Multispectral Imaging for Astroplant

The current version of the Astroplant kit has a built-in camera in the top panel. Next to just taking regular photos for visual inspection of the plant in the kit some extra information about the plant can be gathered through multispectral imaging. That means capturing the various spectrums a plant reflects into the camera. Here, we make use of the NDVI (Normalized Difference Vegetation Index) to monitor the health and growth of the plant in the kit over a period of time.

An NDVI photo or measurement requires two components, the reflectance in the red channel and the reflectance in the near-infrared (NIR) spectrum. Regular cameras don’t pick up NIR light, but a modified version of the Raspberry Pi camera module does. NDVI measurements are usually made using sunlight (which has NIR components) and filters, but since the AstroPlant kit uses an artificial light source to grow the plants, this approach will not work. Instead some additional light sources (small band LEDs) with specific ‘colors’ of light are used to provide the red and NIR light needed for the photos. This has both benefits and drawbacks. One on hand the environment is in a really controlled state, which allows for repeatable, reliable measurements, that should make sense on a longer time scale. On the other hand, light from sun is very consistent on different parts of the plant, because it is so far away, while in the kit there are large light intensity differences due to some parts of the plant being closer to the light source then other parts. Reflections also play an important (negative) role in the final image.

This research is new in a sense that filterless approaches / multi-spectral imaging solutions have yet been very expensive (1000s of dollars) and therefore only accessible for large organisations and labs. Providing access to useful open source instruments to do plant analysis is an important goal of AstroPlant. Granted, the possibilities for such machines are generally far larger than this module, but some of the more common multispectral identities should be achievable with this project. The controlled environment that is the Astroplant kit is a nice bonus to this project, further enabling consistent measurements over prolonged periods of time.

Preliminary multispectral image of a mint plant inside the Astroplant kit. Top left shows a regular image of the plant. Top right shows a NDVI image of the same plant (higher NDVI roughly equals healthier plant), note that the negative values (-1 to 0) are clipped to 0, since they do not represent anything of importance right now. Bottom left shows the NIR image of the plant, where | as expected | the reflectance is high. Bottom right shows the red image of the plant, note here that the reflectance is low, as should be expected for most healthy vegetation. Some challenges still lie ahead, as can be seen in the picture. Direct reflections on the plant distort the NDVI value and some reflections on the plant container still show up in the NDVI image, which ideally would not be the case.
1 month ago