Lenses for remote sensing fall into three main categories: commercial drones, stratospheric imaging and low earth orbit (LEO). Our lenses for drones are most often used for agriculture and construction applications, while our lenses that are mounted on weather balloons and carried into the stratosphere are used to capture high resolution images for urban planning. We have only worked on one LEO system so far: an imaging spectrometer for detecting methane emissions. For each category there are different factors that should be considered during initial project planning.
Lenses for Remote Sensing
Drone Lenses
For commercial drone lenses, size, weight, aperture and resolution are the most important factors. Because there are many different sizes of drones, there are no standard size and weight requirements, but commercial 35mm camera lenses are typically too large and heavy. When they are light weight, this is all too often achieved by “vignetting” (the corners of the image are dark). Vignetting also allows off-the-shelf lenses to claim large apertures (low F/numbers) even though this claim is only met near the center of the field of view. Resolution also drops off in the corners of the image. Although we cannot violate the laws of physics, we can provide lenses for commercial drones that are the best possible compromise between size, weight, aperture and resolution.
For an in-depth explanation on how to define your lens requirements for UAV-based imaging, visit our Drone Lens Specification Guide.
We’ve also created an interactive Remote Sensing Calculator to help translate project requirements like altitude, swath, and pixel size into optical parameters such as focal length, F/#, and GSD.
Download the app here: link
If you’re ordering drone lenses in volume, or your specs go beyond what a catalog lens can hit, our Custom Lenses for Drones page covers the optical and mechanical tradeoffs (CRA, distortion, IP rating, mounting) and what design, prototypes, and production actually cost.
Stratospheric Observation Lenses
Lenses for stratospheric observation are typically carried aloft by weather balloons. The FAAs FAR 101 regulates high altitude balloon flights, but if the payload is less than 6 pounds (2.7kg) many of the restrictions do not apply. This is a good incentive to keep the payload weight and thus the lens weight as light as possible. Another constraint is that the lens must perform as well on the test bench at STP as it does in the stratosphere at 0.1 Atm and -40°C. This means that a lens must be designed to be athermal (insensitive to temperature) and must be focusable to compensate for changes in pressure. At Eckhardt Optics, we have designed an 800 mm F/10 lens that weighs under 1kg and provides diffraction-limited imagery.
Lenses for Low Earth Orbit
Lenses for LEO can be used for visible-light, NIR or SWIR, or hyperspectral imaging. They must be mechanically stable enough to survive launch and athermal so they don’t go out of focus as the temperature changes due to solar loading. For CubeSats, size is also a common restriction. Another consideration is radiation browning of the glass. Lenses for LEO applications must collect enough light and have high enough resolution to give sharp images. We are familiar with all of these issues and have solutions for each of them.
If you’re exploring spectral imaging, building an optical system for a CubeSat, or planning your next mission, here are some resources you may find helpful:
Our new Remote Sensing Calculator connects project inputs to optical outputs: focal length, F/#, entrance pupil diameter, and GSD. It’s interactive, downloadable, and built to help you move from idea to design with more clarity.
Technical Background
Resolution of Remote Sensing Systems
One of the most important specifications when designing a remote sensing system is the system resolution limit. It is very important to consider the different factors that can affect resolution in a system. Ground Sampling Distance (GSD), Ground Resolution and Spatial Resolution are terms used when specifying remote sensing system resolution. They are all used to define the smallest feature a remote sensing system can resolve.
Mechanical Constraints of Remote Sensing Systems
Each application within remote sensing has its own restrictions on size and weight, so they will be discussed in detail on these individual pages: Commercial Drones, Stratospheric Imaging and Low Earth Orbit (currently works in progress, thank you for your patience).
Athermalization of Remote Sensing Systems
Remote sensing lenses are used in many different temperatures. Almost all materials change size over large temperature ranges, and this must be accounted for in the design process. The coefficient of thermal expansion (CTE) for each material defines the rate at which its dimensions change with temperature.
Because of these changes, lenses that are not designed for varying temperatures (non-athermalized) will lose focus as the temperature changes. An athermalized lens system is specifically designed to maintain sharp focus through a defined temperature range. This is done by carefully selecting glass and mechanical materials that have complimentary CTEs resulting in a minimized focus shift. In addition, most lenses also need to operate over a range of wavelengths (achromatic). Since chromatic aberration correction requires the use of different glass types, it makes the problem a bit more difficult to have a lens that is designed to be achromatic and athermal.
We have successfully athermalized lenses from -50°C to +30°C while maintaining some of the highest resolutions seen from the stratosphere.
Wavebands for Remote Sensing
Three factors determine what wavelengths of light are useful for remote sensing from high altitude and space. The atmosphere transmits light in specific wavebands between 0.3 and 14 microns, while most other wavelengths are absorbed. Detectors made of crystals have specific bandgaps, so they also have specific wavebands in which they are sensitive to light. Finally, the materials used to make lenses or coat mirrors also have transmission or reflection that depends on wavelength. The combination of these factors determines which wavebands are useful for remote sensing.
For more information, visit our Remote Sensing Wavebands page.
If you have a remote sensing project and are interested in learning more, please contact us. We would be happy to discuss the details of your project.