HUD and It's Principles

   The Head-Up Display (HUD) can project information on the windshield, tell you where to turn, notify you of lane markings, identify how close the car in front of you is and even give you restaurant information all right in front of your eyes -- and while you're actually able to see those objects or places. Such display
technology offers various opportunities for different types of
information presentation. Information in regularly available
HUDs is shown in symbolic style, but emerging contact analog HUD technology allows for a wider range of information presentation: virtual 3D information can be interactively
superimposed in the driving environment through Augmented
Reality (AR). ARs can integrate with GPS systems, infrared cameras, the Internet and mobile apps to turn your car's windshield into an on-board information screen. 

   A HUD should not obstruct the user's view. This is typically achieved by the use of projected or reflected transparent displays in line-of-sight. However, with display miniaturization, non-transparent but unobtrusive screens can provide similar functions.

Some of the earliest HUDs were used in military vehicles to assist in navigation and targeting. The first generation of that type of HUD used reflected CRT display. The second generation used solid-state light sources like LED to back-light LCD projection – this is the technology commonly used in commercial aircraft. The third generation uses wave-guided optics. The forth uses scanning lasers to project all types of images and video.

Basically HUDs are split into four generations reflecting the technology used to generate the images.

• First Generation—Use a CRT to generate an image on a phosphor screen, having the disadvantage of the phosphor screen coating degrading over time. The majority of HUDs in operation today are of this type.
• Second Generation—Use a solid state light source, for example LED, which is modulated by an LCD screen to display an image. These systems do not fade or require the high voltages of first generation systems. These systems are on commercial aircraft.
• Third Generation—Use optical waveguides to produce images directly in the combiner rather than use a projection system.
• Fourth Generation—Use a scanning laser to display images and even video imagery on a clear transparent medium.