Real time method and apparatus for adjusting contrast ratio of liquid crystal displays.

The present invention relates to display apparatus of the kind having a display panel which has a brightness versus voltage characteristic which is a function of temperature, comprising means for generating various voltages across the panel for producing a display on the panel.

In today's aviation industry, it is common for a single aircraft to be subjected to several extreme thermal conditions in a relatively short time interval. It is not uncommon for an aircraft to be operated with internal avionics equipment having dynamic temperature ranges of more than 70°C. With the current aspirations for trans-atmospheric aircraft, these extreme vicissitudes in the ambient temperature will continue to confront avionics engineers with perplexing problems of increasing difficulty and importance.

One particular problem that is exacerbated by these temperature oscillations is the continual change of contrast and brightness of liquid crystal displays during take-off and landing of an airplane. Typically, the transmittance versus voltage curve of a particular liquid crystal display is a function of the temperature of the liquid crystal material and, to some degree, a function of the age of the liquid crystal material. The transmittance versus voltage curve characteristics for liquid crystal displays are extremely important when grey scaling is attempted. The voltage across the liquid crystal material which is necessary to produce a predetermined level of transmittance of light through the liquid crystal material can vary widely over the typical operating temperature range of a display. This creates a situation where the contrast and brightness of the display are constantly changing, with a concomitant loss of information which is provided by the gray scale.

One method of controlling the brightness of a display is disclosed in U.S. Patent US-A-4,114,366 to Carl H. Renner et al entitled "Digital Brightness Control". This known brightness control system provides for a light sensor which measures the ambient light intensity and generates an analog signal to regulate a display. While this known method can assist in controlling the brightness of a display it suffers from serious drawbacks. The predominant drawback of such a brightness control system is that the brightness of a display is a function of the sensed ambient light intensity, and there is no control over the varying light transmittance through the liquid crystal material at a given voltage as a function of temperature, and therefore it does not eliminate the loss of gray scale information.

In US-A-4 319 237 there is described a brightness adjusting circuit for a liquid crystal matrix panel in which one electrode of each liquid crystal cell is connected to a common electrode that is supplied with a DC voltage that is varied with temperature by the use of a temperature sensitive resistor in a voltage dividing circuit. Hence the manner in which the brightness of the panel varies with ambient temperature depends on the characteristics of the temperature sensitive resistor.

A need exists for an improved apparatus for controlling the brightness and contrast of a liquid crystal display over a wide range of temperatures.

It is an object of the present invention to provide a display with improved gray scaling capability over a wide temperature range.

According to the present invention, the display apparatus of the kind defined hereinbefore at the beginning, in which the display image is formed by light transmitted through a display panel and in which gray scale levels of the image are produced according to the voltage levels across the panel, is characterised by: <ul id="ul0001" list-style="none"><li>(a) means for producing a predetermined light level and transmitting it through the panel;</li><li>(b) means for sensing the light transmitted through the panel and producing a signal representative thereof; and</li><li>(c) means for monitoring in real time the said signal representative of light transmitted through the panel at various voltages applied across the panel and utilizing the said signal in adjusting in real time the gray scale levels of the panel.</li></ul>

A preferred embodiment of the present invention includes a light sensor which monitors the actual transmittance of light through a liquid crystal panel as a function of the voltage across the liquid crystal material of the panel.

The preferred embodiment provides a direct parameter monitoring function in the sense that the transmittance through the liquid crystal material, as a function of voltage, is continuously determined and compared over time to determine any variations in the transmittance and voltage characteristic. The light sensor senses the actual light transmitted through the liquid crystal material and a microprocessor comparator monitors and compares the transmittance versus voltage characteristics in real time.

The means for producing a predetermined light level may in the preferred embodiment comprise a light emitting diode disposed on a first face of the liquid crystal panel. The means for sensing the light transmitted through the liquid crystal panel may comprise a photo diode disposed on the opposite face of the panel with respect to the light emitting diode. The means for generating various voltages across the liquid crystal panel may comprise means for generating a series of different voltages across the liquid crystal panel for producing different transmittance levels through the liquid crystal panel.

A preferred embodiment of the invention may take the form of display apparatus having a liquid crystal display panel which operates in an environment with temperature variations, comprising:    means for generating a series of different voltages across the liquid crystal material in the panel for creating variable transmittance therethrough;    characterised by <ul id="ul0002" list-style="none"><li>(a) the liquid crystal panel having predetermined dimensional characteristics and a first face and a second face;</li><li>(b) a light emitting diode disposed on the first face of the liquid crystal panel for generating a predetermined light level to be transmitted through the liquid crystal panel;</li><li>(c) a photo conductor disposed on the second face of the liquid crystal panel for generating a signal in response to the light incident thereon from said diode;</li><li>(d) means for comparing the photo conductor output for a plurality of the voltages across the panel;</li><li>(e) means for determining the relationship between the variable voltages and the photo conductor output; and</li><li>(f) means for adjusting gray scales in real time in response to the relationship between the variable voltages and the signal generated by the photoconductor.</li></ul>

A preferred embodiment of the invention employs a microprocessor for manipulating the gray scale voltage levels in liquid crystal displays, in order to provide for enhanced image quality, over temperature extremes by monitoring the actual transmittance through the liquid crystal material over time and comparing the information with earlier information.

The invention will now be described by way of example with reference to the accompanying drawings, wherein: <ul id="ul0003" list-style="none"><li>Fig. 1 is a schematic representation of a display device which shows the positioning of several light sensors in the corners of the display device.</li><li>Fig. 2 is a graphic representation of a typical transmittance versus voltage graph for a particular liquid crystal display with the several curves representing the transmittance versus voltage characteristics at several different but constant temperatures.</li><li>Fig. 3 is a schematic diagram of an embodiment of the present invention which provides for the monitoring and comparing over time of the actual light transmittance through a liquid crystal panel as a function of voltage.</li></ul>

In the drawings, Figure 1 shows a display device, which may be a liquid crystal display. In the preferred embodiment of the present invention, which utilizes a liquid crystal display panel, light sensors are placed in the corners of the diplay panel where they are able to measure the relative brightness which is attenuated by the liquid crystal material during its actual use. In general, each sensor looks through the liquid crystal material at a known light source. This allows the operation in all ambient light levels. The outputs of the sensors correspond to the light transmittance of the liquid crystal material as if it were operated with the varying voltages which are intentionally developed at the corners for this monitoring function. This approach will enhance the predictability of the voltage required to obtain different gray scale levels.

Now referring to Fig. 2 there is shown a series of transmittance versus voltage curves for a given liquid crystal display where each curve represents a different liquid crystal temperature. It can be readily seen that the voltage across the liquid crystal material which is necessary to produce a certain predetermined transmittance level varies greatly depending upon the temperature of the liquid crystal material.

Figure 3 shows an apparatus of the present invention which includes a liquid crystal display panel 100 having a first corner 102 with light shielding material 104 disposed thereabouts for shielding the viewer from light generated therein. A light emitting diode 106 for generating a light to be transmitted through the liquid crystal material is positioned on a first surface of the liquid crystal display panel 100 while on the opposite surface is located a photo conductor 108 which is connected by line 110 to a sensor analog-to-digital converter 130. A microprocessor, not shown, is used to control a reference digital-to-analog converter 120 for generating a varying voltage signal for application to the liquid crystal display. This varying voltage signal is input into a video multiplexer 116 where a video signal 118 is combined with it before being output to a liquid crystal display column driver 114 which performs the function of delivering the variable voltages for controlling the liquid crystal display. In operation, the microprocessor (not shown), in combination with the video multiplexer 116 and the LCD column drivers develops a series of different voltages across the liquid crystal display panel 100.

The photo conductor 108 measures the actual light received from the light emitting diode 106 which is transmitted through the liquid crystal display panel 100. The photo conductor signal on line 110 for each of the varying voltage levels is converted by the converter 130 and then monitored and compared by the microprocessor so that a determination of the actual transmittance versus voltage characteristic is carried out in real time.

As the temperature of the liquid crystal display panel 100 changes, the transmittance through the liquid crystal material of the light produced by the light emitting diode 106 changes so that the signals produced by the photo conductor 108 change. This information is utilized by the microprocessor to determine the actual transmittance to voltage characteristic for the liquid crystal display panel at any given time and is utilized therein for setting the voltage levels for the gray scale.

The embodiment of Fig. 3 operates to improve the image quality of the liquid crystal display which operates in environments having temperature variations, by a method comprising the steps of: <ul id="ul0004" list-style="none"><li>(a) determing the transmittance through the liquid crystal panel (100) as a function of voltage; and</li><li>(b) adjusting predetermined gray scale voltage levels in response to the transmittance as a function of voltage determination.</li></ul>

In the apparatus, the light emitting diode 106 serves as a means for producing the predetermined light level and the photoconductor 108 as a means for sensing the light transmitted through the liquid crystal panel. The microprocessor, operating through the DAC 120, multiplexer 116 and driver 114, acts as a means for controlling the voltage across the liquid crystal panel; and, by input through the ADC 130, as a means for monitoring the light transmitted through the liquid crystal panel as a function of the voltage across the panel. The microprocessor futhermore, through units 120, 116, and 114 acts as a means for controlling the gray scale levels.

In operation, generating a series of different voltages across the liquid crystal panel produces different transmittance levels through the liquid crystal panel.

It will be appreciated that, at the corner of the panel 100 represented in Fig. 3, there is a liquid crystal with predetermined dimensional characteristics and having a first side and a second side;    a light emitting diode 106 disposed on the first side of the liquid crystal for generating a predetermined light level to be transmitted through the liquid crystal display.    a photo conductor 108 disposed on the second surface of the liquid crystal for generating a signal in response to the light incident thereon; and    means for generating a series of different voltages across the liquid crystal for creating variable transmittance therethrough.

The microprocessor compares the photo conductor output for a plurality of the voltages across the panel, and determines the relationship between the variable voltages and the photo conductor output. Hence the microprocessor can manipulate the gray scale voltage levels in the liquid crystal display, in order to provide for enhanced image quality, over temperature extremes by monitoring the actual transmittance through the liquid crystal material over time and comparing the information with earlier information.