The Optical Heart of Spaceborne Hyperspectral Imaging Spectrometers

Hyperspectral imaging spectrometers used on Earth Observation (EO) satellites require a precise chain of optical components to enable applications such as ocean-colour monitoring and vegetation health analysis. In this article, we discuss the optical train and the component-level specifications behind it.

 

An EO hyperspectral imaging spectrometer resolves hundreds of contiguous narrow bands from orbit to read Earth’s surface and oceans. Compared to multispectral platforms that capture a small number of discrete bands, hyperspectral varieties measure the spectrum continuously, so detectors pick up an uninterrupted reflectance curve per pixel rather than a few sample points. This allows systems to resolve fine spectral details that fall between broad bands. This is enough to distinguish different phytoplankton groups in ocean observations or stress signatures in crops.

 

The optical train

Inside these spectrometers are a series of optical components helping to collect incoming light, dispersing it into its constituent wavelengths and focusing it onto the detector.

The entrance aperture sits at the front of the optical path, determining what light enters the instrument. Here, it sets the spectral resolution, with a tighter slit resulting in a finer resolution, and defines the spatial line being imaged in an imaging spectrometer. While a narrower slit provides better resolution, it also lets in less light, reducing the signal-to-noise ratio (SNR). In contrast, a wider slit delivers more throughput but coarser resolution. A collimator then takes the diverging light off the slit and turns it into collimated rays before hitting the grating, which disperses the light. This dispersion is only clean if the incoming beam is collimated, otherwise wavelengths smear and resolution suffer.

The heart of the spectrometer is the diffraction grating. Its grooved face splits parallel beams so each wavelength leaves at a slightly different angle, spreading the light into its spectrum. The groove density, which is measured in lines per millimetre, determines how widely light is spread.

At the end of the optical train, either a lens or a mirror (depending on the architecture) then re-images dispersed light and brings each wavelength into focus at a specific position on the detector. Refractive (lens-based) configurations suit more compact systems, such as CubeSats, while all-reflective designs are favoured for larger spacecraft.

Optical component specification considerations

Stray light is a persistent challenge for these instruments, and the harsh in-orbit environment compounds the problem. Scattering from grating grooves, surface roughness, and lens or mirror edges can introduce unwanted light, causing ghosting, raising the noise floor, and reducing spectral purity through out-of-band crosstalk.

This occurs when light from neighbouring wavelengths or overlapping diffraction orders bleeds into the target band. To mitigate these effects, holographic gratings (which produce less stray light than ruled types) and low-scatter or bandpass filters are often employed to suppress out-of-band light and prevent it from reaching the sensor.

The substrates chosen for optics need to meet transmission, thermomechanical stability, and durability requirements. Materials must have transmission bands that cover the application, a low coefficient of thermal expansion (CTE) to maintain shape and performance across temperature swings, and sufficient durability to withstand demanding environments. In aerospace hyperspectral imaging spectrometers, fused silica is often used due to its broad ultraviolet (UV), VIS and near-infrared (NIR) transmission, low CTE and low fluorescence. Sapphire is another strong candidate, as it is extremely hard, scratch- and abrasion-resistant, with broad spectral coverage and excellent toughness.

For refractive configurations, temperature-dependent refractive index (dn/dT) is a particular concern. Because a material’s index changes with thermal conditions, orbital thermal cycling shifts where light focuses and drifts the wavelength calibration, which is one reason larger missions favour all-reflective designs.

Across the optical chain, quality sets the ceiling on data reliability. Working with an optical component supplier that offers low-scatter filters, precision gratings, anti-reflective (AR)-coated optics for VIS to SWIR, and radiation-resistant materials ensure you’re building a system fit for purpose and built to last.

To find out more about our optics for EO hyperspectral imaging spectrometers, contact our technical sales team today.