Fluorescence in Highlighter Pigments

It took me a while to publish these new photographs thanks to work and all those things associated with bad publications. Luckily, my homemade set-ups never stopped during these times and I now managed some time to whip up a quick but interesting post. 

An acrylic carrom striker puck imbued with fluorescent material glows green when exposed to near ultraviolet light. The light source used for this photo was from a commecially inexpensive near-violet LED.

About a month ago, I received a complimentary 5-colour-in-1 highlighter as a door gift from an exhibition. Since I only use yellow and blue for all my highlighting jobs, I decided to use the pigments contained within these highlighters for the so-called UV photography. There are a few types of UV photography techniques but this article in particular focuses on a phenomena called "fluorescence". 

Fluorescence is a form of luminescence. An object is fluorescent if it emits light after absorption of light in another wavelength. Typically, light of shorter wavelength will be absorbed by a fluorescent material and re-emitted as light in longer wavelength. Fluorescent is familiar to us. Most people know shining ultraviolet (shorter wavelength) from commonly available "black-light" on bank notes will reveal strips of glowing colours typically red or orange (longer wavelength). These strips are essentially fluorescent paints that was printed on bank-notes for the purpose counterfeit identification.

Now as far as I am familiar with highlighters, I know they contain fluorescent pigments to exhibit bright colours. Pyranine is a known fluorescent dye that was added into yellow highlighters but I was curious to know what pigment is responsible for other colours or if they could fluoresce at all. So I disassembled the new 5-in-1 highlighter, removed the felt tip and washed all the spongy core with 20 ml of deionized water. It appears water is an excellent solvent for all the colours and I kept them separated with a glass vial for a while. 

Lately I came across a very cheap source of ultraviolet light. Apparently near UV light sources was already introduced commercially in the form of inexpensive light emitting diodes. These LED typically made from mixing different portions of Gallium Nitride (GaN) and Indium Nitride (InN) are able to release continuous near-violet UV light peaked at 390 nm. I bought my light source with MYR 5 from a night market and was able to demonstrate the following:

One colour did not glow under UV. Does it require light of a shorter wavelength to exhibit fluorescence? 

The picture above shows 5 different highlighter colours mentioned earlier. They were diluted to approximately the same concentration to about 3 ml each colour and kept in glass sample bottles (top). When shined with near violet light, only 4 colours appears to fluoresce (bottom). Perhaps blue highlighter needed another light source with wavelength shorter than 390 nm to cause it fluoresce? Needless to say, the photograph describes the relative intensity of fluorescence emission from the other 4 dyes pretty well. Based on visual observation, the green dye could actually share same fluorescent material with yellow. But we will never truly know unless we get a spectroscopic data from these samples.