Fitting dual band filter to Canon A4000

[Ed Blanchard]

I have recently bought a Canon A4000 to convert for taking NDVI images of potato plants during the growing season.

I am planning to mount the camera on Hobby KIng Skyray UAV plane with auto pilot.

The first step will be to remove the IR blocking filter and replacing it with a more suitable filter.

I have both the infrablue and red filters and I was thinking of using the dual pass 660-850 filter from Mid Opt

What are your suggestions ?

[Chris Fastie]

Ed,

@Claytonb has had impressive success reproducing the NDVI results of professional NDVI cameras by using the MidOpt 660-850 filter in a PowerShot: https://publiclab.org/notes/Claytonb/07-09-2016/plant-health-ndvi-consumer-camera-vs-professional-multispectral-camera. So that approach is highly recommended. Calibrating the process is an important part of the workflow.

Chris

[Ed Blanchard]

Thanks Chris,

I have checked out Clayton's note.

Clayton mounted his filter where he removed the internal IR filter.

Where can I order the dual pass filter from to be delivered to Australia and what size ?

[Chris Fastie]

Ed,

You can get any filter in any size from midopt.com. You can order a custom size of this http://midopt.com/filters/db660850/ to go inside your camera. You have to get a quote from them.

The PowerShot S100 and some other PowerShots have an IR cut filter inside that is about 8 mm by 9 mm. I don't know what the size of the IR cut filter is in the A4000.

To make the cost of that filter worthwhile, you will probably want to get targets for calibrating the process. If you don't do that, it might make more sense to use a simpler red filter like the Wratten 25.

Chris

[Ed Blanchard]

Thanks Chris,

Do you know how much mid opt charge for an 8x9mm filter ?

I have been following your and Ned Horning's notes on using targets for calibration and I have organized a series of cards, black, white, red, blue and green.

Do I use these once to calibrate the camera or each time I take a series of photos in the paddock ?

[Chris Fastie]

Ed,

To use Ned's calibration plugin, you must enter the reflectance values for visible and NIR light for each calibration target you include in the photos. So each calibration target must be characterized with a spectrometer-type instrument. Obtaining these characterized targets is the largest obstacle to calibrating the process. There is a discussion of working around this issue in the comments to this note: https://publiclab.org/notes/carolccarvalho/07-15-2016/raspberry-noir-cam-blue-filter, but I have not heard that anyone has tried it and I don't know how well it would work. The targets should be included in some photos from every photography session.

I don't know how much MidOpt's filters cost.

Chris