Washington: A new optical prescription for automobile side-view mirrors may eliminate the dreaded “blind spot” in traffic without distorting the perceived distance of cars approaching from behind.
Objects viewed in a mirror using the new design appear larger than in traditional side-view mirrors, so it’s easier to judge their following distance and speed, researchers explained.
Today’s motor vehicles in the United States use two different types of mirrors for the driver and passenger sides.
The driver’s side mirror is flat so that objects viewed in it are undistorted and not optically reduced in size, allowing the operator to accurately judge an approaching-from-behind vehicle’s separation distance and speed.
Unfortunately, the optics of a flat mirror also create a blind spot, an area of limited vision around a vehicle that often leads to collisions during merges, lane changes, or turns.
The passenger side mirror, on the other hand, possesses a spherical convex shape. While the small radius of curvature widens the field of view, it also causes any object seen in it to look smaller in size and farther away than it actually is.
A simpler design for a mirror that would be free of blind spots, have a wide field of view, and produce images that are accurately scaled to the true size of an approaching object—and work for both sides of a vehicle—has been proposed by researchers Hocheol Lee and Dohyun Kim at Hanbat National University in Korea and Sung Yi at Portland State University in Oregon.
Their solution was to turn to a progressive additive optics technology commonly used in “no-line multifocal” eyeglasses that simultaneously corrects 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,” said 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,” the researcher explained.
Lee stated that a driver’s side mirror manufactured with his team’s new design would feature 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,” Lee said, “while the image sizes in the inner and outer zones are not changed.”
The horizontal progressive mirror, Lee said, does 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). These minor errors are a positive trade off, the researchers feel, to gain a mirror with a greatly expanded field of view, more reliable depth perception, and no blind spot.
To prove the merits of their design, the researchers used a conventional glass molding process to manufacture a prototype horizontal progressive mirror. They were able to produce a mirror with more than double the field of view of a traditional flat mirror.
The researchers claim that the manufacturing cost of their proposed mirror design would be cheaper than the mirror design with the added small wide-angle viewing section.
The new design was described in a paper published in the Optical Society’s (OSA) journal Optics Letters.
First Published: Tuesday, January 29, 2013, 19:08