False colour green in UV-images (Baader U filter)

[Nico]

A recent thread by Dave:

http://www.ultraviol...olour-spectrum/

 

and Andrea's reply to it:

"Seems like with the Baader-U we get mostly blues, yellows and some very dark greens in UV False Colour Land - or maybe they are very dark cyans, not sure which. It is all a bit tricky nailing down the false colours because of little differences between editors, sensors, bayer filters, lenses, lens coatings and so forth. Then, of course, that colour list can change quite a lot once you stray from the Baader-U.

The greyed-blues always look lavender/purple to me. And greyed-yellows look more greenish. Never have gotten any bright greens with the Baader-U."

 

reminded me of some images that I had taken from Scilla bifolia. Unlike most of the times when leaves are pretty dark in the UV-image, they appear cleary (dark) green.

WB was done against PTFE like in my other UV-images.

 

I've no explanation why these leaves turn green and I have not seen this effect again since then ...

 

Scilla bifolia. Images taken near Biblis, Germany. 4 March 2013. Images taken with the Panasonic Lumix G1 broadband modified and the EL-Nikkor 80 mm/f5.6.

 

 

Visible light image: Baader UV/IR cut filter

Image reference:NCH-1070564

 

 

UV-Image: Baader U filter.

Image reference: NCH-1070567

[JCDowdy]

Nico,

 

That is very interesting, especially with other green leaves in the imediate foreground and background of your visible image not rendered similar in UV.

 

Let me be certian I understand, when you say ".... I have not seen this effect again since then ..." do you mean with this species?

[DaveO]

Nico,

 

I will scour through my more recent work to see if I can find any UV-green leaves, I have a vague feeling I may have seen some...

 

What intrigues me is when a visible-yellow becomes UV-yellow as well as visible-blue becoming UV-blue

 

I have seen Klaus' false color palette on his blog and wondered if the false-UV-colors do always represent the same wavelength or if, as occurs in visible light, refraction or other physical effects also produce different UV-false colors.

[JCDowdy]

DaveO & Nico,

 

Yes, I recall some discussion about a month and a half ago on this very topic

(http://www.ultravioletphotography.com/content/index.php?/topic/734-som-berthiot-cinor-1715-c-mount-lens/)

see 04Apr14

 

From what I can glean from his postings, Klaus' work is based on correlation between color rendition in his photos and reflectance spectroscopy.

http://forum.mflense...ion-t51237.html

 

However, it is not just Dr Schmitt's false color palette that suggest a basis for calling ~360nm UV-yellow and ~340nm UV-Green. There are also other false color responses seen using the "sparticle" filter arrays:

http://photographyof...lower-leaf.html

& http://www.savazzi.net/photography/uvtestfilters.html

& http://www.savazzi.net/photography/35mmuv.html

 

It is a tempting hypothesis that comparable out of band UV Bayer array responses combined with comparably standardized white balance technique, lighting and filtration could yield some insight into the spectral reflectance imaged. Your point that refraction or other physical effects could predominate isn't necessary contradictory to that notion.

 

We have been shown that flowers can reflect different false color signatures depending on developmental stage or viability. Perhaps the UV-green Scilla bifolia has a transient 330-345nm reflectance at some point in it's life cycle or due to some environmental variable yet again encountered?

 

Fascinating!

- John

[Andrea B.]

Leaves mostly photograph in grey/black false colours when shot close-up. Sometimes you get some false green. Landscape photographs at medium to long distances frequently will show some green false colours.

 

Just a slight change in the area chosen for the white balance operation may either bring out or reduce the false greens. This is why it is wise to shoot a CC Passport or WB Disk with every subject and not rely on one white balance preset.

 

The most interesting false dark greens are those that occur on a flower.

 

I also note for the record that I tend to get slightly more false dark greens with the Capture NX2 editor than with other editors.

 

In short, be wary of your false colours and do not read too much in to them. Too many variables are uncontrolled!!! Everything plays a role in false colour: Bayer filter, sensor, lens, coatings, filters, illumination, editor, white balance algorithms, etc.

[nfoto]

Nico,

 

I will scour through my more recent work to see if I can find any UV-green leaves, I have a vague feeling I may have seen some...

 

What intrigues me is when a visible-yellow becomes UV-yellow as well as visible-blue becoming UV-blue

 

I have seen Klaus' false color palette on his blog and wondered if the false-UV-colors do always represent the same wavelength or if, as occurs in visible light, refraction or other physical effects also produce different UV-false colors.

 

If this assertion would be true, this requires the relationship between expressed colour and wavelength to be unique 1:1 (ie its inverse exists and is unique as well). There is to my knowledge no evidence that this required condition does exist. We already know that the camera (and processing software) must emulate any given colour by mixing components from different channels, and their spectral response is not single-peaked or independent from the others. Thus, while it is tempting to deduct spectral property from colour(s) present, it is a risky business and likely to be highly unreliable.

[Andrea B.]

As an example, the camera will see a mix of R+G as yellow and will see a pure yellow wavelength as yellow.

So, based on our false colour photographs only, we cannot "invert" the recorded false colour

and claim a particular UV wavelength has been photographed.

 

Also, we have not yet seen any such mappings which take into account the steep drop off of UV from 400nm down to 300nm.

 

Although certainly these attempted false colour mappings and sparticle boards are giving us interesting clues.

[Nico]

Nico,

 

That is very interesting, especially with other green leaves in the imediate foreground and background of your visible image not rendered similar in UV.

 

Let me be certian I understand, when you say ".... I have not seen this effect again since then ..." do you mean with this species?

 

No, I mean in general. This species is an early spring flower that I haven't photographed again since then. I’ll repeat the shoots when I get a chance, next spring.

[JCDowdy]

No, I mean in general. This species is an early spring flower that I haven't photographed again since then. I’ll repeat the shoots when I get a chance, next spring.

Good, that is the most interesting answer I was hoping for! To bad you have to wait so long to repeat the experiment. I know nothing about this plant, is it to late to sprout some more?

[Nico]

Good, that is the most interesting answer I was hoping for! To bad you have to wait so long to repeat the experiment. I know nothing about this plant, is it to late to sprout some more?

 

Scilla bifolia is flowering only in very early spring ...