Terahertz imaging, which is already familiar from airport security checkpoints, has a number of other promising applications — from explosives detection to collision avoidance in cars. Like sonar or radar, terahertz imaging produces an image by comparing measurements across an array of sensors. Those arrays have to be very dense, since the distance between sensors is proportional to wavelength.
In a digital camera, the lens focuses the incoming light so that light reflected by a small patch of the visual scene strikes a correspondingly small patch of the sensor array. In lower-frequency imaging systems, by contrast, an incoming wave — whether electromagnetic or, in the case of sonar, acoustic — strikes all of the sensors in the array. The system determines the origin and intensity of the wave by comparing its phase — the alignment of its troughs and crests — when it arrives at each of the sensors.
As long as the distance between sensors is no more than half the wavelength of the incoming wave, that calculation is fairly straightforward, a matter of inverting the sensors' measurements. But if the sensors are spaced farther than half a wavelength apart, the inversion will yield more than one possible solution. Those solutions will be spaced at regular angles around the sensor array, a phenomenon known as "spatial aliasing."