Light amendments

I’m writing in reply to Jon Hemmings on the subject of headlights (22/29 December 2021). I wish to correct some of his misunderstandings. I work in light metrology, photometry and the testing of vehicle lighting.

In Europe, car headlights must conform to UNECE regulations. These define limits to the maximum illuminance (lux level), or brightness. So it’s not possible for a vehicle to be type-approved unless the shape of its headlight beam and its low-beam cutoff comply with the maximum values. These limits apply to any light tech, be it incandescent, high-intensity gas discharge (xenon) or LED.

The colour temperature of a light is the shade of white light emitted. In daylight levels of illumination, we perceive lower-temperature green/ yellow-tinted light as brightest. This is known as the photopic response of the eye. However, at lower levels of illumination, the eye transitions towards a scotopic response, and in these conditions we favour light with a higher colour temperature, which means more blue-tinted white light is more effective at illuminating the road and reducing driver fatigue. So there’s no such thing as “too white”.

Regardless of all that, though, if the light beam isn’t properly aimed, oncoming drivers will be dazzled. Robert Yeo

Director, Pro-lite Technology