Broad-band green phosphor screens as a light source for head up displays in moving platforms

Abstract

A broad-band green light source for a head-up display is presented. To our knowledge, this is the first report of a green phosphor screen being excited by a blue LED as a backlight for monochrome HUDs. The phosphor screen not only generates the green light but it acts as a diffuser to give a homogeneous illumination. A microlens array focuses the emissions from LED sources on to the diffusing screen eliminating halo effects from the individual LEDs. The purpose of using a green phosphor is to exploit the fact that the eye is sensitive to more shades of green than any other colour. In uses where there are elements of danger such as automobiles (in busy areas), vehicles on construction sites and military vehicles in war zones, green displays have obvious attractions. This paper presents a discussion of the green phosphors that can be used in green screen fabrication, the deposition of the phosphor powders on the screens, the influence of the thickness of the phosphor powder on the screen brightness. In addition, the factors that influence the CIE coordinates of the light emitted from the screen are considered. The importance of choosing the optimum LED emission wavelength along with the general construction of the HUD is discussed. The merits of using a green screen are compared to those when using full colour displays based on white phosphor screens in which yellow emitting phosphors are excited by blue LEDs. Heat management in these HUD devices is achieved by pulsing the LEDs and rastering rows within the LED array.