Abstract:
An image display apparatus including an active display device, which is divided into picture elements, and a passive display device, which is divided into picture elements and which is superposed upon the active display device. The image display apparatus has-a high luminance resolution and a high definition.

Description:
This is a Continuation of application Ser. No. 07/564,515 filed Aug. 9, 1990 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a flat type image display apparatus. This invention particularly relates to a flat type image display apparatus which has a high luminance resolution and a high definition. 
     2. Description of the Related Art 
     CRT display devices have heretofore been known as a means for displaying images. The CRT display devices have a high luminance resolution and a high definition, i.e. fine resolution of picture elements, and therefore have heretofore been used widely. However, the CRT display devices have the drawback in that they are large and heavy. 
     Therefore, recently, flat type image display devices, such as a liquid crystal display device (LCD), a plasma display panel (PDP), and an electrochromic display device (ECD), have been utilized in some fields of application. 
     However, none of the conventional flat type image display devices simultaneously satisfy the requirements with regard to a high luminance resolution and a high definition. For example, an LCD and an ECD have a comparatively high definition, but have a markedly lower luminance resolution than a CRT display device. By way of example, the luminance resolution of an LCD or an ECD is 16 levels of gradation. Also, a PDP has a lower luminance resolution and a lower definition than a CRT display device. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide a flat type image display apparatus which has a high luminance resolution and a high definition. 
     Another object of the present invention is to provide a flat type image display apparatus which displays an image having very good image quality. 
     The present invention provides a flat type image display apparatus comprising an active display device, which is divided into picture elements, and a passive display device, which is divided into picture elements and which is superposed upon the active display device. 
     An LCD or an ECD may be utilized as the passive display device. As described above, such a passive display device has a comparatively high definition and a comparatively low luminance resolution. With the flat type image display apparatus in accordance with the present invention, the active display device is located at the rear of the passive display device. Therefore, the luminance resolution can be increased synergistically. Specifically, when a passive display device having, for example, 16 levels of gradation is combined with an active display device having four levels of gradation, the total number of levels of gradation of the flat type image display apparatus becomes equal to 16×4=64. 
     It is desirable that an active display device having the same number of picture elements as the passive display device be utilized. However, it is difficult to obtain such an active display device. Therefore, an active display device having a smaller number of picture elements than the passive display device be utilized. In such cases, a plurality of adjacent picture elements of the passive display device are exposed to an equal amount of light emitted by a single picture element (a single light emitting element) of the active display device. However, in the case of an ordinary image, the difference in image density between the adjacent picture elements is very small. Therefore, even when a plurality of adjacent picture elements of the passive display device are exposed to an equal amount of light, the gradation reproducibility does not deteriorate to such an extent that the gradation become imperceptible. 
     Accordingly, an active display device should preferably be utilized which is suitable primarily for improving the luminance resolution. Specifically, an active display device having more levels of gradation should preferably be utilized. The number of picture elements of the active display device may be smaller than that of the passive display device. By way of example, an electroluminescence panel (EL panel), a PDP, or a light emitting diode panel (LED panel) is suitable as the active display device. 
     As described above, with the flat type image display apparatus in accordance with the present invention, the passive display device, which is comparatively easy to increase the density of picture elements, is superposed upon the active display device. Therefore, the flat type image display apparatus in accordance with the present invention has a high definition and a high luminance resolution. Accordingly, with the flat type image display apparatus in accordance with the present invention, an image having a very good image quality can be displayed. Also, because the flat type image display apparatus in accordance with the present invention is thin in thickness and light in weight, it is applicable in various fields. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are a side view and a plan view showing an embodiment of the flat type image display apparatus in accordance with the present invention, and 
     FIG. 3 is an explanatory graph showing the luminance resolution of the embodiment shown in FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinbelow be described in further detail with reference to the accompanying drawings. 
     FIGS. 1 and 2 are a side view and a plan view showing an embodiment of the flat type image display apparatus in accordance with the present invention. The flat type image display apparatus comprises a flat plate-like substrate  10 , an electroluminescence panel (EL panel)  20 , which serves as an active display device and which is secured to the substrate  10 , and a liquid crystal panel  30 , which serves as a transmission type passive display device and which is secured to the EL panel  20 . The EL panel  20  comprises a plurality of EL elements  21 ,  21 , . . . , which are located in the matrix form. Each of the EL elements  21  constitutes a single picture element. By way of example, the liquid crystal panel  30  is of the thin-film transistor (TFT) switch matrix laminate type. The liquid crystal panel  30  of this type comprises a glass plate  31 , a TFT switch matrix  32 , a liquid crystal layer  33 , a common transparent electrode  34 , and a glass plate  35 , which are stacked in this order from the side of the EL panel  20 . The part of the liquid crystal layer  33 , which is located between one of the picture element electrodes of the switch matrix  32  and the common transparent electrode  34  constitutes a single liquid crystal cell  36 , which serves as a single picture element. 
     In this embodiment, the size of each picture element of the liquid crystal panel  30 , i.e. the size of each liquid crystal cell  36 , is approximately 250×250 μm. The size of each picture element of the EL panel  20 , i.e. the size of each EL element  21 , is approximately 1×1 mm. Therefore, sixteen (i.e. 4×4) liquid crystal cells  36 ,  36 , . . . face each EL element  21 . 
     A control circuit  40  of the flat type image display apparatus receives a digital image signal Sd and generates an EL panel control signal S 1  and a liquid crystal control signal S 2  in accordance with the digital image signal Sd. The EL panel control signal S 1  and the liquid crystal panel control signal S 2  are fed respectively into an EL panel operating circuit  41  and a liquid crystal panel operating circuit  42 . The EL panel  20  and the liquid crystal panel  30  are respectively operated by the EL panel control signal S 1  and the liquid crystal panel control signal S 2 . The image signal Sd takes 64 values ranging from “000000” representing the minimum luminance (maximum image density) to “111111”representing the maximum luminance (minimum image density). The image signal Sd corresponds to the number of picture elements of the liquid crystal panel  30 . The liquid crystal panel control signal S 2  controls the light transmittances of the liquid crystal cells  36 ,  36 , . . . of the liquid crystal panel  30  independently of one another. Therefore, an image displayed, each of the liquid crystal cells  36 ,  36 , . . . constitutes a single picture element of an image displayed on the flat type image display apparatus. The light transmittance of each liquid crystal cell  36  can be controlled such that it takes one of 16 levels. The liquid crystal panel control signal S 2  controls the light transmittance of each liquid crystal cell  36  such that it takes one of 16 levels in accordance with the value of the four least significant digits of the image signal Sd, which value ranges from “0000” to “1111”. 
     As shown in FIG. 2, the EL panel control signal S 1  designates the value of each EL element  21  in accordance with the representative value (e.g. the median value, or the mean value) of the values of the image signal Sd corresponding to sixteen picture elements P 1  through P 16 , which face the EL element  21 . Each EL element  21  is capable of emitting light having one of intensities L 1 , L 2 , L 3 , and L 4  (L 1 &lt;L 2 &lt;L 3 &lt;L 4 ). The EL panel control signal S 1  activates each EL element  21  to emit light having the intensities L 1 , L 2 , L 3 , and L 4  respectively when the value of the most significant digits of the representative value (which is constituted of eight digits) is “00”, “01”, “10”, and “11”. 
     FIG. 3 shows the relationship between the luminance of each picture element and the value of the image signal Sd when the EL panel  20  and the liquid crystal panel  30  are operated respectively in accordance with the EL panel control signal S 1  and the liquid crystal panel control signal S 2 . As illustrated, the intensity of back light emitted by each EL element  21  is controlled such that it takes one of the levels L 1 , L 2 , L 3 , and L 4 . Also, at each level of the intensity of back light, the light transmittance of each liquid crystal cell  36  is controlled such that it takes one of 16 levels. Therefore, the flat type image display apparatus has a total of 64 (i.e. 4×16) levels of gradation. 
     In the embodiment described above, the EL panel  20  is employed as the active display device, and the liquid crystal panel  30  is employed as the passive display device. However, in the flat type image display apparatus in accordance with the present invention, the active display device and the passive display device may be selected from various other types of devices. Also, in the aforesaid embodiment, the ratio of the number of picture elements of the active display device to the number of picture elements of the passive display device is 1:16. However, the ratio of the number of picture elements of the active display device to the number of picture elements of the passive display device may be set to any of other values, including 1:1.