NDVI(normalized difference vegetation index) is a simple graphical indicator of plant health with near-infrared imagery.
It starts with disassembling of the tablet to get access to the front facing camera. Using pliers and some force the glue around camera lens can be easily broken and lens unscrewed.
The problem is that lens has an IR blocking filter(thin piece of glass with red-ish hue when seen from the side). Some people use solvents etc. for removal of this filter, but since it lies about a millimeter above the actual lens there is a quicker solution. Using small Philips-head screwdriver you lean it on the center of the IR filter and hit the screwdriver. The filter glass will crack. Now use tweezers to remove all the peaces of the cracked glass from the lens and clean the lens by blowing the remaining pieces off.
Using small scissors cut blue filter slightly bigger than threaded hole for the lens in camera(cameras are usually "roomier" on the inside) and insert it in to the camera itself. Then attach lens back in the camera.