Fraunhofer FEP presents anti-reflective coatings for commercial holographic displays
The German-based Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology (FEP) has developed a coating, which is required to expand the diameter of a laser beam by more than a factor of one hundred. With this coating, backlighting for holographic displays can be realised in the future. The main aim of the project is to provide an approach for 3D television sets that work without annoying 3D glasses. Holographic displays would be a giant step in this direction. Moreover, medical scientists could inspect spatial images of the inside of the body and observe detailed movements of organs.
The company SeeReal Technologies in Dresden works on such displays. Holographic displays use certain properties of laser light for the complete three-dimensional display of images. Therefore, an expansion of the laser beam to the display size is necessary. It is obvious that a laser beam with a diameter of a television display is difficult to realise. A conventional option would be large lens systems, but these are clunky and can only be manufactured complexly and at very high costs.
In a joint project with SeeReal, the scientists of Fraunhofer FEP have now developed coatings that enable usage of low power lasers for illumination. The laser is directed in at a very flat angle into a glass plate (here 5 degrees, respectively 85 degrees against the vertical). Similarly to the shadow of a person, which is extending in the setting sun, and whose projected area on the ground also extends, the diameter of the laser beam increases. A small spot becomes an elongated ellipse. In a second step, the elongated ellipse impinges again on a second glass plate at five degrees, whereby the second direction of the ellipse, the “short axis”, is elongated. Thus, the laser spot is expanded to a circle that is large enough to illuminate the entire display. However, if one shines with a laser on an uncoated glass plate at such a flat angle, about 73% of the beam is reflected. In case of two “expansion steps”, more than 90% of the original intensity would be lost.
„We have developed an anti-reflective coating which increases the part of the transmitted light significantly”, says Dr. Daniel Glöß, head of the “Dynamic coatings” department at FEP. “By means of magnetron sputtering, thin layers are deposited on glass. These layers consist of two different materials with varying optical density. Even complicated optical functions can be achieved via multilayer systems, which, for instance, let only certain colours of the light pass through and reflect the others.”