Panasonic's sensor detects distant small objects in dark

Tinuku - Panasonic Corp developed a range image sensor that can take an image of a 10cm object located 250m away in the dark. For ranging, the sensor uses ToF (time of flight), which emits infrared light and calculates a distance based on the time that it takes the reflected light to return.

To increase the accuracy of measuring a remote place that makes the reflected light weak, Panasonic used a high-sensitivity light-receiving device. The sensor has about 260,000 (688 x 384) pixels so that recognition with image analysis becomes easier. The sensor will be used for monitoring the surrounding situation of a vehicle as well as for monitoring cameras. Panasonic expects to start to ship samples in fiscal 2019 and begin volume production in fiscal 2021.

Tinuku Panasonic's sensor detects distant small objects in dark

In the field of autonomous driving, the company considers that the sensor can supplement the functions of existing sensors because the new sensor (1) supports a longer distance than LiDAR (light detection and ranging), which enables to obtain range images, and (2) can take images in the complete dark unlike CMOS image sensors.

Measurement carried out several times for each distance range

For taking range images, the new sensor uses a principle similar to the principle of flash-type LiDAR. In other words, near-infrared-light pulse (wavelength: 940nm, output: 1,200W, pulse width: 10ns, GaAs-based laser device in the case of the prototype) is applied to the entire imaging area.

Reflected light is received by an array of light-receiving elements, and the ToF of each pixel is measured. The features of the new sensor are its ranging method and the structure of its light-receiving device.

A distance is measured as follows. The array of light-receiving elements is capable of detection only during the time it takes for reflected light to return from objects located within a certain distance range, and, for each pixel, whether the sensor has received the reflected light during the time range is judged.

The reception of the reflected light is judged with a photon counter embedded in the light-receiving device. When the number of photons is larger than a preset number, the reception is confirmed. Such processes are repeated for multiple distance ranges.

For example, for measurement within a range of 0-250m, measurement with a 10m range is carried out 25 times. Objects can be detected for each distance range and each pixel, enabling to obtain a range image. This method limits the detection time of the pixel array. So, it is not affected by incoming noises.

Though imaging needs to be done several times to obtain a frame of range image, it does not seem to be a serious bottleneck in measurement time. Even with a distance of 250m, from which it takes the longest for reflected light to return, the measurement time is only about 1.7μs.

In theory, at the time of obtaining several tens of frames per second of data, measurement can be done several thousand to several tens of thousand times with each frame. With the prototype, near-infrared pulse is emitted with a cycle of 167μs to measure distance for each distance range.

Tinuku.com Panasonic's sensor detects distant small objects in dark

Moreover, this method was combined with a technology to detect weak reflected light coming from a remote place with a high accuracy. Based on a calculation conducted by Panasonic, when the viewing angle of the prototype is set at 20, the number of photons coming from a distance of more than 100m away and entering one pixel is 1 or less.

In other words, even when there is reflected light, some pixels receive it while the others do not. Therefore, in the case of a distance from which the number of incoming photons becomes 1 or less, measurement is carried out several times for the same distance range. When a photon is detected, it is considered that the light has been received by the pixel.

Vertically-stacked APDs

For the light-receiving device, Panasonic integrated avalanche photo diodes (APDs), whose sensitivity can be easily increased, with a high density. APD has a multiplication function that generates a large number of electrons with one incoming photon. However, it had a large pixel size due to the multiplication function, etc.

Panasonic reduced its area by vertically stacking the incident part, the part that realizes the multiplication function, etc. While the previous (equivalent) APD had a size of about 20 x 20μm, the new APD has a size of 11.2 x 11.2μm. When 260,000 pixels were integrated together with peripheral circuits, the area of the sensor was reduced by more than half from 12 x 18mm to 8 x 12mm.

The stacking of APDs does not require a special manufacturing process, Panasonic said. There are stacked models of CMOS sensors in volume production, and general-use processes of semiconductor contract manufacturers can be applied to the production of the new sensor.

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