The optical performance of an ultra high speed ICCD camera
An intensified CCD ultra high speed camera is mostly used to take images under residual light conditions or with extremely short exposure times. Although the ICCD camera provides an image intensifier, in some cases the resulting image quality will depend on the absolute light level available.
If there is no light, there will be no image at all. If there is only a very small quantity of light, it appears that light consists of individual particles. There will not be a continuos illumination but a "hail like" bombardment by single photons. Thus, at very low illumination levels there will not be enough photons for the human eye to form an image. Increasing the illumination will increase the number of photons and a noisy image will appear. However, it will still not be possible to identify very small details until the illumination increases even more as shown in the diagram. This means the image quality, i.e. the resolution of the image, generally depends on the available light level in low light conditions. These working conditions are called the low light level regime or shot noise limited regime and the image quality is described by the signal to noise ratio SNR.
If there is enough light, the noisiness will disappear. The image quality will be much better and will no longer depend on the illumination intensity. As can be seen from the diagram, the optical resolution in linepairs/mm remains constant in high light level conditions. Therefore, the image quality may be described by this limiting optical resolution, by means of the modulation transfer function MTF or the contrast transfer function CTF.
Please note: Granulous image quality under very low light conditions is not a deficiency of the ICCD camera system, but the manifestation of the quantum character of light. This may only be overcome by increasing the available light level, e.g. by using longer exposure times or, even better, by using sophisticated frame adding techniques as the STANFORD COMPUTER OPTICS Dynamic Range Expansion System.