TECHNICAL FIELD

The present invention relates to a head-up display in which a display unit is protected from damage caused by light entering from the outside and a method for its operation.

BACKGROUND

A head-up display generates a virtual image in the windshield that is visible for the driver. This image is usually generated on a display unit, for example a liquid-crystal display (LCD) and reflected to the windshield through a hole in the instrument panel by means of mirrors.

Head-up displays are often protected from incoming solar radiation by complex and expensive measures such as polarization membranes, dichroic reflectors and the like in order to protect the display unit from overheating. The costs of the countermeasures, in particular the costs for polarization membranes or dichroic reflectors, are quite high. In addition, these measures only delay the damage to the display unit for a certain time. When countermeasures that are based on polarization are used, such as an image may disappear if the driver uses polarizing sunglasses. Additionally, polarization membranes that are applied to the head-up display shimmer and are therefore may be regarded as irritating.

A head-up display has a display element whose image is projected onto a mirror unit of a vehicle at a critical angle by means of an optical path, and is reflected from there, overlaid on an image of the surroundings, in the direction of the driver. Its display element is protected against damage from sunlight entering directly in that a light-sensitive sensor detects the incoming sunlight and, when the intensity is too high, reduces the luminous intensity of a lighting unit for the display element or switches the lighting unit off entirely. The total quantity of light incident on the display element is thereby reduced, which increases its service life. The intensity of the incident sunlight is not, however, limited, as a result of which damage to the display element is delayed, but not prevented.

A head-up display in which the rise in temperature of a display element is limited has an estimation which is performed as to whether the current position of the sun is such that there is a risk that too much sunlight falls onto the display element, such that its temperature is therefore raised too much, and that it may thereby be damaged. In this case, the light falling onto the display element is reduced by a filter that is inserted into the optical path. However, the estimation may be incorrect, in particular if it is based on an incorrectly determined geographical position of the vehicle, an incorrect time of day or if, as a result of the weather, little direct sunlight is falling onto the display element. In that case the optical filter also reduces the quantity of light of the image of the head-up display that is directed in the direction of the driver, and therefore it must forgo an optimally bright image of the display even though unnecessary.

A reliable protection of the display element of a head-up display from damage from ambient light, without limiting the operation of the head-up display unnecessarily, is desirable. It is also desirable to achieve this as economically as possible.

SUMMARY

A head-up display has a sensor that detects ambient light which enters the optical path coming from the mirror unit at a critical angle. A control unit is provided, at the input of which the signal of the sensor is present, and whose control output is connected to a parking device for the head-up display. In the event of strong light entering at the critical angle, this is detected by the sensor, whereupon the parking unit, driven by the control unit, moves the head-up display into a parking position in which the light entering at the critical angle does not fall onto the display element. The display element provided here may be a liquid-crystal element, also known as an LC display or LCD, an OLED element that comprises organic light-emitting diodes, or another display element that reacts sensitively to certain frequencies of the incoming light or in general needs to be protected from overheating caused by incoming light. The windshield, or a so-called combiner, a partially reflecting mirror that is arranged between the driver and the windshield, is generally used as the mirror unit.

The vehicle may be a land vehicle, a watercraft or an aircraft. The driver, in whose direction the image of the display element, overlaid on the image of the surroundings, is projected, may be the person who steers the vehicle. The overlaid image may be in addition or alternatively provided for the co-driver or another passenger of the vehicle.

The sensor is an angle-dependent sensor that detects ambient light which is entering the optical path coming from the mirror unit at a critical angle. When the head-up display is in the operating position, all or a high proportion of the ambient light that enters at this angle and at a range of angles surrounding it reaches the display element, possibly damaging it if there is a long period of exposure or even a short period of very high intensity. The head-up display is therefore placed into a parking position in the event of incoming ambient light in this critical range of angles.

Damaging ambient light is, in general, directly entering sunlight, but can also originate from other strong light sources that heat the display element excessively or damage it in some other manner. These are, for example, strong, artificial sources of light for street lighting which enters at the critical angle for a lengthy period when the vehicle is travelling slowly or stationary.

The parking position of the head-up display is reached by closing the cover of the head-up display, whereby the beam path is interrupted. Other means that are suitable for interrupting the beam path are also used according to the invention. Tilting the mirror unit or another mirror located in the beam path thus also interrupts the beam path and prevents light that is entering in the critical range of angles from reaching the display element.

In one embodiment a head-up display comprises a light tunnel that is aligned according to a critical angle, and at the end of which a sensor is arranged. This ensures that only light that is incident at the critical angle, or in the critical range of angles, is registered by the sensor. The sensor itself therefore does not have to have any angle-dependent design. The light tunnel is integrated into the housing of the head-up display or into another component of the head-up display, for example in a plastic component manufactured using injection molding.

According to one embodiment, the light tunnel comprises one or a plurality of absorptive wall surfaces on its inner side. This has the effect that only light that enters at the critical angle passes through the tunnel in a straight line, whereas light entering from outside the critical range of angles is absorbed by the wall surfaces and is thus not detected by the sensor.

One or a plurality of conically shaped, inclined wall surfaces allow the critical angle to be extended to a range of critical angles, for example to +/−5° deviation from the critical angle, in which incoming ambient light of too high an intensity also leads to damage to the display element. The light tunnel is, for example, a circular cone, i.e. having a single, conical, inclined wall surface. A plurality of conically shaped inclined wall surfaces form the surface of a cone with a non-circular cross-section, for example a quadrilateral or hexagonal cross-section, or even an irregularly shaped cross-section. Shapes deviating from a circular cone in this way are suitable for adapting the range of critical angles detected by the sensor to the geometric shape of the display element.

One or a plurality of separating surfaces are arranged between the wall surfaces of the light tunnel. They are arranged essentially parallel to the wall surfaces, and also have the effect that only light from the critical angle or critical angle range falls on the sensor element, even if the cross-section of the light tunnel has been chosen to be relatively large or its length is relatively short in comparison with its cross-section.

In this case, many small light tunnels are formed by the wall surfaces, which likewise ensure a high angular selectivity in spite of a less than ideal ratio between the length and the cross-section of the light tunnel. In general, a plurality of separating surfaces are used; with a high length and a comparatively small cross-section of the light tunnel. However, even one separating surface can be sufficient to achieve the desired angular selectivity. If the light tunnel comprises conically inclined wall surfaces, the separating surfaces may be aligned to the same point of convergence as the wall surfaces of the funnel.

In one embodiment, the light tunnel is arranged at a housing of the head-up display in the vicinity of the light output opening of the beam path. Therefore, the critical angle can easily be determined, and no other effects arise from elements arranged in the interior of the head-up display that make it more difficult for the light tunnel to be arranged at the correct angle. The light tunnel may be designed as one piece with the housing, for example as a plastic part, that is manufactured by injection molding simultaneously with the housing, or as a separate component that is molded into the housing at the time of its manufacture or inserted into an opening in the housing.

In one embodiment, the control unit comprises a delay unit and, as an alternative or in addition to that, an integration unit. The effect of the delay unit is that exceeding a threshold value of the intensity of the light entering at the critical angle for only a short time does not have the effect that the parking position is adopted. Such as, for example, briefly driving over a bump in the ground, driving fast round a curve, sunlight being reflected from a reflecting element located outside the vehicle, for example the glass frontage of a house past which the vehicle drives. Therefore, no interruption of the image of the display of the display element visible for the driver occurs. Only a slight heating of the display element occurs in the case of such briefly occurring high light intensities, and this only leads to a slight damage to the display element or to none at all. Instead of or in addition to the delay unit, an integration unit is provided which integrates the intensity of the incoming light over a certain period, and has the effect that the parking position is only adopted when a certain total energy has been incident within a certain critical period. It releases the parking position again when the total energy, again the total energy incident within a critical period, has again fallen under a critical value. This is helpful if, for example, driving takes place along a tree-lined street where, due to the gaps between the leaves, a high light intensity only falls briefly onto the sensor each time, but over a longer period, however, this nevertheless leads to a critical heating of the display element or to damaging it in some other way.

In one embodiment, the light tunnel has a length that is a multiple of its diameter. Such a light tunnel, with a length that is large when compared to its cross-section, which provides good angular sensitivity.

According to one variant, the head-up display comprises a plurality of separating walls which form a plurality of small light tunnels with a cross-section that is small in comparison with their length. This combines good angular sensitivity with a short structural length.

A conically shaped wall surface and inclined separating surfaces that are all aligned to the same point of convergence are provided according to one embodiment. A sensor is located at or close to the point of convergence. Therefore, light from an angular range defined by the conical shape is taken into account.

The light tunnel may be injection-molded into a housing of the head-up display. This is prefabricated with a suitable precision. Lower requirements for precision are then sufficient when injection-molding into the housing.

If the light tunnel has a square cross-section, this provides an uncomplicated manufacture. If it has a rectangular cross-section, this provides that the light tunnel is adapted to the often-used longitudinal form of the display element.

According to one embodiment, the light tunnel is formed by a recess in an outer wall of the head-up display. Therefore, it is easy to adapt to pre-existing product designs that only need to be changed slightly in order to implement the solution.

According to a further embodiment, the light tunnel comprises conically shaped, inclined wall surfaces that form a truncated cone with a square cross-section.

A method for the operation of a head-up display comprises detecting ambient light that enters the head-up display in a critical angular range. If the intensity of the light that is entering in the critical angular range exceeds a first threshold value, then the head-up display is moved from an operating position into a parking position. If the intensity of the light that is entering in the critical angular range falls below a second threshold value, then the head-up display is moved from the parking position back into an operating position.

Exceeding the first threshold value occurs when the sensor signal exceeds a certain value, regardless of for how long this certain value is exceeded. Alternatively, exceeding the threshold value occurs when the sensor signal exceeds a certain value for more than a certain period. It is alternatively provided that the sensor signal is integrated over a time window, and the parking position is then adopted if the integrated signal exceeds a certain value. The second threshold value is may be lower than the first threshold value.

Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

