Fluorescent Highlighters

Fluorescent highlighters are extra bright in sunlight, but not so different than other highlighters under a desk lamp. This is because two bundles of light energy is being sent to your eyes and both are for the same color. This makes the color more intense–louder–brighter–super-bright.

Let’s consider a yellow fluorescent highlighter. One source of light energy has to do with the yellow pigment in the ink. Like all yellow highlighters, when any kind of light strikes this ink the ink absorbs all the visible colors in the light and reflects the yellow colors. So you see the yellow mark.

The second source of yellow light energy is from the yellow phosphors in the ink. A phosphor is a chemical that absorbs ultraviolet radiation, and then emits visible light. The color of the visible light depends on the phosphor used. In a yellow fluorescent highlighter, the phosphor is one that  emits yellow light. Add this yellow light with the yellow light from the pigments and the mark made by a fluorescent highlighter appear super-yellow.

A mark on white paper with a yellow fluorescent highlighter appears super-yellow ONLY if the light source contain ultraviolet radiation.

1. Incandescent light bulbs basically give off light in the red end of visible light, while ultraviolet radiation is near the opposite or violet end of visible light. Even so, people that are allergic to UV of any amount cannot use incandescent lighting. Is the UV from an incandescent bulb enough to make marks from a fluorescent highlighter look super-bright?
2. According to Westinghouse, fluorescent light bulbs give off a minute amount of UV radiation. Another source indicated that the long bulbs in stores do give off more UV than incandescent bulbs. Is it enough to make the ink from a fluorescent highlighter look super-bright?

Discover for Yourself

1. Use a yellow fluorescent marker to color an area about 1 inch (2.54 cm) square on a white index card.
2. Repeat step 1 using a regular yellow highlighter (not fluorescent) to make a second colored square on the index card.
3. Allow the ink on the card to dry. This should take less than 1 minute.
4. Hold the index card, ink side up, under the light of an incandescent light bulb. Unless you have a room with no windows, perform this step at night so that there is no sunlight or light from fluorescent bulbs striking the card.  Observe any difference in the brightness of the two ink marks on the card.
5. Repeat step 4 using a fluorescent bulb. If you do not have a fluorescent bulb, perform the investigation at a store where these bulbs are used. Just make sure you are away from any doors or windows that let in sunlight.
6. Repeat step 4 using sunlight. Do this by standing outdoors so you face away from the Sun. Hold the card in front of you in the SHADOW OF YOUR BODY.

So, what are your results?