Types of screens
There have been several techniques for transparent HMD. Most of these techniques can be summarized in two main families: based on “curved mirror” (or curved combiner) and based on “wave guide” or “light guide”. The mirror technique was used in EyeTaps, by Meta in its Meta 1, by Vuzix in its Star 1200 product, by Olympus and by Laster Technologies.
Several waveguide techniques have been around for some time. These techniques include diffraction optics, holographic optics, polarized optics, reflective optics and projection:
Diffractive waveguide: inclined diffraction lattice elements (10E-9 nanometers). Nokia’s technique is now licensed to Vuzix.
Holographic waveguide: 3 interleaved holographic optical elements (HOE) (RGB). Used by Sony and Konica Minolta.
Reflective waveguide: thick light guide with a single semi-reflective mirror. Epson uses this technique in its Moverio product.
Virtual Retina Display (VRD) – Also known as Retinal Scanning Display (RSD) or Retinal Projector (RP), it is a display technology that draws a raster screen (like a TV) directly on the retina of the eye, developed by MicroVision. INC ..
CastAR by Technical Illusions uses a different technique with clear glass. The glasses have a projector and the image is returned to the eye through a reflective surface.
Smart sunglasses that can change their light-filtering properties at runtime generally use liquid crystal technology. As lighting conditions change, such as when the user moves from inside to outside, the brightness ratio also changes and can cause unwanted vision problems. An interesting solution to overcome this problem is to incorporate darkening filters into smart sunglasses that control the amount of ambient light that reaches the eye. An innovative liquid crystal component for use in smart sunglass lenses is LC-Tec’s PolarView .  PolarView offers analog dimming control and the dimming level is adjusted by an applied drive voltage.
Another type of smart sunglasses uses the adaptive polarization filter (ADF). ADF type smart sunglasses can change their polarization filtering characteristics at run time. For example, ADF-type smart sunglasses can switch between horizontal polarization filter and vertical polarization filter at the touch of a button.
The lenses in smart sunglasses can be made from multiple adaptive cells, so different parts of the lens can exhibit different optical properties. For example, the top of the lens can be electronically configured to have different polarization filter characteristics and a different opacity than the bottom of the lens. 
Human Computer Interface (HCI) control input
Head-mounted displays are not designed to be workstations, and traditional input devices such as keyboard and mouse are not compatible with the concept of smart glasses. Instead, human-computer interface (HCI) control input should be methods that lend themselves to mobility and / or hands-free use are good candidates, for example:
Touchpad or buttons
Compatible devices (eg Smartphone or control unit) for remote control
Gesture recognition