Multispectral and Photoshop

[Tom Lewis]

I want to use Photoshop to map monochrome images taken with just one of the color channels in my camera into a multispectral image.  I'll be using external filters not in the visible region.  I will use only one of the three color channels in my camera for all three images, so each of the source images will be monochrome.  

 

I want to use Photoshop because I'm already paying for it.  If necessary, I could use Lightroom also.

 

I'm fairly sure I saw a post somewhere on this forum of steps to do this, but I can't seem to find it.  And I'm kinda a novice at Photoshop.  Can anyone point me to a post with these steps, please?

 

Thanks,

Tom

[Andy Perrin]

Yeah we were discussing it (...again, haha, it's a perennial topic) here:

 

[Tom Lewis]

Thanks, @Andy Perrin.  I will review.

 

Tom

[Fandyus]

I have some experience with this. I've successfully stacked 3, 4 and 7 images coherently in Photoshop. I don't think I posted a tutorial here but message me if you need any advice.

https://www.ultravioletphotography.com/content/index.php?/topic/5935-multispectral-fruit-bowl/ - some showcase in this older post of mine.

 

[Tom Lewis]

Thanks @Fandyus.

[Tom Lewis]

I believe I have had success.  Thanks to all.

 

This is a horrid multispectral photograph of a part of my back yard that I took yesterday before it got dark.  I took it then because I had just received my filters!  It was a little windy so you can see the colors as a result of movements between filter changes.  Filters are Midwest Optical BP735, BP810, & BP880.  Camera is Nikon Z 7 converted by Kolari and lens was that little plastic 40mm f/2 Z job set to f/2.8.

 

For all three source images, I only used the data output from the red filter of the CFA.  After assigning the data to BGR color channels, I messed around with the sliders in Properties > Levels to improve the mid-tones.  

 

Tom

[Tom Lewis]

I decided I wanted to be like others and try fruit.  Below is photograph taken with same filters and camera.   This time the lens was my father's old Nikkor-P Auto 105mm f/2.5 and aperture was f/4.  Illumination was sunlight at 4:00 the afternoon.

 

Mapped to blue, green, and red in Photoshop.  Using the color mixer I adjusted red to 137 and blue to 39, while green is unchanged.  I did not attempt any other color balance.  For reference I have also included a photograph taken using visible light.

 

 

 

[Tom Lewis]

If anyone has suggestions about improving colors, please let me know.

[Andy Perrin]

Put something IR-white like PTFE in the picture and white balance the final result. You have a strong color cast there, so that will bring out the differences. 
 

Photoshop’s white balance too may be too limited. You can use Darktable or something probably. 

[Tom Lewis]

Okay, thanks @Andy Perrin. 
 

I ordered a PTFE sheet on the 17th and it’s taking forever to get here. ETA is Tuesday. Never have I anticipated so much getting a piece of Teflon. 
 

It does seem Photoshop is lacking in its white balance capability. I usually use Lightroom for that. Do you think Lightroom’s white balance tool is sufficient for a good result?

 

Tom

[Andy Perrin]

I don’t think Lightroom’s white balance is any different from Photoshop’s. It’s probably even the same code, since they are both Adobe products. 

[Tom Lewis]

I’ll confess here I really don’t know Photoshop well.

[photoni]

@Tom Lewis 

I use Photoshop to retouch, resize and stack images
To develop RAW/NEF I use Capture One, even if the white balance values are out of range it manages to balance the channels

 

Adobe Camera Raw works well in visible and IR (monochrome) doesn't work for UV, can't do white balance.
On this site they recommend Darktable... but I don't know how to use it XD

[Tom Lewis]

Thanks @photoni.  Your comment reminds me that I have some IR related profiles I got from Rob Shea that may help me white balance after I get going with the PTFE.

[Tom Lewis]

I converted the layer to a smart object within Photoshop and then did a color balance in Camera Raw using PTFE tape I grabbed from the potting shed and put on the orange.  The only other processing was to decrease exposure 1.5 stops.  Below is the result taken today:

 

I could not use the special Lightroom color profiles I have from Rob Shea because the images have been rasterized in Photoshop.  The color temp ended up being 100 after the Camera Raw white balance.  Maybe I need to create a profile or LUT in Photoshop for better white balance considering the max is 100?

 

[Edited this reply because image did not show.  Added image in a different way] 

[Tom Lewis]

@Andy Perrin, you mentioned Darktable.  Can you please provide guidance on how best I can use Darktable to improve the white balance here?  
 

I now have defined an Action in Photoshop for building the image from the three monochromes, so it’s automated and fast.  Can I do the same sort of automation in Darktable but with better white balance, or would my workflow be faster by continuing to use my Photoshop Action for combining the three monochromes, and then do white balancing after that in Darktable?

 

Or perhaps the white balance appears okay in the above image I posted in prior comment?
 

Thank you, Sir.

[Andy Perrin]

Hmm. The above image definitely looks improved to me. I think there is still a bit of a cast. I think I would keep your photoshop workflow and maybe just use Darktable to correct the white balance afterward. I’m not myself a big user of Darktable and I mostly know what people have told me about it since I use PhotoNinja instead. I suggested Darktable because it’s free. 
 

Other than WB, I guess you could try playing with the saturation and see if that brings out more color.
 

The pale results you are getting are typical of every attempt I have seen at near IR tricolor images - there just does not seem to be much natural variation in spectrum for most materials in that band. 

[Tom Lewis]

Thanks @Andy Perrin.  I’ll continue to mess with Photoshop for now. 

[Tom Lewis]

This version is where I did white balance and 0.75 stop exposure decrease in camera raw.  Then I adjusted the individual color channels with a curves adjustment layer as suggested by Rob Shea.  When I tried the reverse order I discovered that adjusting the white balance affected the curves adjustments and then I had to go back again and re-do the curves.  

 

I'm fairly satisfied now with the color rendition.  Next is to move to focus; must determine if focus breathing varies according to wavelength for this lens.

 

Thanks @Andy Perrin!!

[Andy Perrin]

Looks good!