Electromagnetic Spectrum- Multispectral Bands
Multispectral data comes in two different forms in the hardware that we provide- Modified and Multiband. To understand the difference between these two types of Multispectral data you must first understand Electromagnetic Spectrum and how these signatures function in a sensor.
A standard sensor collects Red Green and Blue wavelengths of light. Outside of the visual range of colors there are non visible wavelengths that you cannot see with the human eye but is possible to sense with the right hardware.
Above is the full chart of electromagnetic spectrum. As you can see only a small portion of the spectrum is actually visible. Multispectral sensors for the purposes of agriculture typically use some combination of Red, Green, Blue, and Near Infrared (NIR) bands wavelengths of the chart above. The NIR band is key in this Multispectral collection. This additional band allows us to make more accurate calculations around vegetation.
When you look at these bands and how they interact with Vegetation it becomes apparent why the NIR spectrum is so valuable. The Blue and Red bands of light are predominantly absorbed by leaves while the Green bands of the visual spectrum reflect back (Giving living plants their green color). Healthy vegetation will absorb more Blue and Red light to fuel photosynthesis and create chlorophyll in the plant. A plant with more chlorophyll will reflect more NIR energy than an unhealthy plant. Thus, by comparing the absorption of this blue and red light to the reflectance of the NIR wavelength we are able to provide valid information about a plants health.
Modified Multispectral Imagery
A Modified Multispectral sensor is created from a standard Visual sensor. Here at PrecisionHawk we are able to modify the Zenmuse x5 and x5s series of DJI sensors to collect this 3 Band Multispectral imagery.
In the Modified version 3 bands of light are collected at once through the same lens.The modification with this version of sensor is a filter that is placed inside of a standard composite band camera to provide a Multispectral result in the sensor data. With your Multiband data there is a separate lens for each Spectral Band that is collected.
Filters can come in many different formats to display different combinations of Spectral bands. Most often we provide a RGNIR filtered sensor but the BGNIR is an alternative. These band combinations are designed for the purposes of sensing vegetation. The NIR band in this combination gives us a more reliable reading of plant health when using an algorithm like a Vegetation Index.
Multiband Multispectral Imagery
A multiband multispectral sensor collects many different bands separately instead of a single combination of 3 bands collected by the Modified Multispectral sensor. There are only two Multiband sensors that are currently compatible in the PrecisionMapper system- The Parrot Sequoia and the Micasense RedEdge. Each of these sensors has a slightly different variation in the bands that it collects.
Parrot Sequoia- Collects 4 bands of data- Red, Green, Red Edge, and NIR
Micasense Red Edge- Collects 5 bands of data- Red, Green, Blue, Red Edge, and NIR