JPH0566501A - Projection type liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide the projection type liquid crystal display device which projects a high-contrast-ratio image on a screen and easily makes a gradational display and is reducible in power consumption. CONSTITUTION:The projection type liquid crystal display device equipped with a transmission type liquid crystal panel 21, a light source 22 which irradiates this liquid crystal panel 21 with light, and an optical system 23 which transmits the light emitted by the light source 22 through the liquid crystal panel 21 and projects the light on the screen 24 is provided with a brightness detecting circuit 12 which detects mean brightness per picture of a video signal, a video signal modulating circuit 15 which modulates a video signal voltage applied to the liquid crystal panel 21 corresponding to a brightness signal detected by the circuit 12, and a light source brightness modulating circuit 13 which varies the brightness of the light source 22 corresponding to the detected brightness signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection type liquid crystal display device, and more particularly to a projection type liquid crystal display device provided with means for varying the brightness of a light source.

[0002]

2. Description of the Related Art In recent years, a thin and lightweight liquid crystal display device has been attracting attention as a display device used for a display of a personal computer or a small television. However, currently commercialized LCD TVs have a diagonal size of only about 4 inches, and their size and price are far from the present CRT TVs.

Therefore, recently, a projection type liquid crystal display device has been developed which enlarges an image on a screen by using a liquid crystal panel as a light valve and combining it with a lens or a dichroic mirror. This device is smaller and lighter than a three-tube type projector using a CRT, and can sufficiently satisfy the increase in screen size.

FIG. 8 shows an example of a typical structural view of a projection type liquid crystal display device. This device has three liquid crystal panels 81,
It comprises a light source 82 such as a halogen lamp and an optical system 83 such as a lens and a mirror. Then, the light emitted from the light source 82 is divided into three primary colors of red, blue, and green by the dichroic mirror 83a and passed through the liquid crystal panel 81, and then combined again by the dichroic mirror 83b, and the projection lens 83 is used.
It is projected on the screen 84 by c. A signal voltage corresponding to each color is applied to each liquid crystal panel 81.
Therefore, images corresponding to the three colors of red, blue, and green are enlarged and projected on the screen 84, whereby a color image is displayed.

However, this type of device has the following problems. That is, if the screen size is increased, the brightness on the screen decreases and the contrast ratio decreases. If the brightness of the light source is increased to solve this, the power consumption increases and the amount of heat generation also increases.

The voltage-luminance characteristic of the liquid crystal panel itself is as shown in FIG. 9, and the contrast ratio is as low as about 50: 1, and the original contrast ratio of the video signal cannot be displayed. Therefore, when a bright part or a dark part is displayed, the image is crushed and a fine image cannot be obtained. Furthermore, since the voltage-transmittance characteristic is too steep and the voltage difference is small, it is difficult to perform fine gradation display.

When an amorphous silicon (a-Si) thin film transistor (TFT) liquid crystal panel is used for the liquid crystal panel, if the light of the light source is increased in order to brighten the screen, the off current (light leakage current) of the TFT is generated. Increased,
Image deterioration such as a decrease in contrast ratio occurs.

[0008]

As described above, in the conventional projection type liquid crystal display device, the contrast ratio is insufficient due to the characteristics of the liquid crystal panel, and the voltage-transmittance characteristic of the liquid crystal panel is too steep and fine. It is difficult to display different gradations. Further, there is a problem that the power consumption by the light source is large, and when the light of the light source is made stronger, the image deterioration such as the decrease of the contrast ratio due to the increase of the light leak current occurs.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a high contrast ratio of an image projected on a screen, easy gradation display, and low power consumption. An object is to provide a projection type liquid crystal display device that can be reduced.

[0010]

In order to achieve the above object, the present invention adopts the following configuration.

That is, the present invention is a transmissive liquid crystal panel, a light source for irradiating the liquid crystal panel with light, and an optical system for transmitting the light emitted from the light source to the liquid crystal panel and projecting the light on a screen. In a projection type liquid crystal display device having means for supplying a drive voltage according to a video signal to a liquid crystal panel, the brightness of a light source is changed according to the video signal.

As means for changing the brightness of the light source, the average brightness, the maximum brightness and the minimum brightness per screen of the video signal are detected, and the brightness of the light source is changed according to the detected information. Good. Further, not only the light source but also the drive voltage applied to the liquid crystal panel may be modulated according to the video signal.

[0013]

In the present invention, the brightness of the light source is changed according to the video signal. That is, the luminance of the light source is lowered when the entire screen is a dark video signal, and the luminance of the light source is raised when the entire screen is bright. Then, the contrast ratio when a bright screen and a dark screen are compared is improved. Moreover, when the brightness of the light source is lowered, not only the power consumption can be reduced, but also the light leakage of the active elements of the liquid crystal panel can be reduced and the contrast ratio can be improved.

Further, the signal voltage applied to the liquid crystal panel is changed according to the video signal. In other words, the video signal is modulated so that the range of the white level and the black level of the signal voltage-transmittance characteristic of the liquid crystal panel is used as much as possible, and the signal voltage is applied to the liquid crystal panel. Then, when gradation display is performed, since the signal voltage can be divided into fine gradations, gradation display can be facilitated and fine gradations can be adjusted.

[0015]

The details of the present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a schematic configuration diagram showing a projection type liquid crystal display device according to a first embodiment of the present invention. This device is roughly divided into a signal processing unit 10 and an optical structure unit 20.

The optical structure 20 is the same as that of the conventional example, and includes a liquid crystal panel 21, a light source 22, an optical system 23 and a screen 2.
It is composed of 4 etc. The optical system 23 includes dichroic mirrors 23a and 23b and a projection lens 23c, and the liquid crystal panel 21 is normally white. The optical structure unit 20 is not limited to the one shown in FIG. 1, and may be any unit as long as the light emitted from the light source 22 passes through the liquid crystal panel 21 and is enlarged and projected on the screen 24.

The signal processing unit 10 includes a video signal input circuit 11 for receiving a video signal or a radio wave from a broadcasting station and inputting a video signal, and an average luminance, a maximum luminance and a minimum luminance per screen of the video signal. , A light source brightness modulation circuit 13 for changing the brightness of the light source according to the detected brightness signal, a light source power source 14 whose voltage value is controlled by the modulation circuit 13, and a brightness detection circuit 12 The video signal modulation circuit 15 which modulates the original video signal with the luminance signal detected by and produces a drive signal voltage to be applied to the liquid crystal panel.

Here, the light source luminance modulation circuit 13 supplies the power source 14 when the average luminance detected by the luminance detection circuit 12 is dark.
Is set so that the voltage value of the power source 13 is increased when the average brightness is bright. Further, the video signal modulation circuit 15 is set such that when the average brightness is dark, the lower drive signal voltage is lower, and when the average brightness is bright, the higher drive signal voltage is higher. Also,
It is also possible to perform finer control according to the maximum brightness and the minimum brightness.

The result of measuring the video signal voltage of the screen displayed by using the apparatus of this embodiment and the luminance characteristic on the screen is shown by a solid line in FIG. The display was a raster display and the brightness on the screen was measured. In addition, the luminance characteristics of the conventional drive signal are indicated by dotted lines on the same figure. From these comparisons, it can be seen that the contrast ratio and the reproducibility of the video signal in the device of this embodiment are excellent.

Here, the principle of the present invention will be described with reference to FIG. When there is a liquid crystal panel having a drive signal voltage-transmittance characteristic as shown in FIG. 3A, when the luminance of the light source is changed, the drive signal voltage-luminance characteristic is changed as shown in FIG. 3B. When is bright, it goes up, and when the light source is dark, it goes down. That is, the brightness of the light source with respect to the drive signal can be changed within the range of the diagonal lines in FIG.

FIG. 4 is a plot of FIG. 3B with the horizontal axis representing the relative brightness of the light source and the vertical axis representing the brightness on the screen. The top straight line is when the drive signal voltage is small and is the white level of this display device. The bottom straight line is when the drive signal is large and is the black level of this display device. Here, for the sake of simplicity, it is assumed that the brightness of the light source changes only in three steps: (1) standard (100%), (2) brighter than standard (120%), and (3) darker than standard (50%).

As a video signal, V3 to V5 are displayed on one screen.
It is assumed that the signal voltage in the range is applied. When the video signal is applied as it is to the liquid crystal panel as a drive signal for the liquid crystal panel in the range of V3 to V5, when the brightness of the light source is the standard of (1), the brightness on the screen becomes A. At this time,
The brightness of the light source is made darker than (3) standard, and the drive signal voltage is V2.
A'in the same brightness range as A, even if changed in the range of V5
Is obtained. The drive voltage of the liquid crystal in A'is wider than that in A. Therefore, A ', that is, the darker the brightness of the light source is, the finer the gradation can be taken.

Similarly, when a video signal is input in the range of V1 to V3, if it is applied to the liquid crystal panel as it is, the range of luminance becomes B. At this time, if the brightness of the light source is increased to satisfy the condition (2) and a drive signal in the range of V1 to V4 is applied to the liquid crystal panel, the white level is increased and the contrast ratio is increased.

In this embodiment, the liquid crystal panel is a-Si.
Although the TFT liquid crystal panel of was used, as described in the conventional example, when the TFT is irradiated with light, it is in an off state (Vg <0).
Drain current (light leakage current: Ioff) increases, and the retention characteristics of the pixel deteriorate, which causes a decrease in contrast ratio and flicker. FIG. 5 shows the difference in light leak current when the light intensity is changed. This figure is
Shows the respective transistor characteristics when no light is irradiated (shown by the solid line in the figure), when the illuminance is L1 (shown by the dashed line in the figure), and when the illuminance is L2 (shown by the broken line in the figure). ing. However, (L2> L1). It can be seen that the light leakage current is smaller in L1 having low illuminance than in L2 having high illuminance.

In the case of a projection type liquid crystal display device, it is necessary to brighten a light source in order to brighten an image, and the intensity of light applied to the liquid crystal panel reaches 200,000 lux in the conventional example. Therefore, the light leak current of the TFT is also not negligible, and this light leak current causes various adverse effects.

In this embodiment, the brightness of the light source is increased when the image is bright, and the brightness of the light source is decreased when the image is dark. Therefore, the leak current is not constant, and is large when the image is bright and small when the image is dark. On the other hand, the light leakage current causes almost no problem when the image is bright, but when the image is dark, the light leakage current remarkably appears as image quality deterioration. Therefore, if the brightness of the light source is changed as in the present embodiment, the light leak current in a dark image that causes deterioration of image quality can be reduced.

As described above, according to this embodiment, the contrast of the contrast ratio is changed by changing the luminance of the light source according to the average luminance per screen and modulating the drive signal (video signal) applied to the liquid crystal panel. It is possible to realize a projection type liquid crystal display device which has a high price, facilitates gradation display, and has low power consumption.

In the above embodiment, the light source and the drive signal are modulated by the luminance of the video signal, but the modulation by the color signal or
It is also possible to add a signal dedicated to modulation to the video signal. Moreover, the brightness of the light source is not limited to three levels, but can be set in a plurality of levels and can be continuously changed. Further, although the normally white type is used as the liquid crystal panel in the embodiment, the same effect can be obtained by using the normally black type. In this case, the voltage applied to the liquid crystal panel may be black and white and opposite to that in the embodiment.

FIG. 6 is a schematic block diagram showing the second embodiment of the present invention. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. The present embodiment is different from the above-described embodiments in that two lamps are used as the light source 22.

The light sources 22a and 22b in this embodiment are metal halide lamps. Since the metal halide lamp is a discharge type lamp, it is characterized by high luminous efficiency and high brightness. In the present embodiment, the metal halide lamp 22b is used to keep the brightness constant and to increase the brightness of the display device, and the brightness is modulated using the other metal halide lamp 22a.

The lights emitted from the two lamps 22a and 22b are combined by the prism 23d in order to align the optical axes. Even in this case, the same effect as that of the previous embodiment can be obtained. In this embodiment, two lamps of the same type are combined, but the kinds of lamps may be different and the number may be two or more. Further, the combination of the lamp that changes the luminance and the lamp that does not change the luminance is not limited to the present embodiment.

In the above two embodiments, a polarizing plate is used for the liquid crystal panel, but in the case of the projection type, the light from the light source is strong, and most of it is absorbed by the polarizing plate and replaces heat.
Therefore, in order to protect the liquid crystal panel from heat and to improve the heat radiation efficiency, the polarizing plate 71 is provided on the glass substrate 7 as shown in FIG.
3 is attached with an adhesive 72 and installed at a position separated from the liquid crystal panel 74. In this case, as shown in FIG. 7A, the case where the glass is placed on the liquid crystal panel side,
As a result of comparing the case where the polarizing plate is arranged so as to face the liquid crystal panel side as shown in FIG. 7B, the arrangement as shown in FIG. 7B shows better contrast ratio and color display characteristics.

The reason for this is considered to be the influence of the adhesive 72. That is, in the case of FIG. 7A, the light polarized through the polarizing plate 71 is disturbed by the adhesive 72 and enters the liquid crystal panel 74. On the other hand, in the case of FIG.
The light disturbed by the adhesive 72 is polarized by the polarizing plate 71,
This is because light whose polarization direction is uniform is incident on the liquid crystal panel 74. In addition, the adhesive 72
A method of fixing the polarizing plate without using may be adopted.

[0035]

As described above in detail, according to the present invention, since the brightness of the light source is varied according to the image signal, the contrast ratio of the image projected on the screen is high and the gradation display is possible. It is possible to realize a projection type liquid crystal display device which is easy and consumes less power.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a projection type liquid crystal display device according to a first embodiment of the invention,

FIG. 2 is a diagram showing a video signal voltage-luminance characteristic in the first embodiment,

FIG. 3 is a diagram showing drive signal voltage-luminance characteristics in the first embodiment;

FIG. 4 is a diagram showing a luminance change on a screen with respect to a luminance change of a light source,

FIG. 5 is a diagram showing Id-Vg characteristics of an a-Si thin film transistor;

FIG. 6 is a schematic configuration diagram showing a second embodiment of the present invention,

FIG. 7 is a view showing an arrangement example of polarizing plates,

FIG. 8 is a schematic configuration diagram showing a conventional projection type liquid crystal display device,

FIG. 9 is a diagram showing a signal voltage-luminance characteristic in a conventional device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Signal processing part, 11 ... Video signal input part, 12 ... Brightness detection circuit, 13 ... Light source brightness modulation circuit, 14, 14a, 14b ... Power supply, 15 ... Video signal modulation circuit, 20 ... Optical structure part, 21 ... Liquid crystal Panel, 22 ... Light source, 23 ... Optical system, 23a, 23b ... Dichroic mirror, 23c ... Projection lens, 23d ... Prism, 24 ... Screen.

Claims (1)

[Claims] 1. A transmissive liquid crystal panel, a light source for irradiating the liquid crystal panel with light, an optical system for transmitting light emitted from the light source to the liquid crystal panel and projecting the light onto a screen, and the liquid crystal panel. A projection type liquid crystal display device comprising: a means for supplying a drive voltage according to a video signal; and a means for changing the brightness of a light source according to the video signal.