Low-reflective surfaces – LTBC and other options

Reflective surfaces can cause malfunctions in certain applications and even influence the results, e.g. in quality control or for optically supported measurements. This should be taken into account when selecting a surface. But how can reflections be minimized? This article presents various options for surface treatment and surface coating. Low Temperature Black Chrome Plating, LTBC or also black chrome process, is explained in particular.

Terms - Reflection in general and in physics

Reflection is derived from the Latin word "reflexio", which means “turning back”. In general, reflection means that, for example, waves, light or sound impact, and are then thrown back, by a surface. In physics, reflection refers somewhat more specifically to the change in the propagation direction of a wave. The Law of Reflection applies in this case (from Latin reflectere: turn back). The Law of Reflection states that the incident and the reflected beams as well as the axis of incidence form a common plane or lie on a common plane. The angles of incidence and reflection are equal.

The propagation of the light can be shown in a simplified manner in waveform:

• t - Time unit
• x - Intervals per time = frequency
• a - Amplitude peak. The higher the maximum amplitude peak, the more intense - i.e. brighter - the light appears.
• The higher the amplitude peak and the shorter the intervals, the greater the transported energy.

The light path is also reversible, i.e. when light is incident from the direction of the reflected beam, it is reflected in the direction of the incident beam.

Instead of reflecting, light, waves, and other rays can also be absorbed and transmitted. Absorption occurs when the material impacted by a beam completely absorbs the latter and converts it into a different form of energy, such as heat. Transmission occurs when the beam passes completely through the medium without being reflected or absorbed.

Purposeful use in engineering

Engineers can make purposeful use of reflection, transmission and absorption. For example, the following technologies take advantage of reflection:

• Optical devices: Light reflected by a mirror, e.g. in a camera, can thus be controlled in a purposeful manner.
• Communication technology: Parabolic mirrors, for example., reflect electromagnetic waves. This makes it possible to send and receive signals.
• Solar technology: Mirrors are also used here that specifically concentrate sunlight, thus generating more heat.

Transmission, for example, occurs with ultrasound. Ultrasonic waves penetrate solid materials and provide images of the inner structure.

Absorption is used in optics: Diffuse light can be absorbed and thus minimized by absorbing surfaces, such as black surfaces.

Reflected light as noise factor? The significance of low-reflection surfaces

Reflections are not desired everywhere they occur. In certain applications, reflections can even have a negative influence. For example, reflections can distort images used for quality control. An example is measuring and aligning components. Both applications make use of lasers. If the laser beam is reflected or distorted, the reflected laser beams will interfere with the measurement accuracy. In optical systems such as microscopes, reflections can also negatively influence image quality and make it difficult to evaluate the images.

Low-reflective surfaces are therefore an important component in many systems.

Influence of various surfaces on reflection

The degree of reflection varies depending on the color of the material, the surface finish and previous surface treatment. The light is reflected, scattered and absorbed to varying degrees.

* The illustration shown here is very simplified and does not address all the phenomena that come into play.

Dark surfaces generally absorb more light than bright surfaces and reflect much less visible light to the eye. The less light is reflected, the darker a surface appears.

For example, a shiny, bright material (1a) will reflect light directly. Since bright materials do not absorb light particularly strongly, the incident quantity of light is almost as large as the reflected amount; there is little scatter. This is a little different for a shiny dark surface (1b): The light is also reflected directly, but a part of the light is already absorbed by the dark surface. This means that the energy density of the reflected light is also reduced. The angle of incidence is equal to the angle of reflection.

A combination of direct and diffuse reflection takes place for light and dark semi-glossy surfaces (2a and 2b): In both cases, the energy density of the scattered retroreflective radiation is lower. However, the dark surface greatly attenuates the retroreflected radiation due to the increased absorption. Here, too, the angle of incidence is equal to the angle of reflection.

For matte surfaces (3a and 3b), the direction of the reflected light can no longer be clearly determined; the angle of incidence and angle of reflection vary. Without purposeful alignment, the light may again impact the component surface and can also be reabsorbed there. Particularly on dark matte surfaces (3b), the light, which is partially reflected multiple times, is greatly attenuated by absorption, and a large part of the light is absorbed as a result.

How can reflections be minimized?

Reflections can best be minimized using various types of surface treatment. The surface can be modified, for example, by increasing the roughness. The incident light is scattered and diffusely reflected by increased roughness. Methods for example include: Etching and grinding.

Another option is to coat a surface. These coatings are differentiated into deposition layers or conversion layers. There are various methods for this, which are presented in detail below.

The LTBC method

LTBC coatings are mainly used to improve corrosion protection and to reduce abrasion. However, they offer a further advantage: Due to their black color, LTBC-coated components also have minimized reflection behavior. LTBC plating involves diffusing an approx. 5 μm thick anodized fluoropolymer layer at temperatures below 0°C, thus creating a permanent bond to the material. This forms an alloyed black surface that - due to its material strength - does not affect the original properties of the base material. However, it provides long-lasting corrosion protection and is also low-reflective due to the black color. In many coatings, mechanical wear causes minute delaminations over time. Precisely this problem is avoided with low temperature black chrome plating (LTBC).

Other methods for reducing reflection

The following is an overview of other metal surface treatments that can influence the ability to reflect:

• Black chrome plating: Black chrome coatings consist of chrome depositions in different oxidation stages. The amorphous layer structure causes the surface to appear deep black and thus absorbs a lot of light.
• Black chromating: The metal surface is converted to a chromating layer, which creates a uniformly black surface. It improves absorption behavior and corrosion protection.
• Nickel-plating: Nickel-plating can be deposited either galvanically or chemically. Galvanic nickel plating is primarily used for the optics and the corrosion protection of metal. In both variants, nickel is deposited onto the material as an additional layer. Firstly, a smooth, bright surface can then be generated that results in a controlled reflection, but conversely, a matte coating can also be achieved in combination with a roughened surface that diffusely reflects the light.
• Enamels: Reflections can be greatly minimized, for example by painting the metal. This method can be easily implemented and additionally improves the optics of the component.
• Etching method: The material surface is roughened by the use of chemicals. The light is consequently scattered in different directions, the reflection is reduced.
• Texturing: The goal of texturing is likewise to further scatter the light and thus reduce reflection. In this case, a texture is applied to the surface.
• Surface coating: Similar to texturing, different layers can also be deposited onto the metal surface, e.g. anti-reflective layers, nanolayers or specialized absorption layers.
• Burnishing: An iron oxide layer is deposited onto the steel surface. It is impervious, black and permanently bonded. The surface is additionally dipped in oil to achieve a bright surface. Although this is visually appealing, it increases reflections and has limited protective effect. In combination with other methods, such as texturing, a significant reduction in reflections can be achieved by burnishing (black coloration).
• Black anodizing: The aluminum surface is oxidized by electrolysis. This method deposits black color pigments that inhibit nearly all light reflections.

MISUMI offers a variety of options for surface treatment, see the following table:

Different coatings can also be used on a component, see figure:

Different surface treatments on a precision positioning table

• Base body, left: chemical nickel coating
• Base body, right: LTBC plating
• Adjusting screws: clear anodized

The methods mentioned here represent only a portion of the methods and processing options possible for achieving a low-reflective surface. What method can be used depends not only on the material used, but also on the intended use, the current operating conditions, and the type of application. Components with reduced reflection, such as bases, brackets for construction profiles or clamping rings for shafts are available for a wide range of applications.