During the last decade, CCD based camera systems have made great strides in achieving a low light imaging capability. Standard black and white RS-170 security cameras achieve 100% video at 0.01 footcandles faceplate illumination that, with a fast, low f/# camera lens, corresponds to deep twilight scene illumination. The highest performance low light level cameras available utilize a 'Gen-III' image intensifier optically coupled to a standard CCD chip (Image Intensified CCD or ICCD). These camera systems provide usable video at light levels as low as 10^-5 footcandles -- starlight or lower scene illumination. In most high performance ICCD systems, a fused fiberoptic images the output of the image intensifier onto the CCD array. In this approach, a low light scene is imaged on the image tube photocathode generating photoelectrons which are proximity focused onto a microchannel plate (MCP) where they are multiplied. A voltage potential accelerates the amplified electron signal from the MCP output onto a phosphor screen where the image is converted back to light. A glass fiberoptic element couples the phosphor screen image out of the tube. Additional fiberoptic elements coherently relay the intensified image to a CCD chip where the optical signal is converted back to an electrical signal and read out for image processing and display. At each stage of the process, as light is converted to electrons, back to light, and finally once again to electrons, image quality is lost .

For the ICCD system described above, the image is sampled at four interfaces: 1) at the microchannel plate, 2) at the phosphor screen on the fiberoptic output window, 3) at the interface between the image tube's fiberoptic window and the fiberoptic coupler, and 4) at the interface between the fiberoptic coupler's output and the CCD. The optical quality of each interface is strongly dependent upon the fiber size, the orientation, and the position of the fiberoptic array. The combined degradation of the electro-optics, the microchannel plate, the phosphor screen, and the fiberoptic elements compromises resolution. Moiré patterns, blemishes, and fiber array discontinuities ('chicken-wire') accumulate in the electro-optical path and are imaged as 'fixed pattern' noise by the CCD. Moreover, scattering in the optical interfaces and within the fiberoptic further degrades the modulation transfer (MTF) capability of the sensor. This leads to 'washed-out', poor quality images.

The transmission loss of the fiberoptics, the inefficient collection by the fiberoptic of the near-lambertian phosphor output of the intensifier tube, the inefficiency of the image tube phosphor, and the mismatch between the spectral emission from the phosphor and the CCD spectral responsivity all decrease the sensor gain. These losses require that the image intensifier operate at high gain. Operating the image tube at high gain reduces the STN performance of the tube and increases the scintillation noise or 'snow' which degrades image quality under low light conditions. Another ICCD sensor limitation is that, due to unreliable adhesion of the glass fiberoptic element to the CCD surface, the fiberoptic element may delaminate from the CCD .

An Electron Bombarded CCD (EBCCD) eliminates the complicated image transfer chain by inserting a thinned back-illuminated CCD into the image intensifier tube. The back-illuminated CCD forms the anode of the EBCCD sensor. It replaces the MCP, the phosphor screen, and the fiberoptic coupler found in a conventional image intensified tubes.

The photoelectrons emitted from the EBCCD photocathode are proximity focused directly onto the electron sensitive CCD, the silicon dissipates the incident photoelectron energy in the form electron-hole pairs, and electron bombarded semiconductor (EBS) gain occurs. The EBS process is significantly lower in noise than the electron gain obtained using a MCP. By imaging the electrons from the photocathode directly with the CCD, the EBCCD avoids the inefficient and image degrading process of converting visible light into electrons at the photocathode, back into light at the phosphor screen and then back into electrons in the CCD. Due to the reduction in the number of image conversion steps and the significantly greater signal to noise performance (STN) the EBCCD has higher contrast and resolution than does the ICCD.

Eliminating the fiberoptic couple of the image intensifier tube to the CCD reduces the EBCCD sensor size and weight. Excluding the weight of the high voltage power supply, a typical 25 mm ICCD weighs in excess of 110 grams. In contrast, the EBCCD typically weighs only 39 grams. Whereas the EBCCD requires only a single voltage supply (approximately 1.8 kV) to operate, the ICCD requires three high voltage supplies (approximately 0.9 kV, 0 to 1.2 kV, and 7 kV). When one considers the weight of each sensor's high voltage power supply, the benefits of the EBCCD's reduced weight and size are further advanced.

EBCCD sensor tests demonstrated significant advantages over standard intensified CCD sensors. These advantages include:

1. Increased sensitivity that allows for greater resolution under low light conditions;

2. Superior contrast and resolution that allow for better target identification;

3. Increased dynamic range that allows for better contrast and less blooming;

4. Reduced size and weight that allow for more covert imaging and helmet mounting;

5. Increased mechanical integrity and reliability allow for longer lifetime; and

6. Lower cost.

The term EBCCD is commonly used to describe the detector that embodies the image tube and the back-illuminated CCD as well as to describe the back-illuminated CCD in the image tube vacuum. Whenever possible the authors use the term 'EBCCD' or 'EBCCD sensor' to describe the hybrid consisting of the image tube and the back-illuminated CCD, and have used the term 'electron-bombarded CCD' or 'back-illuminated CCD' when referring to the actual CCD array.