Three low light imaging sensors were fabricated and tested: 1) A SITe model SI502AB back-illuminated CCD; 2) An Intensified CCD fabricated using a SITe SI502AF front-illuminated CCD array fiberoptically coupled with a Shott 1:1 magnification 6 micron pore, type 32A glass window to an Intevac 45 lp/mm GaAsP photocathode image intensifier tube; and 3) An Electron bombarded CCD fabricated using a SITe SI502AB back-illuminated CCD separated by a .055 inch spacing from an Intevac 'extended-blue' GaAs photocathode. Experiments were conducted to determine the ability of each sensor type to resolve various spatial frequency bar targets under photon shot-noise limited performance conditions. Figure 7 exhibits each of the sensor's responsivity characteristics. Also shown in the Figure is the responsivity of a high performance scientific grade front-illuminated CCD.

The experiments used a diffuse 590 nanometer light source to image a 'multi-burst' target on each device. Availability, rather than experiment design, governed the choice of a GaAs photocathode for the EBCCD and a GaAsP photocathode for the ICCD. As is seen in Figure 7, although the GaAsP has 65 percent greater responsivity at 590 nanometers than does the GaAs photocathode, both photocathode materials have reasonable sensitivity levels. Although not optimal, 'ease of manufacturing' dictated the 0.055 inch photocathode to the EBCCD spacing. A closer spacing, less than .018 inches, will result in higher resolution and modulation transfer characteristics.

So that a 'true' comparison could be made between the sensor types. each sensor manufactured for the experiments incorporated the SITe SI502A family of CCD imagers . The SI502A family is of a 512x512 format with 24 micron square pixels. Multi-burst bar target images were obtained using the SITe model SMEC SI502 camera electronics. The SME Series of camera electronics is a versatile low noise (10 electrons rms/sample), 100 kHz, 14-bit module that optimally operates all of SITe's CCD products. The same electronics module tested all three sensors.

Figure 8 depicts each sensor's measured CTF. So that photon statistics would dominate the sensors' noise, the measurements in Figure 8 used 'high light' signal levels. The light levels were chosen for each device to corresponded to 80 percent of the CCD pixel 'full well' and were obtained while operating the device for maximum sensitivity. As was anticipated, the back-illuminated CCD's CTF is superior to the EBCCD as well as the ICCD over all spatial frequencies. The EBCCD's CTF is only slightly lower in contrast than the back-illuminated CCD's CTF and, depending upon the spatial frequency of interest, is 20% to 100% higher in contrast than the ICCD's CTF. At their 'limiting' spatial resolution, unlike CCDs, MCP image intensifier tubes, have very low contrast. Moreover, the image intensified tube's non-linear intra-scene dynamic range degrades the contrast of the ICCD system.

Figure 9 contains a multi-burst target image obtained using an EBCCD and an ICCD for 16.67 millisecond exposures and a 6.6*10^-7 footcandle faceplate illumination. It is apparent from the images that the EBCCD has much lower signal to noise and higher CTF than does the ICCD. Although it is difficult to determine the absolute contrast range from printed images, Figure 10 illustrates the comparative CTF for the EBCCD and the ICCD for a 6.6*10^-7 footcandle faceplate illumination and a 16.67 millisecond exposure.

The figures show that the superior EBCCD signal to noise characteristics will result in a better contrast modulation and limiting resolution. In this respect, the low light EBCCD imagery is comparable in contrast and resolution to daylight CCD imagery, and does not contain the 'washed-out', low contrast characteristics of ICCD sensors. In particular, the low and medium spatial frequencies, frequencies found to be critical for nighttime navigation, are dramatically superior in the EBCCD. As previously was mentioned, a GaAsP photocathode and a closer photocathode-to-CCD spacing will further improve the EBCCD CTF performance over that of the ICCD.

The square-wave transfer function of the sensor is a physical property of the information link between the scene and the observer. Unlike image intensified systems which obtain high resolution information at the expense of gray level information, the EBCCD has high contrast as well as high resolution information. Although the psycho-physics of scene interpretation is beyond the scope of this paper, if one considers the total area beneath the CTF curve as a measure of the amount of scene information transmitted by the sensor to the user, Figure 10 reveals that the EBCCD has greater than 95% more contrast resolution information, as defined herein, than does the ICCD. Figure 11 contains a plot of the ICCD's CTF measured at various exposure levels. Figure 12 contains a similar plot for the EBCCD and shows that at low light levels the information content advantage of the EBCCD over that of the ICCD is even greater. Figure 13 depicts the effects of the acceleration voltage on the EBCCD's CTF. For low acceleration voltages, the electron's radial emission energy is a larger percentage of the acceleration energy. At low accelerating voltages, the biplanar, proximity focused, electron lens optics dictate a wider distribution of collected electrons at the back surface of the EBCCD. Increasing the accelerating voltage decreases the spread of the distribution of accelerated electrons at the back surface and increases the EBCCD's CTF performance.