CCD Read Noise and Dark Current
A CCD camera uses as the imaging sensor a charge coupled device (CCD) that converts light striking each pixel to a digital signal. The amount of signal generated depends on many factors but mostly the quantum efficiency (QE), which is the probability an incident photon is converted to an electron, and the gain factor which specifies the amplification of the measuring electronics. Complicating this process is dark current and read noise, both of which contribute to the background or signal to noise ratio. The read noise is the amount of uncertainty introduced by the act of electronically reading a CCD pixel. Spectral Instruments cameras, used in every SI Imaging system, provide some of the lowest read noise measurements on the market today. Read noise is generally expressed in electrons. Dark current is a thermally generated background that can be reduced by cooling the CCD sensor. The rule of thumb is that the dark current will be reduced by a factor of 2 for every 7C the camera temperature is lowered. For example, reducing the temperature from -25C to -90C the dark current will be reduced by over a factor of 600. Dark current can be subtracted from images but cooling the CCD is required to reduce the noise associated with this subtraction.