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uEye HDR Sensor FX4: Method of Functioning |
uEye Camera Manual Version 3.80
uEye HDR Sensor FX4: Method of Functioning
Photovoltaic Pixel
The FX4 HDR sensor used in IDS uEye cameras works based on a completely different principle. No conventional photodiodes are used in this sensor, rather miniaturized solar cells are used. These are in principle also photodiodes, but are operated differently. While photodiodes generate a linear current proportional to the amount of light, solar cells output a logarithmic voltage based on the amount of light. This means that taking a logarithm of the signal afterward is not necessary. The characteristic is already truly logarithmic. Thus the FX4 achieves a dynamic range of 120 dB.
The signal is amplified in the pixel and output analog. In uEye cameras, an external A/D converter handles digitization with 14-bit resolution. The voltage range of the A/D converter is greater than the maximum output voltage of the sensor to prevent overcontrol of the digital level and to compensate for temperature drift of the sensor.
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Figure 55: 40 W light bulb captured with CCD sensor (le.) and FX4 HDR sensor (ri).
Exposure
The FX4 sensor does not feature an integrative principle, i.e. there is no exposure time. The HDR pixels output a voltage corresponding to their current light amount at all times. This voltage level is polled at the desired frame frequency. For 30 frames per second, for example, the light value currently measured by each pixel is output 30 times a second. The analog voltage values are digitized by a downstream analog/digital converter
Pixel values are read out line by line (rolling readout) with the FX4. Due to the slight time offset between reading out a line n and the next line n+1, the so-called rolling-shutter effect may be seen (see also Shutter Methods). Here, objects that move through the field of view at a high speed may be displayed geometrically distorted. If a square object moves horizontally in relation to the image, for example, it will appear as a parallelogram in the image, as the upper parts of the object are imaged earlier than the lower parts.
The readout duration for an entire image is timage = 1/frame rate; the offset between two lines is thus tline = timage/frame height.
The sensor works in progressive scanning mode, where complete images are read out (in contrast to interlaced scanning, where fields are read out using the interlacing method).
See also:
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