In the automotive lighting research field, there has been much study over the decades of the effects of disability and discomfort glare. Including defining the actual visible space where this light meets the eyes. There are many variables that are taken into account such as driver eye height, vehicle height, etc. The most widely accepted angle and its geometry is *near* the area of 0.5 degrees UP, 3.5 degrees LEFT from a headlamps center point visualized with intersecting vertical and horizontal lines, as seen by the observer behind the headlamps shining. And because vehicles of course are always moving, the eye location moves during that trajectory. I have mapped out this glare window based on field measurements done by other researchers (Figure 1a).
At 50 meters (164 ft) which is the distance to which glare is regularly measured at, the glare window is *roughly* a size of 3 feet high by 10 feet long.
At 7.6 meters (25 ft) which is the common distance for headlamp aiming, the glare window is *roughly* a size of 5 inches by 18 inches.
At 3 meters (10 ft) which is probably how far most people would park away from a garage wall to check out their new lights, the glare window is *roughly* a size of 2 inches by 7 inches.
It's quite obvious that the closer the headlamp is to a screen or wall the smaller the window. You will not be able to visually assess the glare window at distances less than 10 feet because the beam has not fully formed. So anyone that thinks they can tell if a headlamp is glaring too much or glaring very little is mistaken. Figure 1b. illustrates this point because most people who would be facing toward the vehicle and with their backs to the wall would be positioned with their heads directly into the region that is NOT glare. Rather instead the glare area is much smaller and closer to the cutoff line.
The next problem is our eyes. Despite them being one of our most important senses it's also the worst reliably. The federal standard for glare has TWO luminous intensity test points. A lower and upper. The lower point has a maximum of 1000 candela. The upper point has a maximum of 700 candela. Would you be able to tell with your eyes if glare exceeded the maximum intensities, for example, by 25%? Of course not. The only way to properly know that glare is under the legal limit is by measuring with a high accuracy light meter AND at the correct location of the beam.
I didn't want to make this post super long but hopefully I didn't over simplify this topic because glare is one of the most important safety aspects of headlighting. And there are a lot more technical aspects behind it. Don't ever assume that glare from certain lighting upgrades are "ok" or "good enough" simply by only taking a glance at the beam shining on a wall or just standing in front of it.
