Energy efficient transmissive and transreflective display devices are
described. Ambient light from a natural or artificial source is used to
replace and/or supplement light normally supplied by a backlight. This is
done by directing ambient light to the rear of a transmissive display
panel. A window, light tunnel, or a reflective surface located in the rear
or top of a display device may be used to direct the ambient light to the
back of the display panel. A translucent diffuser and/or diffuse reflector
are used to diffuse the ambient light to reduce the chance of bright spots
appearing on the display. Additional energy efficiency is achieved by
using one or more photo-sensors to detect the amount of light, incident on
the front and/or rear of a display panel and by automatically adjusting
the backlight intensity as a function of photo-sensor output. In this
manner, backlight intensity can be minimized while maintaining the
viewability of images shown on the display. In such an embodiment, energy
savings are achieved as compared to devices which use fixed backlight
intensity settings in a variety of light conditions. Control of the power
supplied to a display's backlight in accordance with the present invention
can reduce electrical energy consumption and prolong the amount of time a
portable device can be used before its batteries need to be recharged.
