidence (which is typically a few percent, sometimes below 1%). Often, a wide spectral range is relevant — for example, the full visible range or more, e.g. if a camera sensor also has some responsivity in the infrared. Further, the reflectance may substantially depend on the angle of incidence and the polarization of light. For grazing incidence, the reflectance is often substantially increased.

Some coatings have an extremely low reflectance (e.g. well below 1%) throughout the visible region and may be called super-black.

Note that the optical properties of the coating may change with time; in particular, there can be aging for operation under extreme conditions.

In some cases, the ability of a black coating to withstand high optical intensities is relevant. This is the case for application in power meters, for example. A high thermal stability (chemical stability at high temperatures) is essential.

Some additional properties can be relevant for applications:

• Coatings may not equally well stick to different surfaces. For example, there may be problems when there is a poor match of thermal expansion coefficients of coating and substrate, the coating is not elastic, and the application involves substantial temperature variations. It is sometimes important that they firmly stick even to strongly curved surfaces, for example.
• Some black coatings may need a thickness of 100 μm, for example, while others are effective already for a thickness of only 10 μm. A high uniformity of thickness may be relevant e.g. when equipping narrow slits and pinholes with such coatings.
• There are black coatings with very high mechanical stability, while others are rather sensitive to abrasion and should therefore be protected against accidental touching.
• Some black paints need substantial time for drying after application, while others are finished quite quickly.
• Particularly for coatings used in vacuum devices, it is often important to have a low tendency for outgassing — which is a problem with some paints. Note that gases escaping from a paint can not only spoil a vacuum, but also lead to depositions when they get into a laser beam.
• In some applications, the electrical conductivity is important, e.g. to avoid electrostatic charges.

Types of Black Coatings

Polymer materials can be equipped with various dyes; some combination of dyes can make a material black. Such substances can also be integrated into paints, e.g. based on acrylic. A wide range of such materials is available, covering large parameter regions in terms of reflectance in different spectral regions, required thickness, viscosity before application, adhesive properties, durability, etc. Tentatively, the thermal stability of such organic materials is quite limited, making them unsuitable for absorbing high-intensity laser light, for example. Also, they can have a tendency for outgassing.

There are also black coatings containing carbon nanotubes. They can achieve a very low reflectance of far below 1% over a substantial wavelength range.

For higher thermal stability, one may use black inorganic coatings, which may e.g. be applied in anodizing processes or with chemical etching. For example, anodized aluminum is widely used. Some coatings are basically oxides, but there are also nickel–phosphorus (Ni–P) coatings and others. Such inorganic black coatings can be very durable, but there is a smaller range of material properties available, compared with organic materials.

Available Products

Some suppliers offer paints which manufacturers of optical devices can use themselves to equip parts with black coatings. Other products can be sprayed onto surfaces.

For some applications, one uses black foils which can be glued to metal parts, for example.

There are also coating services, where black coatings are produced on delivered parts. This may be a more practical approach for substances which need to be deposited in a vacuum apparatus, for example.

Frequently Asked Questions

What is the main purpose of black coatings?

Their primary application is to suppress stray light in optical systems by strongly absorbing light. They can also be used to maximize thermal radiation from a surface, as their high absorption is associated with high thermal emissivity.

What are super-black coatings?

These are coatings with an extremely low reflectance, often well below 1%, throughout the visible spectral region and sometimes beyond.

Are black coatings only effective for visible light?

No, many black coatings also exhibit strong absorption in the infrared or ultraviolet regions. However, some specialized coatings are designed to be black only in the visible range while having low absorption (and thus low emissivity) in the infrared.

Where are black coatings commonly used?

They are used inside optical instruments like monochromators and camera objectives to reduce stray light, on thermal detectors and beam dumps for complete light absorption, and on thermal solar panels to efficiently capture solar energy.

What materials are used to make black coatings?

Common types include paints with polymer materials and dyes, coatings containing carbon nanotubes for very low reflectance, and inorganic coatings like anodized aluminum or nickel-phosphorus for higher thermal stability.

Besides low reflectance, what other properties of black coatings can be important?

Other key properties include the ability to withstand high optical intensities, thermal and mechanical stability, strong adhesion to substrates, low outgassing for vacuum applications, and sometimes electrical conductivity.

Suppliers

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AcktarBlack™ Direct Coating Services suppress stray light in high-performance optics with no extra effort using advanced optical black coatings for stray light absorption in ultra-violet, visible light spectrum and infrared wavelengths. Extremely low reflectance, high thermal stability, high absorptance, high emissivity, excellent adhesion to essentially all materials and low outgassing in a few microns of a coating.

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AcktarBlack™ coatings are available not only for direct application to parts, but also as coated foil or film. The coated foils enable the customer to benefit from Acktar coatings performance without sending the parts to Acktar. This is an ideal solution for very large quantities, over-sized part areas and for contaminated environments where the light absorbing surface needs to be periodically renewed. The black foil coating are available with or without an adhesive backing as sheets, die-cuts or roll.

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