The high-gain avalanche rushing amorphous photoconductor (HARP) camera tube achieves ultrahigh-sensitivity by
using the avalanche multiplication. The applications of this tube extend beyond broadcasting into other fields. It is
attracting a great deal of attention especially for radiation diagnosis, such as synchrotron radiation microangiography,
because it can obtain high-resolution and high-contrast images with a low dose of radiation. However, in the present
system, a fluorescent screen and the photoconductive film of the HARP tube are connected optically by a lens-coupling
method, and low light throughput remains a big problem. To improve the light throughput by using a fiber-coupling
method, we applied a fiber-optic plate (FOP) to the substrate of a HARP tube. The FOP consists of three types of glass
that have differing hardnesses and elastic coefficients that make it difficult to flatten the FOP surface enough to form the
HARP film. We thus introduced a new mechanical polishing method and succeeded in realizing avalanche multiplication
in the FOP-HARP tube. The results of shooting experiments by applying the FOP-HARP to the microangiography
showed that a spatial resolution of over 20 line pairs/mm was obtained. Moreover, rat femoral arteries of 150-200 μm in
diameter could be visualized as motion pictures with a one-fourth lower concentration of contrast material than that
needed for ordinary microangiography. Another potential application of the FOP-HARP is an ultrahigh-sensitivity nearinfrared
(NIR) image sensor made by fiber-coupling with an image intensifier (I.I.). The image sensor provides highquality
images and should be a powerful tool for NIR imaging.
We enhanced the photoelectric conversion efficiency of red light in a 15-&mgr;m-thick HARP film without deteriorating
image pick-up characteristics or reliability. To achieve a higher photoelectric conversion efficiency for red light, we
designed a new film structure with an increased amount of doped Te, which has a narrower band gap than that of a-Se.
The thickness of the LiF-doped layer for trapping holes was increased from that of the conventional red-extended HARP
film in order to weaken the internal field that would otherwise be enhanced by trapped electrons in extra doped Te. The
new red-extended HARP film achieved a photoelectric conversion efficiency for red light of about 22.5% at a
wavelength of 620 nm, which is twice that of the conventional red-extended film. We confirmed an improvement in
signal to shot noise ratio of 3 dB and a dramatic improvement in color reproduction when we experimented with an
HDTV camera with a camera tube incorporating the new film.
We developed an ultrahigh-sensitivity camera tube with a 15-μm-thick high-gain avalanche rushing amorphous photoconductor (HARP) film and applied it to an HDTV camera. The camera, called the "New Super-HARP", can achieve about 30 times the sensitivity (62.5 lux, F10) of conventional HDTV CCD cameras. Furthermore, for slow-moving subjects, the camera can dramatically increase the sensitivity in the intermittent read-out mode (for an accumulation time of 4 seconds, about 240 times the sensitivity of a New Super-HARP camera under normal operations). The very-low-dark-current feature of the HARP film results in excellent video images without any fixed pattern noise. We investigated the relationship between the operating temperature of the film and the occurrence of highlight defects in 15-μm-thick HARP films when shooting fixed, strong spot-lights directly. We found that defects could be suppressed by shifting the operating temperature to 35°C from the conventional 25°C. Furthermore, we optimized the concentration of arsenic (As) doped in the film to improve the heat resistance so that the film could be used at temperatures as high as 35°C. Ultrahigh-sensitivity imaging technology using HARP has been attracting considerable interest from many fields outside of broadcasting, such as medicine, biology, and digital film production.
The New Super-HARP image sensor, which relies on avalanche multiplication in a photoconductive film made mainly of amorphous selenium, is ultra-high sensitive. The sensor has already ben used to film very dark scenes, however in such a situation, shot noise due to the quantum characteristics of light becomes a serious problem. Increasing the quantum efficiency of the image sensors can reduce shot noise. The quantum efficiency of the New Super-HARP sensor has been improved to obtain an even better picture. To increase the quantum efficiency for green incident-light two improvements have been made to the amorphous selenium film. The first is doping the film with a suitable amount of tellurium on its incident-light side. The sensor is reducing the lithium- fluoride-doped layer to about 60 percent of the thickness of the conventional film. The improved version of the New Super-HARP Film has higher quantum efficiency. Its quantum efficiency at a wavelength of 540 nm was evaluated to be double that of the conventional film. Shot noise is reduced by three dB, that is, the S/N is improved by three dB.