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Back Lighting vs. Transparency

LCDs are inherently transparent, although the polarizing solution generally blocks almost half the light, or more. They are referred to as light-valves, which can pass or block polarized light. In the construction of LCD displays for mobile devices, PCs, dashboards, and signs, a bright light back panel is placed behind the LCDs. It is the back-panel light that makes an LCD non-transparent. There are also edge-lit, and front-lit solutions, as well as reflective LCDs used in monitors and industrial systems, but not augmented reality headsets.

Early LCD displays used a reflective back panel because at the time the cost, size, and power requirements of an emissive back panel was prohibitive (and still today, large numbers of passive matrix display, clocks/calculators/etc., are reflective). Since then the panels have been made very thin using cold cathode fluorescent lamps (CCFLs), and since about 2010, LEDs have been used for back panel lighting.

Presumably, an augmented reality display being used in nominal ambient light could be a native LCD panel mounted in the lens of the headset or helmet. However, it wouldn’t work well at night or in dark areas, or smoke-filled areas.

However, a ‘guilty little secret’ of many transparent LCDs is that the transistor arrays can act as diffraction gratings and don’t give a clear view of anything that’s distant. That’s why they are often used in showcase applications where the object being viewed is closer to the display than the viewer.

Syndiant’s LCoS moduel (Source Syndiant)

Fig. 8.15 Syndiant’s LCoS moduel (Source Syndiant)

 
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