小型Ｉ．Ｉ．を用いたＨＤ超高感度カメラの開発

抄録

The lowest practical luminous intensity under which conventional video camera can capture images is around 20-lx. The illumination available under a full moon in the night is 0.2-lx, which is approximately two orders of magnitude lower. Although a number of high-sensitivity imaging devices have been proposed including avalanche multiplication device, yet the sensitivity of these devices is still limited up to 30 times higher than that of conventional CCD.
To achieve ultra high sensitivity to film astronomical phenomena, we have been working on an ultra-high-sensitivity HDTV color camera based on a 2.5-inch image intensifier since 1997. This camera can capture clear images of extremely dark objects in space, as well as of comets, nebula, and galaxies. However, its bulky size and heavy weight had been restricting its versatility.
Ultra-high-sensitivity HDTV Color Camera has been developed featuring small-sized image intensifiers with GaAsP photocathodes, which provide very high quantum efficiency. To achieve HD performance and a compact camera body, we combined three 1-inch image intensifiers with a 2/3-inch taking lens and three 2/3-inch CCDs by means of a new optical system capable of enlarging and reducing images.
Our approach well suited for compact implementations is to use a fiber optic plate instead of a relay lens. To use a 2/3-inch CCD, a FOP tapered from 1 inch to 2/3 inch was used to couple the image intensifier tube to the CCD. Because inconsistencies more easily occur due to graphic distortion and other causes with a tapered FOP than with a straight FOP, we had to address the prospect of registration errors in designing our camera. We solved this problem by selecting tapered FOPs from the same production lot and inspecting the best combinations by means of graphic comparison with a computer, so that their characteristics extremely matched.
There is a critical requirement of incorporating a videocassette recorder system into a camera for operators to keep its simple and convenient mobility. It is realized due to the compact size, lightweight and little power consumption of these imaging devices. As a whole this camera weights 6.3-kg and consumes 48-W electric power including a VCR system.
The camera provides excellent color reproducibility even under low light level conditions of 0.2-lx at an iris setting of f/2, with a signal-to-noise ratio of 55 dB at pedestal level. Its sensitivity is about 400 times greater than that of current HDTV CCD cameras, making it particularly well suited for capturing real time images of faint objects in space, aurora, etc., filming the nocturnal activities of animals in their natural settings, and reporting breaking news at night.