WO2008017315A1

WO2008017315A1 - Image reproducing device and operating method therefore

Info

Publication number: WO2008017315A1
Application number: PCT/EP2006/007829
Authority: WO
Inventors: Peter Brandt; Heiko Charle; Daniel Sahlbom; Stevan 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

In an image reproducing device for generating an image in the vision field of a person, comprising an image source and a first adaptive optical element, wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals, an adaptation control unit comprises a signal generation unit that can be controlled to produce at least two different signals wherein the different signals generate different characteristics of the adaptive optical element.

Description

Image reproducing device and operating method therefore

FIELD OF THE INVENTION

The invention refers to an image reproducing device with an optical element which may be used for example for a head-up display for a vehicle. 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 or lead to images that leave to desire an improvement in quality. 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 to reach an optimized quality.

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 and control electronics 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 a ground electrode.

SUMMARY OF THE INVENTION

The image reproducing device for generating an image in the vision field of a person comprises an image source and a first optical element. An advantage of the invention is, that the first optical element is an adaptive optical element which allows for changing of optical characteristics by applying different electrical signals. These electrical signals are generated by an adaptation control unit which is connected to the adaptive optical element and which comprises a signal generation unit. By said signal generation unit at least two different signals can be generated and so at least two different optical characteristics of the adaptive optical elements can be produced. Thereby, the image being reproduced by the device can be changed with regard to the position, optical quality, focal length, optical correction or compensation of optical errors or other parameters.

It is an advantage of a preferred embodiment of the invention that signal characteristics can be adjusted and stored to optimally allow for compensation of tolerances of other optical elements.

It is a further advantage of a preferred embodiment of the invention that different signal characteristics can be stored in storage means of the adaptation control unit or calculated by computing means positioned in said adaptation control unit.

To select appropriate signal characteristics or to calculate a signal characteristic, measurement values from one or more sensors can be delivered to the adaptation control unit.

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 or heating the layer, if short reaction times for the liquid crystal layer of the adaptive optical element are required.

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 imaging-reproducing device shown schematically more in detail,

figure 9 parts of an image reproducing device using a lens-like adaptive optical element,

figure 10 a liquid crystal layer array where the pixels are controlled in a way to produce a refractive index profile.

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 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 all 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 the 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. The form of the windscreen also has a relatively huge tolerance, that is each windscreen of a series shows a different form that leads to different reflection geometries and that have to be compensated. Additionally, the image source itself or the optical element 2 or other optical elements 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 element 3.

Therefore, optical elements have been proposed that allow for a dynamic change of optical characteristics. So, for example during the production of a car, the tolerances of the windscreen form can individually be compensated by adjusting optical characteristics.

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.

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. Thereby problems of incorrect image transfer and bad image quality with the image reproducing device are avoided. No additional heating system is needed. The electrodes that are needed for operating the optical element of the image reproducing device are used in a heating status 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.

Generally, it would also be possible to heat the liquid crystal layer by using one or both of the contact electrode and the ground electrode as a heating shunt and using them for control of the optical characteristics of the liquid crystal layer at the same time.

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.

Figure 8 shows an image reproducing device with an image source 1, a first optical element 29, a second optical element 30 and a windscreen 4 of a car reflecting light to a person 5. The image reproducing device further comprises a temperature sensor 31, a light sensor 32, an adaptive control unit 33 with a signal generating unit 34, storage means 35 and computing means 36. Further the image reproducing device comprises an image control unit 37 with an image generation unit 38, image analysing means 39 and synchronising means 40.

The image reproducing device works as follows: Firstly, an image is generated by the image generating unit 38 and analysed by the unit 39 if necessary. After that, a signal representing the image, that shall be reproduced, is sent to the image source 1 and displayed. The image source 1 in a preferred embodiment comprises a LC-display which is backlit by a strong light source.

The image is sent by the image source 1 to the first optical element 29, which acts as a mirror with a certain focal length and reflects the picture to the second optical element 30 which reflects it to the windscreen 4. The person 5 can see the image produced by the image source 1 in the windscreen and depending on the focal length of the device, and also depending on the focal length of the first optical element 29, the person sees a virtual image behind the windscreen 4 in a certain distance. In the example of figure 8, three exemplary virtual images 41, 42, 43 are represented above the hood of the car.

The focal length of the first optical element 29 which is an adaptive optical element build with a reflecting layer as a reflecting adaptive mirror is controlled by the adaptation control unit 33. A signal generating unit 34 generates electrical signals, that can consist in the preferred embodiment in a one-piece liquid crystal cell, which is not patterned, of a simple AC-voltage of between 5 and 15 volt. To create the appropriate characteristics of the first optical element 29, different modes can be addressed by an appropriate pattern of the AC-voltage.

It should be noted, that here instead of the unpatterned liquid crystal layer, also a pixel array of liquid crystal cells could be used which are controlled to represent a certain profile of grey scale values which represent different phase shift values of light passing the optical element. Thereby, a refractive index profile can be approximated by digital steps between the grey scale values of neighboured pixels.

This is more in detail shown in figure 10 where 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 common 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 also be used as a heating shunt in this case if it is connected to a power source 50.

The computer controls the single liquid crystal cells 45, 46, 47, applying different voltage values to each of the cells. This results in different grey scale values of the pixels and different refractive index values which are shown in the diagram above the LC-layer 44. It is shown in the diagram that the voltage distribution follows a parabolic profile like in figure 6. The refractive index profile is represented by the respective parabolic curve turned upside down. Therefore, an optical element like a reflective mirror or a lens can be modelled or represented by a LC (liquid crystal) cell array 44.

To come back to figure 8, said first optical element 29 is controlled be the adaptation control unit 33. In the adaptation control unit 33, either the input for the signal generation unit 34 comes from a selection of values or a profile stored in storage means 35 or is the result of a computation carried out by computing means 36.

The input for selecting special signal characteristics comes from a temperature sensor 31 or from a light sensor 32 or from the image control unit 37.

An input can also be provided by manually selecting one of the profiles to adapt the adaptive optical element and thereby the image reproducing device to a certain type of car or windshield.

It should be noted, that the device can also only comprise input by one of the sensors or only by said image control unit 37.

If for example a car with the image reproducing device according to the invention build in is standing in the sun light and the sun shines into the device via second optical element 30, either the brightness in the device can be so high, that the light from the image source 1 is too weak to be appropriately distinguishable by the person 5 or the device heats up by the input of strong light. Both conditions can be detected either by the light sensor 32 or by the temperature sensor 31 which is positioned in the neighbourhood of the first optical element 29. As a result, the adaptation control unit 33 can decide to defocus said first optical element 29 in order not to reflect sunlight to the image source 1 and to further heat up the device. For this purpose, it can also be advantageous, to put an adaptive optical element in the place of the second optical element 30 to be able to defocus this optical element and hinder sunlight to be reflected into the device. Also, if the temperature is getting too high to allow for a reliable function of first optical element 29, the device can be shut off or at least the optical element can be defocused to prevent sunlight from invading the device or to reflect the sunlight back out of the device.

The light sensor 32 can also advantageously be connected to the image control unit 37 to increase the light intensity to be delivered by the image source 1 when strong sunlight is detected. Thereby, the visibility of the image can be improved.

The possibilities to control optical characteristics of the first optical element 29 can also be used to produce different virtual images 41, 42, 43 which can be regarded by the person 5 at the same time or one after another. The distance of the virtual images can be dependent on the importance that the respective images have for the driver in a special situation. The distance of a single image can also be changed stepwise or gradually to show changes of importance to the driver. For example, if a certain image is approaching the driver, this draws the attention to this image and tat can be used to warn the driver.

Different images can be selected by said image control unit 37, especially by the image analysing unit 39 and the images can be generated one after another by the image source 1. The synchronisation unit 40 can control changes of the focal length of the optical element 29 to generate each of the virtual images 41, 42, 43 with different characteristics of said first optical element 29. This leads to the result that the person 5 sees the three images one after another in different distances above the hood of the car. This can help to distinguish images with different information content. If the generation of the images is repeated with a high frequency, for example above 10 hertz, the person 5 has the expression to see all images 41, 42, 43 at the same time. So different information can be displayed in different distances from the person 5 at different times or at the same time. For example, the most important information can be displayed positioning the virtual image very close to the person 5 whereas less important information can be displayed in a manner that the virtual image, like in figure 8 image 41, seems to be more distant from the person 5.

The fact, that different images can be displayed at the same time, can also be used for transmitting three- dimensional images to the person 5. Such a three- dimensional image can be generated by the image generating unit 38 of the image control unit 37. The three-dimensional image can be analysed by the analysing unit 39 to break it down into different images to be displayed in different distances. Then the synchronising unit 40 can control the image source 1 to display one of the images after the other at the same time synchronising changes of characteristics of first optical element 29 by the adaptation control unit 35. Thereby, the different images, that are analysed from the three-dimensional image, can be shown at different distances (of the virtual images) from the person 5 who looks at the images. If all the images are repeated with a sufficient repeating rate, the person 5 believes to see all the images at the same time and interprets the different layers as part of one three-dimensional image. So, the person 5 can see behind the windscreen 4 a three- dimensional image.

By changing the three-dimensional image, generated by the image generation unit 38, even moving three-dimensional pictures can be transmitted. So even complex images with high information content can be displayed.

It should also be noted that especially for dynamic control activities as described above a very short response time of the adaptive optical element is required. As liquid crystal elements show shorter reaction times with rising temperatures, the liquid crystal layer could be continuously heated if dynamic control of the optical element is required.

A temperature control can make sure that the element is not overheated.

The invention described above realises different advantages in its embodiments. It can for example protect the image reproducing device from the influence of strong sunlight that would heat up the device by defocusing the adaptive optical element. Thereby no additional hardware is required and the problem can be solved with a cost- effective control method. Also, virtual images can be produced in different distances from a person to make the reading of the display as comfortable as possible and display a lot of complex information in a manner distinguishable by the person.

The characteristics of the first optical element that are selected by the adaptation control unit 33 are also selected by taking into consideration the optical conditions of for example the windscreen 4. Sets of characteristics, that are stored in storage means 35, and can be selected by the signal generating unit 34, can be prestored taking into consideration different geometrical forms of windscreens of different car lines or individually adjusted parameters that have been found in the production process by adjusting a reference image. Thereby, no additional adaptation of the image reproducing device to different car types is necessary and no different hardware is necessary. To make the image reproducing device applicable for different car lines only a software selection is necessary to provide for a certain set of signal characteristics that can be provided by the adaptation control unit 33. Thereby also a very cost-efficient device is presented.

One additional parameter, that can be taken into consideration to select appropriate signal characteristics to be reproduced by the signal generation unit 34, is the position of the head of the person 5. For example, different signal characteristics and different optical characteristics of the adaptive optical element can be chosen for different heights of the position of the head of the person 5 being caused either by different seat positions selected or by different body measures of different persons using the car.

For different positions, different locations on the windscreen are used for the image reflection and thus different optical adaptation has to be provided.

Finally, it should be mentioned that the invention can be used for different sorts of vehicles like cars, aircrafts or other transport means and also for monochrome or colour displays. 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. 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 vision field of a person,

comprising an image source and a first optical element, wherein said first optical element is an adaptive optical element, and wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals,

said image reproducing device further comprising an adaptation control unit connected to the adaptive optical element, wherein said adaptation control unit comprises a signal generation unit that can be controlled to produce at least two different signals.

2. Image reproducing device according to claim 1, wherein said adaptation control unit comprises storage means for storing different signal characteristics and wherein different signals can be selected to be applied to the adaptive optical element .

3. Image reproducing device according to claim 1, wherein said adaptation control unit comprises storage means for storing different signal characteristics, wherein said adaptation control unit is connected to a measurement device and wherein said adaptation control unit comprises selection means to select a signal characteristic from the storage means dependant from measurement values delivered by said measurement device.

4. Image reproducing device according to claim 1, wherein said adaptation control unit is connected to a measurement device and wherein said adaptation control unit comprises computing means for calculating a signal characteristic dependent from measurement values delivered by said measurement device .

5. Image reproducing device according to claim 1, wherein said image reproducing device further comprising an image control unit controlling said image source and being connected to the adaptation control unit, said image control unit comprising synchronizing means synchronizing different images produced by the image source with different optical characteristics to be generated by different signals from the adaptation control unit.

6. Image reproducing device according to claim 1, wherein said adaptive optical element comprises a liquid crystal array.

7. Image reproducing device according to claim 1, wherein said adaptive optical element comprises a non patterned liquid crystal layer.

8. Image reproducing device according to claim 1, wherein said adaptive optical element comprises a mirror.

9. Image reproducing device according to claim 1, wherein said adaptive optical element is transparent .

10. Image reproducing device according to claim 1, wherein said adaptation control unit is connected to a temperature measurement device which is located in the device.

11. Image reproducing device according to claim 1, wherein said adaptation control unit is connected to a light intensity measurement device.

12. Image reproducing device according to claim 1, comprising an image control unit for controlling the image source, said image control unit being connected to a light intensity measurement device.

13. 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, wherein said first optical element comprising a liquid crystal layer, a first electrode layer and a contact electrode wherein the contact electrode is electrically connected to said first electrode layer and 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.

14. Image reproducing device according to claim 13, 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 for controlling said liquid crystal layer.

15. Image reproducing device according to claim 13, 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.

16. 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 and a contact electrode wherein said contact electrode is electrically connected to said first electrode layer 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 means of 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.

17. Image reproducing device according to claim 16, 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.

18. Image reproducing device according to claim 16, wherein said second ground electrode contact is connectible to the contact electrode connected to said first electrode layer.

19. Image reproducing device according to claim 16, wherein said second ground electrode contact is connectible to a contact of said contact electrode.

20. Image reproducing device according to claim 16, 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.

21. 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 contact electrode array and a ground electrode, the cells of the contact electrode array being connectible to a number of voltage sources, wherein said 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 or synchronously to the control of said liquid crystal layer.

22. Image reproducing device according to claim 13, comprising a second optical element, said second optical element being a windscreen of a vehicle.

23. Image reproducing device according to claim 13, wherein said image source comprises a liquid crystal display.

24. Image reproducing device according to claim 13, wherein said image source comprises a liquid crystal display and wherein said liquid crystal display is backlit by LEDs (light emitting diodes) .

25. Image reproducing device according to claim 13, 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.

26. 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, wherein 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 and said contact electrode act together to control the liquid crystal layer and 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.

27. Image reproducing device according to claim 26, comprising an electrical switch that in one switching status connects at least one of 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.

28. 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.

29. 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.

30. Operating method according to claim 29, 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.

31. Operating method according to claim 29, 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.

32. Operating method according to claim 29, wherein instead of said electrode layer and said contact electrode a contact electrode array is used.

33. Operating method according to claim 29, 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.

34. Method of operating an image reproducing device for generating an image in the vision field of a person, wherein the image reproducing device comprises an image source and a first optical element, said first optical element being an adaptive optical element and wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals, said method comprising the step of measuring a temperature in the device, comparing the measured temperature value with a minimum operating temperature of the adaptive optical element and using the electrodes, that are controlling the liquid crystal layer by an electric field for heating the adaptive optical element to a temperature equal or higher than the minimum operating temperature.

35. Method of operating an image reproducing device for generating an image in the vision field of a person, wherein the image reproducing device comprises an image source and a first optical element, said first optical element being an adaptive optical element and wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals, said method comprising the step of measuring a temperature in the device, comparing the measured temperature value with a maximum operating temperature of the device and changing optical parameters of said adaptive optical element, if the measured temperature value exceeds said maximum operating temperature.

36. Method of operating an image reproducing device for generating an image in the vision field of a person, wherein the image reproducing device comprises an image source and a first optical element, said first optical element being an adaptive optical element and wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals, said method comprising the step of changing the image produced by the image source and synchronously changing optical characteristics of the adaptive optical element with the change of the image.

37. Method according to claim 36, wherein the images and respective characteristics of said adaptive optical element are changed periodically with a frequency higher than 10 Hz and wherein the different characteristics of said adaptive optical element lead to generate the single images in different distances from the person.

38. Method according to claim 36, wherein the images and respective characteristics of said adaptive optical element are changed periodically with a frequency higher than 10 Hz and wherein the different characteristics of said adaptive optical element lead to generate the single images in different distances from the person and wherein said single images buildup to a comprehensive three- dimensional representation.

39. Method of operating an image reproducing device for generating an image in the vision field of a person, wherein the image reproducing device comprises an image source and a first optical element, said first optical element being an adaptive optical element and wherein optical characteristics of said adaptive optical element can be changed by applying different electrical signals, said method comprising the step of changing the focus length of the adaptive optical element stepwise or gradually to approach the virtual image to the person in order to draw the attention of the person to the image.