JAI TM-6740GE, Руководство пользователя

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Operation 25
TM-6740GE Series
3.7 Dynamic Range Control
The typical interline transfer CCD has fixed noise levels based on dark current (thermal or KT noise), pattern
noise, and the operating clock speed. In general, the level of the 40 MHz pixel clock CCD at room temperature
is around 20 to 30 electrons. The maximum capacity of CCD charges is limited by the well capacity at
saturation. The range is limited by the structure and the pixel size.
The TM-6740GE uses a CCD with 7.4 μ m x 7.4 μ m pixels and a two-phase vertical shift register structure. The
well capacity is 20,000 electrons. The theoretical dynamic range is 20,000:30 = 666:1 (56 dB).
A typical CCD camera does not use the full dynamic range due to the nominal gain and the output specification
such as RS-170. The typical CCD camera’s gain is set between 16 and 22 dB and the RS-170 video level is 714
mV. Using 20 dB gain for the calculation, CCD output is limited to 714/10 = 71.4 mV. Since the CCD’s saturation
voltage is 400 mV to 500 mV, it uses less than 1/5 of the full dynamic range.
Machine vision and outdoor applications cannot afford to miss image information behind the saturation, which
is why the dynamic range adaptation is critical.
3.7.1 Programmable Look-Up Table (LUT) and Knee Control
The TM-6740GE has a built-in LUT (look-up table) for dynamic range control. At a specific gain setting, the
offset (minimum level.... dark point) and A/D reference top voltage (maximum level... saturation point) are
set to 10-bit A/D input so that the full dynamic range of the CCD is utilized at 10-bit references as the input
and the LUT output is converted into 8-bit to adjust the gamma correction. Notice that the LUT (look-up table)
is not available in the 10-bit grayscale mode.
The look-up table has two knee points (variable gamma selection) that allow the 10-bit input to be segmented
into three regions. The look-up table selection can be made by knee curve direct input.