The present invention relates to an image display for displaying images with image shift by wobbling.
Japanese Patent Disclosure No. 6-324320 discloses an image display, which comprises a vibrating means disposed on an optical path between a display element having a discrete pixel array and an observing position for vibrating the optical axis of light emitted from the display element in predetermined directions. In this display, odd and even field images are successively written in the same pixel of the display element for display, and in synchronism to the fields the vibrating means vibrates the optical axis of light from the display element for predetermined directions, that is, wobbles the display surface of the display element, thus spatially separating the odd and even field images from one another. In this way, equivalent pixels are displayed on a pixel-free black matrix portion of the display surface, thus improving the resolution.
FIG. 22 shows the construction of this prior art image display. The illustrated image display comprises a color liquid crystal panel (hereinafter referred to as LCD) 1 as a display element), having a backlight 1a and a color liquid crystal display element 1b and a vibrating means including a polarization converting element 2 and a double refractor 3 disposed one ahead of the other on the front surface side of the LCD 1. The LCD 1 has, for instance, one half the scanning lines of the NTSC, and as shown in a fragmentary plan view in FIG. 23, has delta arrays of R, G and B pixels. In FIG. 22, a reduced number of, i.e., several, scan lines are shown for the sake of the clarity of the drawing.
As the polarization converting element 2, a twist nematic liquid crystal shutter (hereinafter referred to as TN shutter) is usually used, which is relative inexpensive and is manufactured on the basis of an established technique. As shown in FIGS. 24(a) and 24(b), the TN shutter 2 includes a pair of polarizing members 6 having transparent electrodes 5 and a TN liquid crystal layer 7 sandwiched between the transparent electrodes 5. An AC power source 9 is connected between the pair transparent electrodes 5 via a switch 8. As shown in FIG. 24(a), with an AC voltage applied across the TN liquid crystal layer 7 by turning on the switch 8, the polarization of light incident on the polarization converting element 2 is transmitted without being rotated. As shown in FIG. 24(b), with no AC voltage applied across the TN liquid crystal layer 7 by switching off the switch 8, the polarization of the incident light is transmitted while it is rotated by 90 degrees.
The double reflector 3 is formed from an anisotropic crystal, such as rock crystal (xcex1-SiO2), lithium niobate (LiNbO3), rutile (TiO2), calcite (CaCo3), Chile nitre (NaNo3) and YVO4. As shown in FIG. 25, it transmits incident light of a first polarization as normal light, and transmits incident light of a second polarization at right angles to the first polarization as abnormal light. Denoting the thickness of the double refractor 3 in z-axis direction perpendicular to xy coordinates of the display surface of the color LCD 1, i.e., the direction of incidence of light beam by d and the angle of separation between the normal light and abnormal light by xcex8, the normal and abnormal light beams emitted from the double reflector 3 are spatially separated by dxc3x97tan xcex8.
Thus, with the crystallization axis 3a of the double refractor 3 set in a suitable direction, as shown in FIG. 26, by turning off the TN shutter 2 the polarized light is rotated in the TN shutter 2 by 90 degrees and transmitted therethrough as a second polarized light, and is then transmitted through the double refractor 3 as, for instance, abnormal light. In this way, as shown in FIG. 27, the pixels of the display surface of the color LCD 1 can be observed in black matrix positions obliquely upwardly rightward by substantially one half pixel pitch from the original pixel positions. As shown in FIG. 28, by turning off the TN shutter 2 the polarized light from the color LCD 1 is transmitted through the TN shutter 2 without being rotated but as the input first polarized light itself, and is transmitted through the double refractor 3 as normal light. In this case, the pixels of the display surface of the color LCD 1 can be observed in the original positions as shown in FIG. 23.
In the prior art image display as shown in FIG. 22, the properties of the TN shutter 2 and the double refractor 3 are utilized such that, while odd and even field images of the input image signal are successively displayed on the same pixel of the color LCD 1 under control of an image display control circuit 11, the voltage applied to the TN shutter 2 is on-off controlled fixedly by an TN shutter drive circuit 12 which constitutes a vibrating means. Thus, wobbling, i.e., changing of the pixel position observed through the double refractor 3 according to the direction of polarization of light transmitted through the TN shutter 2, is obtained to improve the resolution. More specifically, in the odd field the TN shutter 2 is held xe2x80x9coffxe2x80x9d, and, as shown in FIG. 29, the observed pixel positions are shifted obliquely upwardly rightward by substantially one half pixel pitch from the original pixel positions (the pixel positions in this case being shown as Ro, Go and Bo). In the even field the TN shutter 2 is held xe2x80x9conxe2x80x9d, and, as shown in FIG. 30, the original pixel positions are restored as the observed pixel positions (the pixel positions in this case being shown as Re, Ge and Be). It is thus possible to permit observation of images with double the pixel number of the color LCD 1.
For the odd and even field images displayed on the color LCD 1, the image signal is sampled at timings different from each other by a time corresponding to the extent of image shift. More specifically, when displaying the odd field images, the timing of sampling of the image signal is delayed behind the timing of when displaying the even field images by a time corresponding to substantially one half pixel pitch. Also, since the color LCD 1 holds the hold image in display until it is re-written by the next field image, one of the pair electrodes of the TN shutter 2 is divided into a plurality of lines, for instance about 51 lines, while the other electrode is used as a common electrode. The voltage application is thus controlled by selecting the divided electrodes according to the timing of the line scanning of the color LCD 1.
However, according to various experiments and studies conducted by the inventor with the prior art image display adopting the wobbling as described above, it has been found that there are many problems. For example, the response characteristic in the rotation of the light from the TN shutter 2 constituting the vibration means has dependence on temperature and is deteriorated in the low temperature, thus sufficient resolution improvement can not be obtained.
FIGS. 31(a) and 31(b) are views for describing the response characteristics in the rotation of the polarized light in the TN shutter 2. Specifically, FIG. 31(a) shows the first polarized light transmittance, and FIG. 31(b) shows the drive voltage. It is assumed that a high frequency voltage is applied as the drive voltage. The TN shutter 2 has a rise response time xcfx84ON when the drive voltage is turned on and a fall response time xcfx84OFF when the drive voltage is turned off. Denoting the maximum and minimum first polarized light transmittances of the TN shutter 2 by Tm and To, respectively, the rise response time xcfx84ON is represented by the sum of a rise delay time tdON from the instant when the drive voltage is turned on till the instant when 10%, i.e., (To+0.1(Tmxe2x88x92To)) is reached by the first polarized light transmittance after the commencement of behavior of the liquid crystal and a rise time tr from the instant of actual rising of the TN liquid crystal upon reaching of the 10% transmittance till the instant of reaching of 90% transmittance, i.e., (To+0.9(Tmxe2x88x92To). The fall response time, xcfx84OFF, on the other hand, is represented by the sum of a fall delay time tdOFF from the instant of commencement of the behavior of the liquid crystal when the drive voltage is turned off till the instant of transmittance fall down to 90% and a fall time td from the instant of actual falling of the TN liquid crystal upon the transmittance fall down to 90% till the instant of the transmittance fall down to 10% again.
In the above response characteristics, the rise time tr depends on the applied drive voltage, while the fall time td depends on material characteristics peculiar to the liquid crystal. These times tr and td as well as the rise and fall delay times tdON and tdOFF, also depend on temperature. Thus, the values of tdON, tr, tdOFF and td vary in dependence on temperature even by applying the same drive voltage to the same TN shutter 2. For example, the response rate which is assumed to be tdON=0.5 ms, tr=1 ms, tdOFF=5 ms and td=5 ms, and hence xcfx84ON=1.5 ms and xcfx84OFF=10 ms, at 30xc2x0 C., is better at 40xc2x0 C. at which tdON≈0 ms, tr≈5 ms, tdOFF=2 ms and td=3 ms, and hence xcfx84ON≈0.5 ms and xcfx84OFF=5 ms, and is worse at 10xc2x0 C., at which tdON=1 ms, tr=2 ms, tdOFF=8 ms and td=7 ms, and hence xcfx84ON=3 ms and xcfx84OFF=1.5 ms.
When the rise and fall response characteristics are bad particularly at low temperatures, the transmittance of one pixel of the TN shutter 2 to the first polarized light with the drive voltage as shown in FIG. 32(b) is as shown in, for instance, FIG. 32(a). Ideally, in the even field only the first polarized light should be transmitted for displaying the data (i.e., image signals Re, Ge and Be) thereof only in the original pixel positions (hereinafter also referred to as even lines). However, during the rise response time tdON, the second polarized light is also transmitted. Therefore, the data of the first polarized light is also displayed in the shifted pixel positions (hereinafter referred to odd lines), at which the data (i.e., image signals Ro, Go and Bo) of the second polarized light are to be displayed. Likewise, during the fall response time tdOFF both the first and second polarized light beams are transmitted, and data (i.e., image signals Ro, Go and Bo) which should be displayed only in the odd lines, are also displayed in the even lines, thus resulting in an observed image as shown in FIG. 33(b).
For the above reason, particularly at low temperature, at which the response characteristics are worse, due to residual light of the preceding field it is impossible to obtain sufficient resolution improvement by wobbling. In addition, since the temperature dependency of the response characteristics usually pronounced during the fall response time xcfx84OFF compared to the rise response time xcfx84ON, a problem of contrast reduction is posed. In the case of FIGS. 32(a) to 32(c) opposite polarity drive voltages are alternately applied to the TN shutter 2 in successive even fields. However, it is also possible to apply a high frequency drive voltage in each voltage application period.
The calculation of the contrast will now be described with reference to FIGS. 34(a) to 34(b), which are enlarged-scale views of FIGS. 32(a) and 32(b). In FIG. 34(a), the response characteristic in the rotation of the polarized light is approximated by a straight line plot for the sake of simplifying the calculation. In FIG. 34(a) denoting the areas of the even and odd fields of the response characteristic by Se and So, respectively, the contrast Cont is given as:
Cont=(Sexe2x88x92So)/(Se+So)xe2x80x83xe2x80x83(1)
Denoting the time of one field by tF, the areas Se and So are given as:
Se=tF=xcfx84NO+(1/2)trxe2x80x83xe2x80x83(2)
and
So=tdOFF+(1/2)td.xe2x80x83xe2x80x83(3)
The contact Cont is thus given as:
Cont=(tFxe2x88x92xcfx84ONxe2x88x92tdOFF+(1/2)(trxe2x88x92td))/(tFxe2x88x92xcfx84ON+tdOFF+(1/2)(tr+td))xe2x80x83xe2x80x83(4)
By substituting tF=16.67 ms (i.e., 1/60 s) as each of the values at 30xc2x0 C., 40xc2x0 C. and 10xc2x0 C. noted above into the equation (4), we have
Cont.[30xc2x0 C.]=0.353
Cont.[40xc2x0 C.]=0.649
and
Cont.[10xc2x0 C.]=0.121,
it was found that the contrast is reduced with reducing temperature.
Such contrast reduction with temperature changes is also pronounced in the case when image to be displayed on the color LCD 1 is preliminarily corrected in the odd and even fields to compensate the resolution reduction due to the response characteristic in the TN shutter 2.
Where the response characteristic and the temperature dependency of the TN shutter 2 have been described, where the color LCD 1 is used as display element as shown in FIG. 22, its constituent, for instance a TFT LCD, also has a response characteristic. Therefore, when the TN shutter 2 is on-off controlled for shuttering as the same timings as the timings of switching of the odd and even field images to be displayed on the LCD, even in the case that the TN shutter 2 has an ideal response characteristic and selectively transmits the first and second polarized light beams at the same timings as the image switching timings, residual light of the preceding field is also generated due to the response characteristic of the LCD, In this case, sufficient improvement of the resolution by wobbling can no longer be obtained, thus giving rise to the problem of the contrast reduction.
FIGS. 35(a) and 35(b) are views for describing the response characteristic of a positive type Crossnicol LCD. Specifically, these Figures show the light blocking factor of the LCD and the drive voltage applied thereto when white at a certain brightness and black are displayed alternately for each field on a pixel of the LCD. Referring to FIG. 35(a), denoting the rise delay time from the instant when the drive voltage is turned on till the instant when the light blocking factor of the pixel reaches 10% by TdONxe2x80x2, the subsequent rise time till the reaching of a light blocking factor of 90% by trxe2x80x2, the rise response time represented by the sum of tdONxe2x80x2 and trxe2x80x2 by xcfx84ONxe2x80x2 the fall delay time from the instant when the drive voltage is turned off till the instant when the light blocking factor is reduced down to 90% by tdOFFxe2x80x2 the subsequent fall time till the instant when the light blocking factor is reduced down to 10% by tdxe2x80x2 and the fall response time represented by the sum of tdOFFxe2x80x2 and tdxe2x80x2 by xcfx84OFFxe2x80x2, with an LCD using TN liquid crystal we have tdONxe2x80x2=2 ms, trxe2x80x2=10 ms, tdOFFxe2x80x2=2 ms and tdxe2x80x2=10 ms, and hence xcfx84ONxe2x80x2=12 ms and xcfx84OFFxe2x80x2=12 ms. The response characteristic is not so satisfactory.
Here, the contrast Contxe2x80x2 obained as in the case of FIGS. 34(a) and 34(b) by setting the field time to be tF=16.67 ms and the areas in the even and odd fields to be Sexe2x80x2 and Soxe2x80x2 is
Cont.xe2x80x2=0.160.
With such unsatisfactory response characteristic of the LCD, by turning on and off the TN shutter 2 in synchronism to the field the contrast is deteriorated even when the TN shutter 2 has an ideal characteristic.
As descried before, in the case of using an LCD as the display element, the use of DC as the drive voltage leads to characteristic deterioration due to internal electro-chemical changes. Usually, therefore, an AC drive method is adopted, in which a high frequency voltage is applied or the polarity of the applied voltage is inverted for every field.
Considering now a pixel in driving the TN shutter by wobbling, the polarity inversion of the applied drive voltage for every field results in a deviated pixel display position. For example, the applied voltage is always positive (or negative) in the odd fields and always negative (or positive) in the even fields, in which the pixel is displayed in the original pixel position. However, as shown in FIG. 36 which shows the waveform of image signal when a pixel is considered, the center potential Vc (shown by phantom line) of the applied AC drive voltage and the common electrode voltage Vcom (shown by dashed line) of the LCD do not coincide with each other, but may, for instance, be Vc greater than Vcom. Therefore, when image data of the same brightness is to be displayed in the successive fields, even though the absolute value Va of the applied drive voltage is the same in the odd and even fields of the image signal, with Vb=(Vcxe2x88x92Vcom) the absolute value of the actual drive voltage applied to the LCD is Vo=(Va+Vb) in the odd field and Ve=(Vaxe2x88x92Vb) in the even fields, that is, the drive voltage absolute value Ve in the odd fields is greater than the drive voltage absolute value Ve in the even fields.
Due to the deviation of Vc and Vcom from each other, the inter-field drive voltage waveform is no longer symmetrical. Thus, the brightness is changed even when the same brightness image is displayed. For example, in the case of an LCD of the positive type, in which the display is the blacker the higher the absolute applied voltage value, with Vo greater than Ve as noted above, the display is dark in the odd fields and bright in the even fields. In other words, image irregularities are generated in the wobbled image due to repeated generation of dark and bright fringes in the image, thus deteriorating the image quality.
In the meantime, a binocular display such as a head mounted display (hereinafter referred to as HMD) may use the image display as shown in FIG. 22 as each of the displays for the left and right eyes. In such a binocular system, if the direction of pixel shift by wobbling is the same obliquiely rightward direction, a frequency space as shown in FIG. 37 can be obtained concerning the resolution. Specifically, when the wobbling is not executed, the frequency space, in which an LCD having a delta pixel array can display images, is an area as defined by xc2x1Px and xc2x1Py. With horizontal and vertical pixel pitches ax and by of the delta pixel array, are area is defined by (Px=1/ax) and (Py=1/by).
When a pixel shift is executed in this binocular system by wobbling in the same obliquely rightward direction by, for instance, one half pixel pitch in both the horizontal and vertical directions, the frequency space is now an area defined by xc2x1Pxxe2x80x2 and xc2x1Pyxe2x80x2 in the obliquely rightward direction. Compared to the case when the wobbling is not executed, this area is broader in correspondence to the pixel pitch reduction to one half. In this case, however, the frequency area is increased in the sole obliquely rightward direction noted above. Therefore, the images displayed on both the let and right image displays, which are viewed as a merged iamge, can not be observed as natural image.
The present invention was made in view of the various problems described above, and it has an object of providing an image display capable of effective improvement in the wobbling effect reduction due to the temperature dependency of the response characteristic of the vibrating means and displaying images at a high resolution.
Another object of the present invention is to provide an image display capable of effective improvement in the wobbling effect reduction due to the responce characteristic of the display elements and displaying images at a high resolution.
A further object of the present invention is to provide a image display capable of effective prevention of image irregularities and displaying images at a high resolution and of a high image quality.
A still further object of the present invention is to provide a binocular image display with a left and a right image display capable of observation of natural merged images from the two image displays by displaying high resolution images thereon by wobbling.
According to an aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields on the display element, and a vibrating means for vibrating the optical axis of light emitted from the display surface in predetermined directions in synchronism to the switching of images by the image display control means, wherein the image display further comprises a temperature control means for the temperature of the vibrating means.
With the provision of the temperature of the vibrating means, it is possible to effectively improve the wobbling effect reduction due to the temperature dependency of the response characteristic of the vibrating means.
The temperature control means includes a heating means for heating the vibrating means for improving the optical axis vibration response characteristic of the vibrating means at low temperatures.
According to another aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields of the display element, and a vibrating means for vibrating the optical axis of light emitted form the display surface in predetermined directions in synchronism to the switching of images by the image display control means, wherein the vibrating means vibrates the optical axis in accordance with the response characteristic of the display element.
Since the vibrating means for wobbling is adapted to vibrate the optical axis according to the response characteristic of the display element, it is possible to effectively improve the wobbling effect reduction due to the response characteristic of the display element and thus display high resolution images.
The vibrating means includes a polarized light converting means and a driving means for driving the polarized light converting means in accordance with the response characteristic of the display element.
According to other aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields on the display element, and a vibrating means for vibrating the optical axis of light emitted from the display surface in predetermined directions in synchronism to the switching of images by the image display control means, wherein the image display control means includes a polarity inverting means for inverting the polarity of an image signal applied to the display pixels of the display element for every two instants of image switching.
According to still other aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields on the display element, and a vibrating means for vibrating the optical axis of light emitted from the display surface in predetermined directions in synchronism to the switching of images by the image display control means, wherein the image display control means includes a polarity inverting means for inverting the polarity of an image signal applied to the display pixels of the display element for each frame.
Since the image display control means for displaying different images different in successive fields on the display element includes a polarity inverting means for inverting the polarity of an image signal applied to the display pixels of the display element for every two instants of image switching or for each frame, even when the center potential of the AC drive voltage and the common electrode potential on the display element fail to coincide with each other in the polarity inversion, it is possible to effectively prevent the generation of image irregularities and display high resolution and high image quality images by wobbling.
According to further aspect of the present invention, there is provided an image display comprising a left and a right display elements each having a display surface with a regular array of a plurality of pixels, a left and a right display control means for displaying different images different in successive fields on the display surfaces of the respective display elements, and a left and a right vibrating means for vibrating the optical axes of light emitted from the display surfaces in predetermined directions in synchronism to the switching of images by the image display control means, wherein the left and right vibrating means vibrate the optical axes in different directions.
In binocular observation image display in which wobbling is executed in both the left and right image displays, the left and right vibrating means for wobbling are adapted to vibrate the optical axes in different directions, and it is thus possible to permit observation of high resolution images obtained as a result of wobbling as natural images.
The left and right vibrating means vibrate the optical axes in symmetrical directions with respect to a vertical axis.
According to still further aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields on the display element, and a vibrating means or vibrating the optical axis of light emitted from the display surface in predetermined directions in synchronism to the switching of images by the image display control means, the image display including a twist nematic liquid crystal shutter (TN shutter), a double refractor and an TN shutter drive circuit for on-off controlling the TN shutter, and the temperature control means permitting the temperature of the TN shutter to be held at a fixed temperature.
The temperature control means is one of a sheet-like heater disposed around the TN shutter, a transparent electrode of the TN shutter as a heater, a Peltier element, and a transparent heater pattern for generating heat on a glass substrate of the TN shutter. The image display further comprisies a cooling means thermally coupled to the TN shutter. The temperature control means further controls temperature of the display elements.
According to other aspect of the present invention, there is provided an image display comprising a display element having a display surface with a regular array of a plurality of pixels, an image display control means for displaying different images different in successive fields on the display element, and a vibrating means for vibrating the optical axis of light emitted from the display surface in predetermined directions in synchronism to the switching of images by the image display control means, the image display including a twist nematic liquid crystal shutter (TN shutter), a double refractor and an TN shutter drive circuit for on-off controlling the TN shutter in responsive to a synchronizing signal of an image signal, wherein the TN shutter is controlledf on the basis of the display element and the TN shutter.
The TN shutter is controlled such that the transmittance to two polarized lights are substantially 50% at the instant when the time tA has passed from the instant of the field switching, tA being one half the arithmetic mean of the rise time from the instant when the light blocking factor is 0% till the instant of reaching of 100% light blocking factor.
The image display further comprises, a field detecting circuit for generating a field synchronizing signal on the basis of the synchronizing signal from the display element, a first and a second delayed signal generating circuits for delaying the field synchronizing signal by a first and a second times, respectively, on the basis of response characteristic of the TN shutter, TN shutter drive signal generating circuit for generating, in response to receipt the outputs of the delayed signal generating circuits, a TN shutter drive signal, the TN shutter being controlled such that the transmittance to two polarized lights are substantially 50% at the instant when the time tA has passed from the instant of the field switching, tA being one half the arithmetic mean of the rise time from the instant when the light blocking factor is 0% till the instant of reaching of 100% light blocking factor.
The two outputs of the first and a second delayed signal generating circuits are used as a set signal and a reset signal for generating the TN shutter drive signal.
Other objects and features will be clarified from the following description with reference to attached drawings.