Understanding NDVI Cameras

NDVI is therefore calculated as the difference between the reflected light in the near-IR and the visible spectrum.

Digital cameras typically employ a repeating pattern of blue, green, and red filters over individual pixels called a bayer pattern. These filters modify the response of the pixels so that the blue pixels detect light in the spectrum between 400nm and 500nm, the green pixels detect light between 500nm and 600nm, and the red pixels detect light between 600nm and 700nm. However, these filters are not perfect and the blue, green and red pixels also detect light in the near-infrared above 800nm. The figure below shows an example of the frequency response for the Onsemi AR0134CS color image sensor, a common sensor used in industrial camera applications.

Internal Transmittance is a measure of how much light passes through the filter. The Schott BG38 allows over 80% of the light in the visual range from 400nm to 650nm to pass through the filter before attenuating, or blocking, the flight beyond 650nm so that no near infrared light passes through the filter. When installed in front of the color image sensor, the blue, green, and red pixels now only detect light in their corresponding wavelengths as shown in the figure below.

The presence of the IR cutoff filter presents an opportunity to modify the camera’s optical response by replacing the IR cutoff filter with another filter with different properties. All NDVI conversion cameras employ this technique but the effectiveness varies significantly as I will discuss next.

Replacing the IR cutoff filter with Schott BG3 exposes the camera sensor to light in the blue (400nm-500nm) and near-IR (700nm-1000nm) portions of the spectrum. The result, shown in the figure below, is that the green and red pixels detect primarily near-IR light, while the blue pixels detect both blue and near-IR light.

To calculate NDVI the blue pixels are used as the value for the visible portion of the NDVI calculation, while the value for red, since it responds more to near-IR than the green pixel does, is used for the near-IR part of the equation. Therefore NDVI is calculated as:

This custom filter has the advantage of transmitting more of the blue light below 500nm than Schott BG3. Since healthy vegetation reflects very little blue light, which is used as the visible portion of the NDVI calculation, contrast is improved by increasing the amount of blue light detected by the image sensor. The NDVI calculation is the same as for the Schott BG3 filter.

To fully evaluate this filter we have to look at it together with the image sensor as shown below.

With this filter the red pixels detect almost exclusively light around 850nm while the green pixels do the same but with slightly more detection of light in the blue visual spectrum. The blue pixels, on the other hand, detect blue light and light around 850nm. Therefore, with this filter we would calculate NDVI the same way as we would with the Schott BG3 filter.

With this filter, the user should consider the following points:

Overall, I believe the Midopt Blue+850nm NIR filter has the potential to significantly improve NDVI performance compared to transitional Schott BG3 conversion. When you are shopping for an NDVI camera for your drone I suggest that you ask your potential vendor for the filter properties and ask yourself if that NDVI conversion will meet your needs.

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