As indicated in the absolute performance characterization detailed above, low noise, back-illuminated, low light level CCD (BCCD) cameras offer superior low light level performance to image intensified approaches. With wideband spectral response, a better spectral match to the night sky than GenIII image tubes, over 14-bits of dynamic range, high MTF, and anti-blooming protection, these images offer better sensitivity than image intensified approaches. This results in greater transmission of information to the user.

The back-illuminated CCD uses a dramatically different approach to low light level imaging. Rather than relying on gain, which degrades the signal to noise of the imager, the BCCD increases the responsivity with respect to the night sky radiance, fill-factor, contrast, and MTF of the imager to improve the signal to noise of the resulting target image – especially at high spatial frequencies. This allows for superior resolution and contrast low light level imaging at low light levels. With the combination of high MTF and high sensitivity, the BCCD video sensor provides imaging of scenes that cannot be achieved with conventional CCDs and for which ICCDs suffer from low contrast and resolution. Thus, the BCCD is a superior sensor to ICCD approaches.

Figures 7.1 and 7.2 show a side-by-side comparison of a back-illuminated CCD low light level imager and a GenIII image intensified imager (images courtesy of FLIR Systems [Portland, OR]).

The boat is at a range of approximately 520 meters. As can be seen in the images, the running lights and the reflections off surfaces of the boat cause the image intensified imager to bloom – making it useless for identification purposes. The low light level back-illuminated CCD imager clearly identifies the resolution target on the back of the boat, as well as identification markings. In tests by FLIR Systems and USAF Phillips Laboratories, range to recognize 6-inch letters was shown to be just under 600 meters, and 1100 meters for 12-inch letters.

Figures 7.3 and 7.4 show a similar test of bar targets clearly illustrating the superiority of the back-illuminated CCD for low light level imaging of high resolution targets at a range of 400 meters.