Comparison of IR Absorber Coating Methods

Infrared radiation characterized by wavelengths ranging from 0.700 – 1050μm is no exception.

Absorption and reflection of IR radiation on a specific surface can be altered with the help of coating.

This article describes applications and characteristics of IR absorber coating in optical systems and a few other industries.

A clear comparison has been made among the different methods available to apply IR absorber coating along with their optical
and mechanical properties and when each method is most suitable.

We have also presented three guiding rules to help you easily select the correct method based on your specific application
and requirements.

Applications of IR absorbers

Light absorption is one of the essential requirements in a wide variety of optical, automotive, space, defense, medical and
solar applications.

Absorbers are primarily used to suppress stray light in optical systems.

Optical applications of IR absorbers include sensing and imaging, passive heating and cooling, laser systems, mobile phone camera parts, and much more.

Below are some of the applications of coating from different spectrum within IR:

1. The Short-Wave Infrared (SWIR: 1.4–3 µm) band is reflected by human skin for medical disease sensing and diagnosis.
   The sensing is achieved with the information detected as differences between reflected and absorbed wavelengths.
2. The Mid-Wave Infrared (MWIR: 3–8 µm) band of wavelengths are useful in remote-monitoring of earth, ocean and climate changes as well as tissue health in a human body.
3. The Long-Wave Infrared (LWIR: 8–15 µm) instruments sense thermal emission instead of reflection, and have application in military imaging devices and identifying tumors in a human body.

IR absorbers can be used to identify a forged document. IR radiation from the sun can provide power to the space satellite. Black coatings and foils are used when facing the challenges of stray light, scattered light, and low signal to noise ratio.

IR Absorber Coating Methods

1.IR Absorbing Paint

IR absorbing paints can be applied to materials to eliminate the reflection of IR wavelengths. They are used in low precision optical housings, surveillance cameras and etc.
It is usually fast and easy to apply the paint on the surface when no precise masked area required.

However, paint coating has a bunch of disadvantages when it comes to optical applications. Many of them are prepared in forms of oxides and then mixed with carriers before application. Bonding of IR absorbing paint with the substrate can be problematic. It has a limited adhesion and may take several days to dry.

IR absorbing paints can be applied by spraying, and their thickness varies in the range of 60-100 microns with a thickness tolerance of +/-20 microns.

Such thick layers with low tolerance and lack of control of thickness might not be suitable for critical applications such as space satellites and high precision optics.

One of the disadvantages of using IR paints for sensitive applications is high organic contamination with high outgassing, and they are typically non-conductive.

2. IR Absorbing Foils and Films

IR absorbing foils and films are used when you have large quantities and over-sized part areas. It can be used for clean room
applications or in contaminated environments where the light-absorbing surface needs to be periodically renewed.

They are generally used in apertures and baffles, beam dumps, mobile phone camera parts, laser safety screens as well
as automotive cameras and HUDs.

Also, when it comes to small spacecraft applications, miniaturized thermal management systems are needed.

Films and foils enable passive thermal management for spacecraft, and their optical properties can be altered based
on specific requirements, making it more cost-effective.

For example, inside the fully-enclosed spacecraft, conductive heat transfer is dominating whereas outside in the space by thermal radiation, so the coating is altered accordingly.

IR absorbing foils and films are available in the form of Sheets, Rolls, Die-Cuts.

Metal Velvet and Spectral Black coated foils deliver the lowest in-the-industry reflectance across the spectrum from the EUV thru the VIS to the FIR.

3. IR Absorber For Grazing Angles

If you are looking for IR absorber coating for grazing angles (40° – up to 88°AOI), eliminating reflectance and enhanced performance between UV through to MWIR FIR, Hexa Black Light Trap Sheets exhibit low hemispherical and specular reflectance at large angles, high abrasion resistance, and low outgassing properties.

For absorption at normal angle as well, Hexa Black Light Absorbing Panels should be used.

4. Coating Services for IR Absorption

Based on application requirements, the coating can be designed for a specific IR spectrum while considering environmental conditions and the substrate.

Example, any optomechanical part made of any substrate (metal, glass or polymer) can be coated to absorb 99% of EUV-UV, VIS, NIR-FIR.

Sensing and imaging applications are sensitive to signal-to-noise ratio, and coating can provide excellent stray light suppression.

Space application further demands thermal stability, vacuum compatibility, almost zero outgassing, and sustainability in the space radiation.

Acktar’s Vacuum Black coatings enables such a robust performance of optical system components at low cost in space applications.

When you require a coating that offers low reflectance across the spectrum from the UV through LWlR, you can rely on
Fractal Black coating service. It has been rigorously tested and was used in a variety of space programs by NASA.

Compatible with virtually all substrates, Nano Black coating is completely inorganic and used in satellite for solar thermal
collectors and solar panels as well as for passive thermal management.

Litho-Black coating offers tailorable electrical resistivity and is applied as a wafer-level patternable light-absorbing overcoat on wafers.

Space applications require very challenging optical system design and the coating on various apparatuses needs to have a longer life cycle, almost no maintenance, and zero outgassing in addition to high optical performance. Acktar’s Black Coatings are qualified for space applications.

How to Choose the Correct IR Absorber?

To help you quickly pick the correct IR absorber material and method for coating, we have listed three criteria below:

1. Mechanical Properties

Some of the critical mechanical properties you can consider when choosing the right IR absorber include coating thickness and it’s tolerance and degree of control, bonding to coating, adhesion, particulate, and organic contamination.

Type of substrate on which coating will be made also plays a vital role in deciding which coating methods mentioned above will be used.

2. Optical Properties

Reflectance, and absorption, and emissivity values are the essential criteria you should think about for selection of correct IR method.

Besides the angle of incidence (normal or grazing angles) and a specific spectrum in IR range should be considered as well.

3. Application-Specific Requirement and Environment

Every application presents a unique set of requirements and environmental conditions.

Example, if you are using IR absorber for remote sensing targets, selective emissivity plays a crucial role, whereas in safety and IR cameras in automotive applications black material with very low reflectance at grazing angles is required.

Moreover, when it comes to space applications, requirements, and environmental conditions are entirely different, where low outgassing, thermal management, and low
maintenance are paramount.

Nowadays, it is easier and essential to control and enhance the IR absorption of a surface. We discussed the common methods of coating, along with how to select a specific method for your project.