Fluorescent catnip

Digital Photography For What It's Worth—always 100% chad-free!

Optics bench, Exploratorium, San Francisco, CA. Click to see 1600x1200; all rights reserved. [C-2000Z]

Fluorescent catnip—chlorophyll fluorescence under an incandescent black light

Last updated July 27, 2004

A discussion of chlorophyll fluorescence on RPD led to this simple demonstration of the phenomenon with my daughter's 75W incandescent black light bulb, a live potted catnip plant and a cluster of silk leaves borrowed from a fake ficus.

Under ordinary tungsten lighting, the live and silk leaves both looked medium green to me, the live ones a bit lighter. The photo at right was taken under the same lighting with tungsten white balance (WB). To my eye, the live and silk leaves were both slightly lighter than shown here, but the colors are fairly accurate otherwise.

The silk leaves served as controls lacking chlorophyll and, in all likelihood, any kind of fluorescence. They photographed pretty much as I saw them under both ordinary and black light illumination.

Catnip and silk leaves in visible (tungsten) light with tungsten WB [C-2020Z]

Under the black light alone, the live catnip leaves turned a rich dark magenta to my eye, while the silk leaves simply became darker green with no magenta to speak of. I'm confident that the dark magenta I witnessed under the black light was due largely if not entirely to the fluorescence of chlorophyll a and b, which together absorb UV-A through blue-green photons (375-430 nm at full width half maximum) and emit far red photons (640-680 nm at FWHM), as shown in Figure 1 of SCUFA's informative online PDF.

At right are the same live and silk leaves, now illuminated only by a 75W incandescent black light.

To the camera, the catnip leaves became a golden yellow mauve as far removed from the deep magenta my eyes saw under the black light as from their green color under visible light. Here I used flash WB because it came closest to restoring the white of the background chair, but the live catnip leaves came out about the same with all my WB settings.

Catnip and silk leaves under an incandescent black light with flash WB [C-2020Z]

I'm not yet sure how to interpret the eye vs. camera discrepancy in the black light live leaf colors. As the chair shows, the black light photo clearly has a lot of residual purple, but purple + deep magenta doesn't make yellow. My hunch is that the difference stems in part from the camera's sensitivity to UV-A reflected by the subject, but WB and IR contamination may also play a role at this range.

Someday, I'll explore this step-up with my 18A UV pass and hot mirror IR cut filters.

References and Links

Measuring Chlorophyll a — SCUFA's online PDF offers valuable details on chlorophyll fluorescence, especially the absorption and emission spectra in Figure 1.