Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1998-10-30
2001-03-27
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C315S086000
Reexamination Certificate
active
06207943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a backlighting system for a LCD type display. More specifically, the present invention relates to a backlighting system which monitors the brightness of the backlight illumination and which gradually increases the power supplied to the backlight in order to compensate for the degradation of the light source lamp with the passing of time.
2. Description of the Related Art
Display technology is rapidly changing with the advent of better and brighter liquid crystal displays (LCDs). As is known, cathode ray tubes (CRTs) are being rapidly replaced by LCDs due to the size and power advantages that are possible with such arrangements.
A few short years ago, the brightness of LCDs was 2.5 times less than that possible with CRTs. However, advances in backlighting technology have enabled the development of LCD displays that now exhibit brightness that rivals that possible with CRTs. These advances have involved the use of brightness enhancing films which focus the viewable light being emitted from the screen, color polarizers which maximize the usable light coming from the backlight, and more efficient light guides.
However, one undesirable tendency in this quest for brighter displays, is to either drive the light sources at their fill rated power, or even to overdrive the same from the outset. This of course, increases the brightness that is produced, but markedly speeds up the rate of degradation of the light sources and drastically reduces the life of the lamp or lamps being used.
Accordingly, one advantage that CRT-type displays have over LCDs is that the CRT type display exhibits more consistent display brightness over its lifetime. More specifically, a CRT will decrease in brightness about 10% 15% during its lifetime due to factors relating to the electronic gun filament, phosphor degradation and the driving electronics. Depending on the type of CRT, one or all of these factors may be responsible for the reduction in brightness with the passing of time. An LCD, on the other hand, will decrease in brightness by about 50%. This reduction is, however, is directly related to the degradation of the backlighting system associated with the liquid crystal structures which actually produce the image(s).
In most instances, the backlighting for LCD type displays is provided by one or more cold cathode fluorescent lamps (CCFLs), a power inverter and a light guide. The CCFL is the only component that degrades significantly with the passing of time. Accordingly, as the user must manually adjust the level of brightness to that which is preferred, it quickly becomes apparent to the user that the rate of reduction in the brightness in LCD type displays is more pronounced than with those which are based on CRT.
Additionally, the change in brightness in a LCD equipped display, which follows a service wherein the light source(s) is replaced, tends to be much more apparent to the user than in the case wherein a new CRT is installed.
Hence, in order to make LCD displays acceptable alternatives to CRTs, it is necessary to reduce the degree by which the brightness of LCDs are perceived to vary and reduce the passing of time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a backlighting arrangement for an LCD or the like type of display which enables the level with which the brightness of the display can be maintained for protracted periods of time and thus rival the stability which has been responsible for the widespread use of CRT's.
It is a further object of the present invention to provide a backlighting arrangement for an LCD or the like type of display, which increases the working life of the light sources which are used therein while automatically maintaining the level of illumination essentially constant.
As is well known, driving CCFL of backlighting system at or above 100% of their rated power is such as to induce relatively rapid degradation which shortens the mean time before failure, or MTBF (i.e., the operational hours required for a display's brightness to drop by half). The present invention therefore is such to drive the light sources at less than 100% of their rated power specification. This not only facilitates the control that the invention seeks to provide, but also prolongs MTBF markedly. That is to say, the relationship of power to life is not linear but rather is geometric. A 50% reduction in the power used to drive a light sources can result in a five fold increase in the MTBF. Therefore, a CCFL with a rated MTBF of 20,000 Hrs. will be increased to about 100,000 Hrs. with a 50% reduction in driving power.
The present invention features the use of a photocell type sensor to measure the brightness of the CCFL. This cell is used to monitor the reduction in brightness produced by the light source and to accordingly increase the amount of power that is supplied thereto. This increase in power is selected to result in the brightness being maintained at its original level.
This technique also demands that the original power setting be less than 100% of the rated power so that the amount of power can be increased with the passing of time. This initial low power setting, of course, tends to increase the life of the light sources so that with the gradual increase in the amount of power, which is induced in accordance with the present invention, an MTBF on the order of 10,000 hours is considered readily attainable.
In more specific terms, a first aspect of the present invention resides in a device for use with a LCD comprising: a light source for providing a backlight for the LCD; means for measuring a level of brightness of the light source; means for comparing the level of brightness to a reference level of brightness; and means for increasing the power to the light source when the level of brightness falls below the reference level of brightness and for maintaining a constant level of brightness for the LCD.
In the above-mentioned device, the light source comprises at least one cold cathode fluorescent lamp and the means for measuring brightness comprises a photocell.
In addition, the means for comparing the level of brightness comprises at least one comparator circuit and the means for increasing the power to the light source comprises at least one potentiometer.
A second aspect of the invention resides in a method of maintaining a constant level of brightness for a LCD comprising the steps of: measuring a brightness level of a light source using a photocell; comparing the brightness level of the light source to a reference level of brightness; increasing the power of the light source when the brightness level falls below the reference level to maintain a constant level of brightness for the LCD.
In the above mentioned method, it is possible to drive the light source at less than 100% of its rated power so as to increase the life of the light source. For example, 75% rated power can be supplied to produce a brightness level of 150 Nits.
A further aspect of the invention resides in a backlight device for an LCD comprising: a light guide located behind the LCD; at least one light source located behind the light guide; means for measuring a brightness level of the light source, the measuring means being disposed proximate to the light source; means for comparing the brightness level with a reference level of brightness, the comparing means being electrically connected to the measuring means; means for increasing the power to the light source when the brightness level of the light source falls below the reference level of brightness and for maintaining a constant brightness level, the power increasing means being electrically connected to each of the comparing means and the light source.
Yet another aspect of the invention resides in a method of controlling the brightness of a display comprising the steps of: illuminating a light source by supplying the light source with electrical power; monitoring illumination brightness and producing a signal indicative of the am
Allen Stephone B.
Baker Electronics Inc.
McDermott & Will & Emery
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