On moving vehicles, it is life-and-death important to have the right kind of mirror made from the right materials with the right curvature. Understanding the effects of size and curvature is crucial to getting the right mirror for the right application.
A Mirror’s Design Should Match Its Needs
A mirror’s size and curvature depends on the requirements of its function. Mirrors on ATM machines, for example, are designed to let customers watch behind them for nosy onlookers peering over their shoulder. They don’t need a big image reproduction so the mirror only needs to be 25 x 50 mm (1″x2″) to function.
On the other hand, the driver of an 18-wheeler needs to be able to see down the entire length of his truck, whether he’s looking for passing vehicles on the highway at 110 km/h (70 mph) or backing up into a tight loading zone. 18-wheelers in the USA have 53-foot trailers with another dozen or so feet past the hinge point and driver’s cab. Only a mirror of 250 – 300 mm (10 or 12 inches) diameter will give the driver sufficient image size and field of view he requires.
The Bigger the Image Size, the Smaller the Field of View: The Great Curvature Trade-off
Mirrors can be flat, or they can have a rate of curvature all the way up to an entire hemisphere. Like size, the curvature of a mirror is directly determined by its application. When it comes to curvature, there is a direct trade-off between image size and field of view.
When mirrors are curved — like on circular safety mirrors that help people see around corners in hospitals, or on security mirrors that let clerks watch shoppers in stores — the field of view widens, but the images are smaller. When a transportation mirror is flatter — like on that 18-wheeler — images are big, but the field of view is narrow.
If you ever see a truck that has a small, curved, circular mirror stuck onto the surface of a large, flat, square mirror, the driver is trying to achieve both a large image size and a wide field of view at the same time.
Bus drivers don’t need to see the sky and the road at the same time when they glance into their side view mirrors. But they do need to see the side of their bus and perhaps into the lane next to them. Buses therefore have large, round mirrors with slightly more curvature than in trucks.
Unique Mirrors for Special Jobs
In Europe, where streets are often narrow and curved, they mount huge mirrors on roadside poles so drivers can see what’s coming around the bend. Drivers moving at a high rate of speed need to be able to see other drivers who are also moving quickly while both are fairly far away from the mirror. These mirrors need to be very large and almost completely flat to get an image that is useful.
Mirrors on off-roading vehicles such mining trucks or military vehicles are driven by specification, meaning that they are designed around their unusual requirements. These kinds of mirrors get banged and battered on a regular basis. Therefore, sturdy polycarbonate is often chosen over acrylic for these mirrors because of the sheer amount of impact they will sustain on a regular basis.
Shape: Why Certain Mirrors are Round, Rectangular or Square
Whether it’s the round mirrors on buses, the rectangular mirrors on forklifts or the square mirrors on trucks, a mirror’s shape, well, takes its shape for two reasons. Part of it is functionality and the achievement of appropriate field of view, but it also has to do with tradition, simple aesthetics and what people are used to seeing. There is a functional aspect to shape, but shape is also driven by convention.
Getting the right mirror for the right job is crucial, so the size, shape and curvature of a mirror are all chosen with regard to how the mirror will be used. First determine your application and then design your mirror, not the other way around. In this way you will be sure to have the right size, shape and curvature mirror for your needs.