Observing the earth from satellites using imaging devices provides valuable information for various applications. The fundamental principle of optical earth observation is to capture images from a distance using wavelength discrimination. This approach delivers unique knowledge on a global scale by gathering data from the electromagnetic spectrum.
When it comes to optical detection, optical filters are essential. They increase the signal-to-noise ratio by providing a stronger signal and reducing background noise. This results in a clear signal being transmitted to the detector or imager without any unwanted optical background noise.
Atmospheric Window Matter in Optical Filter Design
Optical observations of the earth are subject to variations in atmospheric transparency, which can vary greatly depending on the wavelength of the electromagnetic radiation. The transparency of the atmosphere for radiation from the earth's surface ranges from almost zero to nearly full transparency.
Image Credit: Iridian Spectral Technologies
The "atmospheric window" for earth observation is roughly between 300 nm to 15 µm, covering the visible spectrum and parts of the infrared.
The ability to detect emissions from the earth's surface or atmosphere depends on the electromagnetic transmission characteristics of the atmosphere. For example, the spectral band between 6 um and 8 um is largely opaque due to atmospheric absorption.
Designing optical filters for earth observation must take into account the specific spectral regime of interest, as well as other functional requirements. An optical designer will use this information to propose solutions that meet the desired specifications.
There are many different types of optical filters available for different use cases, including single-band, multispectral, and panchromatic imaging. In this context, we will focus on single and multi-band detection to provide a general understanding of the design of optical filters for earth observation.
Choosing Single-Band or Multi-Band Detection
In some earth observation applications, it is necessary to detect signals from only one spectral region. For these cases, single band-pass filters are often sufficient. The options available will depend on the specific spectral signature of the emission being monitored.
Optical designers may recommend a narrow line filter or a broader pass-band filter, but there is no one-size-fits-all solution. Different detector sizes require filters of different dimensions, and this presents additional design challenges.
For example, larger format filters must maintain high uniformity to prevent variations in layer thickness from causing significant wavelength shifts in the spectral detection window across the filter, which can lead to inaccurate results in earth observation data.
In other cases, earth observation requires the detection of multiple spectral lines at once to extract as much information as possible with a single instrument.
Combining multiple optical band filters in a single, multi-zone filter array is a practical approach given the weight and dimension restrictions of satellite equipment, as well as the challenges of the harsh space environment.
This approach allows a single detector to be converted into a multi-spectral imaging device. The principle is based on selectively applying different filters to specific pixels on the detector, effectively turning the imaging device into a mini spectrometer with no moving parts.
Earth Observation Filters
Iridian specializes in the production of robust, space-ready optical filters for earth observation applications. The company offers both single-band and multi-zone filters. Multi-zone filters are assemblies comprising individual bandpass filters.
Iridian utilizes two manufacturing approaches for multi-zone filters: the butcher block assembly and the photolithographically patterned monolithic array. Both methods have their unique advantages, but the most suitable approach for a specific earth observation application will depend on the specific requirements of the user.
This information has been sourced, reviewed and adapted from materials provided by Iridian Spectral Technologies.
For more information on this source, please visit Iridian Spectral Technologies.