Fluorescence on the porch

 kw: photo essays, fluorescence, ultraviolet, spectra, diffraction gratings

This is a look into a bag of plastic rings taken from cottage cheese containers, illuminated with a UV "party light", or "black light". A number of food products are sold with such rings sealing the tops. They are polypropylene, which is not fluorescent by itself; manufacturers add a fluorescent tracer for reasons known only to them.

I happened upon this when I saw a glint of blue on a sunny day, in the grass near a Dunkin Donuts shop. It turned out to be a piece of plastic from a soup container. Later I found that the plastic rings that seal my favorite brand of cottage cheese are the same material. I like fluorescent stuff, so I've been collecting these ever since, as UV detectors. As you may conclude, I eat cottage cheese a lot; I have some with my breakfast at least twice weekly.

You may have heard of UV-detecting beads. They slowly change color in sunlight or when exposed to a UV lamp. There are several colors, but they all are clear once they've been away from a source of UV for a few minutes. The bonus of the fluorescent plastic is that it responds instantly.

Here I have a piece of the plastic hanging from a curtain rod in our sun porch. Some UV-sensitive beads are hanging with it, but their color change was rather subtle at the time this picture was taken. The blue tint of the plastic ring is evident…not overpowering, but evident.

This is a fresh ring. I noticed that, after six months in the sunny window, the ring that was there wasn't showing such bright fluorescence, and appeared more greenish. I replaced it with this new one and took it into my workshop to evaluate.

This photo under UV light shows the old ring on the right and a new one on the left. To the camera, they look very similar, but they look quite different by eye. This is at least partly because the camera sensor's color response is different from that of our eyes, and partly because the sensor can record UV, which is invisible to us. Also in the photo is a syringe I use for lubricating small parts, and also a reflection of the illuminating lamp, seen at the top of the photo. The desktop is dark brown Formica.

I decided to take a crude spectrum of these two pieces of plastic under UV illumination. I used this diffraction grating from Rainbow Symphony (the current price is $15 for a pack of 25). 

This grating has 500 lines/mm, which corresponds to 12,700 lines/inch. I have an old sheet of grating material from Edmund Scientific with 13,600 lines /inch, or 535 lines/mm, but it is harder to handle than these convenient slide mounts. Rainbow Symphony also sells gratings with 1,000 lines/mm, for those who want to photograph spectra at higher resolution.

This is the spectrum of the party light, tilted so that a sliver shows past the lampshade, at the left, with the main spectral lines noted. The filter isn't perfect, so visible lines of mercury come through. The very strong line pair centered on 406 nm gives the lamp its distinctive bluish-violet look, but it appears pure blue to the camera. There is also a faint orange-red line that I surmise is fluorescence from the filter material inside the lamp; mercury doesn't have a 620 nm line. Note how the 578 nm line pair appears yellow-green in the photo; to the eye it appears yellow-orange.

The UV wavelength that makes the plastic fluoresce is 365 nm. The camera can see it, as the photo shows. It is not visible to the eye, unless you have had a cataract operation; the natural lens of the eye is yellowish and filters it out. The strongest spectral line of mercury is 256 nm, but that won't pass through glass. A phosphor in the lamp filter fluoresces at 365 nm, increasing the efficiency of the lamp.

For reference, sunscreen and UV blocking sunglasses need to filter out all wavelengths shorter than 400 nm, or even 450 nm (mid-blue). In commercials, UVA refers to wavelengths near 365 nm, and UVC refers to 256 nm and nearby wavelengths. The middle range, called UVB, in the 320 nm range, doesn't have a convenient mercury spectral line to use for testing. Sunlight has a continuous spectrum of light that makes it through the atmosphere, from 180 nm in the deep UV (UVD) to 2,000 nm (mid-infrared).

This is a montage of the spectra of the two rings. I could have sharpened them by adding a slit above, but these spectra serve the purpose. The upper part shows the fresh plastic ring's spectrum. It fluoresces in the range from blue to green, with blue being brighter. The mix of blue and green makes the fluorescence of the ring look blue-white rather than deep blue.

The lower part shows the spectrum of the old, faded ring. There is less blue, but the green is unchanged. Thus, to the eye it appears greenish and dimmer when sitting in sunlight. That gives me a good reason to keep a bagful of these rings. About twice yearly I need to replace the "UV detector" in my porch window with a fresh ring.

These are useful to me, to remind me to wear sunglasses outside. I already have cataracts, but they aren't so bad that I need cataract surgery…yet. The more diligent I am to protect my eyes from UV, the longer I can do without it.