Tuesday, March 23, 2010

High dynamic range cameras based on solar cell technology

The image shows a 40W light bulb, imaged on the left with a conventional CCD sensor and on the right with the FX4 HDR sensor.

A new range of cameras from Stemmer Imaging use a sensor based on solar cell technology to allow imaging from scenes which simultaneously contain both very light and very dark areas. This makes them ideally suited to use in environments with a very high dynamic range, or where there are strong and unpredictable brightness fluctuations. The new FX4 HDR (High Dynamic Range) sensor from IDS produces a logarithmic signal output. This enables fine differences in brightness to be imaged even in very bright scenes, without saturation, in a similar way to the human eye. Most of the USB and all of the GigE versions (HE, RE and SE) of the IDS uEye family of industrial cameras feature an FX4 HDR sensor version.

The FX4 HDR sensor features a patented pixel structure that provides a truly logarithmic output with effective suppression of fixed noise and gives a dynamic range of 120 db. This equates to a 1000 times greater brightness ratio (ratio of the highest brightness value to the lowest brightness value that can be imaged in a scene) compared to conventional linear CCD sensors, which typically have a dynamic range of 60 db.

The logarithmic response not only means that large changes in brightness in light areas of a scene cause only small changes in image brightness, but also prevents saturation in the image and ‘blooming’. Conventional linear CCD sensors can become saturated, then additional light leads to overexposure, which causes blooming, where charge from overexposed pixels ‘overflows’ into neighbouring pixels, causing whole areas of an image to appear white with the loss of image data.

High dynamic range applications include automotive/traffic, welding, paints/glossy finishes and payment kiosks. In traffic applications the sensor may need to identify detail in the dark interior of a vehicle while the headlights are on. In welding, details of a weld seam or the object being welded can be imaged even in the presence of the bright weld arc. Reflections and glossy spots on reflecting surfaces such as paints require increased dynamic range sensors. Payment kiosks for petrol stations or ATM withdrawals are frequently poorly illuminated and therefore require high dynamic range sensors to adequately image people using the machines. Applications involving very strong fluctuations in brightness are often related to security and monitoring, such as monitoring on a sunny day with fast-moving cloud cover.

The sensor provides a full 12-bit colour depth output to take full advantage of its reduced noise characteristics. This allows contrast to be adjusted to enhance the visual display of images in applications where the dynamic range of the scene is less than that of the sensor. For added versatility, the sensor can be set to give a linear output similar to conventional sensors for conventional imaging. The FX4 sensor does not use integration methods and so operates completely in real time at 30 frames/sec.

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