This invention relates to dichroic liquid crystal displays. More particularly, this invention relates to liquid crystal displays which are back lit with incandescent lights.
Liquid crystal displays provide a lightweight, low cost, low power display technology which is adaptable for a variety of usages. The most common application is that which is worn on the wrists of millions of people, the LCD wrist watch display. Actually, there are a wide variety of different types of liquid crystal displays, each with its own specific liquid crystal type with its respective optical properties. Most of the liquid crystal displays, however, share a common operating principle. Designated areas of the liquid crystal are sandwiched between two electrodes which produce an electric field therebetween, which in turn causes a change in the optical property of the liquid crystal when compared to its ground state. Dichroic displays operate on an absorption instead of a polarization priciple required by liquid crystal displays using twisted nematic materials. The dichroic liquid crystal display is essentially a light valve. It is opaque in the "OFF" state, appearing black when exposed to incident light. With an applied voltage, the liquid crystal display takes on a transmitting state. The energized bars will present the color of the reflective material. The operation of the dichoic display involves a phase change from the cholesterol state, in which the dye absorps light to the non-absorbing nematic state. The usual application is to form the so-called seven-bar display which is capable of forming all of the numbers or alphanumeric displays capable of forming letters and numbers by selective driving of the individual bar segments. Most all liquid crystal displays are quite easily read in daylight or other high ambient light conditions. Since the liquid crystal display is an optically passive device which requires an outside light source in order to be read, independent lighting means must be provided for low light conditions.
One such low light application is for the displays which are found in the cockpit of an aircraft. During daylight, the high ambient light condition within the cockpit provides sufficient light for the liquid crystal displays to be read easily. However, during night flying conditions, there is little or no ambient light within the cockpit in order the preserve the pilots' night vision. Therefore, the liquid crystal displays would normally be back lit by small incandescent bulbs which are dimmable in much the same manner as are the dashboard instruments lights in an automobile. Unfortunately, these small incandescent lights tend to produce an optical output spectrum in which the longer wavelengths of the visible spectrum (the reds) predominate. This predominantly red light when passed through a liquid crystal element with a white reflector produces a muddy brown appearance of the display instead of the white bar and black background display found under high ambient light conditions and severely degrades the contrast of the liquid crystal display between the energized bars and background. In other words, the prior art incandescent back lit optical display of the liquid crystal appears as light brown bars against a redish brown background with unacceptable results.
It is therefore an object of this invention to provide for a liquid crystal display which may be back lit with incandescent bulbs in a low ambient environment to provide a high contrast display and remarkable improvement in display color.