This post was written by Keith Dawson for UBM Tech’s community Web site All LED Lighting, sponsored by Philips Lumileds. It is archived here for informational purposes only because the All LED Lighting site may go dark at any time. This material is Copyright 2013-2015 by UBM Americas.

2014-07-18

Modeling Discomfort Glare from LED Sources

Everyone talks about glare from LED lights, and now researchers in China and the Netherlands have done something about it. In fact glare in general has been well studied, and characterized, for incandescent lighting. But there are reasons to believe that LEDs produce glare that human vision reacts to differently. The light source is smaller, of course, and depending on the luminaire design, that could concentrate the glare. The surplus blue in the spectrum of phosphor-converted LED light may also strike the human visual system as brighter than light sources with less blue.

Researchers led by Yandan Lin in China wanted to put numbers on the discomfort glare resulting from LEDs, in particular from LED street lights.

Let's note the distinction between glare that is merely uncomfortable, and that which is disabling. The latter causes physiological changes great enough to reduce vision temporarily and may damage the eye. The former may cause blinking, eye watering, and the urge to turn aside -- which are all bad enough if you are, say, driving. Discomfort glare can build up and result in mental fatigue, headache, and tension, reducing attention and safety.

If you think about it a minute, it's not hard to figure out what factors contribute to the uncomfortable feeling you get when a too-bright light is shining too close to your line of sight. It's going to be worse if the light source is larger in your field of view, is intrinsically brighter, contrasts more strongly with the background brightness, and is closer in angle to the direction you are looking.

Those factors are accounted for in the general equation at right, in which the exponents a, b, c, and d are to be determined from laboratory measurements. As mentioned, this has been done (beginning with work by deBoer in 1967) for incandescent lighting, and the results have guided light placement and characterization for years. What these researchers have done is to determine appropriate values of a, b, c, and d for LED illumination, then validate their model with further experiments both in the laboratory and in the street.

Here is the bottom line after a good deal of experimentation and calculation. The researchers say that with their model they can predict with 87% accuracy what level of discomfort a person will report when subjected to LED light glare of known characteristics.

This research was funded by Philips Asia. Two of the authors are affiliated with that company (a division of which sponsors this site). The other three authors are from the Institute for Electric Light Sources at Fudan University in Shanghai, China.

— Keith Dawson Circle me on Google+ Follow me on Twitter Visit my LinkedIn page, Editor-in-Chief, All LED Lighting