Thanks Chris!

I initially found this site while searching for automatic insect recognition because I'm interested in the potential for robots to help us by shooting pests with lasers. I'm super new to all of this but I've been reading for a few hours and after seeing these two posts http://publiclab.org/notes/cfastie/10-11-2013/oaktober http://publiclab.org/notes/cfastie/11-27-2013/cryptogams as well as this one I am left wondering if one camera systems can give meaningful NDVI data. It seems to me that if the system can't detect a difference between a brown or red leaf and a green one then we aren't getting the kind of info that we want from NDVI. The Wratten25A almost gets it but like on the cryptogram page the fern and the brown leaves have about the same value. Maybe a different formula would have to be used like where R=NIR+Red, G=XNIR, and B=YNIR, where X and Y are coefficients that correspond to how much the G and B sensors pick up the NIR so that NIR=(G/X+B/Y)/2 then NDVI=((G/X+B/Y)-R)/(R) . Of course the white balance will change the values of X and Y. If you can get the B sensor (or G) to match the NIR picked up by the R sensor then you can simplify the equation to NDVI=(2B-R)/R. You could test this if you got a red filter and compared the R and B values for an IR source. Or maybe you could just set your white balance based on looking at an IR LED with all visible light filtered and set the white balance so R and B have the same pixel values for that source. Anyways I don't know if this makes sense or if it was helpful in any way. If I can make a request, personally, I'd like to see some close ups of sick leaves and also plants with insects on them. Keep up the amazing work!

Hi Kendrick90, You're right that single camera systems have their limitations. The Wratten 25A camera produces a very good NIR channel (blue) with little contamination by visible light, but the red channel has lots of NIR in it. Corrections could be made, but the result will never be as good as a two camera system. Even two camera systems make crude versions of satellite NDVI because the bands are so wide (130+ nm vs. 40 to 100 nm). I'm intrigued by the idea of subtracting some brightness from a channel to account for contamination, but I still haven't figured out how to do it.

Some of the limitations are with NDVI itself. For example, dead leaves still have pigments which absorb selectively and can mimic the signature of healthy leaves. NDVI is a crude index and can't be expected to distinguish all differences in plant health.

Ned Horning is working on a promising protocol to calibrate the NDVI from these systems, and I am working on a two camera system with narrower bands.

And here is a narrower band pure-NIR GIF of plasterer bees (Colletes sp.) on chickweed (Stellaria media). They are collecting pollen from tiny white flowers which are indistinguishable from the green leaves in this NIR band (ca. 850-1050 nm). These are native pollinators so please don't zap them with lasers.

@cfastie What should be the formula of NDVI for NIR camera as it is blocking red light.

With a blue filter which blocks only red light, the red channel will capture NIR. Use the red channel for NIR and the blue channel for visible light. NDVI is the difference between NIR and VIS divided by the sum of NIR and VIS, so (red-blue)/(red+blue).

With a blue filter which blocks only red light, the red channel will capture NIR. Use the red channel for NIR and the blue channel for visible light. NDVI is the difference between NIR and VIS divided by the sum of NIR and VIS, so (red-blue)/(red+blue).

@cfastie Thank you for your time. I have two more doubts in my mind, 1.) What are the steps that I need to apply for image calibration? 2.) What is the importance of image calibration?(I am using image of agriculture farm taken by drone for plant health analysis.)

Here is my discussion of calibrating NDVI images: https://publiclab.org/notes/cfastie/05-01-2016/calibration-cogitation

And Ned's most recent notes explain how his Photo Monitoring plugin works: https://publiclab.org/profile/nedhorning

@cfastie On an agriculture land, How can we specify the targets for calibration?

@cfastie sample image is like this,

Finding inexpensive, do-it-yourself calibration targets is an obstacle to NDVI calibration. You need targets for which the spectral reflectance is known for the wavelengths you capture and use for the NIR and VIS bands. Ideally these targets would have the SAME reflectance for ALL wavelengths, but that usually requires commercial targets like these: http://labsphere.com/labsphere-products-solutions/materials-coatings-2/coatings-materials/permaflect-1/

You can use common materials with published spectral response, but these don't have the same response for all wavelengths, so you have to know the exact wavelengths you are capturing. Below are some sources of spectral data for common materials:
http://speclab.cr.usgs.gov/spectral-lib.html, http://speclib.jpl.nasa.gov/ http://spectrallibrary.utep.edu/, http://www.frames.gov/partner-sites/assessing-burn-severity/spectral/

You can use a spectrometer to describe the response of any material as Ned did here:
https://publiclab.org/notes/nedhorning/05-01-2014/improved-diy-nir-camera-calibration

Some initial results using your white balance settings are impressive, however in this particular picture the roof of the house is showing high photosynthetic activity. The camera i have used is Mobius Modified Infrared Camera from Public Labs with red filter gel and wide angle lens , the photo was taken at a height of 150 feet. I think i should follow the procedure by Ned where he asked his friend to scan the spectroradiometer to create my own custom white balance. Do you think i need the targets of known spectral reflectivity in the same bands that will be used to compute NDVI or it can be tested with any materials as mentioned by Ned.

Also, do you have any idea how could i possibly validate my final end results of NDVI ? Do i need to take spectroradiometer measurements in the field as well to validate ? If you have validated it can you explain the process of validation.

Best Regards, Suman

Hi -- well, there may be methods like that; just to put in another idea, although you probably won't want to do it; Dorn Cox from Farm Hack has actually mowed, dried, and weighed biomass from different test plots to validate. There is some on this under the #ifarm tag, from a few years ago.

Do you think i need the targets of known spectral reflectivity in the same bands that will be used to compute NDVI or it can be tested with any materials as mentioned by Ned.

Any target will have reflectance (or none) at each wavelength. Ideally you would know the reflectance at each wavelength, including the band you will use to compute NDVI. If you measure the reflectance at higher and lower wavelengths, you could interpolate the values in between. The ideal target material will have the same reflectance at all wavelengths (a flat curve) so the same value can be used regardless of the band used to compute NDVI. But inexpensive target materials rarely have flat reflectance curves, so it is important to have a measurement in the band you will use for NDVI. If the reflectance curve for the target is not close to flat within the band you plan to use for NDVI, more than one measurement within that band might be important.

There are lots of types of validation, and different ways to validate. One way is to find recent satellite (e.g., Landsat) NDVI data for a big agricultural field near you and compare your results. Or you can measure the radiance (light energy) in the wavelength bands you use for NDVI and compare the resulting NDVI with your camera system. Or compare your results with a camera system or other NDVI system that has been calibrated.

Chris

Thanks for your response warren and Chris...I highly appreciate it..I was also interested to know about the vignetting effect and how public labs is accounting the vignetting correction in the mobius action camera?

I look forward to your response.

Best Regards, Suman

There are a few different approaches to this too, but one which definitely works is documented alongside an in-progress web-based approach, both under the tag #lens-distortion.

Thanks for your prompt response warren.....highly appreciable.... Cheers

Thanks for your prompt response warren.....highly appreciable.... Cheers

Dear Warren, I checked about the lens vignetting correction under the tag Lens-distortion as you mentioned. However, i didn't find anything related to lens vignetting. Can you please send the link of the specific page.

Thank you in advance

Best Regards,

Suman

Ah, i've just learned that I didn't know the definition of vignetting. But I do now -- apologies for mixing this up! I haven't heard anything about dealing with that, unfortunately. But if you do, please share!

Dear Warren, I am currently looking some literatures on Image Vignetting Correction. I will definately post back after i come up with a conclusion.

Thanks for your prompt reply as always.

Cheers