1. Why Highlighter Glow Disappears When You Photocopy a Page
When you use a color copier, the bright glow of a highlighter often does not appear on the copied page. According to physics explanations from an American university, the copied lines do not contain the chemicals that create fluorescence. Because these chemicals are missing, the special brightness cannot be reproduced on the copy [3].
Copiers work by mimicking the appearance of colors. They print ink that looks like the original color. However, the brightness of a highlighter comes from something other than just the color itself. If the source of the brightness is different from the color pigment, it makes sense that it disappears when copied. That "different source" is the special material inside the highlighter ink.
2. How Fluorescent Ink Turns Invisible Ultraviolet Light into Visible Light
Highlighter ink contains fluorescent substances. These materials change ultraviolet light, which we cannot see, into visible light [1]. Pilot, a major pen manufacturer, explains that when light containing ultraviolet rays hits the ink, the substance absorbs the energy from the ultraviolet rays. Then, it releases that energy as visible light [2].
Let us look closer at how this works. Inside the material, electrons absorb the energy from the ultraviolet light. This raises the electrons to a higher energy state. When they return to their original state, they release the extra energy as light. During this process, some energy is lost as heat. Because of this loss, the light that comes out has a longer wavelength and lower energy than the light that was absorbed [5]. This is how invisible ultraviolet light becomes visible light.
It is important to note that highlighters do not glow by themselves. They do not produce their own light. Pilot explains that they only appear to glow in environments where light containing ultraviolet rays is present [2]. This is different from bioluminescence, such as the light produced by squid, which comes from chemical reactions inside their bodies.
3. How Highlighter Ink Adds Extra Light to the Light It Reflects
Normal colors are seen because they reflect some of the light that hits them. With fluorescent colors, something else happens. In addition to the reflected light, the visible light created by fluorescence is added to the reflection. Physics explanations state that under light sources like sunlight, which contain both visible light and ultraviolet rays, fluorescent colors become physically brighter than normal colors [3].
The Institute for Future Engineering explains that the light from a highlighter line is the total of three things: the light emitted by the fluorescent dye, the light reflected by the pen’s pigment, and the light reflected from the surface. In the visible light range, there is more light than if there were only reflection, which makes the color look vivid [4].
This also explains why the brightness disappears on copies. The copied lines do not have fluorescent substances, so the part of the light that is "added" by fluorescence is missing.
4. Why Some Highlighter Colors Glow Brighter Than Others
Not all highlighter colors glow with the same intensity. Mitsubishi Pencil Company explains that the fluorescent substances themselves produce only yellow and pink colors. Other colors are created by combining inks, so the degree to which they appear to glow differs by color [1]. Pilot states that yellow and pink are especially easy to see as glowing, and orange also glows well [2].
According to chemistry explanation sites, examples include compounds used for these colors. For example, pyrene-based compounds are used for yellow, rhodamine-based compounds for pink, and coumarin-based compounds for orange. However, this is just one example. The ingredients vary depending on the manufacturer and the specific product [5].
Have you noticed that text under a highlighter line is still visible? Pilot explains that this is because the amount of the color base is less than in a normal pen [2].
5. How Fluorescent Brightening Agents Make White Clothes Look Whiter
The same mechanism used in highlighters is used in laundry. Kao, a Japanese company, explains that fluorescent brightening agents are used in the manufacturing of white clothing. These agents absorb invisible ultraviolet light and change it into bluish-white light. This increases the perceived whiteness of the fabric. Since washing can remove these agents, they are also added to laundry detergents [6].
This is the same "absorb ultraviolet, change to visible light" mechanism found in yellow and pink highlighters. The color of the emitted light is different, but it is not a different phenomenon [2][6].
Kao also notes that fluorescent brightening agents are not used in natural-colored or pastel-colored clothing. Washing these items with detergents containing fluorescent agents can change their color tone [6].
6. How the Switzer Brothers Made Fluorescent Paint in the 1930s
In the 1930s in America, the Switzer brothers, Robert and Joseph, are said to have created fluorescent paint by mixing fluorescent substances with shellac. They conducted many experiments in their home bathtub and demonstrated their invention at a magician’s convention in 1934. Robert obtained a patent in 1936 [7].
The brothers founded a company in 1946 and changed its name to "Day-Glo" in 1969. They are also known for producing bright fabrics used for signaling during World War II [7].
Regarding highlighters specifically, the German company Stabilo introduced the "Boss" in 1971 as the world’s first highlighter. However, this claim of being the "world’s first" comes from the company’s own introduction and has not been verified by other sources [8].
7. An Experiment: Comparing Highlighter Colors in Different Lights
You can try an experiment to see how light affects these colors. Draw a line on the edge of a piece of paper with a highlighter. Place it next to a white shirt sleeve. Compare how they look near a window during the day versus in a dim corner of a room. Pilot explains that fluorescence is visible under light that contains ultraviolet rays [2]. From indoors, compare how the colors look near a window and in a dim corner of the room.
Results will vary based on the weather, the time of day, and the room’s lighting. Do not draw a conclusion after just one look; try observing them multiple times.
SAFETY NOTE: Never look directly at the sun. Do not use ultraviolet light lamps for this experiment. Always follow these safety rules.