Fun with mirrors

Once again something obvious that everybody has probably mused about briefly, but one man decided to study: wide angle mirrors with little distortion was patented.

R. Andrew Hicks (Optics Letters Vol 33 Number 15 page 1672, 1-Oct-2008) wrote up the requirements for calculating the curved surface needed to project a fan of light 45 degrees wide into a driver's eye without a great deal of distortion.

Hicks can customize his creations with exquisite precision. He achieves the desired effect first by characterizing the problem with sophisticated equations. He then programs a computer to spit out the coordinates for tens of thousands of points on the mirror's surface - each one tilted differently to reflect light in just the right way.

The data are then sent to B-Con Engineering, a Canadian firm that uses a milling machine to make the mirrors out of aluminum. The metal is ground and polished with a diamond bit until the smooth, curvy surfaces emerge - a process that takes up to a day for Hicks' unusual free-form designs, company president Brian Creber says.

Like Perline's bike mirror and the mirrors of ancient Turkey, Hicks' new automotive mirror is convex. This provides a wide, 45-degree field of view, instead of the narrow 15 or 20 degrees afforded by a typical flat driver-side mirror.

Objects in the prototype mirror also appear to be farther away; if more things are crammed into the picture, obviously, they have to appear smaller.

US law allows curved mirrors (with a warning) on the passenger's side but not the driver's side, so this will probably wind up being a popular add-on item. If I can figure some way of attaching it I'll invest in one. Or two. Though I don't think I can use his "vampire mirror."

One good reason people haven't worked much with this before is that shaping mirrors isn't trivial. You can get flat, parabolic, or spherical surfaces pretty easily. But other shapes need special grinding, as described above--and that is only really feasible with modern computer-controlled milling machines. Once you have the shape you can make a die or mold and make new ones much more easily, but the prototypes are very difficult. Notice that "up to a day" and imagine how much that costs.