CCD Imaging 101Doug George
Raw CCD images are exceptional but not perfect. Due to the digital nature of the data many of the imperfections can be compensated for or calibrated out of the final image through digital image processing. This page contains some basic definitions used in CCD imaging that you will encounter from time to time:
Composition of a Raw CCD Image
A raw CCD image consists of the following signal components:
IMAGE SIGNAL – The signal from the source.Electrons are generated from the actual source photons.
BIAS SIGNAL – Initial signal already on the CCD before the exposure is taken. This signal is due to biasing the CCD offset slightly above zero A/D counts (ADU).
THERMAL SIGNAL (Dark Current) – Signal (Dark Current thermal electrons) due to the thermal activity of the semiconductor. Thermal signal is reduced by cooling of the CCD to low temperature.
Sources of Noise
CCD images are susceptible to the following sources of noise:
PHOTON NOISE – Random fluctuations in the photon signal of the source. The rate at which photons are received is not constant.
THERMAL NOISE – Statistical fluctuations in the generation of Thermal signal (Dark Current Noise). The rate at which electrons are produced in the semiconductor substrate due to thermal effects is not constant.
READOUT NOISE – Errors in reading the signal; generally dominated by the on-chip amplifier.
QUANTIZATION NOISE – Errors introduced in the A/D conversion process.
SENSITIVITY VARIATION – Sensitivity variations from photosite to photosite on the CCD detector or across the detector. Modern CCD’s are uniform to better than 1% between neighboring photosites and uniform to better than 10% across the entire surface.
REDUCING NOISE – Readout Noise and Quantization Noise are limited by the construction of the CCD camera and can not be improved upon by the user. Thermal Noise, however, can be reduced by cooling of the CCD (temperature regulation). The Sensitivity Variations can be removed by proper flat fielding.
CORRECTING FOR THE BIAS AND THERMAL SIGNALS – The Bias and Thermal signals can be subtracted out from the Raw Image by taking what is called a Dark Exposure. The dark exposure is a measure of the Bias Signal and Thermal Signal and may simply be subtracted from the Raw Image.
FLAT FIELDING -A record of the photosite to photosite sensitivity variations can be obtained by taking an exposure of a uniformly lit ‘flat field”. These variations can then be divided out of the Raw Image to produce an image essentially free from this source of error. Any length exposure will do, but ideally one which saturates the pixels to the 50% or 75% level is best.
The Final Processed Image
The final Processed Image which removes unwanted signals and reduces noise consists of the following:
Final Processed Image = (Raw – Dark) / Flat