The Bee, the Chicken, the Human and the Camera Look at a Sunflower

[Andrea B.]

^{EDIT 24 Jan 2017: I have added further explanations.}

 

Introduction

This flower color template attempts to illustrate the reflected color appearance of a flower as seen by humans, a UV-pass camera, a bee having trichromatic UV/B/G vision and a chicken having tetrachromatic UV/B/G/R vision.

 

I made this template to illustrate a point elsewhere. But I think it might be useful to give it its own thread so that others could use the idea to work out what a particular subject might look like if viewed by an animal with vision differing from the human RGB.

 

Warning: This attempt at illustrating insect or animal vision is very much oversimplified. I cannot emphasize that enough! The physiology and neurology and behavioral characteristics of bee vision or chicken vision is so very complex that we have no real way to show what the creatures see. We can only model small parts of it and then stand back and admire its magnificence.

 

Notation:

R = red

G = green

B = blue

UV = ultraviolet

IR = infrared

 

Reference

Chittka, L., Shmida, A., Troje, N., & Menzel, R. (1994) Ultraviolet as a component of flower reflections, and the colour perception of Hymenoptera. Vision Research, 34, 1489-1508.

 

Color Reflectance & Absorbance Notation

In the preceding paper the authors use the following notation indicate the absorbance or reflectance

of a particular color X by a subject.

• +X = colour X is all or mostly reflected
  
• -X = colour X is all or mostly absorbed
  
• +x = colour X is partially reflected
  
• -x = colour X is partially absorbed
  

We are always applying a judgement call as to how much we think a particular colour is absorbed or reflected by any subject. But for a simple diagram, it's best not to overthink it.

 

Human-Invisible Colour Notation: UVcolorName

Any combination of one reflected RGB colour and reflected UV is a human-invisible colour, of course. I've been writing names of these human-invisible colours using a compound colour name: UVred for +R-G-B+UV, UVgreen for -R+G-B+UV, UVblue for -R-G+B+UV, and so forth. This or similar naming is common in the scientific literature. In the diagram I've indicated these human-invisible colours by adding patterning to the underlying reflected colour. This is not particularly scientific, but it serves the purpose. (We could alternately add fine striping or even use contrasting colours not in the animal's spectrum.)

 

Assumptions

The bee has green, blue and ultraviolet visual receptors, so how does it see a color such as reflected yellow? In additive light, reflected yellow may be a combination of reflected red and green light with blue absorbance, +R+G-B. But yellow may also have its own reflected wavelength, not a combination of red & green. For this diagram I have simplified the matter by assuming that both reflected +R+G-B yellow and a yellow wavelength would stimulate only the bee's green receptor. Other colors are treated similarly:

• reflected yellow wavelength or yellow as +R+G-B = +G, green to the bee
  
• reflected violet wavelength or violet as +r-G+B = +B, blue to the bee
  
• reflected orange wavelength or orange as +R+g-B = +g, dark green to the bee
  
• reflected brown wavelength or brown as +r+g-B = +g, dark green to the bee
  
• reflection of all RGB wavelengths +R+G+B = white = +G+B, cyan to the bee
  
• magenta which can only be +R-G+B = +B, blue to the bee
  

Diagram

First row, left: Human Vision

This is how we humans would see a flower such as a Sunflower which has yellow or orange-tinged yellow rays and a dark brown central disk.

 

First row, center: Camera Vision, UV-only

This is the flower as seen by a UV-capable camera fitted with a UV-pass (only) filter on the lens. In only UV light, the flower ray tips reflect UV and the bases of the rays absorb UV. The flower's central disk also absorbs UV. Any color seen in a UV photograph is false colour which is a by-product of the fact that the UV light passes through the dyes of a Bayer filter and is thus recorded in the camera's color channels.

• The key point to make is that strongly reflected UV is recorded as whites or very light greys or very bright false colours in such a photograph while absorbed UV is recorded as blacks or dark greys or very dark false colours. Moderately reflected (or absorbed) UV is of course somewhere in between these two extremes.
  

First row, right: Camera Vision, UV/B/G filter stack

If the lens is fitted with a filter stack which passes large amounts of reflected UV together with small amounts of reflected blue and reflected green light, then we begin to take a step in the direction of emulating trichromatic UV/B/G bee vision. This filter stack maintains the areas of UV absorbtion and UV reflection seen in the UV-only photograph, but there may alterations in the tonality in UV absorbing areas if such areas happen to reflect some blue or some green as a component of the flower colour.

 

Be sure to note that with this filter, the colours recorded by the camera should also be considered false colours because we cannot know exactly how the UV signals will combine with any blue and green signals when passing through the camera's Bayer filter.

 

Another assumption here is that in order to correctly derive any blue and green colours passed under the UV/B/G filter stack, we must make a white balance setting either in the camera or in the raw file converter. The white balance setting would affect both actual colors and false colors.

• The flower's dark brown central disk reflects small amounts of red and green light while absorbing UV and blue light. After passing through the UV/B/G filter stack, passing through Bayer filter and being subjected to a white balance adjustment, the reflected light is recorded as a dark green.
  

• The flower's yellow ray bases reflect red and green light while absorbing UV and blue light. After passing through the UV/B/G filter stack, passing through Bayer filter and being subjected to a white balance adjustment, the reflected light is recorded as a slightly brighter green.
  

• CHECK THIS The flower's yellow ray tips reflect red, green and UV light while absorbing blue light. After passing through the UV/B/G filter stack, passing through Bayer filter and being subjected to a white balance adjustment, the reflected light is recorded as bright yellow.
  

Second row, left: Bee Trichromatic UV/B/G Vision

To step to a better colour modeling of bee vision, we must consider how to handle UV-reflecting areas recorded under the UV/B/G filter stack. Remember, from above, that when UV is reflected along with some other RGB colour, we have the human-invisible colours like UVblue, UVgreen or UVcyan. And remember that we are assuming for simplicity that reflected yellow stimulates the bee's green receptor. Thus UVyellow will be considered as UVgreen.

 

There is more than one way of modeling, for example, a bee's UVgreen. In this diagram I've chosen to add a kind of patterning to the areas which reflect both UV and green. This modeling might fail with any subject which has similarly patterned areas! But so far, for flowers, we are OK using this.

 

[Andrea B.]

Visible

 

Ultraviolet

^{The colours are false.}

 

UV + Blue + Green, version one.

This representation of bee vision is only approximate, of course. Why do we get the false yellow (=R+G in cameras) on the reflective tips of the rudbeckia flowers when all red is blocked by the filter stack? Because most of the UV light is recorded in the red channel. An additional white balance adjustment also helps pull the colours to what is seen here.

 

UV+Blue+Green, version two.

Here I extracted the raw recorded colours (no white balance). Then I shifted yellow to green. The result is another approximation of bee vision under the assumption that yellow cannot be seen by the bee. This version also represents a more neutral background for the grassy lawn and more neutral leaves. It is thought that this is the way the bee might see green leaves and green grass. This version is not sharpened or leveled. Ran out of time!

[Andrea B.]

Dandelion = Taraxacum officinale

 

Visible

 

UV

 

UV + Blue + Green

This was made using color separation rather than a UV/B/G-pass filter. There is a bit of edge bleed which could be cleaned up.

[Andrea B.]

Rudbeckia hirta

 

Visible

 

UV

 

 

 

Nasturtium

 

 

Visible

 

UV

I think this flower is particularly interesting in UV for the way 3 "star" outlines are revealed at the base of the lower petal just in front of the fringe. That star shape is not at all evident in the Visible photograph.

[Andy Perrin]

I love all the detail you can see in the petals of the black-eyed Susan! Andrea, what camera and lens did you use for these?

[Andrea B.]

Dandelion: Nikon D600-broadband + Coastal Optics f/4.0 UV-Vis-IR Apo Macro

unpublished

 

Rudbeckia hirta: Nikon D600-broadband + Nikon 105mm f/4.5 UV-Nikkor

http://www.ultravioletphotography.com/content/index.php/topic/496-rudbeckia-hirta-prairie-sun-gloriosa-daisy/

 

Tropaeolum majus: Nikon D300-broadband + Carl Zeiss 60mm f/4.0 UV-Planar]

http://www.ultravioletphotography.com/content/index.php/topic/451-tropaeolum-majus-alaska-mixed-garden-nasturtium/

[Andy Perrin]

My rudbeckia didn't have those subtle color differences in the petals! Mine were just yellow. Is that because you're getting more of the shorter (and longer) wavelengths than I do, perhaps? This was with the Noflexar and my 330WB80, which cuts off above 380nm. The Noflexar cuts off 330nm and smaller I think.

 

[Andrea B.]

My Rudbeckia above was a cultivar named 'Prairie Sun' which is unusual in having a green central disk. I don't know why its rays have a false colour appearance in UV which is different from the usual false-yellow rays we see in other Rudbeckias, but I suspect it's because there are some kind of differences throughout its pigments aside from the obvious green disk flowers. Also, when we get the information about a cultivar like this from the accompanying plant tag, we can't always be sure is 100% accurate. So the plant might be some kind of variant R. hirta and the variety was just not given on the tag. I'll go Google around for Prairie Sun to see if I can find out anything.

[Andy Perrin]

Ooh, interesting. It did not occur to me that not all members of a species would have the same coloring, although given this is (very obviously!) true of humans, perhaps it should have.