NDVI questions

[baelcin]

Hi everybody.

i need to some information about ndvi with  modified cameras. i read many things but still i need some answers.

1. we need to deattach standart filter from camera (bayer filter). and we have to attach another filtre (same place bayer filtre) for capture near infrared.

So why we don't use standart filtre for lens (not front of sensor )

2. if we modify camera can i see near infrared wawelenght in histogram?

3. which wawelenghts  are visible with this modification? just one near infrared wawelenght like 850 nm or is this a interval wawelenght like 720nm-1000nm?

sorry about my english. if you understand me please answer:)

thanks and i need some source about ndvi (for like this basic questions)

[Chris Fastie]

Hi baelcin,

These are good questions.

1. The filter we remove from inside the camera is not the Bayer filter array, it is another filter in front of the sensor that blocks all near infrared light (but passes visible light). After that filter is removed, another filter has to be installed. This can be installed either inside the camera or in front of the lens. 
2. Yes, the standard histogram will represent the near infrared light. In two-camera systems with one camera capturing only near infrared light, all the data represented in the histogram is near infrared light. In one-camera systems with one color channel capturing near infrared light, only the histogram for that channel will represent near infrared light. The histogram includes no information about which wavelengths are captured. It represents the number of pixels with each brightness value (from 0 to 255). Brightness values are along the x axis.Number (of pixels) is on the y axis.
3. Camera sensors (with intact Bayer filter arrays) vary in their sensitivity to near infrared light, but most seem to record wavelengths up to about 1000 nm. There seems to be much less sensitivity above about 850 nm, and very little above 950 nm. All of these wavelengths are combined in the image as one value for each pixel. Depending on the filter used, the near infrared wavelengths can be mixed with some visible wavelengths. 

There are lots of discussions of NDVI elsewhere in this Google group: https://groups.google.com/forum/#!searchin/plots-infrared/ndvi

Chris

[baelcin]

Thanks for answers Chris. 

i have to ask two more questions about your answers and you don't have to answers these ones:)

1. which filter you recommend for ndvi (in front of lens) i don't want to pass visible lights from my filter.

2. which cameras better for ndvi? tetracam or modified cameras?

3. can you suggest site or any document for explain all ndvi process. (like how can i modified cameras,one camera systems, 

two camera systems, how can i capture ndvi maps etc.)

Thanks for answers again.

[Chris Fastie]

Hi baelcin,

1. It's easy to buy glass filters to block visible and pass NIR with a cutoff between 700 nm and 950 nm. Probably a good filter for a two camera system has a cutoff around 720nm.
   
2. Tetracam makes several models with multiple or single cameras. The single camera I know about is very low resolution and very expensive. But you get their software which does proprietary calibration and post processing which might be good. The quality of the photos produced with a $4000 Tetracam are probably far inferior to those from a $50 PowerShot and Wratten 15 filter. But a Tetracam system might enable you to get calibrated NDVI values without thinking about it.
3. I don't know of any single document which explains how to do this. There are multiple cameras systems (single and multiple camera), different filters (blue, red, yellow, 720 nm), calibrated and uncalibrated, different post processing options (Fiji, infragram.org, Tetracam), different end goals (NDVI, CIR), different applications (agriculture, forestry, backyard), etc. And much of the information is rather recent. So searching the web is your best bet for learning how to do it. Here are my research notes at Public Lab about NDVI. A couple of these might be helpful.

Chris