Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2000-06-09
2002-04-16
Tarcza, Thomas H. (Department: 2713)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005050
Reexamination Certificate
active
06373557
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for picking up a three-dimensional range image of spatial objects.
Three-dimension projection and processing sensor systems are becoming more and more important for a variety of tasks in industrial technology. Known optical radar systems such as laser radar are based either on the principle of measuring laser pulse transit times or on the determination of the phase difference of modulated laser light for the purpose of deriving the object's distance. Additional mechanical scanning devices are necessary in order to build a three-dimensional imaging system. This leads to a relatively expensive electronic and mechanical outlay, which limits the use of such three-dimensional systems to a few specific applications.
There are known methods that employ a CCD camera (Charged Coupled Device), for which cameras the TV standard is applied. Relatively long readout times, however, are results of these methods.
SUMMARY OF THE INVENTION
It is the object of an invention to provide a method for recording a three-dimensional range image and an apparatus with which a rapid and economical method can be provided for obtaining a three-dimensional range image for spatial objects without costly mechanical mechanisms.
According to an aspect of the present invention, a method is provided for picking up a three-dimensional range image of spacial objects using an optoelectronic sensor with pixel resolution having electronic short-time integrators for each pixel element within the sensor and wherein an integration time can be adjusted. The method includes the steps of illuminating an object having a plurality of object points with one or more light pulses each having a predetermined period &Dgr;
L
. Light pulses are then sensed with the sensor that have been backscattered by object points of the object at corresponding pixels of the sensor within a predetermined short integration time &Dgr;
A
, where &Dgr;
A
≦&Dgr;
L
. Additionally, a time essence for the beginning of the predetermined short integration time &Dgr;
A
proceeds incidence of the first backscattered light pulse at the sensor, which corresponds to a nearest object point. Next, intensities of each of the sensed light pulses that have been backscattered by the object points are registered and distance values are computed from different registered intensities of the backscattered light pulses resulting from their different transit times.
According to another aspect of the present invention a method for picking up a three-dimensional range image of spacial objects using an optoelectronic sensor with pixel resolution includes the steps of first picking up and integrating the sensor signal of the sensor from the beginning of the picking up and integration to a defined integration time T
2
. This integration represents dark current and environmental light. Next, an object is illuminated by an illumination device simultaneous to the beginning of the picking up and integration of the sensor signal at the sensor. The integration occurs during a light intensity rise of the light received at the sensor up to an integration time T
1
where T
1
≦T
2
. The object is then repeatedly illuminated by the illumination device with simultaneous starting of the picking up and integration of the sensor signal at the sensor, wherein integration occurs within the light intensity rise of the light received at the sensor up to the integration time T
2
the respectively integrated value of the sensor signal for all pixels is readout and stored at times T
1
and T
2
. A transit time T
0
of the light from the illumination device to the sensor via the object and a corresponding distance value based on the stored integrated values is calculated for each pixel.
According to yet another aspect of the present invention, an apparatus for picking up a three-dimensional range image is featured including an illuminating device that emits light pulses onto an object via a first optical system. An optoelectronic sensor with a second optical system is configured to sense received light pulses backscattered by the object within an adjustable integration time and is comprised of a plurality of pixel elements to provide a pixel resolution, the pixel elements being randomly readable and configured to adjust the integration time pixel by pixel. A triggering mechanism is included that is configured to provide time synchronization between the illumination device and the sensor. Finally, a computing unit is included to calculate a three-dimensional image from corresponding charges of pixel elements of the sensor that have been charged by the received light pulses.
The present invention is based on the recognition that an extremely fast registration of a three-dimensional range image is possible using a randomly readable optoelectronic sensor with pixel resolution whose integration time can be adjusted point by point. To this end, an object is illuminated with one or more very short light pulses, whereupon light impulses of the same length are backscattered by the object. These backscattered light pulses are conducted to the optoelectronic chip via a corresponding optical system. Owing to the difference in the distances of different points of the object from the sensor, backscattered light pulses that correspond to respective locations will arrive at the sensor at different times. A time measuring window is opened for ranging whose duration corresponds to a predeterminable integration time. The integration time is less than or equal to the length of the emitted and, thus, of the reflected light pulses. Hence, is guaranteed that a uniform cutoff of the backscattered light pulses occurs at the sensor at the end of the integration time. The light pulses of each pixel element that arrive with a time delay are cut off in back, so that the different transit times can be converted into charge differences based on the different charges in the raster of the optoelectronic sensor. A three-dimensional range image can be computed in this way.
According to another embodiment of the invention, instead of a light pulse with a defined length, a mere light intensity rise having a steep edge is used, which is correspondingly registered and evaluated at the sensor. In this way, the measurement result becomes independent of the course of the trailing edge of the light pulse. On the other hand, the influence of a dark current, which is generated by the operating heat of a sensor element, and the environmental light (unwanted light) portion can be exactly compensated for each pixel. First, the dark current and the environmental light are acquired by a total of three consecutive measurements. Then the light quantities that are reflected by the object and received at the sensor are integrated in the form of a senor signal in the context of an illumination, this process then being repeated with a longer integration time. From this, the transit time of the light can be computed for each object point by a corresponding interpolation. This allows the possibility of using lower light powers while at the same time affords more precise measurement of the transit time and, thus, the distance to the object.
In an preferred embodiment of the invention, all light pulses are registered simultaneous with the above described measurement process using a very long integration time or are registered after this with their full length at a time offset. This is used for normalizing, so that differences in the reflection behavior of the object can be detected and compensated.
The essential advantages of the invention are that mechanical shutters are are not used, for example. Extremely short image pick-up times can, thus, be realized. The utilized optoelectronic sensor is generally referred to as a CMOS sensor, though this is merely the technical term for the semiconductor component. Using this type of sensor, minimum integration times of 50 to 30 nsec can be realized (jitter at less than 0.1%). Accordingly, technical development progresses wit
Doemens Günter
Mengel Peter
BakerBotts LLP
Mull Fred H.
Siemens Aktiengesellschaft
Tarcza Thomas H.
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