On Glare

Controlling dangerous glare is a job that falls on those downstream in the lighting business: luminaire designers and lighting designers.

I was coming home Friday on the ferry from Bridgeport to Port Jefferson. The week was finally coming to a close. I had just finished up some important meetings in Bridgeport, and I planned on getting some work done on the ferry. The ride is a little more than an hour -- enough time to write a report, fill out an expense report, or create some calculation tables in Excel.

(Source: Flickr.com, basykes, used under a Creative Commons license)

As I'm sure is true with everybody, my work loads are never evenly distributed. The last couple of weeks have been crunch time, with several deadlines looming more or less concurrently. I needed to take advantage of every opportunity to be productive to keep up. So I boarded the ferry, picked a table and seat by the window, and set up my laptop.

I always pick a window seat; it's a habit I've had since I was a kid. I love to see the scenery and what's going on outside. This time, it was a bad decision. I didn't give it any thought when selecting my seat. I just stayed on the side of the vessel where I had boarded. I should have thought a little.

It was 2:00 p.m., and the vessel was heading southwest, with the sun setting in the west on a clear day. It was a prime setup for solar glare. Sure enough, regardless of how I positioned myself and my computer at the table, I could not read my screen. In one direction, with the back of the computer facing the sun, the reflection of my shirt and surroundings washed out the screen, not to mention that I had the sun in my eyes. Facing the other direction, with the sun shining directly on my screen, was even worse. It was impossible to read anything. I fumbled, rotated, squinted, and tried every trick I could think of, but nothing helped.

I reluctantly conceded that I wasn't going to get any work done. I might as well just sit back and enjoy the ride, even if it kills me. Fortunately, it did give me inspiration for this article.

Glare defined

In the Society of Aerospace Engineers standard ARP6161, Flight Compartment Glare, glare is defined as "the sensation produced by brightnesses within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to reduce contrast and cause annoyance, discomfort, or loss in visual performance and visibility and could potentially be distracting to the observer."

Glare is a challenge when developing modern LED lighting systems. In the early days of white LEDs, the outputs were less than stellar. Glare wasn't a significant concern, even when you were looking directly into the LEDs. In fact, in those early days, salespeople would proudly demonstrate their LEDs by turning them on in direct view of customers to show how bright they were. Now, however, with high-flux devices and small emission surfaces, viewing the LED directly is intolerable. Consequently, controlling glare in LED fixtures has become an important consideration. It is particularly a concern in applications such as street lights, aircraft flight decks, or the engineer's compartment in trains, where visual distraction can impose a significant safety risk.

It is obviously important to prevent the direct view of LEDs, but it is also important to consider the possibility of specular reflections from the light source that could result in glare.

Unfortunately, glare can never be completely eliminated from these environments, because it depends on the sensitivity of the observer, the exact position of the observer's eyes, the location of people and reflective surfaces, the contribution of outside light sources, etc. However, it can be controlled to a large extent by controlling the luminaire's cutoff angle, mounting location, orientation, and optics.