The present invention relates to a liquid crystal device and an image forming apparatus utilizing a liquid crystal device and more particularly to an image forming apparatus with a printer head of the type in which a light path is opened or closed by means of an optical modulator utilizing a ferroelectric liquid crystal so as to produce light signals.
With recent remarkable advances in information processing technologies, there have been increasing demands for high information or packaging density and high speed of image forming apparatus. Furthermore, there has been a strong demand for high printing quality. To satisfy these and other demands, electrophotographic devices, laser beam printers and optical fiber tube printers have been developed and put into commercial practice. However, these image forming apparatus are very expensive and complicated in construction and it is difficult to make them compact in size and light in weight. Therefore, recently there have been proposed less expensive and smaller image forming apparatus utilizing PLZT or optical shutters of liquid crystal or image forming apparatus such as LED printers utilizing light-emitting diodes. Of these image forming apparatus, liquid crystal shutter printers utilizing electrooptical effects of liquid crystal are considered promising as image forming apparatus capable of obtaining images with less expense and high information density.
As for a liquid crystal used in a head of a liquid crystal shutter printer, a method for driving a twisted nematic crystal by a two frequency system is disclosed in, for instance, Japanese Laid-Open patent application No. 94377/1981. According to a printer head of this system, a liquid crystal composition which exhibits a positive or a negative dielectric anisotropy in response to different frequencies of applied voltage is used, and the operation principle is based on the fact that when applied frequencies are selectively varied, the liquid crystal is optically distinguished between the state in which liquid crystal molecules are oriented in the direction of electric field and the state in which liquid crystal molecules are oriented in the direction perpendicular to the electric field. In general, the higher the applied voltage is, the faster the response speed becomes. Accordingly, as liquid crystals oriented in one direction produce a bright state while those oriented in the other direction produce a dark state, switching between the bright and the dark state can be effected by the forced application of a voltage so that high-speed response becomes possible if as high a voltage as possible is forcibly applied. However, the response time is of the order of one microsecond at the shortest and is considerably longer than the response time of the order of tens of nanoseconds of LED printer heads so that the head of a liquid crystal shutter printer has not been employed as a printer head requiring a high response speed. As for an LED printer head, it is difficult to form an LED array luminance. Therefore, when an electrostatic latent image formed by receiving this light-emitting luminance is developed with a developer comprising a toner having a charge of opposite polarity to that of the electrostatic image, there arises a defect that the optical density becomes nonuniform from dot to dot.
Meanwhile, ferroelectric liquid crystals with spontaneous polarization have been discovered and it is well known that they have considerably fast response as compared with the conventional liquid crystals because electric dipoles of liquid crystal molecules can respond in about one microsecond to the external electric field. When such a ferroelectric liquid crystal is used to form a cell of a thickness of one to two microns and used as an optical shutter, the brightness-to-darkness contrast of 1:20 can be obtained. Accordingly, research and development of high speed liquid crystal shutter printers has been carried out so as to replace the printer heads utilizing the conventional liquid crystal modes.
It is known however that when these ferroelectric liquid crystals operate as liquid crystal shutters, they are in a chiral smectic C phase (SmC*) or in a chiral smectic H phase (SmH*) which appears at relatively high temperatures (for instance, about 60.degree.-90.degree. C.) as compared with room temperature. Accordingly, there arises a problem that it is difficult to apply these ferroelectric liquid crystals to an image forming apparatus in which light signals are produced by a printer head utilizing such a ferroelectric liquid crystal and illuminated over a photosensitive drum of, for instance, an electrophotographic copying machine. More particularly, for the sake of the normal operation of an image forming apparatus, the liquid crystal in an optical modulator of a printer head must be always maintained at a temperature between 60.degree.-90.degree. C. so that the liquid crystal is kept in the SmC* or SmH* phase, whereby additional power is consumed. Moreover, there arises a problem that when the liquid crystal in the SmC* or SmH* phase is heated in excess of the above-described temperature range, smectic A phase (SmA) appears so that high speed response cannot be obtained.