A new optical design for motor vehicle side-view mirrors may end the famous “blind spot” in traffic. And all without distorting the perceived distance of cars approaching from behind, making it easier to judge distance and speed. The idea was proposed by researchers Hocheol Lee and Dohyun Kim at Hanbat National University in Korea and Sung Yi of Oregon’s Portland State University.
Credit: Optics Letters
Modern cars use two kinds of mirrors, one for the driver’s side and one for the passenger side. The driver’s side mirror is flat so things seen in it are not reduced in size optically or distorted, letting the driver correctly gauge an approaching-from-behind vehicle’s separation distance and speed.
The problem is, that a flat mirror’s optics also create a blind spot, an area of limited vision around a vehicle that often leads to collisions during merges, lane changes, or turns.
Passenger side mirrors have a spherical convex shape. The small curvature radius widens the field of view, but it also makes any object observed in it look smaller in size and farther away than it actually is. Hence the safety warning, “Objects in mirror are closer than they appear.”
In Europe, both driver and passenger side mirrors are aspheric (the curvature bulges more to one side than the other, creating two zones on the same mirror). The inner zone, nearest the door, has a nearly perfect spherical shape. The outer zone, farthest from the door, becomes less and less curved toward the edges. The outer zone of this aspheric design also produces a similar distance and size distortion seen in spherical convex designs.
Simpler and Better
Some automotive manufacturers install a separate, small wide-angle mirror in the upper corner of side mirrors to try and fix the problem. It’s a slightly domed square that gives a wide-angle view like a camera’s fisheye lens. But many drivers find this system distracting as well as an unneeded added expense.
The proposed simpler design for a mirror that would have a wide field of view, be free of blind spots, and allow for accurately scaled images on both sides of a vehicle was to use progressive additive optics. This is a technology regularly used in no-line multifocal eyeglasses that both correct myopia (nearsightedness) and presbyopia (reduced focusing ability).
“Like multifocal glasses that give the wearer a range of focusing abilities from near to far and everything in between, our progressive mirror consists of three resolution zones: one for distance vision, one for close-up viewing and a middle zone making the transition between the two,” says Hocheol Lee. “However, unlike glasses where the range of focus is vertically stacked [from distance viewing on top to close-up viewing on bottom], our mirror surface is horizontally progressive.”
A driver’s side mirror manufactured with the team’s new design could have a curvature where the inner zone is for distance viewing and the outer zone is for near-field viewing to compensate for what otherwise would be blind spots.
“The image of a vehicle approaching from behind would only be reduced in the progressive zone in the center, while the image sizes in the inner and outer zones are not changed,”
Lee explains.
Twice the Field of View
Horizontal progressive mirrors do have some problems with binocular disparity (the slight difference between the viewpoints of a person’s two eyes) and astigmatism (blurring of a viewed image due to the difference between the focusing power in the horizontal and vertical directions). But the researchers feel these errors are a fair trade-off to get a mirror with an expanded field of view, more reliable depth perception, and no blind spot.
The researchers used a conventional glass molding process to fabricate the prototype horizontal progressive mirror and were able to produce a mirror with more than double the field of view of a traditional flat mirror. Since mirror specifications are controlled by national automobile regulations, the new design would need to be approved for use in the United States before appearing on cars there.
The photo above (a) shows a standard aspheric mirror, a blue line indicates the boundary between the two zones. You can see the distortion between the two zones. In (b), the progressive solution shows the improved transition between zones that eliminate blind spots but gives an undistorted view of objects at a distance. (c) is the regular flat side view mirror compared with (d) the wider field of view of the progressive mirror.
Original study: “Horizontally progressive mirror for blind spot detection in automobiles” H. Lee, D. Kim, S. Yi, _Optics Letters_, Vol. 38, Issue 3, pp. 317 – 319 (2013)
