In the ink: why are blue highlighter markers less bright than others?

Why do blue highlighter markers never seem to have the high luminosity of pink, yellow, orange and light green highlighters?

Guy Cox, Sydney, New South Wales, Australia

There are two reasons for this. One is that our eyes are much less sensitive to blue light – peak sensitivity is in the yellow-green region of the spectrum, so if all markers were of equal intensity, the yellow and green ones would always seem brighter to us. Sensitivity falls in the red region, too, so red markers can never match the brightness of yellows and greens either.

The second reason is inherent in the way these markers work. They seem so bright because they contain fluorescent dyes that absorb short-wavelength light and re-emit it at their specific colour. Blue is at the short end of the visible spectrum, so a blue fluorescent pigment can only be excited by ultraviolet light, which is usually in short supply indoors.

Yellow-green dyes can be excited by blue light and UV, so they are excited more efficiently. Red-emitting dyes can be excited by green light as well and this helps to compensate for the eye’s lower sensitivity to their colour.

Eric Kvaalen, Les Essarts-le-Roi, France

Our eyes are more sensitive to light in the middle of the spectrum, which means orange, yellow and green. For longer wavelengths (red) or shorter wavelengths (blue), the sensitivity drops off, falling to zero for infrared and ultraviolet.

Pink is a mixture of red and white, which makes it more luminous than just red. One can also make a mixture of blue and white, which will also be fairly luminous. Some languages, such as Russian and Hebrew, have a different word for this blue/white mixture, just as we have different words for pink and red.

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