WO2008017316A1

WO2008017316A1 - Thermally tunable liquid crystal based optical correcting element for a display

Info

Publication number: WO2008017316A1
Application number: PCT/EP2006/007830
Authority: WO
Inventors: Peter Brandt; Heiko Charle; Daniel Sahlbom; Steven A. Stringfellow
Original assignee: Gm Global Technology Operations, Inc.
Priority date: 2006-08-08
Filing date: 2006-08-08
Publication date: 2008-02-14

Abstract

A thermally tunable liquid crystal cell (15,17,18) functions as an optical correcting element for a display system. The optical correction element acts as a switchable lens or mirror for a display system (such as a head-up display on a car windscreen or a projection display). Thermal tuning of the element allows for the adjustment of the virtual image of a flat panel display in response to changes in environmental factors (e.g. temperature or viewing distance). The liquid crystal cell comprises electrode layers (13, 14, 16) arranged on either side of a liquid crystal layer (15). At least one of the highly resistive transparent cell control electrodes (14) also acts as a heater, either simultaneously with the operation of the cell or when this is switched off. In operation, the electrode resistance distribution causes a parabolic variation in the electric field applied to the liquid crystal layer which then produces a corresponding variation in the liquid crystal refractive index profile. In this way, the element provides a phase change distribution over the cell for light passing through it. Instead of a single unpatterned control electrode, an array of pixels can be individually addressed. A temperature sensor in the cell (100) measures the temperature of the liquid crystal layer and a comparator (101) determines the amount of heating required so that the liquid crystal layer temperature is above a certain minimum operating temperature or can be switched with a faster response time. A voltage source (U3) is connected across the high resistance electrodes to provide the heater power supply.

Description

THERMALLY TUNABLE LIQUID CRYSTAL BASED OPTICAL CORRECTING ELEMENT FOR A DISPLAY

FIELD OF THE INVENTION

The invention refers to an image reproducing device with an optical element. Image reproducing devices are nowadays widely spread for example in the form of displays, projectors that produce an image on a screen, or projectors that produce a virtual image and other devices, where a person can look at an image via one or more optical elements.

TECHNICAL BACKGROUND OF THE INVENTION

Modern technical equipment enables to adapt the image reproducing devices to different environmental conditions, for example can images be zoomed in or out, or changed in colour or brightness dependant on daylight intensity.

For transferring and forming images optical elements are used that can cause optical errors. Also, special conditions can require optical measures to optimize images. Optical errors can be corrected or special conditions can be compensated by influencing at least one optical element of the image reproducing device. Known elements that can be used to correct or compensate optical errors are for example deformable mirrors, mirror arrays that can be controlled by computers and liquid crystal units that allow control of the phase modulation of light transferred by the units.

The control of the refractive index of a liquid crystal layer can for example be achieved by choosing an array of addressable pixel LC-elements or by using an unpatterned liquid crystal layer and an electrical field gradient generated by an electrode to create a profile of the diffractive index over the extension of the liquid crystal layer.

For producing respective electrical fields to control the phase shift profile of light over the extension of the liquid crystal layer said liquid crystal layer can be positioned between a field electrode layer and ground electrode and typically an AC voltage signal is applied.

For an optimal function of a liquid crystal cell, especially for short response times in dynamical applications, a certain minimum working temperature of the cell must be kept. Response times of liquid crystals to electrical field influence become shorter with rising temperatures .

Respective Liquid crystal elements are known from prior art, for example US 6317178 Bl. There, it is disclosed to heat a liquid crystal layer by special heating elements in a liquid crystal cell to reach optimised temperature conditions .

SUMMARY OF THE INVENTION The use of image reproducing devices comprising optical elements with a liquid crystal layer can be made easier and more reliable by securing that the temperature of the liquid crystal layer is in an optimum operating temperature range. As the behaviour of liquid crystal nematic materials is non linear and temperature dependant, operating conditions can be widely improved by controlling the operation temperature. One advantage of the invention is that the temperature of the liquid crystal layer can be controlled with a minimum of additional hardware.

A method for operating the image reproducing device comprises the steps of measuring the temperature at the liquid crystal layer and heating said layer, if the temperature is below a predetermined operating temperature .

The liquid crystal layer can also be heated dependant on the operating mode which is used to control the liquid crystal layer. When the adaptation of the optical element has to be controlled with high dynamic and short response times, the element should be heated to higher temperatures .

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the disclosed invention can be understood from the following detailed description in conjunction with the drawing.

The drawing shows in

figure 1 a schematic illustration of a person sitting in a car regarding a virtual image produced by an image reproducing device, figure 2 a schematic cross sectional view of an optical element with a liquid crystal layer,

figure 3 the distribution of phase shifting with variation of voltage in a liquid crystal layer,

figure 4a a schematic cross sectional view of an LC- layer,

figure 4b a top view on an electrode layer with a ring shaped contact electrode,

figure 5 a schematic model representing an electrode layer and a liquid crystal layer by electronic elements,

figure 6 a curve representing the electrical potential profile provided by the configuration of figure 5,

figure 7 an optical element comprising a liquid crystal layer with a temperature control unit,

figure 8 an optical element with a basis electrode and a contact electrode array to control liquid crystal pixel cells.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following detailed description and in the several figures of the drawing alike elements are identified with like reference numerals.

Figure 1 shows as a preferred embodiment of the invention a typical image reproducing device in the form of a head- up display for a car. The shown type of head-up display generates a picture of for example a speedometer, a clock or other instruments typical for a vehicle and reflects the picture by the windscreen of the vehicle to the eyes of the driver. For the driver, the picture appears more or less horizontally in front of his eyes and as the focal length of the optical elements can be set up appropriately, the picture/image appears not on the windscreen but behind the windscreen in some distance for example above the hood of the car. Thereby the driver is no more obliged to take his view from the road when reading the car's instruments which gives the device the name "head-up display" because all indicators can be read by the driver without moving his view down to the dashboard. Also, by choosing the right distance of the virtual image, the driver does not have to accommodate his eyes from an infinite distance to the very close distance between his head and the dashboard. The accommodation activity of the driver' s eyes is thus reduced which can motivate the driver to look at the instruments more often and which may reduce traffic risks.

Figure 1 shows more in detail an image source 1 which in the example is a liquid crystal display backlit by light emitting diodes. Liquid crystal displays are becoming more and more widely spread and are currently used not only for televisions and computers but also for cell phones, handhelds and as micro displays for video beamers . Technology development has led to smaller and higher resolution displays which are also available as mobile devices with little current consumption.

The image source is fed with data that describe the technical status of the car like speed, fuel consumed, fuel still available, temperature, status of multi media devices in the car and so on. Additionally, other information that is convenient for a driver, like information about the road or environment, can be displayed. The image source generates a picture where some items can be seen by the driver either one after another to be selected by the driver or preferably some of them at the same time.

The image from the image source 1 is transferred by some optical elements like lenses, mirrors and the like to the windscreen.

In the example of figure 1, there is provided an optical element in form of a mirror 2 transferring the light from the image source 1 to the optical element in the form of mirror 3 which transfers the light to the windscreen 4. On the inner or outer surface of the glass windscreen, the light is reflected to the driver 5. To avoid double- reflection images caused by the thickness of the windscreen, polarisation effects can be used for example by polarising the light before the reflection at the windscreen and reflecting the light near the brewster angle so that one polarisation direction is reflected with less intensity than the other. This can be combined with a special material in the windscreen glass which changes the direction of polarisation for the share of light that is passing through the glass.

One problem occurring with an image reproducing device according to figure 1 is that the reflection area of the windscreen is not necessarily flat but usually will show a complex curvature in different directions and tolerances of the geometrical form occurring during production.

Additionally, the image source itself or the optical element 2 can show optical errors that have to be compensated. All the optical errors can be compensated or corrected by a special geometric form of the first optical element 3 which can be calculated and produced. This would however only be a solution for a very specific configuration, typically for one type of car with a special form of the windscreen. As commercial head-up displays should be applicable for different types of cars and also, the configuration of the optical elements can change for example to change the focal length of the image production, it is not efficient to care for all occurring optical errors by a special static geometrical form of the first optical compensation element 3.

Therefore, optical elements have been proposed that allow for a dynamic and adjustable change of optical properties .

One of the types available that allow for a control of optical parameters is an LCD array which allows by addressing the single pixels of the array to control the refraction index over the area of the element whereby a sort of an adaptive lens or mirror is modelled.

The use of such array-configuration requires an electronic control of the pixels. Usually, there is one basic electrode covering the array and an array of separately controllable contact electrodes to control the single pixels. One of the aspects of the invention is, that the basic electrode can be used as a heating shunt while the array is controlled by electrical signals of the contact electrodes or while the adaptive optical element is temporarily not used. Another type of controllable optical elements, which may be selected for the first optical element 3, is a sort of liquid crystal layers that are not subdivided into pixels but unpatterned and controlled by an electrical field. Thereby, liquid crystal lens like or mirror like elements are provided that can be changed with respect to focal length or other optical characteristics by applying an AC-voltage.

The basic construction of a liquid crystal element of the type described above is shown in figure 2. It is characterised by a high-resistance transparent electrode layer of typically ITO (Indium Titan Oxide) 6 and a ground electrode 7 that is connected to ground potential. Between the electrode layer 6 and the ground electrode 7 a liquid crystal layer 8 preferably of constant thickness is positioned. A first contact 9 connected to the electrode layer 6 is also connected to a voltage-source 10 which provides an AC-voltage in the range of typically between 5 and 15 volts.

There will be a voltage drop across the length of the liquid crystal layer 6 because of the high resistivity of the electrode layer 6 consisting of ITO and having a resistivity of some MΩm.

The respective curve showing the drop of the voltage is shown in figure 3 where the first curve 11 shows the voltage between the first contact layer and the ground electrode in dependence of the distance form the first contact in form of a strip whereas a second curve 12 shows the rise of the shifting of phase angle of light passing through the liquid crystal layer. In the area where the highest voltage applies, the nematic crystals are oriented rather in parallel to the direction of light crossing whereas in the region of low voltage or low electric field strength respectively the crystals are oriented perpendicular to the direction of light passing. At the latter end the phase shift of light passing through the LC-layer is bigger than at the former end.

By this simple configuration, an optical element like a glass prism is modelled by an LC-layer.

In figure 4a, another configuration is shown which can model a glass lens or a concave mirror by applying a ring shaped contact electrode 13 with a very low electrical resistance connected to the electrode layer

This leads to the effect that around the LC-layer 14 in the contact electrode 13 there is everywhere the same electrical potential. The shape of the potential that is generated in the liquid crystal layer 15 will therefore be cylindrically symmetrical.

The contact electrode 13, the electrode layer 14, the liquid crystal layer 15 and the ground electrode 16 are sandwiched between glass plates 17, 18 like in the configuration in figure 2 for reasons of stability and to protect the configuration from environmental influences.

Figure 5 shows a model that is the electrical equivalent of the LC-layer, the first high resistivity electrode layer and the ground electrode shown in figure 4. Voltage (AC) is applied to the high-resistance ITO-layer 14 which is represented as a chain of resistors 19, 20. The series of capacitors and conductances 21, 22 represent the liquid crystal layer. The whole model resembles to the modelling of a transmission line.

The voltage or potential profile across the circular shaped liquid crystal layer is described by a second order partial differential equation with the voltage, the sheet resistance of the ITO-layer, the capacitance and the conductance of the liquid crystal layer as variables.

An approximate solution of the equation is that of a parabolic potential bowl that describes the potential difference across the LC-layer. The phase shift of light is represented by a respective parabolic curve rotated about 180°. The potential curve is shown in figure 6. It can in a simple manner be explained by two different potential curves 23 falling from left to right and 24 rising from left to right where each of the curves represent an approximate solution taking into account the presence of only one side of the first contact where the two curves combined lead to the parabolic curve 25.

With the above explanations it may have become clear that by choosing physical parameters appropriately the characteristics of such optical elements with liquid crystal layers can be controlled.

An additional parameter that can be influenced is the frequency of the applied AC-voltage. The control of the frequency allows for the so called modal control.

The model shown in figure 5 shows that the respective differential equation will have cylindrically symmetrical periodical functions as solutions that have as one variable time. Therefore by including special frequencies and harmonics of basic frequencies in the voltage fed to the first contact and applied to the LC-layer, the different modes can be controlled separately and thereby the characteristics of the optical element can even more specifically be controlled. Figure 7 shows a preferred embodiment very similar to that of figure 4 with a liquid crystal layer 15 covered by a first electrode layer 14 of ITO, a contact electrode 13, a ground electrode 16 with a first ground electrode contact 26 connected to ground potential 27, a second ground electrode contact 28, glass layers 17, 18 covering the first electrode layer 14, the contact electrode 13, ground electrode 16 and ground electrode contacts 26, 28. In figure 7, switches Sl and S2 are closed (off- position) , switches S3 and S4 are open (on-position) . All switches Sl to S4 are operated by a common operating element driven by the common control unit, for example a mechanical actuator 102. If the switches Sl and S2 are open, and S3 and S4 are closed, the second ground electrode contact 28 is connected by switch S4 with first contact 13a and through switch S3 with voltage U3. In this configuration high current through the ground electrode 16 heats up the ground electrode and current through the contact electrode 13 heats up the contact electrode. Thereby heat is transferred to the liquid crystal layer, heating it to a temperature at least above the minimum operation temperature of the LC layer.

In the other configuration of the switches, which is shown in figure 7, switches S3 and S4 are in off-position and switches Sl and S2 in on-position thereby securing an AC-voltage Ul to be applied to the ring shaped contact electrode 13 and to the electrode layer 14 to provide an inhomogeneous electrical field in the LC-layer between said first electrode layer 14 and ground electrode 16. Thereby the profile of the refractive index in said liquid crystal layer 15 is appropriately controlled.

By the temperature measurement device 100, for example an NTC element or a thermo element, temperature is measured regularly, for example periodically in the immediate environment of the LC-layer. The temperature measurement device is connected to a temperature comparison unit 101 where the measured temperature is compared to a predetermined operating temperature, for example the minimum operating temperature that is stored in the comparison unit. If the measured temperature is lower than the minimum operating temperature the actuator 102 is activated to move the switches Sl, S2, S3, S4 from the position shown in figure 7 to a position where the switches Sl and S2 are open and S3 and S4 closed. After a certain time that maybe constant or dependant on the measured temperature, the actuator 102 may be moved back to change the configuration back to the constellation in figure 7.

Thereby it can be made sure that the first optical element which can be for example either the optical element 2 or 3 in figure 1 with the liquid crystal layer 15 is only operated when the temperature conditions are admissible.

It should be noted, that the heating can also take place while the liquid crystal layer is being controlled by the contact electrode and the ground electrode. Heating current and control signals led to the contact electrode and to the ground electrode do not interfere or can be appropriately adapted.

Especially when a dynamic control of the liquid crystal layer with short response times is required the liquid crystal element should be heated to an optimum operation temperature because liquid crystal elements show shorter response times at higher temperatures. So, also problems of incorrect image transfer and bad image quality with the image reproducing device due to low temperatures and bad performance of the liquid crystal cell are avoided. According to the invention, no additional heating system is needed. The electrodes that are needed for operating the optical element of the image reproducing device are used for heating the optical element.

The heating efficiency in the heating configuration is especially optimized when said second ground electrode contact 28 is connected to the first contact 13a at one end of said contact electrode 13 whereas the second contact 13b at a second end of said contact electrode 13 is connected to the voltage-source U3. Thereby it is secured that in the heating configuration current is flowing through the whole contact electrode 13, and through the whole ground contact 16 so that development of heat by electrical current is distributed between the LC-layer 15, contact electrode 13 and ground electrode 16.

According to figure 8 an array 44 of liquid crystal cells 45, 46, 47 is connected by single contacts with a computer 48. A ground electrode 48 can be either a common basis electrode for all pixels or can be realized by single ground electrodes for each pixel. The common electrode, be it the ground electrode or the contact electrode, can be used as a heating shunt as shown, when two different contacts of the common electrode are connected to an electrical power source 50. This can be done at the same time as the optical element is used applying electrical signals to the contact electrode array or in a moment, when the optical element is not used.

It should be understood that the features of the invention described above can be combined in any combination to realize as many advantages as possible for a special application case. The image reproducing device described above can be used for all application cases where an adaptive optical element is used for reproducing an image from an image source in the field of vision of a person, i. e. in head- up displays for vehicles, aircrafts or in any other form of terminals. It also goes by itself that by the LC-layer and by the first optical element respectively a lens like element or a mirror like element can be modelled depending on the characteristics of the ITO-layer and the ground electrode. At least one of them can be reflective so that a refractive mirror can be modelled by said optical element. If all layers are transparent, a lens like element is modelled.

Claims

What we claim is:

1. An image reproducing device for generating a virtual image in the field of vision of a person

comprising an image source and a first optical element, first optical element comprising a liquid crystal layer, a first electrode layer, a contact electrode electrically connected to said first electrode layer, the contact electrode being connectible to a first voltage source,

said first optical element further comprising a ground electrode, the liquid crystal layer being positioned between said first electrode layer and said ground electrode, wherein said contact electrode and said ground electrode act together to control said liquid crystal layer and wherein at least one of said contact electrode and said ground electrode is connectible to an electrical power source to act as a heating shunt.

2. An image reproducing device according to claim 1, wherein at least one of said contact electrode and said ground electrode is connectible to an electrical power source to act as a heating shunt while said contact electrode and said ground electrode are acting together to control said liquid crystal layer.

3. Image reproducing device according to claim 1, comprising an electrical switch that in one switching position connects one of said contact electrode or ground electrode to said power source and comprising a temperature control module with a sensor sensing the temperature of the liquid crystal layer, said temperature control module comprising a temperature comparison unit and a unit activating said electrical switch.

4. An image reproducing device for generating a virtual image in the field of vision of a person comprising an image source and a first optical element,

said first optical element comprising a liquid crystal layer, a first electrode layer, a contact electrode electrically connected to said first electrode layer, the contact electrode being connectible to a first voltage source,

said first optical element further comprising a ground electrode, the liquid crystal layer being positioned between said first electrode layer and said ground electrode, said ground electrode being connected to a ground potential by a first ground electrode contact, wherein said ground electrode is connected to a second ground electrode contact which is connectible to an electrical power source and which is located distant from first ground electrode contact and wherein said ground electrode can act as a heating shunt.

5. Image reproducing device according to claim 4, comprising an electrical switch that in one switching position connects said second ground electrode contact to said power source and comprising a temperature control module with a sensor sensing the temperature of the liquid crystal layer, said temperature control module comprising a temperature comparison unit and a unit activating said electrical switch.

6. Image reproducing device according to claim 4, wherein said second ground electrode contact is connectible to a contact electrode connected to said first electrode layer.

7. Image reproducing device according to claim 4, wherein said second ground electrode contact is connectible to a contact of said contact electrode.

8. Image reproducing device according to claim 4, wherein said contact electrode is connected to a first contact and a second contact, said first contact being located distant from said second contact and wherein said second contact is connectible to an electrical power source.

9. An image reproducing device for generating a virtual image in the field of vision of a person comprising an image source and a first optical element, first optical element comprising a liguid crystal layer, a contact electrode array and a ground electrode, the cells of the contact electrode array being connectible to a number of voltage sources, wherein said the liquid crystal layer being positioned between said contact electrode array and said ground electrode, wherein said contact electrode array and said ground electrode act together to control said liquid crystal layer, said ground electrode being connected to a ground potential by a first ground electrode contact, wherein said ground electrode is connected to a second ground electrode contact which is connectible to an electrical power source and which is located distant from first ground electrode contact and wherein said ground electrode can act as a heating shunt either alternatively to the control of said liquid crystal layer or synchronously.

10. Image reproducing device according to claim 1, comprising a second optical element, said second optical element being a windscreen of a vehicle.

11. Image reproducing device according to claim 1, wherein said image source comprises a liquid crystal display.

12. Image reproducing device according to claim 1, wherein said image source comprises a liquid crystal display and wherein said liquid crystal display is backlit by LEDs (light emitting diodes) .

13. Image reproducing device according to claim 1, wherein said electrode contact is connected to a first contact and a second contact distant from said first contact and wherein by applying a voltage, a current is induced between first and second contact to heat up the LC-layer.

14. Image reproducing device for generating a virtual image in the field of vision of a person, comprising an image source and a first optical element, said first optical element comprising a liquid crystal layer, a first electrode layer, a contact electrode electrically connected to said first electrode layer, the contact electrode being connectible to a first voltage source, said first optical element further comprising a ground electrode, the liquid crystal layer being positioned between said first electrode layer and said ground electrode, said ground electrode being connected to a ground potential by a first ground electrode contact, wherein said contact electrode is connected to a first contact and a second contact distant from said first contact and wherein different electrical potential can be applied to the first and second contacts to generate a current through said contact electrode.

15. Image reproducing device according to claim 14, comprising an electrical switch that in one switching status connects said first and second contacts to said power source and comprising a temperature control module with a sensor sensing the temperature of the liquid crystal layer, said temperature control module comprising a temperature comparison unit and a unit activating said electrical switch.

16. Operating method for operating a virtual image reproducing device according to claim 1, wherein at least one of said contact electrode and said ground electrode is connected to an electrical power source to act as a heating shunt while said liquid crystal layer is being controlled by said contact electrode and said ground electrode.

17. Operating method for operating a virtual image reproducing device for generating an image in the field of vision of a person wherein said device comprises an image source and a first optical element which comprises a liquid crystal layer, a first electrode layer adjacent to said liquid crystal layer, a contact electrode electrically connected to said first electrode layer, said contact electrode being connectible to a first voltage source, said first optical element further comprising a ground electrode, said liquid crystal layer being positioned between said first electrode layer and said ground electrode, wherein said contact electrode and said ground electrode act together to control said liquid crystal layer, said ground electrode being connected to a ground potential by a first ground electrode contact, said ground electrode being connectible to an electrical power source by a second ground electrode contact distant from said first ground electrode contact wherein the operating method comprises the steps of connecting said second ground electrode contact to said power source, heating up said ground electrode and transferring heat to said liquid crystal layer.

18. Operating method according to claim 17, wherein the temperature of the liquid crystal layer is measured and compared to a predetermined operating temperature and said second ground electrode is connected to said power source if the measured temperature is lower than said predetermined operating temperature.

19. Operating method according to claim 17, wherein said second ground electrode is connected to said power source, when the control of said liquid crystal layer requires short reaction time of said liquid crystal layer.

20. Operating method according to claim 17, wherein instead of said electrode layer and said contact electrode a contact electrode array is used.

21. Operating method according to claim 17, wherein said second ground electrode contact is connected to one of the first and second contacts of said contact electrode and one of said first and second contacts is connected to a power source.