NDVI stands for "Normalized Difference Vegetation Index". NRG stands for "Near-infrared / Red / Green". NDVI and NRG are both ways to visualize the amounts of infrared and other wavelengths of light reflected from vegetation. Because both these methods compare ratios of blue and red light absorbed versus green and IR light reflected, they can be used to evaluate the health of vegetation. It's a snapshot of how much photosynthesis is happening. This is helpful in assessing vegetative health or stress. (Read more here: https://www.agronomy.org/publications/jeq/articles/36/3/832) ## Do-It-Yourself These techniques for vegetation analysis were developed for satellite imagery, but at Public Lab, we've been working a lot on capturing infrared imagery using our DIY [near-infrared camera](/wiki/near-infrared-camera) setup, and combining it with visible bands to produce NDVI images such as the one above. ## What these images mean What exactly are these images we're trying to make? What do they tell us about vegetation, and why? These diagrams should help to understand what it is we're doing and why these are good ways to analyze plant life. ## The NDVI equation [](/i/44723) **NDVI = (Near Infrared - Red)/(Near Infrared + Red)** NDVI is a ratio which tries to emphasize photosynthesis while filtering out sun glare. The above equation is run for every pixel, using source data from an infrared photo and a visible light photo, like this pair: [](https://publiclab.org/system/images/photos/000/021/771/original/5390895115_c9d4d38fec_o.jpg) The result can be false-colored to make the high-photosynthesis areas more clear, and used to examine where plants are and how healthy they are. [](https://publiclab.org/system/images/photos/000/021/770/original/PetVISNDVIcomp.png) _Figure above: Normal color photo (right) and normalized difference vegetation index (NDVI) image (left). NDVI image was derived from two color channels in a single photo taken with a camera modified with a special infrared filter. Note that tree trunks, brown grass, and rocks have very low NDVI values because they are not photosynthetic. Healthy plants typically have NDVI values between 0.1 and 0.9. -- @cfastie_ ### Activities Here are a range of activities you can do to produce and interpret your own NDVI imagery, whether downloaded from a satellite imagery provider or [collected yourself using a DIY technique](/wiki/multispectral-imaging) [activities:ndvi] ****   Most DIY converted cameras today (those from Public Lab) use RGN instead of NRG, so the blue channel represents infrared instead of the red channel. That looks like this: [](/i/45468?s=o) **** ## NRG imagery Some people are also interested in producing NRG imagery (like the below image), where `Near-Infrared, Red, and Green` are used to compose a picture instead of the usual `Red, Green, and Blue`. [](/i/25064) This diagram explains the swapping, which allows us to 'see' infrared as if it were a normal color: [](/i/25063) **In NRG images, the deeper and clearer the red color, the denser and healthier the vegetation (more or less).** ### Questions [questions:ndvi] ### Other examples of DIY NDVI imaging From around the internet: Begin watching at 2 minutes to see the resulting imagery: *This topic is part of the [Grassroots Mapping Curriculum](/wiki/mapping-curriculum) series.* **** [](/i/25066) [](/i/25065) ...
| Author | Comment | Last activity | Moderation | ||
|---|---|---|---|---|---|
| WhiteRabbit | "I found this: http://www.vernier.com/products/sensors/col-bta/ which is interesting because unlike other low-cost spectrometers/colorimeters its s..." | Read more » | over 10 years ago | |||
| mathew | "I tried acetone, ethanol, and hot air, none moved the glue. " | Read more » | over 10 years ago | |||
| warren | "Wow chris the barcode scanner filter idea is great!!! " | Read more » | over 10 years ago | |||
| WhiteRabbit | "Would acetone or some other solvent dissolve the glue holding on the IR blocker so that it can be removed without shattering it? Is it known what ..." | Read more » | over 10 years ago | |||
| cfastie | "The Public Lab desktop spectrometer is quite capable of making a spectral graph of sunlight reflected from grass. The curve will differ from the pr..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Ned, Thanks very much for posting this research note. I found it to be helpful background for a related topic (http://publiclab.org/notes/WhiteRa..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Here's a write-up that I'm finding quite helpful: http://publiclab.org/notes/nedhorning/11-01-2013/why-a-red-filter-should-work-well-for-ndvi Eyeb..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Sounds like gel filters would be tricky at best, so I guess ixnay to that. How about this one? http://www.ebay.com/itm/Optical-Interference-Filter..." | Read more » | over 10 years ago | |||
| cfastie | "I have not seen a cheap filter like Rosco that is truly a narrow band pass filter. It might be possible to combine filters to get a fairly narrow t..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Cool! Awesome post. I suppose it's wishful (and probably foolish) thinking, but given the the low price of a filter swatch book and the seemingly..." | Read more » | over 10 years ago | |||
| cfastie | "A filter switcher is a viable solution, especially for small cameras. Cameras as large as Powershots might require a big, precise device in front o..." | Read more » | over 10 years ago | |||
| cfastie | "The only filter I tried that worked was Wratten 25A. Two blue filters worked poorly. @mathew has tried Rosco #19 which seems to work well. I don't ..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Thanks, Chris, for your thoughtful and very informative post. If I'm reading it right, perhaps using two monochrome cameras, each with the appropr..." | Read more » | over 10 years ago | |||
| cfastie | "It should be possible to distinguish between stressed and non-stressed lawn grass using NDVI. I suspect that the $900 FieldScout performs more or l..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "" I can try different filters to learn how to make the best plant health images." Which filters have you found that make the best plant health ima..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "Interestingly enough, I did just now find this $889 turf grass color meter which does, indeed, use NDVI! http://www.specmeters.com/nutrient-manage..." | Read more » | over 10 years ago | |||
| WhiteRabbit | "How are you finding NVDI to be a helpful way to look at your plants? " | Read more » | over 10 years ago | |||
| WhiteRabbit | "Thanks for the links. Supposedly grass also changes color as the soil loses moisture (IIRC, it becomes less green and more blueish?), and people c..." | Read more » | over 10 years ago | |||
| warren | "I think a lot of the challenge of using an Infragram camera well is the need for comparison. Comparing an area treated in one way with a "control" ..." | Read more » | over 10 years ago | |||
| mathew | "I was using the (Near IR - Red) / (Near IR + Red) in my try. I was also using Max 4.5, because I bought a license ~11 years ago and haven't grabbe..." | Read more » | over 10 years ago | |||
| warren | "lucenk - hmm, maybe you're not using the live video version of the browser version? You should definitely be able to remix channels on live video; ..." | Read more » | over 10 years ago | |||
| lucenk | "warren, I will most absolutely share whatever I come up with. I was actually having trouble using the browser version with my mac. Also, in Max, yo..." | Read more » | over 10 years ago | |||
| mathew | "i tried and failed to make a max patch like this last year. The issue I was facing was that the calculations need to 1) allow for negative numbers..." | Read more » | over 10 years ago | |||
| warren | "OH, i see - you have NDVI running, but want to do NRG. Well, NRG is typically showing red and green in the green and blue channels, and in the curr..." | Read more » | over 10 years ago |