Display unit subject to ambient light having light-reflective and light-emitting display elements

Illumination means for a display unit of the kind visually displaying data, as for example in an aircraft instrument or the like, and being of the kind in which both light-emitting elements and light-reflective display elements are concurrently operative and in which both said light-reflective and light-emitting elements are at times subject to ambient light impinging thereon, said illumination means comprising:

• first illumination means comprising at least one lamp so disposed as to illuminate substantially only said light-reflective elements,
• a first light detector for providing an output which is a manifestation of only the intensity of ambient light incident on said display unit,
• a second light detector shielded from ambient light,
• second illumination means having illumination characteristics substantially the same as said first illumination means,
• said second light detector being illuminated substantially only by said second illumination means to thereby cause said second light detector to provide an output which is a manifestation of only the intensity of illumination provided by said first illumination means on said light-reflective elements,
• adjustable control means for varying the intensity of illumination provided by said first illumination means,
• and means jointly responsive to both said first detector in accordance with the level of ambient light and also to said second detector in accordance with the level of illumination provided by said second illumination means to control the level of illumination of said light-emitting elements.