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What makes a "fluorescent" highlighter marker so bright?
Category: Chemistry
Published: May 15, 2015
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Fluorescent highlighter markers are so bright because they are literally fluorescent. When used to describe highlighters, the word "fluorescent" is not a vague term that means "extra bright". Rather, this word is an exact, scientific term indicating that the highlighter ink exhibits fluorescence. Fluorescence is the phenomenon where a material absorbs light of a certain color and then emits light of a different color with a longer wavelength. The most striking type of fluorescence involves the absorption of ultraviolet rays (which humans can't see) and the subsequent emission of light in the visible spectrum (which humans can see). Because humans can't see the original ultraviolet light, a fluorescent object looks like it is glowing mysteriously on its own when it is illuminated only by ultraviolet rays in a dark room . For this reason, ultraviolet lights and fluorescent materials can add an intriguing look to darkened rooms at parties and events. Since highlighters contain fluorescent chemicals, the marks made by highlighters will seem to eerily glow on their own when placed in a dark room with an ultraviolet light (e.g. a "black light").
When a fluorescent object is illuminated by both visible light and ultraviolet light (such as when illuminated by sunlight), the object will still convert the ultraviolet light to visible light. The visible light created by the object's fluorescence gets added to the visible light reflected off the object. As a result, a human observes a fluorescent object that is under full illumination to be unusually bright instead of eerily glowing on its own. Note that this is a physical effect and not a psychological effect. A fluorescent object does not just seem to be brighter. A fluorescent object is physically brighter in the visible spectrum when under full illumination than other non-fluorescent, non-glowing objects.
For example, take a normal yellow marker and a yellow highlighter marker which contains a yellow fluorescent chemical mixed into the ink. Draw with both markers on normal white paper. When visible light and ultraviolet light shines on the paper, such as from the sun or from a normal light bulb, the fluorescent marker ink will always be brighter in the visible-light portion of the spectrum than the normal ink. Furthermore, the fluorescent ink is brighter in the visible spectrum than can be accounted for by the original visible light present. For this reason, fluorescent objects under full illumination appear unnaturally bright. The effect of highlighter ink appearing unnaturally bright under normal illumination and the effect of highlighter ink glowing eerily when illuminated by an ultraviolet light in a dark room are the exact same effect: fluorescence. Fluorescent chemicals are also sometimes added to paper, posterboard, paint, and clothing to make them appear unnaturally bright. Fluorescence in this context is often informally called "neon colors" even though fluorescence has nothing to do with the element neon. A shirt that is referred to as "neon green" should more accurately be described as "fluorescent green".
Note that the extra brightness of a fluorescent object is due to its conversion of ultraviolet light to visible light. As such, a fluorescent object will only appear unnaturally bright if ultraviolet light is present. If normal yellow ink and fluorescent yellow highlighter ink are both illuminated only by a yellow laser in a dark room, they will both be equally bright. Also note that the extra brightness of highlighter ink is due to the fluorescent chemicals that are mixed in. This extra brightness will not be reproduced by systems that have no fluorescent chemicals. For example, a photocopy machine does not contain fluorescent chemicals. This means that when you make a color photocopy of a document containing highlighter marks, the marks in the duplicate document will not contain fluorescent chemicals. As such, the highlighter marks on the duplicate document will not look unnaturally bright. Making a color photocopy of a document containing highlighting marks is an easy and striking way for you to see the effect that the fluorescent chemical has on the ink's appearance.
On the molecular scale, fluorescence is caused by an electron making several downward transitions after making a single upward transition. When an electron absorbs a bit of light, it transitions to a higher energy state inside the molecule. When an electron transitions down to a lower energy state, it must lose some energy and can do so by emitting a bit of light. The frequency, and therefore color, of the light that is absorbed or emitted by the electron is a function of how far the electron transitions along the energy scale. A large transition downward means that the electron must get rid of a lot energy. Thus if it emits light, the light must have high energy, which corresponds to high frequency (more towards the blue/violet/ultraviolet end of the spectrum). A small transition downward means that the electron only needs to get rid of a little bit of energy, so that the light it emits is low energy/low frequency (more towards the orange/red/infrared end of the spectrum).
For regular materials, an electron in a molecule absorbs a bit of the light shining on it, causing it to transition upwards. Then the electron transitions right back down to where it started, making just as big of a leap downwards on energy scale as its original upwards leap. As a result, the light it emits is the same color as the light that hits it. We refer to this effect as standard reflection. (Some of the incident colors can also be absorbed, so that the reflected colors equals the incident colors minus the absorbed colors.) For fluorescent materials, the electron absorbs a bit of high-energy light such as ultraviolet, and therefore it makes a large transition up the energy scale, but then it loses some of its energy to increasing the vibrations of the molecule before it has a chance to transition back down and emit light. As a result, when the electron finally does transition down and emit light, it has less energy to lose, it makes a smaller leap down, and it therefore emits lower-energy/lower-frequency light. In this way, electrons in fluorescent materials such as highlighter ink are able to transform high-energy bits of ultraviolet light into low-energy bits of visible light by converting some of the energy of the incident ultraviolet light into molecular vibrations, which ultimately becomes heat.