This is the invisible border. Invisible? Yes, of course - I made it visible! This is the region of a tiny Bidens petal where all of a sudden the UV reflection changes from bright to very dark at around 10x magnification. And that cannot be see at all in visible light; all we see is a beautiful bright yellow!
Shot using off the shelf D70, UV Rodagon 60mm lens, Baader U-filter, UV High Power Xenon Flash lamp.
[click on image yields a larger one]
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