Liquid crystal display with independently activated backlight sources

A liquid crystal display 10 is backlit by more than one light source 20 which is operated independently. Selective usage of the light sources extends the useful operating time of batteries when the liquid crystal display is used with a portable computer system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid crystal display. More 
particularly the invention relates to a liquid crystal display (LCD) which 
may be used with a portable computer and to a method and apparatus for 
extending the useful operating time of a battery in an LCD portable 
computer. 
2. Discussion of the Prior Art 
Liquid crystal displays operate on the principle that when an electric 
charge is passed between two parallel, transparent surfaces that contain 
liquid crystal, the crystal will align itself with the surfaces. In known 
LCDs a grid or matrix of electrodes is located across the display area to 
control orientation of the crystals. LCDs are also known in which a grid 
or matrix of electrodes is located around the edge of the display and 
enables current to be applied to predetermined pixel areas of the display 
to selectively orientate the crystals. In both types of LCD it is 
necessary to illuminate the display panel. 
One such example of such a display is used with the IBM ThinkPad portable 
computer. This computer typically uses a 10.8 Volt 2.9 AH battery which 
achieves a useful operating life of about 3.8 hours. This operating time 
includes full power management techniques. The average operating power 
using the battery is 8.25 Watts compared to 25 Watts when connected to an 
external power supply. Of this 8.25 Watts up to 4 Watts can be consumed by 
the LCD backlight and even more with colour versions of the display. 
SUMMARY OF THE INVENTION 
Extending the useful operating time and reducing battery weight is a major 
factor in the design of portable computers. Accordingly, viewed from one 
aspect the present invention provides a display unit comprising: a liquid 
crystal display panel; at least two light sources for backlighting the 
liquid crystal display panel; characterised in that at least one of the 
light sources may be activated independently of the other light sources 
thereby enabling portions of liquid crystal material in the display panel 
to be illuminated selectively. 
A typical application is for the user of a portable computer to type in 
minutes of a meeting, or to type a report or a short memo. Such 
applications can save the user time when he gets home or back to his 
workplace and ensure that thoughts are entered while they are fresh on the 
mind. However the majority of the time spent in this sort of application 
is just power typing, and in this activity the user will only need to see 
the whole page when reviewing the document. So although only a small 
section of the LCD display is actually needed, the whole surface is active 
and being illuminated by expensive battery power. 
The present invention provides a power management technique which 
recognizes "non useful" power states, and turns facilities off until 
really needed. "Power Typing" is an application state to which power 
management may be applied.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a schematic view of a conventional liquid crystal display. In 
operation an electric charge is passed between the two parallel surface 
plates of the screen 40 which contain liquid crystal material to cause the 
crystal to align itself with the surfaces. The crystal has a natural 
polarizing effect on the light and a 90 degree twist--which happens when 
the current is passed through the surrounding surfaces--causes the light 
to twist. This twisting effect is combined with a static polarizing layer 
to affect the amount of light which passes through the crystal. The effect 
of the crystal twisting 90 degrees is that it blocks light from passing 
through the crystal and causes that area to change from white 
(transparent) to black. The angle of polarization may be varied by varying 
the voltage thereby enabling increments of light or greyscales to be 
created. This is known as the twisted nematic effect of liquid crystals. 
The present invention is also applicable to Super-Twisted-Nematic (STN) 
LCDs. This technology uses birefringent crystals that have the ability to 
twist through even wider angles. Birefringent crystals can twist through 
between 180 and 260 degrees. An advantage of STN displays are that they 
have lower power consumption. 
The typical LCD panel 10 shown in FIG. 1 is backlit by a single lamp 20. In 
some LCDs the lamp is located centrally and in others it is located at the 
side. It is also known to utilize more than one lamp. Light output from 
the lamp or lamps is normally spread out evenly over the LCD surface by 
some form of spreader or diffuser 30. If there is more than one lamp they 
are always connected in parallel so that the whole LCD surface is evenly 
illuminated. 
FIG. 2 shows an LCD panel in accordance with the present invention wherein 
more than one lamp is provided for backlight. Four lamps 20 are shown in 
the embodiment of FIG. 2 but the minimum number should be two. At least 
one of the lamps is independently controlled from the others. The 
independently controlled lamp has a separate power line 55 to the battery 
(not shown), whereas the other lamps share a power line 50 to the battery. 
The lamps may be electroluminescent panels, incandescent sources, light 
emitting diodes or even plasma discharge panels to mention a few examples. 
In accordance with a preferred embodiment of the present invention an 
application input mode is defined (for example by a window icon or a 
function key) as "Power Typing" and in this mode three of the backlight 
lamps are turned off leaving only one lamp to illuminate 1/4 of the LCD 
screen and hence typically show 10 lines of text to the user. It is 
necessary to tailor the application to synchronize to this mode so that 
just the bottom ten lines for example of the screen are used until the 
normal mode is re-commanded. 
In accordance with another embodiment of the invention the lamps could be 
switched by an extension of conventional circuits. The power line 50 from 
the bank of the three lamps is taken to a switch on the front panel of the 
display to allow the user to control the function. Application windows can 
easily be resized to the lower height needed when only one light is in 
use. An advantage of this manual mode of operation is that no changes are 
required to the microprocessor circuits within the computer or to the 
software. 
From this basic embodiment, various degrees of automation can be devised. 
Examples includes: 
1. Programming a function key, by the application, by a device driver or by 
BIOS, to initiate the lamp switching. 
2. Extending the function key approach to also resize the application 
window. 
3. Having a facility in the application (or operating system) to use the 
minimize button--or a new button--to resize the window and, at the same 
time, to signal via a micro output port to switch the lamps. 
4. Supplying a "miniature application" via a device driver to place an on 
screen icon to initiate window resizing and lamp switching. 
In the ThinkPad 700 example discussed previously, if the backlight power 
was reduced from 4 to 1 Watt during "Power Typing" mode, then the useful 
operating life would extend from 3.8 hours to nearly 6 hours while in this 
mode.