Principles of CCD-Imaging

Gain in CCD imaging

What is gain ? Think of a CCD sensor capturing an image. The output of a CCD pixel is proportional to the amount of light falling onto it. If we draw a graph with the light intensity on the x-axis and the resulting CCD pixel value on the y-axis we get a straight line starting in the origin for an ideal CCD model.

To allow imaging with shorter shutter times or in low light conditions when a longer shutter time is required but not possible the CCD-camera features an amplifier. The amount of amplification is called gain. The gain increases the slope of the light/output graph. Unfortunately the use of gain raises noise in the resulting image.

Too high gain settings will result in clipping. Pixels are saturated and have the maximum value, image information is lost.



Basically each pixel-cell of a CCD device converts photons to electrons during exposure. Each pixel cell can store a maximum number of electrons called Full Well Capacity which depends on the CCD type. What happens if a pixel is "full" of electrons and more photons hit the cell ? The electrons of a saturated pixel will flow to the neighbour cells, if these cells are saturated the electrons will flow to the next adjacent pixels... This process of oversaturating the CCD is called blooming. The direction of the electron-flow depends on the CCD construction, usually it happens on one axis. The blooming can cause fully saturated streaks all over the image destroying pixel information along it's way.

Example of a star causing blooming on the y-axis.

Blooming can be prevented by an anti-blooming-gate between the pixels preventing electrons of saturated pixel from polluting adjacent cells. Such an anti-blooming-gate will limit the pixel size and the resulting possible pixel sensitivity and will destroy the linear relationship between photon hits and pixel saturation.

Hotpixels, Coolpixels

Hotpixels are defective single pixels with a sensitivity over the standard pixels of a CCD resulting in bright always saturated pixels or at least pixels with increased brightness.
Coolpixels are the opposite, single pixels with a lower than standard sensitivity resulting in darker pixels or totally black pixels at worst.