Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1994-03-24
2002-05-14
Lao, Lun-Yi (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S097000
Reexamination Certificate
active
06388649
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spatial light modulator (hereinafter abbreviated as an SLM) used in a projection-type display, a holography television, or an optical computer.
2. Description of the Related Art
An optically addressed SLM is positively developed as an essential part of a projection-type display apparatus having a large screen size which is an alternative to a thin film transistor (TFT) liquid crystal panel or a cathode ray tube (CRT).
An SLM performs a light amplification of a two-dimensional image pattern. The SLM includes a photoconductive layer and a light-modulating layer as main components thereof. When image information with low luminance enters the SLM from the side of the photoconductive layer, the electric characteristics of the photoconductive layer are modulated depending on the luminance of the image information. As the result of the modulation, the optical characteristics of the light-modulating layer are modulated. Then, reading light with high luminance enters the SLM from the side of the light-modulating layer, whereby amplified image information can be output.
As the material of the photoconductive layer, CdS, crystalline silicon (Si), amorphous Si (a—Si:H), or the like is used. Among them, a—Si:H is widely used, since a—Si:H has various superior characteristics such as high sensitivity to writing light, a low dark conductivity, wide variations of film formations, and the like. The erasure of the written image is performed without using special erasure pulse light, so that the photoconductive layer is often provided with rectification.
As the material of the light-modulating layer, electrooptic crystals or liquid crystals are used. Among such crystals, surface stabilized ferroelectric liquid crystals (SSFLC; hereinafter referred to simply as PLC) are positively used since the FLC has a faster response (about 100 &mgr;sec.) as compared with the conventional liquid crystal of the twisted nematic (TN) type.
It is conventionally known that the FLC has bistability (the binary characteristics). Specifically, the direction of spontaneous polarization of FLC is changed depending on the polarity of the applied electric field, so that the FLC has two different optical states, i.e., ON (or UP) and OFF (or DOWN). The bistability is described in detail in, for example, Appl. Phys. Lett., vol. 36 (1980) pp. 899-901.
In the case where the FLC is used practically in a display or the like, the FLC must be able to display a half-tone state between the ON state and the OFF state. In order to realize the half-tone state, the amount of applied charges is controlled, rather than the externally applied electric field. Since the FLC has the spontaneous polarization which is represented by the spontaneous polarization charge P
s
, it is that the aging due to the decomposition of the FLC molecules can be prevented. On the other hand, the driving waveform has the following problems. The voltage of the writing pulse (characterized by a writing period and a writing voltage)
202
is a very high negative voltage, so that the FLC polarization is in verted (electric field switching) even in a condition without writing light. As a result, the contrast is deteriorated. Also, since the duty ratio of the output light is ½ at most, this causes a loss of brightness. The driving method using the voltage waveform in FIG.
2
(
b
) was proposed by the inventors of this invention. In the driving method, the obtained duty ratio can substantially be 1. However, also in this method, it is impossible to prevent the contrast deterioration due to the electric field switching during the application of the writing pulse
205
.
FIG. 3
shows a driving pulse waveform used for solving the above problems. Such waveforms are used, for example, in SID Digest (1991), pp. 254-256 and U.S. Pat. No. 5,178,445. The driving pulse includes a short erasure pulse
301
with a high voltage and a long writing pulse
302
with a low voltage. In the driving pulse, the absolute value of the voltage of the writing pulse
302
is set to be smaller than the absolute value of the voltage of the erasure pulse
301
, so that it is possible to prevent the electric field switching. In addition, since the period of the writing pulse
302
is long, the duty ratio of the reading light can substantially be 1. Thus, the driving method is suitable for the application to a projection-type display or the like. The latter reference specifically mentions the driving voltage conditions for the driving with high contrast. However in both cases, the employed FLC cannot stably have states other than the ON and OFF states. In the former case, the half-tone display is realized by using the above-mentioned multi-domain gray-scale. The latter case uses, as the writing light, pulse light (for example, the emitted light from CRT having phosphors with shorter emission time than the length of one driving period of the SLM).
When the image on a CRT is written on the SLM, the period of the driving pulse voltage signal of the SLM is generally synchronized with one display period of the CRT.
FIG. 4
shows the timing chart for driving the CRT and the SLM. In
FIG. 4
, the timing chart (a) indicates the synchronization pulse voltage of the CRT, and the timing chart (b) indicates the fluorescence intensity from a phosphor on the CRT screen. Also, the timing chart (c) indicates the voltage of a driving pulse of the SLM which is synchronized with one display period of the CRT, and the timing chart (d) indicates the intensity of an output light from the SLM. As is shown in the timing chart (c), a unit driving pulse voltage signal
404
includes an erasure pulse
402
and a reading pulse (characterized by a reading period and a reading voltage)
403
, and the signal
404
is synchronized with a synchronization pulse
401
of the CRT display shown in the timing chart (a). The SLM is driven by the driving pulse voltage signal
404
. As is shown in the timing chart (b), at a certain timing in the reading period of reading pulse
403
of the driving pulse voltage signal
404
, a phosphor of the CRT emits a writing light pulse
405
. The light is received by the photoconductive layer of the SLM, and the light-modulating layer of the SLM is switched into the ON state. As a result, as is shown in the timing chart (d), the intensity
406
of the output light from the SLM rises. When an erasure pulse
402
of the next unit driving pulse voltage signal
404
is applied, the light-modulating layer is switched into the OFF state, so that the intensity
406
of the output light from the SLM becomes 0. By repeatedly performing the above operations, the output light from the SLM can be observed. This method has an advantage in that the duty ratio of the reading light (the ratio of the ON state period of the reading light to one driving period) can be increased even in the CRT screen having phosphors which emit fluorescence with a short decay time. In general, in the case where a negative voltage of a large value is applied to the SLM, the SLM may erroneously be switched into the ON state due to the electric field caused by the negative voltage even if the writing light is not incident. However, if the driving signal having a waveform shown in the timing chart (c) is used, such a negative voltage having a large value is not applied to the SLM, so that the light-modulating layer of the SLM cannot be erroneously switched into the ON state. In addition, the deterioration of the contrast ratio of the image which may be caused by such erroneous switching can be prevented. It is also reported that, by varying the fluorescence intensity from a phosphor on the CRT screen, it is possible to realize the half-tone display of the output light of the SLM.
If an SLM is applied to a projection-type display or a holography television, it is necessary that a stable half-tone display with good contrast and with good controllability can be performed.
The conventional SLM could stably have only two optical states as the light
Akiyama Koji
Asayama Junko
Kuratomi Yasunori
Ogawa Hisahito
Takimoto Akio
Lao Lun-Yi
Matsushita Electric - Industrial Co., Ltd.
Renner Otto Boisselle & Sklar
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