Device and assembly for generating images on a projection surface

Abstract

Device (200, 300) for generating images on a projection surface (202), with at least one light source (204) for generating illumination light; a liquid crystal unit (208) with at least two segments (214a, 214b, 214c), each comprising two electrodes (400, 402a, 402b, 402c) and a liquid crystal (316) arranged between the two electrodes, each of which can be transilluminated by the illumination light and whose respective transparency can be controlled by means of alternating voltages applied to the electrodes (400, 402a, 402b, 402c); an imaging optics (210) which images the illumination light passing through the segments (214a, 214b, 214c) onto the projection surface (202) to generate the images; and a control unit (222) configured to determine the temperature of the liquid crystal unit (208) and to control the frequencies and the potential differences between the alternating voltages applied to the electrodes (400, 402a, 402b, 402c) as a function of the temperature of the liquid crystal unit (208); wherein the light source (204) is designed such that the luminous flux density on a surface (226) of the liquid crystal unit (208) facing the light source (204) has a value between 7 lm / mm 2 and 80 lm / mm 2 , especially between 36 lm / mm 2 and 48 lm / mm 2 , has, and wherein the control unit (222) is designed to control the level and duration of the potential difference between a first voltage applied to a first electrode (400) of the liquid crystal unit (208) and assigned to all segments (214a, 214b, 214c), and at least one second voltage applied to a second electrode (402a, 402b, 402c) of the liquid crystal unit (208) and assigned to at least one segment (214a, 214b, 214c), depending on the temperature of the liquid crystal unit (208).

Description

Technical area

[0001] The invention relates to a device for generating images on a projection surface. The invention further relates to an assembly for a vehicle with a device of the aforementioned type. Background of the invention

[0002] In the automotive sector, so-called logo lights are used. These can project multiple static, i.e., still images or a sequence of images to create a short animation onto an area to the side of a vehicle, for example, in front of a vehicle door. These logo lights function similarly to a film projector. To make such logo lights particularly cost-effective and compact, a liquid crystal unit is used to generate the image. The liquid crystal unit has several individually switchable transparent segments. A light source illuminates them like a mask and images are projected onto a projection surface by an imaging lens to generate images.

[0003] To make the individual segments transparent or opaque, a voltage is applied to the segments. Applying the voltage aligns the liquid crystal molecules in the respective segment, making them transparent or opaque to light. The higher the applied voltage, the greater the transparency of the segment. To prevent permanent alignment of the liquid crystal molecules and thus damage to the liquid crystal, an alternating voltage is used.

[0004] The transparency of the segments also depends on the temperature of the liquid crystal unit. Thus, the transparency of a segment increases with rising temperature and constant voltage. In particular, the transparency of non-transparent segments also increases with rising temperature. To achieve consistent contrast even at high temperatures, the voltage applied to the electrodes in known liquid crystal units is reduced as the temperature rises. This voltage reduction prevents crosstalk between the segments and neighboring areas. However, this voltage adjustment requires complex electronic control, which is costly and requires a large amount of installation space. Voltage adjustment may also require additional electronic components.

[0005] Regarding the prior art, reference is made to DE 691 13 417 T2, which discloses a projection device with a liquid crystal display. Regarding the prior art, reference is also made to US Pat. No. 5,317,332 A.

[0006] It is therefore an object of the invention to provide a device for generating images on a projection surface and an assembly with a device of the aforementioned type, which provides a high-contrast image over a wide temperature range and is compact and cost-effective.

[0007] The problem is solved by a device and an assembly having the features of the independent claims. Advantageous further developments are specified in the dependent claims. Summary

[0008] The proposed device comprises at least one light source for generating illumination light, a liquid crystal unit with at least two segments, each comprising two electrodes and a liquid crystal arranged between the two electrodes, each of which can be transilluminated by the illumination light and whose respective transparency can be controlled by means of alternating voltages applied to the electrodes. The device has imaging optics that project the illumination light, which transilluminates the segments, onto the projection surface to generate the images. The device further comprises a control unit configured to determine the temperature of the liquid crystal unit and to control the frequencies and potential differences of the alternating voltages applied to the electrodes as a function of the temperature of the liquid crystal unit.

[0009] The higher the frequency of the alternating voltage applied to the respective segment, the greater the voltage drop across the resistance of the conductor tracks and the lower the effective voltage between the two electrodes of the controlled segments. The two electrodes represent a capacitance which, together with the resistance of the conductor tracks, forms an electrical low-pass filter at high frequencies. The effective voltage applied to the segments can therefore be adjusted by changing the frequency of the alternating voltage applied to the electrodes. Consequently, the transparency of the segments can be adjusted by controlling the frequency. In order to maintain a constant contrast as the temperature of the liquid crystal unit increases, the proposed device simply increases the frequency of the alternating voltage applied to the electrodes.Changing the frequency of the alternating voltage can be achieved with a simpler electronic circuit than changing the voltage levels of the alternating voltage. This allows the proposed device to be manufactured more cost-effectively. Furthermore, the proposed device can be implemented with smaller electronic components, allowing the device to be installed in tight spaces.

[0010] The control unit is particularly designed to control the frequencies and the potential differences of the alternating voltages applied to the two electrodes of each segment independently of one another.

[0011] The light source is designed such that the luminous flux density on a surface of the liquid crystal unit facing the light source has a value between 7.75 lm / mm 2 and 77.5 lm / mm 2 , especially between 36 lm / mm 2 and 48 lm / mm 2At low temperatures, especially below 0°C, the response time of typical liquid crystal units decreases significantly – even to the point of complete freezing of the segments. Integrated heaters are typically used to bring the liquid crystal unit to its operating temperature. However, these require additional space and their own electronic control. In this embodiment, the liquid crystal unit is heated to its operating temperature using the luminous flux.

[0012] In particular, the control unit is designed to control the light source such that the luminous flux density on a surface of the liquid crystal unit facing the light source is increased when the temperature of the liquid crystal unit is below a predetermined value. The heating of the liquid crystal unit to its operating temperature can thus be accelerated at particularly low temperatures, in particular at temperatures below 0°C.

[0013] In a further preferred embodiment, the control unit is configured to set the frequency of the alternating voltage applied to the electrodes to a first frequency value when the temperature of the liquid crystal unit is below a predetermined temperature value, and to a second frequency value when the temperature of the liquid crystal unit is greater than the predetermined temperature value. In summary, in this embodiment, the frequency of the alternating voltage applied to the electrodes, and thus the temperature-dependent transparency of the segments, is controlled in at least two stages. Such control can be implemented simply and cost-effectively.

[0014] In an alternative embodiment, the frequency of the alternating voltage applied to the electrodes is continuously controlled. Such continuous control can be achieved, for example, based on a functional relationship between the temperature of the liquid crystal unit and the frequency of the alternating voltage applied to the electrodes.

[0015] In a further preferred embodiment, the control unit is designed to set the frequency of the alternating voltage applied to the electrodes to the second frequency value when the temperature of the liquid crystal unit has a value between the first predetermined temperature value and a second predetermined temperature value, and to set it to a third frequency value when the temperature of the liquid crystal unit has a value that is greater than the second predetermined temperature value. In this embodiment, the frequency of the alternating voltage applied to the electrodes, and thus the transparency of the segments, is regulated in three stages. This allows control over the contrast of the device, but is still simple and cost-effective to implement. To enable even more precise control of the contrast, more than three stages can also be used.Alternatively, a stepless, continuous control of the frequency value depending on the temperature can also be carried out, in particular on the basis of a functional relationship.

[0016] Preferably, the first predetermined temperature value is in the range between -5°C and 10°C. In this range, the response time of the liquid crystal unit begins to decrease with decreasing temperature. Driving at a frequency higher than the inverse response time would lead to undesirable flickering of the liquid crystal unit and thus a reduction in image quality.

[0017] In a further preferred embodiment, the control unit controls the frequency of the alternating voltage applied to the electrodes such that the frequency has a value between 1 Hz and 40 Hz when the temperature of the liquid crystal unit is between -40°C and 0°C. Starting at a temperature of approximately 0°C, the response time of the liquid crystal unit begins to decrease with decreasing temperature, down to a value of approximately 1 Hz, before the liquid crystal unit freezes up. By selecting these parameters, flickering of the liquid crystal unit at low temperatures is thus prevented.

[0018] In a further preferred embodiment, the control unit controls the frequency of the alternating voltage applied to the electrodes such that the frequency has a value between 40 Hz and 100 Hz when the temperature of the liquid crystal unit is between 0°C and 60°C. In this temperature range, the frequency of the alternating voltage roughly corresponds to a refresh rate that allows a fluid display of an animation to the human eye. Furthermore, the frequency of the alternating voltage is selected such that the effective voltage applied to the respectively controlled segments produces a high-contrast image.

[0019] In a further preferred embodiment, the control unit controls the frequency of the alternating voltage applied to the electrodes such that the frequency has a value between 100 Hz and 100 kHz when the temperature of the liquid crystal unit is between 40°C and 200°C, in particular between 60°C and 130°C. At higher temperatures, the voltage applied to the transparently switched segments must be reduced to prevent crosstalk to neighboring segments. To produce a high-contrast image on the projection surface even in this temperature range, the frequency of the alternating voltage is adjusted accordingly.

[0020] The control unit is designed to control the level and duration of the potential difference between a first voltage applied to a first electrode of the liquid crystal unit and assigned to all segments, and at least one second voltage applied to a second electrode of the liquid crystal unit and assigned to at least one segment, depending on the temperature of the liquid crystal unit. The first electrode is common to all segments; therefore, the first electrode is also referred to as the common electrode. Each segment also has a second electrode. The liquid crystal is arranged between the two electrodes. By generating a potential difference between the common electrode and the individual electrode of a segment, the transmission rate of this segment is controlled.

[0021] Typically, liquid crystal devices are operated with a rectangular alternating voltage. A first rectangular alternating voltage is applied to the first electrode, and a second rectangular alternating voltage is applied to the second electrodes. The difference between the two alternating voltages corresponds to the potential difference applied to the corresponding segment. A phase shift between the two alternating voltages changes the effective voltage applied to the respective segment. Thus, a phase shift can be used in addition to the frequency change to control the effective voltage and thus the transmission rate of the segments. In particular, the phase can be changed individually for each segment.A phase shift is still possible even if the frequency of the AC voltage cannot be increased any further, for example, because the clock frequency of the control unit cannot be increased any further or cannot be changed at all. This allows, in particular, the use of cost-effective electronics.

[0022] Alternatively or additionally, the device comprises a temperature sensor for measuring the temperature of the liquid crystal unit or the surroundings of the liquid crystal unit. The temperature of the liquid crystal unit can be determined from the temperature of the surroundings of the liquid crystal unit, in particular by the control unit. A thermistor is particularly suitable as a temperature sensor. The temperature can be determined very reliably using a temperature sensor. This is a prerequisite for effectively adjusting the frequency of the alternating voltage in order to achieve a consistent contrast. Thus, in this embodiment, a particularly uniform contrast and thus consistent image quality is achieved over a wide temperature range.

[0023] In a further preferred embodiment, the liquid crystal unit is a passive LCD unit. Passive and active LCD units differ in the way they are controlled. In a passive LCD unit, each segment is controlled via an individual conductor track, making control particularly simple and therefore cost-effective.

[0024] In a further preferred embodiment, the device can be connected to a bus system of a motor vehicle, in particular to a LIN bus or a CAN bus. Both LIN buses and CAN buses are widely used field buses in the automotive sector, connecting various sensors and actuators of a vehicle to a central control unit. The device can thus be controlled by the vehicle's central control unit.

[0025] The invention further relates to an assembly for a vehicle, in particular a motor vehicle, comprising a device of the aforementioned type. The assembly is in particular designed such that it can be arranged in a door of the vehicle, a side mirror, a side sill of the vehicle or in an installation space provided for a rear-view camera.

[0026] Alternatively, the assembly is designed such that it can be arranged in a space provided for a vehicle lamp. In this alternative embodiment, the device is particularly designed to project at least one image onto the projection surface that represents a roadworthy signaling function or is part of a roadworthy signaling function, for example, a tail light or a turn signal.

[0027] However, the use of the module is not limited to the exterior of the vehicle. For example, the module can also be installed inside the vehicle as dynamic reading or ambient lighting. Short description of the characters

[0028] Further features and advantages will become apparent from the following description, which explains exemplary embodiments in more detail in conjunction with the attached figures.

[0029] They show: Fig. 1 is a graph showing the dependence of the transparency of an exemplary liquid crystal device on an applied voltage; Fig. 2 shows a schematic representation of a device for generating images on a projection surface according to an embodiment; Fig. 3 an exploded view of the device for projecting images onto the projection surface according to a further embodiment; Fig. 4 is a schematic representation of the liquid crystal unit of the device for generating images on a projection surface; Fig. 5 an equivalent circuit diagram of a segment of the liquid crystal unit; and Fig. 6 a phase diagram to illustrate the functioning of the liquid crystal unit. Detailed description

[0030] Fig. 1 is a diagram 100 showing the dependence of the transparency of an exemplary liquid crystal device on an applied voltage.

[0031] The abscissa 102 indicates the value of the voltage applied to a transparently switchable segment of the liquid crystal unit in volts. The ordinate 104 indicates the transparency of the segment in percent. The diagram 100 comprises two graphs 106a, 106b, each showing the dependence of the transparency of the exemplary liquid crystal unit on the applied voltage for two different temperatures. A first graph 106a is represented by a solid line and shows the behavior of the exemplary liquid crystal unit at a low temperature. A second graph 106b is represented by a dotted line and shows the behavior of the exemplary liquid crystal unit at a high temperature.

[0032] How to Fig. 1, the segment is not completely opaque even at a low applied voltage. For typical liquid crystal devices, both individually switchable segments and the so-called non-luminous regions between individual segments exhibit this basic transparency because the liquid crystal lies completely over the liquid crystal device and is only switched in the area of ​​the segments. In the example shown, the basic transparency is 20% or 30%. Starting at an applied voltage of approximately 1.7 V, the transparency of the liquid crystal device increases rapidly with increasing voltage, asymptotically approaching the maximum value of 100%.

[0033] The transparency of the liquid crystal unit depends not only on the applied voltage, but also on the temperature of the liquid crystal unit. As the temperature rises, the basic transparency of the liquid crystal unit increases. The second graph 106b is compressed compared to the first graph 106a. To create a consistent contrast between the segments and the non-luminous regions at high temperatures, the voltage applied to the segments is reduced in known liquid crystal units. This voltage reduction reduces the crosstalk from a controlled segment to a neighboring segment or a neighboring non-luminous region. Thus, the regions of the liquid crystal unit remain clearly separated from one another even at increasing temperatures.

[0034] As the temperature drops, the response time of the liquid crystal device also decreases. This means that the time required for the segments to switch from an opaque state to a transparent state increases. In typical liquid crystal devices, the refresh rate drops below 1 Hz starting at a temperature of approximately 0°C. At particularly low temperatures, the liquid crystal device may completely freeze, rendering switching between the segments impossible. It is therefore necessary to warm liquid crystal devices to a certain operating temperature. In conventional liquid crystal devices, this is achieved using heating devices.

[0035] Fig. 2 shows a schematic representation of a device 200 for generating images on a projection surface 202 according to an embodiment.

[0036] The projection surface 202 is in Fig. 2 by a dashed line. The device 200 has a light source 204, a condenser optics 206, a liquid crystal unit, and an imaging optics 210. The light source 204 generates illumination light, which is directed onto the liquid crystal unit 208 by a condenser lens 212 of the condenser optics 206. The liquid crystal unit 208 comprises two or more segments 214a, 214b arranged between two glass plates 216, 218. The segments 214a, 214b can be individually and independently switched to be transparent and are each assigned to one of the images. The respectively switched-transparent segment 214a, 214b is illuminated by the light source 204 and forms a mask of the image assigned to the segment 214a, 214b. To generate the image associated with the transparently switched segment 214a, 214b, the segment 214a, 214b is imaged onto the projection surface 202 by an imaging lens 220 of the imaging optics 210.The device 200 can, on the one hand, project multiple images onto the projection surface 202. On the other hand, by successively switching the segments 214a, 214b transparent, a short animation composed of the images can be projected onto the projection surface 202.

[0037] The device 200 further comprises a control unit 222, which is connected to the liquid crystal unit 208 and the light source 204. The control unit 222 is designed to switch the individual segments 214a, 214b of the liquid crystal unit 208 to be transparent by applying an alternating voltage. For this purpose, the control unit 222 is connected directly to the segments 214a, 214b via conductor tracks 224. Alternatively, a multiplexer unit can be used to address several segments 214a, 214b via a conductor track 224. The frequency of the alternating voltage is controlled by the control unit 222 as a function of the temperature of the liquid crystal unit 208. If the liquid crystal unit 208 is operated at a higher frequency than intended, the liquid crystal unit 208 acts like an electrical low-pass filter. This means that more voltage drops across the electrical resistance of the conductor tracks 224 and the effective voltage applied to the segments 214a, 214b decreases.As the frequency increases, the voltage applied to segments 214a, 214b decreases. This is illustrated below using the . Fig. 4 and Fig. 5. The control unit 222 further controls a potential difference between the alternating voltages applied to various electrodes 400, 402 (see Fig. 4) of the segments 214a, 214b. This phase shift can also be used to control the effective voltage applied to the segments 214a, 214b, which is explained below using the Fig. 4 and Fig. 6 is described in more detail.

[0038] The control unit 222 is further configured to control the light source 204 such that a luminous flux on a surface 226 of the liquid crystal unit 208 facing the light source 204 has a specific value. In particular, the control unit 222 is configured to control the light source 204 as a function of the temperature of the liquid crystal unit 208. In other words, the control unit 222 controls the luminous flux density as a function of the temperature of the liquid crystal unit 208. At low temperatures, for example, below 0°C, the luminous flux density is increased. This allows the liquid crystal unit 208 to be heated and a drop in the refresh rate to be prevented.

[0039] To determine the temperature of the liquid crystal unit 208, the device 200 may include a temperature sensor that measures the temperature at the liquid crystal unit 208 or in the immediate vicinity of the liquid crystal unit 208.

[0040] Fig. 3 shows an exploded view of the device 300 for projecting images onto the projection surface 202 according to another embodiment.

[0041] The device 300 comprises a housing 302, which in the embodiment according to Fig. 3 is embodied, by way of example, as a two-part assembly. The housing 302 consists of a first housing part 304a and a second housing part 304b and serves to fix and adjust the remaining units of the device 300. The housing 302 is light-absorbing and thus shields stray light. The first housing part 304a has a light entry opening 306 directed toward the light source 204. The second housing part 304b has a light exit surface 308 through which the imaging optics 210 images the segments 214a, 214b of the liquid crystal unit 208 onto the projection surface 202 to generate the images.

[0042] The light source 204 is arranged below the first housing part 304a. The light source 204 comprises a circuit carrier 310 on which, for example, two LED elements 312a, 312b are arranged. The LED elements 312a, 312b can be individually dimmed, for example, by controlling the switch position of the voltage applied to the LED elements 312a, 312b. Furthermore, the LED elements 312a, 312b can be individually switched on and off. The luminous flux density on the surface 226 of the liquid crystal unit 208 facing the light source 204 can be controlled both by dimming and by switching individual LED elements 312a, 312b on or off.

[0043] The liquid crystal unit 208 comprises a first polarizer 314, a liquid crystal 316, and a second polarizer 318. The first polarizer 314 only allows illumination light of a predetermined polarization direction to pass through. Individually controllable segments 214a, 214b are formed in the liquid crystal 316. The liquid crystal 316 rotates the polarization direction of passing light by 90°. The second polarizer 318 only allows light of a predetermined polarization direction that has passed through the liquid crystal 316 to pass through.

[0044] The liquid crystal unit 208 is connected to an LCD control unit 320 via the conductor tracks 224. The LCD control unit 320 and the circuit carrier 310 of the light source 204 together form a control unit 222 of the device 300. In the embodiment according to Fig. 3, the LCD control unit 320 and the circuit carrier 310 of the light source 204 are each arranged on a separate circuit board. In an alternative embodiment, the LCD control unit 320 and the circuit carrier 310 of the light source 204 can be arranged on a common circuit board.

[0045] In the embodiment according to Fig. 3, the imaging optics 210 comprises three optical elements 322a, 322b, and 322c. The holding elements 324 are formed on the optical element 322a, which is arranged directly on the liquid crystal unit 208. Each holding element is connected to the liquid crystal unit 208 at a support surface. The condenser optics 206 is formed as a single piece.

[0046] Fig. 4 shows a schematic representation of the liquid crystal unit 208 of the device 200, 300 for generating images on a projection surface 202.

[0047] The liquid crystal unit 208 includes the liquid crystal 316, which is arranged between the two glass plates 216, 218. In Fig. 4 below the liquid crystal 316, a first electrode 400 is arranged, which is connected to the control unit 222 via one of the conductor tracks 224. In Fig. 4 Above the liquid crystal 316, second electrodes 402a, 402b, 402c are arranged, each of which is connected to the control unit 222 via one of the conductor tracks 224 and can be individually controlled by the control unit 222. The regions lying between the first electrode 400 and one of the second electrodes 402a, 402b, 402c are called segments 214a, 214b, 214c. The regions not lying between the first electrode 400 and one of the second electrodes 402a, 402b, 402c are called non-luminous regions 404. The segments 214a, 214b, 214c and non-luminous regions 404 are in Fig. 4 are indicated by different hatching. The first electrode 400 is common to all segments 214a, 214b, and 214c; it is therefore also referred to as the common electrode 400.

[0048] By applying an alternating voltage to the first electrode 400 and to one of the second electrodes 402a, 402b, 402c, the segment 214a, 214b, 214c located between the two electrodes is switched to transparent. If the applied voltage is too high, crosstalk may occur, i.e., the non-switched region 404 adjacent to a switched segment 214a, 214b, 214c is also switched to transparent or partially transparent. This leads to a deterioration in the contrast of the device 200, 300.

[0049] Fig. 5 is an equivalent circuit diagram of a segment 214 of the liquid crystal unit 208.

[0050] Two poles 500a, 500b on the left side of Fig. 5 correspond to two poles of the control unit 222. The voltage output by the control unit 222 is denoted by Ue. Two poles 502a, 502b on the right side of Fig. 5 correspond to the electrodes of segment 214a, 214b, 214c. The effective voltage applied to the electrodes of segment 214a, 214b, 214c is denoted by Ua. The liquid crystal 316 is arranged between the electrodes 400, 402. This arrangement forms a capacitor 504, whose capacitance is denoted by C in the equivalent circuit diagram. The electrical resistance of the conductor tracks 224 between the control unit 222 and the segment is Fig. 5 marked R.

[0051] If the control unit 222 outputs an alternating voltage Ue with a correspondingly high frequency, the liquid crystal unit 208, in conjunction with the conductor tracks 224, acts like an electrical low-pass filter. As the frequency of the alternating voltage Ue output by the control unit 222 increases, more voltage drops across the resistor R of the conductor tracks 224. The voltage drop across the capacitance C decreases, which means that the effective voltage Ua applied to the electrodes of the segments 214a, 214b, 214c also decreases. By controlling the frequency of the alternating voltage Ue output by the control unit 222, the effective voltage Ua applied to the electrodes of the segments 214a, 214b, 214c can thus be controlled.

[0052] Fig. 6 is a phase diagram 600 of the alternating voltage applied to the segments 214a, 214b, 214c to illustrate the operation of the liquid crystal unit 208.

[0053] The abscissa corresponds to time, and the ordinates represent the voltage level. A first line 602 represents the voltage curve of a clock signal from the control unit 222. The individual clock pulses are Fig. 6 is illustrated by vertical dashed lines and numbered from 0 to 19. A second line 604 represents the voltage waveform of a rectangular alternating voltage applied to the common electrode 400. Up to and including clock pulse 9, the voltage level of the common electrode 400 is at a high level, typically 5 V for typical microcontrollers, after which the voltage level is reduced to a low level, typically 0 V.

[0054] A third line 606 represents the voltage curve of a rectangular alternating voltage applied to the second electrode of the first segment 214a. Up to and including cycle 8, the voltage level is at the high level, after which the voltage level is reduced to the low level for cycle 9, increased again to the high level for cycle 10, and finally reduced to the low level. This creates a voltage difference between the common electrode 400 and the electrode 402a of the first segment 214a in cycles 9 and 10, and the segment is switched transparently. This voltage difference is shown in a fourth line 608. Measured over the 20 cycles shown, the first segment 214a has a duty cycle of 10%.

[0055] A fifth line 610 represents the voltage waveform of a rectangular alternating voltage applied to the second electrode of the second segment 214b. Up to and including cycle 7, the voltage level is at a high level; afterward, the voltage level is reduced to a low level for cycles 8 and 9, increased again to a high level for cycles 10 and 11, and finally reduced to a low level. This creates a voltage difference between the common electrode 400 and the electrode 402b of the second segment 214b in cycles 8 to 11. This voltage difference is shown in a sixth line 612. Measured over the 20 cycles shown, the second segment 214b has a duty cycle of 20%.

[0056] A seventh line 614 represents the voltage waveform of a rectangular alternating voltage applied to the second electrode 402c of the third segment 214c. Up to and including clock pulse 9, the voltage level is at a low level, after which the voltage level is increased to a high level. The voltage waveform of the second electrode 402c is thus anticyclic to the voltage waveform of the common electrode 400. This creates a permanent voltage difference between the common electrode 400 and the second electrode 402c of the third segment 214c, and the segment 214c remains transparent. This voltage difference is shown in the eighth line 216. Consequently, the third segment 214c has a duty cycle of 100%.

[0057] A phase shift and / or a potential difference between the alternating voltages applied to the common electrode 400 and each of a second electrode 402a, 402b, 402c changes the effective voltage applied to the segment 214a, 214b, 214c assigned to the respective second electrode 402a, 402b, 402c. Thus, a phase shift and / or a potential difference control can be used in addition to the frequency change to control the effective voltage and thus the transmission rate of the segments 214a, 214b, 214c. In particular, the phase can be changed individually for each segment 214a, 214b, 214c. A phase shift and / or a potential difference control is still possible even if the frequency of the alternating voltage cannot be increased any further, for example because the clock frequency of the control unit 222 cannot be increased any further. List of reference symbols 100 diagram 102 Abscissa 104 Ordinates 106a, 106b Graph 200 device 202 projection screen 204 Light source 206 Condenser optics 208 Liquid crystal unit 210 imaging optics 212 Condenser lens 214, 214a, 214b, 214c Segment 216,218 glass plates 220 imaging lens 222 Control unit 224 conductor track 226 area 300 device 302 housing 304a, 304b housing part 306 Light entry opening 308 light exit surface 310 circuit carriers 312a, 312b LED element 314 Polarizer 316 liquid crystal 318 Polarizer 320 LCD control unit 322a, 322b, 322c optical element 324 holding element 400, 402, 402a, 402b, 402c electrode 404 non-luminous areas 500a, 500b, 502a, 502b Pole 504 Capacitor 600 Phase diagram 602, 604, 606, 608, 610, 612, 614, 616 line P Arrow

Claims

[1] Device (200, 300) for generating images on a projection surface (202), with at least one light source (204) for generating illumination light; a liquid crystal unit (208) with at least two segments (214a, 214b, 214c), each comprising two electrodes (400, 402a, 402b, 402c) and a liquid crystal (316) arranged between the two electrodes, each of which can be transilluminated by the illumination light and whose respective transparency can be controlled by means of alternating voltages applied to the electrodes (400, 402a, 402b, 402c); an imaging optics (210) which images the illumination light passing through the segments (214a, 214b, 214c) onto the projection surface (202) to generate the images; and a control unit (222) configured to determine the temperature of the liquid crystal unit (208) and to control the frequencies and the potential differences between the alternating voltages applied to the electrodes (400, 402a, 402b, 402c) as a function of the temperature of the liquid crystal unit (208); wherein the light source (204) is designed such that the luminous flux density on a surface (226) of the liquid crystal unit (208) facing the light source (204) has a value between 7 lm / mm 2 and 80 lm / mm 2 , especially between 36 lm / mm 2 and 48 lm / mm 2 , has, and wherein the control unit (222) is designed to control the level and duration of the potential difference between a first voltage applied to a first electrode (400) of the liquid crystal unit (208) and assigned to all segments (214a, 214b, 214c), and at least one second voltage applied to a second electrode (402a, 402b, 402c) of the liquid crystal unit (208) and assigned to at least one segment (214a, 214b, 214c), depending on the temperature of the liquid crystal unit (208). [2] Device (200, 300) according to claim 1, wherein the control unit (222) is designed to control the light source (204) such that the luminous flux density on a surface (226) of the liquid crystal unit (208) facing the light source (204) is increased when the temperature of the liquid crystal unit (208) is below a predetermined value. [3] Device (200, 300) according to claim 1 or 2, wherein the control unit (222) is designed to set the frequency of the alternating voltage applied to the electrodes (400, 402a, 402b, 402c) to a first frequency value when the temperature of the liquid crystal unit (208) has a value below a predetermined temperature value, and to set it to a second frequency value when the temperature of the liquid crystal unit (208) has a value which is greater than the predetermined temperature value. [4] Device (200, 300) according to claim 3, wherein the control unit (222) is designed to set the frequency of the alternating voltage applied to the electrodes (400, 402a, 402b, 402c) to the second frequency value when the temperature of the liquid crystal unit (208) has a value between the first predetermined temperature value and a second predetermined temperature value, and to set it to a third frequency value when the temperature of the liquid crystal unit (208) has a value which is greater than the second predetermined temperature value. [5] The device (200, 300) according to claim 3 or 4, wherein the first predetermined temperature value is in the range between -5°C and 30°C. [6] Device (200, 300) according to one of the preceding claims, wherein the control unit (222) controls the frequency of the alternating voltage applied to the electrodes (400, 402a, 402b, 402c) such that the frequency has a value between 1 Hz and 40 Hz when the temperature of the liquid crystal unit (208) is between -40°C and 0°C. [7] Device (200, 300) according to one of the preceding claims, wherein the control unit (222) controls the frequency of the alternating voltage applied to the electrodes (400, 402a, 402b, 402c) such that the frequency has a value between 40 Hz and 100 Hz when the temperature of the liquid crystal unit (208) is between 10°C and 40°C. [8] Device (200, 300) according to one of the preceding claims, wherein the control unit (222) controls the frequency of the alternating voltage applied to the electrodes (400, 402a, 402b, 402c) such that the frequency has a value between 100 Hz and 100 kHz when the temperature of the liquid crystal unit (208) is between 40°C and 200°C, in particular between 60°C and 130°C. [9] Device (200, 300) according to one of the preceding claims, comprising a temperature sensor for measuring the temperature of the liquid crystal unit (208) or the environment of the liquid crystal unit (208). [10] Device (200, 300) according to one of the preceding claims, wherein the liquid crystal unit (208) is a passive LCD unit. [11] Device (200, 300) according to one of the preceding claims, wherein the device (200, 300) is connectable to a bus system of a motor vehicle, in particular to a LIN bus or a CAN bus. [12] Assembly for a vehicle, in particular a motor vehicle, with a device (200, 300) according to one of the preceding claims.

Images