Integration time also affects the dynamic range of a CCD, as illustrated in Figure 1. As the clocking frequency is increased, the number of dark current and shot noise electrons is correspondingly decreased and less bandwidth is required by the output amplifier and video-processing electronics. The noise level is also proportional to the bandwidth of the read-out amplifier, which is influenced by pixel transfer rate and is thus affected by the clock frequency. The amount of dark charge collected in each pixel is dependent not only on the device temperature, but also on the integration time and the storage time before readout. At higher temperatures, dark current is dominant, while at lower temperatures, dynamic range is determined by the noise of the output amplifier. Dark current is strongly influenced by temperature (Figure 1), doubling every 8 to 10 degrees Centigrade. The dynamic range of a particular CCD is dependent upon several variables. To fully utilize the potential of this CCD, a 17-bit A/D converter having 131,072 grayscale levels should be employed (although a 16-bit A/D converter having 65,536 grayscale levels would also suffice). Coupled with a readout noise root-mean-square ( rms) level of three electrons at 20 kilohertz (when cooled), the CCD39-01 is capable of yielding a dynamic range of approximately 100,000 :1.
ANALOG TO DIGITAL VIDEO CONVERTER FOR SUPER 16 FULL
The full well capacity of this device can reach a level 300,000 electrons. As an example, the Marconi Applied Technologies CCD39-01 sensor is a back-illuminated, frame-transfer CCD having a square pixel size of 24 microns with a split output register allowing the utilization of quad output amplifiers.
Higher performance cooled CCD sensors designed with low noise output amplifiers and suitable for use in slow-scan imaging of photomicrographs often have lower read noise and an extended dynamic range. Controlling the size of the read and dark noise is a critical factor in maintaining a high dynamic range in these devices. In order to utilize the full range of grayscale levels available with this dynamic range, the camera should have a 12-bit analog-to-digital ( A/D) converter capable of resolving 4096 gray levels. At a typical readout rate of 1 MHz, the read noise for this CCD is about 10 electrons/pixel, which yields a dynamic range of 44,900/10 or 4,490. Thus, a CCD with 6.7 × 6.7 micron photodiodes should have a maximum charge storage capacity (a full-well capacity) of about 44,900 electrons (or holes). In a high-performance cooled CCD camera, the well capacity is proportional to the size of the individual photodiode, such that the maximum number of electrons stored is about 1000 times the cross sectional area of each photodiode. Where N(sat) is the linear full well capacity stated as the number of electrons and N(noise) is the total value of the read and dark noise, also expressed as the number of electrons.
0 kommentar(er)
