Dark Current Noise Dark current, or thermally generated charge, can be measured and subtracted from data, but its noise component cannot be isolated. Dark current noise is a particular concern in low-light applications.
To reduce dark current, CCDs can be chilled to approximately –50°C with thermoelectric cooling. For extremely long exposures, liquid nitrogen is used to cool the CCD to as low as –120°C. At lower temperatures, CCD performance may be degraded due to poor charge transfer efficiency.
MPP Operation Multi-pinned-phase (MPP) or inverted operation reduces the rate of dark current generation by a factor of 20 or more and thus relaxes CCD cooling requirements to the level where a thermoelectric cooler is sufficient for most applications.
Most of the dark current in a CCD is generated by interface states at the silicon– silicon dioxide interface just below the parallel gate structure. In MPP mode, this dark current component is significantly reduced by biasing all of the parallel register gates into inversion. However, this causes the potential wells essential for operation to disappear, allowing charge to spread up and down columns. Efficient CCD action can be ensured by processing CCDs with a built-in potential step that restores the potential wells when the parallel gates are biased at the same voltage. Only CCDs thus processed can be operated in inverted mode.
Tradeoffs In a given situation, the available light level determines the integration time required to arrive at an acceptable SNR. Acceptable SNRs vary with the application; the tradeoffs between light level and integration time must be considered for each circumstance.
When the light level is high enough that photon statistics are the dominant source of noise, preamplifier noise and dark current are not relevant. The CCD data are said to be photon noise limited. Under low-light conditions, where preamplifier noise exceeds photon noise, the CCD data are said to be preamplifier noise limited. When long integration times are used, it is important to ensure that the noise from dark current does not exceed preamplifier or photonic noise from the signal.
Two examples give some insight into the tradeoffs required to maximize the
SNR:
•Solar astronomy is a typical high-light-level CCD application. For this application, it is important to detect small fluctuations in intensity over the area of the sun as a function of time. Because the light source is very bright, a slow-scan CCD camera always operates under photon noise limited conditions.
•Low-light-level conditions, such as those encountered in fluorescence microscopy, present a different problem. For this application, the photon flux is typically low and the excitation exposure must be kept short to avoid bleaching. CCD sensitivity and preamplifier noise are extremely important. If the CCD has a preamplifier noise of 10 electrons, the image data are preamplifier noise limited when the number of photoelectrons in a pixel is less than 100. For signals above 100 electrons, the data are photon noise limited.
