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I have two regions of interest: red (~665nm) and near-infrared / far-red (~720-750nm). (possibly more but I'd like to see what is possible first)

The ratio of the absorbance values for red/far red is important in my work. Think types of vegetation, that sort of thing. I'd like to distinguish between things that absorb in the red (with a bit of far-red) and things that absorb in the red (with more far-red)

I know it is possible to highlight my samples of interest using a hyperspectral camera, by focusing on two channels. However, a hyperspectral camera is rather expensive. I'd like to see if a cheap hack for fieldwork is possible.

My current (very limited) knowledge:

-smartphone cameras may lack a 'hot filter', enabling them to sense near-infrared light

-an external filter may be used to remove unwanted wavelengths (e.g. https://petapixel.com/2019/08/22/how-i-shoot-infrared-photography-with-a-smartphone/)

-it's important to take RAW photos (with all the data). Some phones like the A51 may not have that by default, but require additional apps (e.g. Lightroom) for that.

Where I am currently stuck: -I assume near-infrared pixels will be labeled in most software as 'red'. Would there be any app which enables me to separate (even false-color differently) shorter-wavelength red from longer-wavelength red? I am not quite sure how cameras work and whether this is is even possible from a hardware perspective, never mind software.

Thank you for your time!

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3 Answers 3

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-I assume near-infrared pixels will be labeled in most software as 'red'. Would there be any app which enables me to separate (even false-color differently) shorter-wavelength red from longer-wavelength red?

It's not a matter of “in most software” — the camera sensor will produce three channels of “red”, “green” and “blue”. It is not possible for the software to make more distinctions than our eyes do, because the hardware is not made to do that.

However, you might be in luck anyway. In every phone camera I've personally investigated, infrared (at least from infrared remote control LEDs, typically ~940 nm) is not detected as if it were red, but as if it were a purplish blue color. That is, the blue channel is more sensitive to that wavelength than the red channel is. So, you may be able to obtain a false-color image where red is visible red and blue is infrared, if this holds true for your desired 720-750 nm, and if you can find the right filter or stack of filters which:

  • passes infrared
  • greatly attenuates visible blue and green
  • moderately attenuates red, so that the exposure is in better balance (the sensor will have much less unintended infrared sensitivity than intended red sensitivity)

On the other hand, if your camera's sensor's built-in color filters turn out to treat 720-750 nm more like red than like blue, then you will not be able to discriminate it from red (in a single exposure). Whether this is worth buying a bunch of specialist color filters and exploring is up to you.

And as another answer already noted, you can always take multiple shots with different filters, if your subject is stationary. If you want to combine those to produce a consistent false-colored image, you'll need to lock the exposure on your camera so that it's the same between shots, and if you want to know what that means in terms of the true spectrum, you'll need to calibrate your setup by taking pictures of reference objects and seeing how they look.

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Sensor in regular cameras and smartphones have sensels with either red, green or blue filters in front of them that measure the light that passed through the filter. So they cannot distinguish wavelengths that pass through the same filter and therefore won't be able to distinguish red from near/far infrared.

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A possibility:

Most likely the red pixels will pick both short-wavelength red, long-wavelength red and maybe some near infrared.

If your subject is stationary, you could place a filter in front of the lens that passes only the kind of red you're interested about. Then take few pictures with different filters.

That won't obviously work if you are photographing vegetation in wind, because photographs taken at different times are genuinely different.

You really require a very sturdy tripod for this (phones aren't really designed for tripods). Removing the first filter and placing in the second filter has to be done in such a manner that the tripod doesn't move. You could consider just holding the filter in front of the lens, without touching the phone camera.

Also the white balance of the camera has to remain a known constant, or preferably you should shoot RAW files (if possible with your phone).

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  • \$\begingroup\$ A51 have so named PRO mode for photos which mean RAW files. And in this mode you can set colour temperature (beside other things) \$\endgroup\$ May 3, 2022 at 17:42

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