INTRODUCTION TO Multispectral Imaging
RotorDrone|January/ February 2020
How this new technology is revolutionizing data collection
GUS CALDERON

Originally developed for military target identification and reconnaissance, multispectral imaging uses specific wavelengths of the electromagnetic spectrum to document environmental features of interest. Early NASA imaging of Earth from space incorporated multispectral imaging technology to map details related to oceans, landforms, and vegetation. More recently, modern weather satellites now produce diagnostic imagery using multispectral sensors.

Today, multispectral imaging is used by the U.S. military to detect landmines and underground missiles. By analyzing disturbed soil features with multispectral imaging, different physical and chemical properties can be detected. Likewise, multispectral imaging of the invisible radiation emitted during intercontinental ballistic missile launches can track their trajectories.

Multispectral imaging is also used to interpret ancient papyri and other ancient documents by imaging the documents in the infrared range. Typically, writing on these documents appears to the naked eye as black ink on dark paper. But when viewed with a multispectral imaging camera, the difference between ink and paper is more distinct due to the way ink and paper reflect infrared light.

Natural resource managers are starting to use drones with multispectral sensors to monitor sensitive lands and preserves, including vegetated areas, wetlands, and forests. These data provide unique identification characteristics that can be measured and studied over time.

Farmers are using multispectral sensors on drones to gather data and help manage crops, soil, fertilizing, and irrigation. This is part of a process called “precision agriculture.”

HOW IT WORKS

Multispectral imaging captures light from a narrow range of wavelengths across the electromagnetic spectrum. Multispectral images are captured with special cameras that separate these wavelengths using filters, or with sensors that detect specific wavelengths. These wavelengths include lights from frequencies that are invisible to the human eye, such as infrared and ultraviolet light.

A traditional digital camera uses a filter to block the invisible light, and only captures the visible light that falls onto the sensor. The sensor uses a Bayer filter mosaic to arrange the red, green, and blue colors on a square grid of photosensors. A multispectral camera instead captures information that may either be in the visible part of the spectrum, or invisible to the human eye. For example, at certain wavelengths soil reflects more energy than green vegetation, while at other wavelengths it absorbs more energy.

Multispectral cameras can distinguish various objects from each other by these differences in reflectance. When more than two wavelengths are used to collect data, the resulting images tend to show more separation among the objects. A good example would be to look at different objects through red lenses, or only blue or green lenses.

PRECISION AGRICULTURE

Multispectral cameras mounted on drones capture light in green, red, and near-infrared wavelengths to produce color and color-infrared images of crops and vegetation. Multispectral imaging helps farmers minimize the use of pesticides, fertilizers, and irrigation while increasing the yield from their fields. Changes in reflectance can indicate that crops have become stressed, prompting field teams to investigate and intervene before a small-scale problem becomes more widespread.

There are huge benefits to both the farmer and the environment, especially when the use of pesticides, fertilizers, and water is minimized while the yield from crops is simultaneously increased.

The multispectral image data are processed with specialized software into useful information such as canopy cover, greenness, and disturbance maps. This soil and crop data allow the grower to monitor, plan, and manage the farm more effectively, saving time and money while reducing the costs associated with the use of excess water, pesticides, and fertilizer.

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