JPH1126183A - Light dimming device for liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always automatically adjust brightness of a panel surface of a liquid crystal display and brightness of the ambient light to the relative optimal relationship so that visibility according to a service environment can be obtained. SOLUTION: A difference between brightness of the ambient light and luminance of an image screen is supplied to a comparator 12, and a triangular wave having a constant cycle period is modulated on a pulse width, and a fluorescent tube 2 being a back light of a liquid crystal display is controlled on luminance by a pulse width-modulated light dimming signal. Since a difference between brightness of the ambient light and luminance of an image screen is kept constant, visibility of the liquid crystal display can be optimized in various environments.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dimming device for a liquid crystal display which can control the brightness of a backlight according to the brightness of ambient light and dimming input.

[0002]

2. Description of the Related Art Since a liquid crystal panel is not a self-luminous element for converting electric energy into light, a liquid crystal display using the liquid crystal panel as a display means needs to be illuminated from behind the liquid crystal panel to increase screen brightness. In general liquid crystal displays, a backlight composed of a small and highly efficient cold-cathode fluorescent tube is mounted between a diffusion plate and a reflection plate, and this is used as a surface light source to illuminate from the back side of the liquid crystal panel. Therefore, dimming of the liquid crystal display means dimming of the backlight.

A light control device 1 for a liquid crystal display shown in FIG.
Is such that the brightness of the fluorescent tube 2 is automatically adjusted in accordance with the brightness of the ambient light, and incorporates an ambient light detection photosensor 3 for detecting the brightness of the ambient light. A photo sensor circuit 4 for converting a sensor output into an electric signal such as a voltage or a current is connected to the ambient light detecting photo sensor 3, and a DC voltage signal corresponding to the ambient light output from the photo sensor circuit 4 is output. It is supplied to the dimming control circuit 5. The dimming control circuit 5 compares the DC voltage signal supplied from the photosensor circuit 4 with a triangular wave having a predetermined period, and according to an input DC voltage value, a signal period ratio between a high potential level and a low potential level of a rectangular wave ( The pulse width modulation output whose duty is changed is supplied to the inverter 6 as a dimming signal. The inverter 6 receives the dimming signal from the dimming control circuit 5, generates an AC voltage having a cycle and a voltage according to the dimming signal, applies the AC voltage to the fluorescent tube 2, and drives the fluorescent tube 2 to light. The inverter 6 operates when the output of the dimming control circuit 5 is at the high potential level and stops operating when the output is at the low potential level. Therefore, the inverter 6 performs an intermittent operation according to the output duty of the dimming control circuit 5, The brightness of the fluorescent tube 2 is adjusted. In the case of this example, as the surroundings of the liquid crystal display became brighter, the brightness of the fluorescent tube 2 as the backlight also became brighter, and visibility comparable to ambient light was obtained.

[0004]

Generally, compared with incandescent lamps and high-pressure discharge lamps, the luminous flux variation due to the ambient temperature is most remarkable in a fluorescent tube, and the maximum is that the mercury vapor pressure in the tube changes according to the ambient temperature. Has been the cause. Generally, a general fluorescent tube is designed so that the luminous flux becomes maximum when the ambient temperature is 20 ° C., for example, and it is said that the practical use range is 5 to 30 ° C. for practical use. For this reason, when used in a cold region in winter, sufficient heat retention measures are required. However, in the case of an in-vehicle liquid crystal display in which the use environment may greatly change with the movement of the vehicle, for example, the fluorescent tube 2 , The display brightness of the liquid crystal display is liable to decrease, and there is a problem that the display becomes dark and difficult to see when used in a warm place. On the other hand, contrary to cold regions, the brightness of the fluorescent tube 2 is higher in a tropical region than in a warm region, so that the display has a problem that the display is too bright and dazzling especially at night. .

In the conventional dimmer 1 for a liquid crystal display, the brightness is adjusted in accordance with the brightness of ambient light.
When the brightness of the fluorescent tube 2 changed due to the influence of the ambient temperature, the operation mode of the inverter 6 could be manually switched to adjust the brightness in, for example, about two steps, but it could be finely adjusted to the user's preference. There is a problem that brightness control cannot be performed, and therefore it is difficult to ensure optimum visibility according to the use environment.

Japanese Patent Application Laid-Open No. Hei 7-64057 "Liquid crystal display device" includes a backlight based on the fact that the light emission characteristics of the backlight change over time and the light transmittance characteristics of the liquid crystal panel change over time. And a device that independently monitors the characteristics of the liquid crystal and determines the replacement time, and as a prerequisite technology of this device, the visibility of a display screen corresponding to a change in the lighting environment is determined by the visibility. Is evaluated using
An apparatus has been disclosed that controls the luminance of a display screen to ensure visibility in consideration of changes in the characteristics of the human eye.

In the latter device, the ratio of the brightness contrast between the object being viewed and its background is compared with the minimum brightness ratio that can be recognized by the human eye in the visual environment where the object is being viewed. The optimal brightness is determined from the screen brightness and the brightness level setting signal so that the visibility representing the constant takes a constant value, and the backlight brightness and the liquid crystal transmittance are variably controlled. However, the visibility calculated by the ambient light receiving unit is a value calculated based on a number of parameters such as dark portion light emission luminance, screen luminance, or equivalent light curtain luminance and contrast, and the calculation is extremely complicated. The visibility threshold used for setting the target of the screen luminance range is also arbitrarily selected from the range of 7 to 10, and the control mode is likely to be inevitably determined because there is much room for selection. Although there is an improvement effect of suppressing a decrease in the brightness of the light and conversely, suppressing an increase in the brightness of the backlight when the surroundings of the screen are dark, the visibility of the display screen corresponding to changes in the lighting environment is always best. There was a problem that it could not be secured in the state of. It was clear that this was a technical limitation due to securing the visibility after extending the life of the backlight and saving power.

The present invention has been made to solve the above problems, and automatically adjusts the brightness of the panel surface of the liquid crystal display and the brightness of the ambient light to a relative optimal relationship so that visibility according to the use environment can be obtained. adjust.

[0009]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides an ambient light detecting means for detecting the brightness of ambient light and outputting a DC signal which has been converted substantially linearly, and a liquid crystal display. Screen brightness detection means for detecting brightness and outputting a substantially linearly converted DC signal; difference calculation means for calculating a difference between a detection output of the screen brightness detection means and a detection output of the ambient light detection means; A triangular wave generating circuit for generating a triangular wave having a repetition cycle of: a comparator for comparing the level of the triangular wave with the output of the difference calculating means and outputting a pulse width modulated dimming signal; an output dimming signal of the comparator And brightness control means for controlling the brightness of the backlight of the liquid crystal display.

Further, in the present invention, the backlight is a fluorescent tube, and the luminance control means transmits a high-frequency drive signal to the fluorescent tube only during a period in which the pulse width modulated dimming signal takes one potential. Is applied to the inverter.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
This will be described with reference to FIG. FIG. 1 is a block diagram showing an embodiment of the dimming device for a liquid crystal display of the present invention. FIG. 2 is a signal waveform diagram of each part of the device shown in FIG. 1 when the brightness of ambient light changes. .

The light control device 11 for a liquid crystal display shown in FIG. 1 supplies a signal supplied to a comparator 12 in the PWM light control circuit 5 used in the conventional device 1 to both the brightness of ambient light and the screen brightness. To be variable according to
The configuration is such that the difference between the brightness of the ambient light and the screen brightness is always kept constant. Specifically, in addition to the ambient light detection photosensor 3 that detects the brightness of the ambient light and emits a DC signal that is substantially linearly converted, the DC sensor that detects the brightness of the screen of the liquid crystal display and detects the linearly converted DC signal A screen luminance detecting photosensor 13 for outputting a signal is provided. As with the photosensor circuit 4 connected to the ambient light detection photosensor 3, a photosensor circuit 14 for converting a sensor output into an electric signal such as a voltage or a current is connected to the screen luminance detection photosensor 13. A subtraction circuit 15 is connected to the photosensor circuits 4 and 14, and the subtraction circuit 15 calculates a difference between the brightness of ambient light and the screen luminance.

The subtraction circuit 15 is connected to one input terminal of the comparator 12, and the other input terminal of the comparator 12 is connected to a triangular wave generation circuit 16.
The triangular wave generation circuit 16 supplies a triangular wave having a fixed repetition period to the comparator 12, so that the comparator 12 compares the level of the triangular wave with the output of the subtraction circuit 15 and outputs a pulse width modulated dimming signal. The inverter 6 as a brightness control means receives the dimming signal from the comparator 12 and controls the brightness of the fluorescent tube 2 as a backlight of the liquid crystal display.

In the present embodiment, since the repetition period of the triangular wave is fixed to a predetermined period (design value) in the design stage, the dimming signal obtained from the comparator 12 is the difference between the brightness of the ambient light and the screen brightness. The pulse width modulation output changes the signal period ratio (duty) between the high potential level and the low potential level of the rectangular wave according to the magnitude. That is, this pulse width modulation output is supplied to the inverter 6 as a dimming signal. The inverter 6 receives the supply of the pulse width modulation dimming signal from the comparator 12, applies a high frequency AC voltage to the fluorescent tube 2 as a driving signal only during a period in which the dimming signal takes a high potential, and The fluorescent tube 2 which is a backlight is driven to be turned on.

By the way, it is assumed that the brightness of the ambient light decreases stepwise at a certain point in time in a state where the luminance of the fluorescent tube 2 is constant, as shown in FIG. In this case, the output waveform of the photo sensor circuit 4 changes stepwise as shown in FIG. That is, the output of the photosensor circuit 4 shows a change in the polarity opposite to the change in the brightness of the ambient light, but this change may be of any polarity as long as it has a linear relationship with the ambient light. Therefore, FIG.
The waveforms shown in (A) and (B) may have a relationship of changing with the same polarity.

The screen luminance of the fluorescent tube 2 is initially constant as shown in FIG. 2C, and the output of the photosensor circuit 14 connected to the screen luminance detecting photosensor 13 is also shown in FIG. ), It is initially constant. For this reason, the output waveform of the subtraction circuit 15 slightly steps up as shown in FIG. 2E in accordance with the change in the brightness of the ambient light. As a result, the comparator 12
In (F), two signal waveforms of a triangular wave indicated by a solid line and a differential output indicated by a chain line are input. In this case, the portion where both signal lines intersect when both signal waveforms are overlapped is a falling point at which the output of the comparator 12 changes from the high potential level to the low potential level, or from the low potential level to the high potential level. It will be a rising point that changes to.

For this reason, as shown in FIG. 2 (G), the output waveform of the comparator 12 is such that the ratio of the high potential level period to the low potential level period starts when the brightness around the liquid crystal display becomes dark. The ratio is reduced. The inverter 6 is connected to the comparator 1 as shown in FIG.
Since the high-frequency AC voltage is applied to the fluorescent tube 2 only when the output of the fluorescent lamp 2 is at the high potential level, the screen brightness of the fluorescent tube 2 decreases in accordance with the decrease in the brightness of the ambient light.

When the screen luminance of the fluorescent tube 2 decreases, the output of the screen luminance detecting photosensor 13 also decreases with the decrease in luminance. As a result, the output of the photo sensor circuit 14 is slightly stepped up as shown in FIG. For this reason, the subtraction output of the subtraction circuit 15 slightly decreases, and the output of the comparator 12 changes in a direction to suppress a decrease in luminance of the fluorescent tube 2. In this way, the repetition of the cyclic control such that the decrease in the screen luminance causes the control to suppress the luminance decrease is continuously performed, and finally, the output waveform of the subtraction circuit 15 gradually approaches a stable value, and the comparator 12 The output waveform also settles to a stable value. Thus, the luminance of the fluorescent tube 2 is settled in a stable state in which the screen luminance is relatively constant with the screen luminance with respect to the brightness of the ambient light, and the optimal visibility is maintained regardless of the change in the brightness of the ambient light. Secured.

As described above, according to the dimming device 11 for a liquid crystal display, the difference between the brightness of the ambient light and the screen brightness is supplied to the comparator 12, and the triangular wave having a constant repetition period is pulse width modulated. Since the brightness of the fluorescent tube 2 as the backlight of the liquid crystal display is controlled by the pulse width modulated dimming signal, the difference between the brightness of the ambient light and the screen brightness can always be kept constant, and the brightness of the ambient light can be maintained. If the brightness decreases, the brightness of the fluorescent tube 2 automatically decreases accordingly, and conversely, if the brightness of the ambient light increases, the brightness of the fluorescent tube 2 automatically increases accordingly. Thus, the screen brightness can be automatically controlled according to the ambient light. In addition, even when the ambient light does not change, when the screen luminance changes due to the influence of the ambient temperature, autonomous screen luminance control is performed.For example, the brightness of the panel surface of the liquid crystal display is darker than in a warm place. Even in cold climates where it is easy to get, the optimal brightness according to the brightness of the ambient light can be obtained and the same visibility as in a warm climate can be obtained, while the brightness of the panel surface is brighter than in a warm climate. Even in tropical areas that tend to be vulnerable, it is possible to obtain optimal brightness according to the brightness of ambient light and obtain visibility equivalent to that of warm areas, so that it can be used in various temperature environments It can be applied to liquid crystal displays for personal use. In this case, a constant visibility can always be obtained, and the backlight luminance control is performed based on a signal obtained by subtracting the two inputs of the ambient light input and the screen luminance input in the subtraction circuit 15 into one unit. Therefore, the circuit configuration can be simplified and the manufacturing cost can be reduced.

The backlight is constituted by the fluorescent tube 2,
Since the brightness control means is constituted by the inverter 6 for applying a high-frequency drive signal to the fluorescent tube 2 only during a period in which the pulse width modulated light control signal takes one potential, the pulse width modulated light obtained from the comparator 12 is obtained. The inverter 6 is intermittently operated by the optical signal, and the amount of light emitted from the fluorescent tube 2 can be changed according to the length of the intermittent operation period, whereby the brightness of the backlight can be easily and simply adjusted according to the brightness of the ambient light and the screen brightness. Brightness control can be performed reliably.

[0021]

As described above, according to the present invention,
A configuration in which the difference between the brightness of the ambient light and the screen brightness is supplied to a comparator, a triangular wave having a constant repetition period is pulse width modulated, and the backlight of the liquid crystal display is controlled in brightness by a pulse width modulated dimming signal. As a result, the difference between the brightness of the ambient light and the screen brightness can always be kept constant, and if the brightness of the ambient light decreases, the brightness of the backlight automatically decreases accordingly, and vice versa. If the brightness of the ambient light increases, the brightness of the backlight automatically increases accordingly, so that the screen brightness can be automatically controlled according to the ambient light and the ambient light does not change Even in such a case, when the screen brightness changes due to the influence of the ambient temperature, autonomous screen brightness control is performed.For example, the brightness of the panel surface of the liquid crystal display becomes darker than in a warm region. Even in cold climates, it is possible to obtain the optimal brightness according to the brightness of the ambient light and obtain the same visibility as a warm climate, while the brightness of the panel surface is brighter than in a warm climate. Even in a tropical area where it is easy to obtain the optimal brightness according to the brightness of the ambient light, it is possible to obtain the same visibility as in a warm area, so that it can be used in various temperature environments for vehicles It can be applied to a liquid crystal display with peace of mind, in which case a constant visibility can always be obtained, and furthermore, a subtraction circuit subtracts two inputs, an ambient light input and a screen luminance input, into a united signal. Since the brightness of the backlight is controlled, excellent effects such as simplification of the circuit configuration and reduction of the manufacturing cost can be achieved.

Further, according to the present invention, the backlight is a fluorescent tube, and the luminance control means includes an inverter for applying a high-frequency drive signal to the fluorescent tube only during a period in which the pulse width modulated dimming signal takes one potential. Therefore, the inverter can be intermittently operated by the pulse width modulated dimming signal obtained from the comparator, and the amount of light emitted from the fluorescent tube can be varied according to the length of the intermittent operation period, whereby the brightness of the ambient light can be improved. There are effects such as that the brightness of the backlight can be easily and reliably controlled in accordance with the screen brightness.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a light control device for a liquid crystal display of the present invention.

FIG. 2 is a signal waveform diagram of each section of the device shown in FIG. 1 when the brightness of ambient light changes.

FIG. 3 is a block diagram showing an example of a conventional light control device for a liquid crystal display.

[Explanation of symbols]

 2 Backlight (fluorescent tube) 3 Ambient light detection photosensor 4 Photosensor circuit 6 Brightness control means (inverter) 11 Liquid crystal display dimmer 12 Comparator 13 Screen brightness detection sensor 14 Photosensor circuit 15 Difference calculation means (Subtraction circuit) 16 Triangular wave generation circuit

Claims (2)

[Claims] 1. An ambient light detecting means for detecting the brightness of ambient light and outputting a DC signal substantially linearly converted, and detecting the brightness of a screen of a liquid crystal display and outputting a DC signal substantially linearly converted. Screen luminance detecting means, a difference calculating means for taking a difference between the detection output of the screen luminance detecting means and the detection output of the ambient light detecting means, a triangular wave generating circuit for generating a triangular wave having a constant repetition period, A comparator that compares the level of the triangular wave with the output of the difference calculation unit and outputs a pulse width modulated dimming signal; and a luminance control unit that controls the luminance of a backlight of a liquid crystal display by the dimming signal output from the comparator. A light control device for a liquid crystal display, comprising: 2. The method according to claim 1, wherein the backlight is a fluorescent tube, and the luminance control unit applies a high-frequency drive signal to the fluorescent tube only during a period when the pulse width modulated dimming signal takes one potential. The dimming device for a liquid crystal display according to claim 1, wherein