Question: Question: Can the Public Labs Desktop Spectrometer analyze light from a patch of grass?

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WhiteRabbit asked on July 27, 2014 17:11
1,004 | 1 answers | #10984


What I want to do or know:

Can I generate a reflectance curve like that shown for lawn grass at the start of research note http://publiclab.org/notes/nedhorning/11-01-2013/why-a-red-filter-should-work-well-for-ndvi by simply pointing the Public Lab desktop spectrometer at a patch of grass on a sunny day? Would it be that simple, or would I need some optics to bring enough light into the desktop spectrometer for that to work? Or would it not work even then?

Background story:

Trying to find a way to analyze and track grass properties, initially using the method described in http://publiclab.org/notes/WhiteRabbit/07-25-2014/can-infragrams-provide-useful-measurements-of-thirsty-my-lawn-is, but now I'm wondering whether the Public Labs Desktop Spectrometer might be a shorter and possibly better path toward extracting data about a lawn's condition.



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1 Answers

The Public Lab desktop spectrometer is quite capable of making a spectral graph of sunlight reflected from grass. The curve will differ from the professional reflectance curve for several reasons:

  1. The professional reflectance curve was made with a calibrated source of white light, or the curve of the source light was subtracted from the curve of the reflected light. The reflectance curve from the Public Lab spectrometer will be influenced by the color of the sunlight.
  2. The Public Lab curve is the average of the three curves from the three color channels in the consumer camera. The average always retains artifacts (e.g., three humps) from the R, G, and B curves.
  3. The consumer camera will weight the three color channels based on some white balance algorithm that is probably irrelevant to the spectrometry task. So the red or blue end of the spectral curve will be inflated arbitrarily.
  4. Although the response of the sensor is probably linear, the in-camera processing to make a jpeg image will probably alter the values in some way (not know to me) so that a value that is twice as great as another value will not necessarily relate to twice as much reflectance.
  5. For some reason, maybe related to one of the above, the Public Lab spectrometer seems not to be as sensitive to infrared light as it is to visible light. So values at the infrared end of the spectrum are rarely very large. For example, these spectra of lawn grass show lower values in the infrared region than in the visible region: greengrass02, backyardgrass.
  6. Have I missed any other differences?

Most of these differences could be corrected if you tried hard enough.


One reason why the plab spectrometer is less sensitive in the infrared wavelength is the ir filter as part of the lens assembly. There are research notes on how to remove. Check mine.

Stef


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