Most of us are still using the reducing prismatic rear-view mirror designs in our cars that we move to a different position at night. This adjustment greatly reduces the glare from the headlights of automobiles behind us. With the advent of xenon headlights, the illumination increased up to threefold making the glare even worse. Many cars now incorporate leveling systems to help reduce this problem but night glare is still a serious safety issue.
At night on a dark highway the eyes pupil is dilated to it’s maximum size allowing more light to enter and more optical aberrations (curvature irregularities and imperfections) to degrade your vision when a car with bright headlights pulls up behind you. The recovery time after the night rod cells in the retina have had their pigment bleached out by all of the light can be daunting when driving 75 mph on a pitch black road. Also, with aging, the optics of the eye have become altered and degraded.
In the 16th century the Venetians invented the method of making mirrors out of plate glass (glass that has parallel sides and does not change the size of what you are viewing). They achieved this by applying a thin film of mercury to the back of the glass. Today, automotive mirrors are clear glass with non toxic reflective coatings on the back.
Today, reducing prismatic rear-view mirror designs utilize a prism shaped piece of glass in front ot the mirror. This means the glass is thicker at the top than the bottom. In daytime alignment almost all of the light goes through the glass and reflects off the mirror giving you a clear view of the area behind you. This is a vast improvement from the early years of automobiles when hand held mirrors were carried at times. At night fall, the mirror is flipped so it is reflecting away from the eye allowing the lower level of reflections that arise from the back of a clear pane of glass to show up with greatly reduced intensity. The fact that these reflections exist in the first place is why we highly recommend anti-reflective coatings on most eye glasses today.
Last year I discovered a flaw in the current technology we use. In not anticipating the stoppage of traffic I was caught in the middle of a major intersection and could not tell for sure if there was a car behind me (more on that in a minute). After getting barked at by a police officer who had obviously had something else bothering him beside me blocking his progress I was given a choice. I could leave or I could go park on the shoulder and he would call someone from that precinct to come and haul me off. I politely declined his offer to wait. Several months later I was backing up in a parking lot and tapped another car. So lightly in fact that I didn’t feel it, but my daughter insisted I had. We drove around the block and came back. As it turned out, somehow I had only bumped the tire of the car, no harm done.
Pondering these two incidents I had an epiphany; glare reducing rear mirrors used improperly can be hazardous. In fact, they may do a lot to make us more comfortable but I would like to find long term good studies on how they have effected accidents rates.
The issue in both instances involved a stopped vehicle with night mirrors and cars pulled up close enough behind that their headlights were not visible. I have tested this at night and it is a frequent occurrence. The dimness of the refection from the glass plate which often is combined with a dirty rear window from winter sanded or muddy roads makes it extremely difficult to see the vehicle without the lights.In essence, night mirrors may actually create a blind spot under the right conditions.
The solution I use is to automatically flip the mirrors back to day when I park, and in high traffic, dark areas with slower traffic flow I may do the same. There is a trade off between glare recovery and overall reduction of vision that needs to be balanced.
The advent of electrochromic windows that automatically adjust is promising, and the new rear view camera systems will eliminate all of these pitfalls in the future. Until then, I highly recommend you flip your night mirror back to day when you stop,so you can see if you pull out again later in the night.
The best thing you can do for safe night driving is keeping your eye glasses or contact lenses prescription up to date with quality anti-reflection coatings and keep the car windows clean. See your eye doctor / optometrist for regular preventative eye care and drive until you are 100!