Flowers in the UV

A marsh marigold flower seen at 365nm in the UV. A visible light picture of the same flower (larger version)

nb. Some of this is described in an article in 'Hasselblad FORUM 98/1' which is reproduced here in a draft form.

Honey bees have eyes with spectral sensitivitiy in the yellow, blue and ultraviolet (UV). Amongst these colour groups, the power to distinguish shades of colour is sharpest in the UV. Many flowers which are pollinated by bees show patterns caused by the absence of UV in certain regions which act as guide marks to help the bee find the nectar and so brush the pollen. While about a third of flowers exhibit patterns in visible light, an additional quarter show patterns only in the UV - an example is the marsh marigold shown above. A more dramatic comparison between visible and UV appearance is shown by a bunch of flower heads. These pictures illustrate the adaptation of the flower to the bee's preference for UV-dark central regions.

Less dramatic differences are seen in flowers like the Freesia. A montage of white (top row), purple (centre) and yellow (bottom) flowers shows visible images on the left and the corresponding UV images on the right.

These pictures were taken with the setup shown here. I use an 80mm lens with a 56mm extension tube directly onto type 667 (3000 ASA) Polaroid print film. For maximum depth of focus the lens is set to f/22 and, with visible illumination, the exposure is very short. I use a 400W mercury arc lamp encased in UV filter envelope as a source of UV light. The arc is focussed onto the flower with a thin singlet lens and the alignment adjusted using a fluorescent screen (white paper) to examine the shadow of the flower. To avoid any fluorescent light entering the camera, I use a second UV filter (2mm of Schott UG2) over the lens. Even though the arc is powerful, exposures of around 60 sec at f/22 are necessary. The effective wavelength is determined by the strong mercury emission lines at 365nm and I have not noticed any degredation of image quality - at least with the grainy Polaroid film. I make no adjustment of focus for the UV image.

In collaboration with Didier Pelat, I had performed this experiment in 1979 using a simpler setup with a singlet lens to form the image.

For several references, see "The Pollination of Flowers" by Proctor and Yeo, Collins, 1973, ISBN 0 00 213178 1.