Public Lab Research note


Initial experience with Wratten 25 filter on A490

by patcoyle | January 30, 2014 23:19 | 135 views | 6 comments | #9991 | 135 views | 6 comments | #9991 30 Jan 23:19

Read more: publiclab.org/n/9991


What I want to do

Change external filter on A490 from Rosco 2007 to Wratten 25. Try it for NDVI.

My attempt and results

Photos of conversion and finished change: filter material:

IMG_4165.JPG

IMG_4168.JPG

A490 with external filter:

IMG_4172.JPG

First testing image examples, were shot after custom white balance in shade using red file folder:

IMG_4173.JPG

Test photos of the community garden at Asbury United Methodist church in Livermore CA.

as shot:

IMG_7940.JPG

processed with Ned Horning Fiji tool:

IMG_7940_NDVI_Color.jpg

processed with Infragram web tool:

7940_canvas.png

Additional test images show comparable NDVI images in my front yard, shot with Rosco 2007, with improved white balance, and the new Wratten 25 filter; processed with Ned Horning tool with the Chris Fasti lut.

Rosco 2007, with improved white balance:

IMG_7927_NDVI_Color.jpg

new Wratten 25 filter:

IMG_7931_NDVI_Color.jpg

Questions and next steps

Tune white balance or other parameters to improve results.

Why I'm interested

Continue to explore as tool to assess vegetative health.


6 Comments

Nicely done. I love the old beat up Kodak packages (but check out this one). Did you get it on eBay? How much? You must have a lot left over for doing internal conversions.

It's probably okay to white balance in direct sunlight, because the more red the better. But your photo looks good. Here is the histogram. Good separation of blue (NIR) and red: Pathist.JPG

I think you processed the Wratten image like an infrablue image. Here is an NDVI image using the blue channel for NIR and red for red:

PatWratten251k.jpg

Infragram.org does not have an easy way to process Wratten 25 (SuperRed?) photos. I couldn't figure out how to do it at the new and improved Infragram sandbox.

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Thanks Chris, I'll try to tick through your points, so the discussion is available for all.

Got it on E-bay for $9.99. Yes, I do have lots left for other conversions. As I cut it with scissors, trying to avoid fingerprints, I also realized I don't have a feel for how vulnerable the material is to scratches and handling problems. I'd be more comfortable with it mounted internally, to avoid handling scuffs.

Based on your experience with both, would you go with this Wratten 25 over the Schott glass filter you got us, to convert an A1200 with internal filter?

For a Rasberry Pi noir camera, I recall Mathew Lippincott posted a glue technique he's used. I need to check that. Some of the trimmed pieces may be large enough for that purpose.

How do you get the histogram you show? In PS, I see a number of other views, but not that one.

Another remedial question: Yes, I guess I just processed the Wratten image like an infrablue image using Ned's tool. What is the workflow you use to to use the blue channel for NIR and red for red? Does it scale to a directory of images. I've had trouble, and see others have too, with PS actions on multiple images.

For infragram.org, I've missed some calls. Is an easy way to process Wratten 25 (SuperRed?) photos a topic under discussion?

Thanks for all your work in this area!

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I would definitely choose a red filter over blue for NDVI. The comparisons I did in the fall suggest that using a red visible band for visible light in NDVI calculations does a better job discriminating between plants and non-plants. Ned's work convinced him of the same thing. It also reduces the haze problem and makes comparisons with legacy NDVI more interpretable. Using a blue band for visible in NDVI calculations (as in infrablue) can also be revealing, for example it seems to allow identification of unusual pigments that absorb at the red end of the spectrum, but mostly when you can compare infrablue with superred. The Wratten 25A gel filter I put inside the A2200 did not seem to cause much more optical degradation that the glass BG3 filter.

That histogram is from PS CS5, and is slightly different from CS3. You can find it under "Window."

Ned's plugin allows you to select the camera channel (RGB) to use for both the visible and NIR band in NDVI calculations. This is true in both the single camera and dual camera tools. So when using the standard two camera setup, you have three choices of NIR bands (from an NIR converted camera) and three choices of visible bands (from the unmodified camera). That means you can simulate the NDVI computed from single cameras and start to learn about the impact due to, e.g., the visible band from an infrablue or superred camera always being contaminated with some NIR (unlike in unmodified cameras). If you take simultaneous photos with both an unmodified and infrablue camera, you have two choices of visible blue band (one from either camera), and you can learn about the impact of the NIR contamination of the infrablue blue band.

Infragram.org does not have an option to process superred (e.g. Wratten 25) photos. The infragram sandbox originally had the capability to enter any formula for NDVI, but I couldn't get it to work in the new version. If you were really committed to using Infragram.org to process superred photos, you could use Photoshop to swap the red and blue channels (Image/Adjustments/Channel mixer). That turns superred into infrablue. To do this automatically, record an "Action" (Alt-F9). You could then use Bridge to process an entire directory.

It might be pretty easy to add superred processing to Infragram.org, but a higher priority might be to add histogram stretching. It's really hard to simulate legacy NDVI without it.

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Thanks Chris, excellent feedback and very helpful tips and suggestions.

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Sir @chris,what software did you use to select a part of the image and check its histogram?

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Photoshop. Also easy with Fiji. Probably also possible with many photo editing programs.

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