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Camera Sensors Boost Safety for Japanese Railway

Photonics Spectra
Sep 2004
Anne L. Fischer

Passenger safety is a primary concern for railways and, although everyone is aware of the use of cameras for security, their use in train stations may not be as well-known. Cameras and monitors are put in place to help conductors see that passengers have safely boarded or disembarked from the train, and that doors are open or closed.

But in Japan, the railway's cameras were not doing the job. The conductors, who usually sit in the last car of the train, were having difficulty seeing the images on the monitors on the platforms where the last car stops. The monitors are intended to give the conductors a full view all the way to the front of the train.

Camera Sensors Boost Safety for Japanese Railway
Lighting conditions at the railroad station faced some serious challenges for imaging, such as the shadows of moving trains, and cameras facing backlit railroad cars and the sun.

They also were having trouble seeing the yellow path that the railroad companies lay as a caution for the visually impaired. In addition, lighting conditions are not optimal on the train or in the stations. Lights and shadows change as trains pass along the tracks. Some of the cameras may face into the sun, while others face a backlit railroad car. The challenge is to be able to display a clear, accurate image in these variable lighting conditions.

Accuracy is paramount because, if the conductor detects a problem, he may react by halting the closing of the door, thus delaying the start of the train and relaying incorrect status to the station. All of this takes time, and every little mistake in judgment can wreak havoc on a schedule on which the railway system prides itself.

Camera Sensors Boost Safety for Japanese Railway
At a Japanese railroad company, the cameras that were in use for safety concerns were unable to provide high-color-fidelity images of this strip designed for vision-impaired passengers. New cameras with a digital pixel system deliver the quality required.

To address the safety issues, the railway company searched for a better system. It installed cameras from JVC in Japan that have a sensing technology that reproduces accurate colors and compensates for variations in backlighting.

The digital cameras are based on a technology developed at Stanford University in the 1990s that has since been licensed by Pixim Inc. of Mountain View, Calif. Pixim refined the method and made it into a chip set and software targeted for security cameras. The unique chip set is manufactured using a 0.18-µm CMOS process, which makes possible the small size and relatively low cost of the chips.

The railway system previously used conventional CCD cameras, but the color reproduction capability was not up to its standards. It moved to JVC cameras, with their Digital Pixel System, because of the application's demand for high-fidelity images. This new technology sets the Pixim chips apart from CCD and CMOS sensors by including an analog-to-digital converter within each pixel of the sensor.

The main difference is how light is captured. With CCDs, for example, images are captured on a grid made up of millions of pixels. The pixels fill with photons and are emptied through a diode that converts the light to an electrical charge. The charge is funneled through one analog-to-digital converter, which outputs information in digital values that run through a microprocessor and that are converted into an image. The CCD method allows degradation because the pixels fill with light at different rates and yet are dumped to the diode at the same time, subjecting the analog signals to noise along the way.

Traditional CMOS sensors work in a similar manner, although when the active pixel sensor technique is employed, an amplifier is placed on each pixel to boost the signal before it's digitized. The image quality of CMOS sensors can approach that of CCDs, but they suffer from a limited dynamic range and noise.

Multi-image capture

The Digital Pixel System can be thought of as the eye and the brain of the camera working together to achieve an optimal exposure and image. The sensor uses a patented multi-image technique that captures many images between the beginning and end of a field capture, and rather than interpolating, as some high-dynamic-range cameras do, the Pixim sensor provides the real measured data. Each pixel captured can act as its own camera with a different exposure time, providing a greater dynamic range. It's possible to extend the range by two orders of magnitude because the information about the lights and darks is captured at different exposure times.

The multi-image capture technique also helps boost the image quality, making what the train conductors see on the monitor close to natural. Noise is reduced because the light signal is turned into a digital value as soon as it is captured, so that it doesn't have to go through analog circuitry. New electronic shuttering techniques used in the chip set help train conductors get instant feedback. The pixels can be sampled at 10,000 fps at full NTSC or PAL resolution.

The new cameras provide clear images under any lighting conditions. Although passengers may not be aware that such state-of-the-art optics are ensuring their safety, railway workers know that they are seeing fast, accurate representations of activities on the platform and train. Such technology is equipping them with the knowledge they need to make quick decisions, ensuring safety and adhering to a strict timetable.


GLOSSARY
diode
A two-electrode device with an anode and a cathode that passes current in only one direction. It may be designed as an electron tube or as a semiconductor device.
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