SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 112

but if the tint does not change—as in ordinary sunglasses—then the user
has difficulty indoors. Photochromic lenses avoids the need for two pair
of prescription eyeglasses, one for inside and one for outside, because a
photochromic lens is dark only outside.
But photochromic lenses are not perfect. Since car windshields
block most ultraviolet radiation, a lot of photochromic eyeglasses will
remain clear when the wearer is inside a car, even though sunglass-like
darkening would be preferred while driving during the day. Even out-
side, the angle of the sun's ray can sometimes affect the extent of dark-
ening, adding an undesirable variability.
To correct these problems, researcher Chunye Xu and her col-
leagues at the University of Washington in Seattle are developing adjust-
able lenses. These lenses are not photochromic—they do not respond
to lighting conditions—but instead are electrochromic, which means
the lenses change color in response to varying electrical conditions. An
early version of these glasses uses an organic compound that changes
when a small voltage is applied. Because of the great variety of organic
compounds, the researchers have been able to find compounds that not
only turn dark but also different colors, such as blue, red, and yellow.
A battery supplies the voltage, and the wearer can adjust the shade by
turning a dial.
AdAPTIvEAndInTEllIgEnTSySTEMS
Using smart materials in objects such as eyeglasses means that the func-
tion can change depending on environmental conditions or the needs of
the user. Smart materials can also be critical components of systems—
assemblies of interacting parts—that adapt to varying conditions. Such
systems are known as adaptive or intelligent systems.
Adaptive systems contain three types of components. There must
be a sensor, a component that detects or measures the varying con-
dition. If the conditions warrant a change in the system's function, a
component called an actuator will perform the adjusting. An actuator is
often a motor, such as the motor in an electric drill, which causes or cre-
ates motion. The component called the controller determines when and
how the system's function should change. This component is usually a
small computer, which receives input from the sensor and switches on
the actuator, if needed.