Much like a
dark frame, a bias frame is an image obtained from an optical CCD or CMOS sensor, with no actual exposure time. The image so obtained only contains noise due to the electronics that elaborate the sensor data, and not noise from thermal radiation caused by the sensor itself.
Since a dark frame contains noise made of a fixed component, which corresponds to the bias frame, and a component depending on thermal radiation, which is proportional to the exposure time, it is possible to obtain an image representing only the thermal component by subtracting a bias frame from a dark frame. The resulting image allows obtaining an "artificial" dark frame when multiplied by a factor depending on exposure time and then added back to the bias frame. Although this technique is less accurate than shooting a dark frame for each specific exposure, it has the advantage of drastically reducing the time needed to obtain dark frame data.
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