category, which might eventually allow direct interaction with the display. For instance, a fog display using multiple projectors can render a 3D image in a volume of space, resulting in a static-volume volumetric display. A technique presented in 2006 does away with the display medium altogether, using a focused pulsed infrared laser (about 100 pulses per second; each lasting a nanosecond) to create balls of glowing plasma at the focal point in normal air. The focal point is directed by two moving mirrors and a sliding lens, allowing it to draw shapes in the air. Each pulse creates a popping sound, so the device crackles as it runs. Currently it can generate dots anywhere within a cubic metre. It is thought that the device could be scaled up to any size, allowing 3D images to be generated in the sky. Later modifications such as the use of an neon/argon/xenon/helium gas mix similar to a plasma globe and a rapid gas recycling system employing a hood and vacuum pumps could allow this technology to achieve two-colour (R/W) and possibly RGB imagery by changing the pulse width and intensity of each pulse to tune the emission spectra of the luminous plasma body. In 2017, a new display known as the "3D Light PAD" was published. The display's medium consists of a class of photoactivatable molecules (known as spirhodamines) and digital light-processing (DLP) technology to generate structured light in three dimensions. The technique bypasses the need to use high-powered lasers and the generation of plasma, which alleviates concerns for safety and dramatically improves the accessibility of the three-dimensional displays. UV-light and green-light patterns are aimed at the dye solution, which initiates photoactivation and thus creates the "on" voxel. The device is capable of displaying a minimal voxel size of 0.68 mm3, with 200 μm resolution, and good stability over hundreds of on–off cycles. Human–computer interfaces The unique properties of volumetric displays, which may include 360-degree viewing, agreement of vergence and accommodation cues, and their inherent "three-dimensionality", enable new user interface techniques. There is recent work investigating the speed and accuracy benefits of volumetric displays, new graphical user interfaces, and medical applications enhanced by volumetric displays. Also, software platforms exist that deliver native and legacy 2D and 3D content to volumetric displays. Artistic use An artform called Hologlyphics has been explored since 1994, combining elements of holography, music, video synthesis, visionary film, sculpture and