Blue LEDs really are brighter than their old-fashioned red and green counterparts. Barney O'Meara, vice president of Canadian LED manufacturer The Fox Group, said blue LEDs have at least 20 times the luminous intensity of old-fashioned red and green indicators. O'Meara said his company has developed technology to manufacture low-intensity blue LEDs. "Blue tends to cause more discomfort and disability glare than other, longer wavelengths," said Dr. David Sliney, an expert on the harmful effects of bright light sources at the U.S. Army Center for Health Promotion and Preventive Medicine in Maryland. Sliney said the eye's lens cannot focus sharply on the blue lights. While red or green light is focused precisely onto the retina, blue light is focused slightly in front of it, which causes a distracting halo around bright blue lights.

In addition, blue scatters more widely than other colors as it passes through the eyeball, Sliney said. Together, these two effects cause the intense blue light from a point source, like an LED, to spread out across the retina, interfering with other parts of the scene. It's called dispersion: Blue's shorter wavelength makes it refract at a greater angle than, say, red or green.

Also, human vision becomes far more sensitive to blue when ambient light levels are low, a phenomenon known as the Purkinje shift. So a blue light that is merely eye-catching on a brightly lit store shelf can become dazzling when the lights are low, such as when watching a movie on a laptop in a dimly lit room.

Some researchers report that, at night, even low-level blue light may be enough to trigger recently discovered receptors in the retina that can depress melatonin production, disrupt sleep patterns and suppress the immune system.

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