Many current 'heads-up' display systems use a mirror or beam-splitter to project an image directly into users' eyes, making it appear that the display is hovering in space somewhere in front of them. However, such systems are extremely limited in terms of their angle of view; the eyes must be in exactly the right position in order to see the image at all.
Other transparent displays use electronics directly integrated into the glass: organic light-emitting diodes for the display, and transparent electronics to control them. But such systems are complex and expensive, and their transparency is limited.
With a new system, developed by researchers at MIT, the image appears on the glass itself, and can be seen from a wide array of angles. Nanoparticles, embedded in the transparent material, can be tuned to scatter only certain wavelengths of light, while letting all the rest pass through. That means the glass remains transparent enough to see colours and shapes clearly through it, while a single-colour display is clearly visible on the glass.
To demonstrate the system, the team projected a blue image in front of a scene containing cups of several colours, all of which can clearly be seen through the projected image.
While the team’s demonstration used silver nanoparticles — each about 60 nanometers across — that produce a blue image, they say it should be possible to create full-colour display images using the same technique. Three colours (red, green, and blue) are enough to produce what we perceive as full-colour, and each of the three colours would still show only a very narrow spectral band, allowing all other hues to pass through freely.
“The glass will look almost perfectly transparent because most light is not of that precise wavelength [that the nanoparticles are designed to scatter],” says lead researcher, MIT's Professor Marin Soljacic. That scattering allows the projected image to be seen in much the same way that smoke in the air can reveal the presence of a laser beam passing through it.
Soljacic says that his group’s demonstration is just a proof-of-concept, and that much work remains to optimise the performance of the system. Silver nanoparticles, which are commercially available, were selected for the initial testing because they are 'simple and cheap'.
The particles could be incorporated in a thin, inexpensive plastic coating applied to the glass, much as tinting is applied to car windows. This would work with commercially available laser projectors or conventional projectors that produce the specified colour.