The science of seeing from space │Inspiration Hub │Bühler Group

Space flight has changed the way we see the world. The earliest photographs of Earth from space were taken in the middle of the 20th century. Then in 1972, the crew of the Apollo 17 mission captured the “Blue Marble” image of our planet – one of the most famous photographs ever taken.

Today, the ability to capture images from hundreds of kilometers above the Earth has evolved from the extraordinary to the everyday. By 2023, there were around 1,300 Earth observation satellites in orbit, and more have been launched every year since then. The information they transmit is used in a vast range of applications, from mapmaking and weather forecasting to intelligence gathering and national security.

Data from space plays a critical role in the global food and agriculture industry. Earth observation systems help make agriculture more sustainable by enabling smarter farming practices. Satellite instruments can measure crop health, water stress, and nutrient status, allowing farmers to optimize irrigation, reduce fertilizer and pesticide use, and increase yields per hectare. At the same time, weather satellites improve storm forecasting and drought monitoring, helping producers plan harvests and logistics more effectively, reducing spoilage and supply-chain disruption.

Space-based sensing also contributes to more resilient food systems. Spectrometers aboard satellites can track greenhouse gases such as CO₂ and methane, as well as monitor air quality and pollution. The resulting data help guide policies and operational decisions that protect soil health, support crop productivity, and strengthen long-term food security.

Coatings matter

The optical instruments used to generate this data are not simple cameras. Many Earth observation tasks depend on the ability to differentiate between specific wavelengths of light reflected or absorbed by different materials. Sensors that track soil and crop moisture levels, for example, may infer the water content of an area by comparing the relative amounts of red light and near-infrared light reflected into the sky.

To make such measurements, space-based instruments must be able to distinguish between different wavelengths with exceptional accuracy. They rely on sophisticated optical filters designed to pass only specific wavelengths of light through to the instrument’s sensor.

These filters are complex nanoscale structures that are created by building up alternating layers of different materials, each only a few nanometers thick. The final coating stack in an advanced filter may contain more than 200 individual layers. As a producer of thin-film coating equipment, Bühler helps its customers to translate demanding requirements of the space industry into stable, repeatable deposition processes.

Bühler Leybold Optics’ coating machines produce optical filters in the same vacuum that is found 100 km above the Earth – so when the object they are mounted on arrives at its planned orbit, the optical filters are already well suited to the environment.

Why space is hard

Research scientist Julien Lumeau is head of the Optical Thin Films Research Group at the Institute Fresnel in Marseille, France. His group develops specialist coating solutions for a broad range of applications that includes not only space optics but also filters for microscopes, high-powered lasers, and even for the jewelry and watch-making industries.

“Typically, our customers know the wavelengths they want to study, and they’ll approach us with a specification,” he says. “We need to translate that into a filter design that can be fabricated reliably and that is sufficiently durable.”

Dr. Julien Lumeau is Head of the Optical Thin Films Research Group and Deputy Director of the Institute Fresnel, as well as Research Director at the French National Center for Scientific Research (CNRS). Prior to joining the Institute Fresnel in 2012, he spent eight years in the United States as a researcher at the College of Optics and Photonics at the University of Central Florida and as an engineer at OptiGrate.

Those durability challenges begin long before instruments leave the ground, he explains. “An instrument might be manufactured in Europe, then transported to a launch site in South America,” he says. “It might stay there in a hot and humid environment for some time before riding a rocket to a place that is mostly cold and very dry.

Filter designers must ensure that their systems perform exactly as intended once they reach orbit. Those filters must continue to function for years or even decades while enduring dramatic temperature fluctuations and high levels of radiation. Satellites in low Earth orbit face additional challenges from the outer traces of the Earth’s atmosphere, including atomic oxygen, which can rapidly erode some materials and alter the optical properties of others.

Years of research characterizing the performance of different coating systems through terrestrial testing and orbital experiments have led space optics designers toward a set of proven materials and manufacturing approaches. “We typically favor oxide materials for their stability, and the Bühler machines we use can produce hard, dense coatings,” says Lumeau.

Weight is money

Hyperspectral imaging is one of the applications for which sophisticated optical filters are required. This hyperspectral image of the Amazon River in Pará, Brazil, was captured by NASA’s Earth Surface Mineral Dust Source Investigation (EMIT) on June 30, 2024. It shows vegetated land in tan and yellow, water in blue and turquoise, and clouds in white. Courtesy NASA/JPL-Caltech.

For mainstream instruments, which typically operate in the visible spectrum and nearby wavelengths, filter design and fabrication are now well understood, says Lumeau. Current challenges focus on making sensors more capable while also making them smaller and lighter. “In most applications, you are interested in a range of different wavelengths, so you need multiple filters,” he says. Traditionally, this has been achieved using a rotating wheel or similar mechanism to move different filters in front of a single sensor. These multispectral instruments collect data in several broad, disconnected wavelength bands.

A more advanced alternative is hyperspectral imaging, which uses filters whose optical characteristics vary across their surface. Such filters can capture precise data across a broad spectrum of wavelengths, albeit with greater fabrication and data-processing complexity.

“In space, weight is money,” says Lumeau. “The high cost of putting payloads into orbit creates a strong incentive to make instruments lighter and more compact.” Optics engineers are responding to this by pushing fabrication capabilities further. This includes the development of ultra-compact hyperspectral filters that are suitable for miniaturized instruments.

At the extremes

Others are working to push space optics into entirely new territory. Marcus Trost is Executive Vice President and Space Project Lead at optiX fab in Jena, Germany. The company, a spin-out from the Fraunhofer Institute, specializes in coatings for extreme ultraviolet (EUV) applications.

On Earth, these ultra-short wavelengths are critical for producing the tiny features used in the most advanced semiconductor devices. In space, they can reveal phenomena that cannot be measured in any other way.

One major target of research is our Sun – the closest star – where strong extreme ultraviolet emissions can provide early warning of solar storms capable of bathing Earth in charged particles, disrupting communications, damaging satellites, and knocking out electricity grids.

Working with EUV light introduces a different set of design and fabrication challenges. “Even though the Sun is very bright to us, when you are looking at single emission lines, you reduce the amount of available light drastically,” says Trost. “This leads to a photon-starved detection scenario requiring optical systems with very high optical throughput.”

EUV instruments use mirrors, not lenses, and those mirrors tend to be both large and extraordinarily precise. “For the current generation of instruments, you are talking about mirror than 300 millimeters, with coating thickness control in the range of single-digit picometers,” says Trost. At that scale, individual atoms can make a decisive difference to performance.

To complicate matters further, robust oxide materials such as those used for applications in the visible spectral range strongly absorb light at EUV wavelengths. Engineers therefore rely on metallic layer stacks, which have a greater tendency to react with one another, requiring highly dedicated interface engineering.

Dr. Marcus Trost is Executive Vice President at optiX fab, Germany, which he joined in 2023. He is responsible for highly customized coatings for extremely short wavelengths such as the Multi- Slit Solar Explorer (MUSE) mission. Prior to that, he headed the group for surface and thin film characterization at Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena, Germany, where he focused on light scattering and analysis of flight hardware.

Meeting the challenge

For Harro Hagedorn, head of R&D at Bühler Leybold Optics, creating coating machines that are capable of meeting the needs of the most advanced space missions is not fundamentally different from supporting customers in other highly demanding industries.

“Our machines produce optical filters in the same vacuum you get 100 kilometers above the Earth,” he says. “Normally, you face the problem that properties can change once they encounter air. If you launch the filter into space, it is going back to where it started in a way.”

Harro Hagedorn, Head of R&D at Bühler Leybold Optics

Bühler machines continuously measure the optical properties of filters as they are built up layer by layer, helping ensure that the finished product meets specification. Verifying that performance can be maintained after extreme temperature cycling and radiation exposure depends on a combination of laboratory testing by customers and decades of operational experience from satellites already in orbit.

And filters are not the only elements in space optical systems that rely on advanced coating technologies. Instruments also use a wide range of specialist coatings, from highly reflective mirror surfaces to highly absorbent coatings that suppress stray light within the instrument. Other coatings help protect systems from temperature fluctuations that could otherwise damage or distort delicate components.

Looking ahead, Hagedorn expects demand for some space-optics systems to increase dramatically, creating new requirements for faster and lower-cost production. “There are perhaps 10,000 satellites of all types in space today, but that may grow in the coming years to a million or more. Many of those satellites will need optical coatings, not just for imaging but also for laser-based communication,” he says. “To reach those volumes, you need to think much more carefully about component costs and the time required to develop, qualify, and manufacture them. And that is what we are working on every day at Bühler.”

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