How Can Light Scattering Measurements Improve Aerospace Optical Projects?

Marion Gaboriau Gil

Sep 25, 2023 / 3 min read

Instruments used on space missions are equipped with the most advanced optics available. Even the slightest degradation – on a molecular scale – can lead to a deterioration in optical properties (e.g., loss of reflection from mirrors, loss of transmission from lenses, increased scattering on the optical path, etc.) that will have a direct impact on the mission. It is therefore essential to be able to characterize the optical performance as accurately as possible.

The Challenges

This characterization presents two key challenges. First, the optics are often highly specular, with a very low level of scattering to detect. To answer this challenge, you need an instrument with a high dynamic range and excellent resolution. The second challenge is that the wavelengths of interest are in the far infrared range. We're talking about lasers and wavelengths that need to be handled with great care. Overcoming this challenge requires an advanced level of expertise.

The Solution: The Synopsys High Specular Bench

Co-developed with the Center for National Scientific Research (CNRS), the Synopsys High Specular Bench is well suited for backward and forward light scatter characterization for all types of materials and objects. The system measures BRDF and BTDF, which perfectly represents the way any surface scatters incoming light in 2D space.

Synopsys High Specular Bench

The high specular bench is 10 meters long, giving it a minimum resolution of 0.02° from the specular direction. It is also equipped with synchronous detection, which permits us to obtain a very high dynamic range (around 1013) and to access a very low level of scattering.

In addition, it allows measurements for a wide range of wavelengths, from ultraviolet to far infrared, including sources at 3.39µm and 10.6µm. 

In addition, the high specular bench is situated in a class 100 clean room (ISO 5). This limits the risk of contaminating the samples, which are also handled with all the necessary protective equipment (gloves, gown, mask, etc.).

Its far-infrared measurements, high dynamic range, and high resolution make it ideally suited to the measurement of the spatial variation of BSDF across samples.

The following table shows the specifications of the bench: 

Synopsys High Specular Bench Specifications Table

This system is for SmartStart Measurement Services only and is not part of our equipment offerings.

Measurement Examples

The following examples present two measurements realized on samples used for space missions: one on a very specular sample and the other on a very black sample. 

Specular Sample

Contamination of space systems can lead to a real degradation of their performance. It is therefore important to be able to detect the presence of contamination in these space systems. The measurements presented here were carried out within the framework of the study of contamination of optics intended for space use. The aim was to understand the impact of molecular contamination on stray light.

Specular Sample Measurements

Thanks to the exceptional dynamic range of the bench, we can detect a very low level of scattering and therefore “quantify” the impact of the spatially localized contamination on stray light. The contamination induces an increase of two decades of BSDF on the scattered part compared to the cleaned sample. 

Black Sample

The aim of this study was to compare the behavior of visible illumination and a far infrared illumination of black samples.

For this kind of sample, all of the dynamic range and performance of the specular bench are not necessarily required, but we took advantage of its capability to enable measurements in the far infrared. 

Black Sample Measurements from the Synopsys High Specular Bench

We clearly see that not all the dynamic range is needed, but we have a very interesting result. The behavior is different between visible and infrared. For a visible wavelength (638nm), the sample is nearly Lambertian. However, at far-infrared wavelengths, the sample is much more specular.

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