• Ultraviolet Photography

Verbena bracteata [Bigbract Verbena]

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#1 Andrea B.

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Posted 08 April 2021 - 19:47

Blum, A.G. (2021) Verbena bracteata Cav. ex Lag. & Rodr. (Verbenaceae) Bigbract Verbena. Flowers photographed in ultraviolet and visible light. https://www.ultravio...gbract-verbena/

El Dorado at Santa Fe, Santa Fe, New Mexico, USA
10 June 2020

  • Verbena bracteosa Michx.
  • Verbena bracteosa var. brevibracteata
  • Verbena imbricata Woot. & Standl
  • Verbena squarrosa
  • Verbena subuligera
Other Common Names:
  • Bracted Vervain
  • Carpet Vervain
  • Prostrate Vervain
  • Prostrate Verbena
The spreading mat of V. bracteata is quite pretty against its dusty background. I was struck by the extremely UV-dark foliage of this plant.

1. SEINet Arizona-New Mexico Chapter (acc 22 Mar 2021) Verbena bracteata.
This is a southwestern biodiversity organization making use of the Symbiota portal software.
2. Wildflowers of New Mexico (acc 22 Mar 2021) Verbena bracteata.
Website published and maintained by George Oxford Miller.
3. Allred, Kelly W., Jercinovic, Eugene M., Ivey, Robert DeWitt (2021) Flora Neomexicana III: An Illustrated Identification Manual, Second Edition, Part 2, Verbena Key, page 773. Print on demand at lulu.com.
4. Littlefield, Larry J., Burns, Pearl M. (2015) Wildflowers of the Northern and Central Mountains of New Mexico. Weed Verbena, page 291. University of New Mexico Press, Albuquerque, New Mexico.

Equipment [Nikon D610-broadband + Nikon 105mm f/4.5 UV-Nikkor]
unless otherwise noted

Click on the photo for a larger view.

Visible Light [f/5 for 1/500" @ ISO-64 in Sunlight with D850 + Micro-Nikkor 60/2.8G AFS]
Bigbract Verbena has a flat, spreading habit.
Attached Image: verbenaBracteataHabit_20200610aveEldorAveTorreon_7244pn.jpg

Visible Light [f/16 for 1/2.5" @ ISO-200 with Onboard Flash and Baader UVIR-Block Filter]
The tiny flowers can be lavender, pinkish or bluish.
Relative to the flowers, the bracts surrounding them really are big
Attached Image: verbenaBracteata_vis_flash_20200610aveEldorAveTorreon_21019pn01.jpg

Ultraviolet Light [f/16 for 20" @ ISO-200 with SB-14 UV-modified Flash and BaaderU UV-Pass Filter]
A long exposure is used to permit multiple UV flashes.
Everything is UV-dark except for the reflective hairs along the edges of the leaves.
Attached Image: verbenaBracteata_uvBaad_sb14_20200610aveEldorAveTorreon_21033pn01.jpg

Visible Light [f/11 for 1/15" @ ISO-200 EV+1.7 with Onboard Flash and Baader UVIR-Block Filter]
Details of the stem and bracts.
Attached Image: verbenaBracteataStemLeaves_vis_flash_20200610aveEldorAveTorreon_21054pn0101.jpg
Andrea G. Blum
Often found hanging out with flowers & bees.

#2 Stefano


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Posted 08 April 2021 - 20:33

That is a really nice deep blue. As far as I know, UV-blue occurs in the longest UV wavelengths, like 390-400 nm.

#3 Andrea B.

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Posted 09 April 2021 - 19:07

Probably that is OK to assume here. But we have to be careful. :lol:

The visible rim colour of that flower when I was looking at it was reflected pale violet-bluish, more on the blue side, not too saturated, moderately bright. The center reflected almost all visible colours given it was near-white.

In UV the little flower was quite UV-absorbing, but with a very slight difference in its raw color from the raw color of the bracts surrounding it. The flower might have had a bit more UV reflectivity than the bracts? I will go get the raw composite and we will look at that. (Added: As it turns out, the flower and the bracts seem to have similar absorption/reflectivity, i.e. similar tonalities.)

Anyway, it would make sense to me that the UV reflectivity could be in the high UV band and thus translate -- after white balance -- to a false blue. From what we know of the absorption curves of flower petals, the shift from being UV-absorbing to UV-reflective is not a sharp, steep shouldered transition. So this little flower's violet-blue reflectivity likely continued to gradually shift (and decrease) into some UV reflectivity between 390-400 nm. (It is difficult to properly describe what I am attempting to convey.)

Does this explanation work for the white center of the flower? The white center reflectivity shifts gradually into some reflectivity in the 390-400 nm band?

A proper analysis of what we are seeing with this flower must also take into account its structure and its pigments.

Crop from Raw Composite from Raw Digger.
We must interpret the raw compositie brightness relatively because another 1/3-1/2 stop in exposure or application of a stronger or weaker "gamma" curve during demosaicing would change the measurements. But the point is that the petals and the bracts are UV-absorbing. The bract in shadow (top measurement) is naturally a bit darker.
Note also that the petals have a different reflected raw color than the bracts. On the color wheel 0° (alternately 360°) is pure red. The 342° and 348° are just before pure red (arriving from red-purple) while the 8° and 10° are just after pure red (headed towards orange). We will skip for the moment any discussions of how different color profiles might affect this analysis and also simply interpret the raw colors relatively.

Top line on each color circle Hue, Saturation, Brightness.
Bottom line R, G, B values.
Attached Image: verbenaBracteata_uvBaad_sb14_20200610aveEldorAveTorreon_21033rawComp.jpg

What I do NOT have is a chart of the RGB transmission curves of my D610 sensor which would help place those two recorded raw colour areas into context. There are sometimes places on those charts where the RGB lines can cross in such a way as to give a neutral tone or where the same colour can represent two different wavelengths. So it is only based on experience (I hope!) that I think we are reasonably safe in assuming the dark false blue comes from a UV reflection closer to 400 nm than not. I cannot factually state that this is so from the evidence at hand because I don't have everything I need to make this judgement.

Here is a de-colored version of the Raw Composite crop. Hard to tell any differences here.
Raw Digger reported underexposure only in some shadowed areas and mostly in the blue channel.
Attached Image: verbenaBracteata_uvBaad_sb14_20200610aveEldorAveTorreon_21033rawCompBW.jpg

Here is a de-colored version of the false color UV photo posted above. Again, I don't know how we could determine anything about what portion of the waveband is represented from this version. The flower looks as dark as the bracts. So is it possible that the false blue of the flower in the finished photograph is just a by-product of the conversion and editing process and does not represent anything meaningful except for a very slight difference in UV-absorption/reflectivity.
Attached Image: verbenaBracteata_uvBaad_sb14_20200610aveEldorAveTorreon_21033pnNoColor01.jpg
Andrea G. Blum
Often found hanging out with flowers & bees.