Mexican Zinnias ("Creeping Zinnia", Sanvitalia procumbens) are quite fascinating little flowers, since they sport a very prominent UV pattern which can't be seen with the bare eye.
This is the visual appearance [click on image to see it larger]
whereas in UV the flower shows that prominent pattern (like a heli landing platform for bees?)
Now I was curious to find out what the spectrometric reflection response would be, since that pattern must also show there. I used a Xenon full spectrum light source, an USB2000 UV-NIR spectrophotometer and a suitable fiber optic UV-NIR reflection probe.
The light pink curve shows the response of the flower center, not very strong, just reaching about 10% in the visual range and no UV response [for the sake of simplicity I will not comment on the NIR responses for now, this had to do with the used white reflection normal]. The petals however show something different. The ground of the petal (which appears dark in UV) only shows quite a strong visible response in the in the 500nm region and above (green curve), whereas the petal tip (which appears bright in UV) indeed shows quite some strong UV (and blue) reflectance between 330....430nm (dark pink curve) with a maximum at about 360nm. Interesting since one of the receptors have its maximum at about 340nm (300...400nm). But also the visible response of the petal tip is some 15% stronger than the base! For comparison I also recorded the green leaf response which is shown as the light blue curve, much weaker, only reaching a bit more than 25% in the green.
Now what's that violet curve now? The reflection of a white Bumble Bee "rear end". Remember what I showed a few days ago? This is the sythesized bee vision version of a bumble bee sitting on a Mexican Zinnia flower with a prominent UV pattern.
Interesting how that Bumble Bee's (Bombus terrestris) abdomen reflection pattern evenly covers the whole spectrum, but exactly starting at the UV maximum of 360nm of that petal tip up to deep into NIR. More research has to be done on that, it can't be just coincidence, or as Einstein put it, "nature does not play dice"....
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos