Lights have gotten smaller over time, and “any given intensity appears brighter if it’s emitted by a smaller apparent surface versus a larger one,” said Daniel Stern, chief editor of Driving Vision News, a technical journal that covers the automotive lighting industry.

“Tall pickups and S.U.V.s and short, small cars are simultaneously popular,” he added. “The eyes in the low car are going to get zapped hard by the lamps mounted up high on the S.U.V. or truck every time.” (Almost half of the 280 million registered passenger vehicles in the United States are S.U.V.s or pickup trucks.)

LED and high-intensity discharge headlights can appear more blue in their output spectrum than halogens, and they often provoke “significantly stronger discomfort reactions” than warm white or yellowish lights, Mr. Stern said.

“Blue light is difficult for the human visual system to process because blue wavelengths tend to focus just ahead of the retina rather than on it,” he said.

Mark Baker, the founder of an activist group called Softlights, said that, while the blue LEDs might be among the best for nighttime driving, that did not mean they were good for everyone.

“It’s true that blue will allow you to illuminate farther,” he said. “If you choose to say, ‘I’m going to make the biggest, baddest light I can,’ you’re not paying attention to the receptors of another driver coming at you.”

“Brightness” is not a term generally recognized by scientists and researchers, who refer instead to lumens, or the output of a light. Halogen lights put out 1,000 to 1,500 lumens, while high-intensity discharge lights and LEDs can measure 3,000 to 4,000 lumens.