CCD Noise Sources
Dark noise and read-out noise are two major sources of noise in CCD and other digital cameras for microscopy. Although great improvements have been made over the past few years in the reduction of CCD dark noise at room temperature, cooling the chip further reduces the noise tenfold per 20° C decrease.
The tutorial initializes by displaying a square wave signal that represents the ideal output from a CCD circuit. To view the effect of various noise sources on the output signal, make a new selection from the Choose A Source pull-down menu. Noise sources vary in digital cameras. Photon noise, dark current, fixed pattern noise, and photo response nonuniformity are generated on the CCD itself, while reset noise, I/f noise, and quantization noise occur during amplification and conversion of the analog signal to a digital output. Read-out noise is generated in the amplifier on the CCD chip that converts the stored charge of each photodiode (i.e., pixel) into an analog voltage to be quantified by A/D conversion. Read-out noise may be viewed as a "toll" that must be paid for reading the stored charge. The size if this toll has decreased steadily in the past few years to 5-10 electrons/pixel because of improvements in CCD design, clocking and sampling methods. Read-out noise increases in proportion to read-out speed. The cost of going faster is more noise and hence, more uncertainty in the voltage determination and lower number of bits of resolution. This is why slow-scan cameras generally exhibit lower read-out noise than faster output detectors and have higher number of useful bits. Digital came4ras range from those with 8-12 bit depth at 30 frames per second output to 16-bit depth at 1-2 frames per second.
One solution to the speed/read-out noise problem is the use of multiple output amplifiers (taps) on a large CCD. Instead of reading the stored charge from the entire CCD through one output amplifier, the sensor is divided into four or eight sections each of which has its own amplifier. The image is read out in parts and then stitched together in software at rates of several frames per second. The required speed and associated noise of each amplifier are reduced accordingly.
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.