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05 September 2003

Geologist for a Day, Part 2: Digitally Photographing Fluorescent Rocks

by Mark Streitman

This is the second part to the article Geologist for a Day: Trip to a Zinc Mine

When we were in the mine we were allowed to take one fair sized rock from a bin and hold it under a Short-wave Ultraviolet (SW UV) light.  I'd only seen fluorescent rocks in museum display cases. Holding what looks like an ordinary rock under the light in the dark and seeing it glow is very cool.  So of course I had to have a SW UV.  After the tour of the mine, I went to their store and just by an amazing coincidence they happen to sell them in the gift shop.  Along with some other goodies, I bought a SW UV light.

SW UV lights start at $42 and go up to several thousand.  I bought the $42 one.  It's from a company called UPV  Model UVG-4.   It's perfectly good and it's battery powered.  The only real difference is the size. The bigger and more expensive the lamp, the bigger the area it can light up and the brighter it is.  Plus some have bigger batteries, can do short and long wave, etc.  You can use long wave light, but my experiments worked well with the SW.  According to some web sources, long wave does work better on some rocks, especially diamonds, so when I find some diamonds I'll remember to buy a LW light.  Anyway, the one I bought works great. Since it's battery powered, it'll be great for field trips.  Here's a picture below:

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The window that the UV comes out of  is facing up.   The light has a warning on it, "DO NOT EXPOSE TO EYES OR SKIN". This is because the short UV rays can be dangerous.  I don't know if this light is really powerful enough to be dangerous, but why take the chance?  I try to be careful with the light and especially not allow the UV to shine into my eyes.  

If you look at the applications of UV lights, you'll see there are a lot of them.  That's because a number of chemicals fluoresce under UV that might be invisible in visible light (that sounds like a contradiction, doesn't it?). However I'm not going to get into it now; the UPV site has some nice information. A google search will get even more.   Suffice it to say that this is a very good tool for scientific investigations and science education.

After the mine we went to the Franklin Mineral Museum  .

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I don't have any pictures of the interior.  They have hundreds of minerals on display.  It's very interesting to see, but display cases make dull pictures.  After the tour, we went out back to the rock dump.  This is a 3-acre area of rocks that were dumped there from the mine operation (a mine different from the Sterling Mine).  Hundreds of different minerals have been found in this area over the years.  This area is also called the fluorescent mineral capital of the world. We were given some small bags and allowed to collect rocks (this was included with the price of admission).   After we filled our bags we took them over to a little shed that had a UV light in it to see if we had some fluorescent rocks.   In the end we only collected a few different minerals. Some of my results are shown below.

I took these pictures with a Kodak EasyShare DX4900 Digital Camera.  It's a 4 Mega Pixel camera, but nothing special.

Here is a calcite (CaCO₃), mostly, nothing  here is 100% pure) rock that I picked up out of the mine dump:

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And here is the same specimen under SW UV:

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If you enlarge each image you can see what parts are glowing and what parts are not.  Even though the two photos are each at a slightly different angle, you can compare the rocks. I'm not an expert on mineral identification, but the orange glow is a really good indicator for calcite.  

If the last example showed UV for mineral identification, then this set is for identification of mixed fluorescents. I got this sample from the Sterling Hill mine.

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It's very difficult to see in normal light the different minerals in or on the rock, but under UV you can clearly see that there are several different kinds.  Actually, you can see more in the photo that you can with the naked eye. Just looking at the rock in UV, all I can see are the green dots.  The other colors don't come out until I do a long (8 seconds) exposure with the camera. The green dots are Willemite (Zn₂SiO₄),  the orange should be calcite and blue is probably Fluorite (CaF₂).  The fluorite and calcite are probably just dust contamination from being in the same bag as the other two rocks.  If I were to carefully wash the rocks and retake the pictures, I could tell if it's just dust or part of the sample.  However the mixed colors make a great picture so I kept this one.  I would also think that if you could compare the spectrum of the light returned  (under UV) to known samples, you might be able to tell the exact minerals that are there.  This might be standard fare for geologists, but it's all new to me.  

Here is the last set and this is sort of cheating because my daughter bought this Fluorite (CaF₂) rock in the gift shop.  We didn't know at the time that it would fluoresce so nicely.

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Again, the purple specks (Orange + Blue = Purple) are just more calcite dust contamination.   After handling the rocks, my hands also have glowing orange specs under the UV.  This is a good example of being able to identify traces of material using the UV light.  

So now the question is "How did I take these pictures?"   It's very easy; you just need four things, a SW UV light, which I mentioned above, a decent digital camera, an external lens adapter and a close-up lens.  The only special feature that the camera must have is a long manual exposure time (I have 0.7" to 16"). I haven't researched many digital cameras to know which ones have this feature, but my guess is that a lot do. The camera also has to have the ability to turn off auto-focus and auto-exposure.  The third and fourth pieces are easy to find on the web. I have a Sakar 37mm close-up lens kit and an adapter for my specific camera.   The close-up lenses look just like filters and they're additive, so the more you put on top of each other, the closer you can get (I'm sure it has limits) to the object.  If you already have a digital camera with the manual exposure settings, then the lenses, adapter and UV light might run about $100 -$200 plus shipping.  Try the manufacturer of your camera to help find the adapter and a close-up lens.  

I did my photography in a bathroom (notice the brown tile in the background) - it's dark when I close the door and it has a nice counter on which to work.  With my adapter and close-up lens attached, I put my rock on a (mostly) non-fluorescent stand about one inch high.   I then placed the camera the distance necessary to get the close-up lens to focus.  The images in normal light were taken with auto-exposure on.  For the fluorescent shots I turned off auto-focus, auto-exposure and the flash.  I then used the manual shutter speed to control the digital exposure of the image.  I then took a series of images starting at 0.7 sec and getting longer as needed.   Since this is digital , I could see how well the image came out and adjust the exposure accordingly.   I didn't write any camera settings down (gasp!) because recent makes of digital cameras save the settings with each picture. For instance, the calcite picture was taken at F/3.4, shutter speed 1 sec.  To actually take the picture I hold the UV light with one hand just above the camera and about 6 inches in front of the rock.  I then press the shutter release on the camera.  The important thing is to keep still while the exposure is taking place.  Since the camera and rock are both on the counter I don't have to worry about a shaky image. All I have to do is keep the light steady.  Mounting the light might be helpful too, but holding it by hand worked OK for me.  After the exposure is done, I turn off the UV light and review the picture.  In an hour, I had taken the pictures, found the right exposure times, downloaded them to my computer and made corrections for several rocks.  This is a far faster method than film-based pictures could be. 

I'm probably not the first person to take these kinds of photos with a digital camera, but from a look at the web it does seem that a lot of people still use film.  To me it seems like I got good results.  The colors and results match the Sterling Hill web site museum pictures.  If anyone needs any more information you can write me at president@njsas.org .