JAI TMC-4200GE Dynamic Range Control, Programmable Look-Up Table LUT and Knee Control

Page 69

TM-4200GE Hardware

9.4 Dynamic Range Control

FIGURE 69. Output and Blooming

Blooming adj. = 13. 5 V

Lens: F=5.6

mV

Max. Digital dynamic range at 3 dB amp

600

Vsub = 8 V

CCD OUTPUT VOLTAGE

Vsub = 10 V

Vsub = 12 V

400

Vsub = 14 V

Vsub = 16 V

200

			Digital saturation at 16 dB amp					Vsub = 18 V
0		Analog saturation at 20 dB amp
0	20	40		60	80	100	120	140	160
					LUMINANCE				FL

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 20 MHz pixel clock CCD at room temperature is around 20 to 50 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-4200CL uses a CCD with 7.4 µm x 7.4 µm pixel and two-phase vertical shift register structure. The well capacity is 40,000 electrons. The theoretical dynamic range is 40,000:30 = 1333:1 (60 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 at 16 to 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.

9.4.1Programmable Look-Up Table (LUT) and Knee Control

The TM-4200CL 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 12-bit A/D input so that the full dynamic range of the CCD is utilized at 12-bit references as the input and the LUT output is converted into either 8-bit or 10- bit to adjust the gamma correction. There is no 12-bit LUT.

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.

Operation