Vehicle camera systems are used, first of all, to generate an image which may be displayed on a display device, for example. In addition, such camera systems are also used to automatically detect and evaluate traffic signs; the traffic information thus ascertained may be displayed to the driver, for instance, or perhaps used automatically in an electronic stability program or driver assistance function.
For dynamic traffic management, variable message signs (VMS) are often installed particularly on expressways, and display different traffic signs, especially speed limits, depending on the traffic or environmental situation.
In order to adapt the variable message signs to the brightness of their surroundings, they are often pulsed, that is, they are switched on and off (active phases and inactive phases) with a signal frequency f of 100 Hz, for instance, or a signal period T=1/f. These switching operations take place so rapidly that they are not discernible to the human eye.
However, in order to reduce motion blur, camera-based driver assistance systems record images with the shortest exposure time possible. Given short exposure times of less than 1 ms, for example, and readout rates (photo frequencies) of, e.g., 30 images per second, the entire photo-taking period of the exposure may be such that the variable message sign is switched off during this time or is in an active phase. Thus, it is possible that a variable message sign will not be picked up by the camera. Sometimes variable message signs are also exposed while being pulsed in some areas, so that a camera may possibly pick up only partial areas of the variable message sign.
In order to reliably detect variable message signs, generally many successive photos are therefore taken, that is, with a high readout rate of the image sensor, so as to completely recognize the variable message sign in one of the images taken.
It turns out, however, that even at high readout rates, problems sometimes occur in detecting variable message signs. For example, the variable message signs may have a period duration of 100 Hz, so that an image sensor having a typical exposure duration of ≦1 ms and a readout rate or photo frequency of, e.g., 30 images per second, i.e., 30 Hz, often does not pick up a variable message sign during its active period of time, especially if the variable message sign is only within the field of view of the camera for a brief time or the vehicle is moving rapidly.
Although the probability of recognizing a variable message sign may be increased when working with higher exposure durations, the image also becomes brighter, so that the image sensor may possibly go into saturation, with the result that details are no longer ascertainable.
The German Published Patent Application No. 10 2010 024 415 proposes that an image sensor sense incoming light constantly or continuously with a maximum readout time; the intention is thereby to prevent the flickering of an LED detected by the camera system. The pause time, during which the image sensor is not converting the light falling on it into electrical signals or is not photosensitive, is selected to be less than one pulse duration with respect to light pulses to be detected, so that no light pulse is “overlooked” by the sensor. To prevent the sensor from becoming overexposed or going into saturation, an attenuation filter or an iris diaphragm is provided which may be adjusted according to the lighting conditions, especially day or night.
Consequently, an additional optical element is used to detect high-frequency signal sources. However, at high frequencies of the signal source, a correspondingly high attenuation is necessary, which in turn may lead to quality impairments for further image acquisition and image evaluation.
The German Published Patent Application No. 10 2005 033 863 describes an image-recording system in which, in addition to a camera, a radiation sensor is provided that ascertains an average brightness level of an image scene and compares it to the brightness of an image taken by the camera. Due to the longer readout time, the radiation sensor also senses pulsed light sources. If a discrepancy is revealed between the two brightnesses, a warning signal is output to the effect that there is a deficiency in the display representation; the exposure phase of the camera and the switch-on phase of the pulsed light source are synchronized, or areas of the images shown on the display are replaced by areas exposed for a suitably longer time.
The disadvantage in this design approach is that a further radiation source is needed which must be synchronized with the camera, thereby increasing manufacturing effort and cost.
In addition, multimodal camera controls are known, especially also as High Dynamic Range (HDR) camera systems, in which successive images are produced using different parameter settings, particularly with different exposure times. German Patent No. 698 08 024 describes such a system. Subsequently, an optimized image may be generated from image areas having different brightnesses by, for example, taking over dark areas from the image exposed a longer time and bright areas from the image exposed a shorter time. To that end, the image sensor may be controlled in such a way that it takes photos during different exposure times in alternation, and thus the image signal output has a plurality of alternating subsequences of frames with different exposure times.