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The new Stingray camera family from Allied Vision Technologies offers a particularly well-thought-out concept for sensor temperature management. Just why is sensor temperature so important? And how can excessive heating of the sensor be avoided?
Sensor Heating and Dark Current Noise
The sensor is heated both through its own operation and through the influence of neighboring components. The heating of the silicon in the sensor chip creates electron flow in the sensor, which is added to the electrons generated by the photons. This current also is present even when the pixels are not illuminated and is therefore referred to as dark current. This current causes background noise in the image, the so-called dark current noise. An additional interference component in the dark image is called fixed pattern noise (FPN). This noise results from the individual response behavior of each pixel in the dark. This component also depends upon the temperature.
The relationship between temperature and dark current noise varies, depending upon the sensor technology. For instance, CMOS sensors are more sensitive to dark current noise than CCDs. Nevertheless, even for CCD sensors, the dark current noise doubles for each 7–9°C rise in temperature. Sensor heating thus has a visible effect on image quality through dark current noise and FPN.
Sensor Heat Management: Passive and Active Cooling
How can sensor heating be reduced or avoided to minimize the development of noise? There are two options: one passive (heat dissipation) and one active (cooling).
Naturally, whether active cooling is necessary at all depends primarily on the applications: In most applications the image processing system can work with the dark current noise up to a certain point.