197?: Shibaden LLSA-15 (HV-15) Low Light 1" Vidicon Camera.
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The LLSA-15 was a silicon target camera based on the Shibaden HV-15.
It was made (retrofitted and modified, really) by a company called Impossible
Electronic Techniques, Inc. at 121 Pennsylvania Avenue in Wayne, PA back
in the 70s.
Impossible Electronic Techniques (IET) was the brainchild of Jesse C. Wagner, and was founded by Jesse and his dad Charlie Wagner, of Wagner Engineering, 121 Coulter Ave., Ardmore PA. Jesse was working as a salesman for Shibaden America, and he was a follower of the Apollo space and moon missions. When a camera in one of these missions panned across the sun and ruined the vidicon tube, he understood the situation. But when silicon target technology solved that problem and several others as well, he really took notice. When silicon target technology was released through RCA in Lancaster, PA, he went to Shibaden management and asked if they would market a silicon target camera. They said no, they were happy with the antimony tri-sulphide technology of the time, a workable but not-so-sensitive solution. Jesse asked if they would mind if he left the company, bought HV-15 cameras, and retrofitted silicon target tubes. Shibaden said no problem, it would be a higher price, higher performance market, and they weren't interested. So, IET was born, some time around 1971. I came on board at IET in April of 1972. I had known Jesse from ham radio in the late 60s, and I was out of work, so he said come on down! I started out installing the existing mods to the HV-15. These were mostly gain adjustments. We added extra video gain, but it added complexity to the design. Later we improved the high frequency peaking to take advantage of the sharpness of the high quality Canon and Wallensak lenses (Wallensak was bought out by Lenzar of Winter Park Florida at that time), and the 700 line resolution of the RCA 4532 1" silicon target tubes we were using. The HV-15 was a really well designed camera, the RCA 4532 was a great silicon target tube, and so our LLSA-15 versions stood up to the vibration and temperature extremes in venturi-cooled enclosures in US Steel's furnaces. The only real flaw in the LLSA-15 was that it did not have "gamma" correction. This caused poor contrast in the shaded areas of the image. The main strength of the LLSA-15 was the "floating" white clipper. Because we ran out of video gain in really dim scenes, we set up the white clipper with a floating clipping level (really a diode to a resistor and capacitor in parallel to ground. At night, the video level was lower, so the capacitor voltage was lower, and the clipping level was lower. The user just turned up the monitor contrast, and the image had good highlight control; this actually worked quite well. A more sophisticated fixed clip level design would have lost the highlight control without heavy video auto-gain increase, which was beyond our technology at the time. Suddenly in about 1974, Hitachi-Shibaden announced that the HV-15 was to be discontinued. They offered the HV-16, I think, but it was designed such that it would be hard to modify as we had been doing with the HV-15. So, IET switched to the Telemation 1" silicon target camera as a base camera for fine-tuning modifications. Telemation (Salt Lake City, Utah) made a nice camera, but it had a couple of design errors that were easily fixed. We modified the Telemation fixed-level white clipper to act as a floating white clipper, and this worked quite well. The Telemation camera had significant video auto-gain control range, and the floating clipper extended the usable light range nicely. We soon discovered that it was useful to decrease video bandwidth when video gain was really pushed; the overall result was a more visible image. A significant feature of the Telemation camera was that it offered "gamma" correction. It was only a one-diode "single-break" correction, but it really made the image more watchable! IET also had a low-cost 2/3" silicon target tube camera (which used the RCA 4833 tube). This camera was known as the LSD500. This camera was also based on a Hitachi-Shibaden camera, I forget which one. The silicon target vidicon cameras had two very-low-light brothers: the SIT and ISIT versions. I forget the model designations for these. The early SIT and ISIT had longer cases specially made by Shibaden to enclose the |
single-intensified SIT and the double-intensified
ISIT. The SIT had a blue-peaked phosphor, but it's high sensitivity
and the slight IR extension of the phosphor made it more sensitive than
a silicon-target tube camera with IR lighting. The ISIT had a seriously
extended IR response phosphor, and it was a true starlight camera.
Pitch dark to the eye produced a snowy 20-30 dB S/N image from the ISIT.
An IR light would really light up a scene. And with a Questar telescope
lens, it was amazing what could be seen! We had similar extended
cases made by Telemation for the later SIT and ISIT we built based on their
camera design.
In the early 1970s, a Dr. Topp in Texas was experimenting with the ISIT for evaluation of the image appearing on the retina of patient's eyes. For this application, he wanted unity gamma, so the HV-15 based camera worked well for him. Around 1976, a local experimenter was building a bottle inspection camera system using a silicon target tube camera, and he also wanted unity gamma. We were using the Telemation camera, and it was strappable for unity gamma, so that's what we did. In the video industry at that time, most video benches were equipped with a "light box" illuminating a gel transparency resolution test pattern. Then the camera was set up with a lens of known focal length at a standard distance, and we would adjust the height and width settings of the camera sweep and check the camera resolution, gray scale, etc. The light box was illuminated by an incandescent lamp on a dimmer. IET later bought an "Optoliner", a high-precision unit that screwed directly into the "C" mount of the camera. The Optoliner was a much more precise resolution pattern source, and it was closely calibrated in light level as well. Finally we could really specify camera sensitivity in strict scientific terms. Dr. Topp noticed an interesting thing with his ISIT. Using his light box and test pattern gel filter, the gray scale appeared to have seriously non-linear response, a very high gamma. He needed the specified near-unity gamma (0.95 spec from RCA), and here we seemed to be giving him major heartburn! I tried this on my own setup, with similar results. How could this be? RCA specified the tube, and they were rock solid on such things. At some point during this test, I happened to change the dimmer setting in the light box. The gamma curve appeared to change! This made me think about the spectrum of the light from the dim incandescent, and the extended red photocathode of the ISIT, and the spectral response of the gel filter resolution pattern. And that was it; the gel was not "neutral density" (flat spectral attenuation) in the near infra-red! Because of this, the attenuation of the gel filter as seen by the camera depended on the spectrum of the light and the spectral response of the camera, and the normally even gray scale steps could really get messed up. And this effect was in turn affected by the color temperature of the light source. This would never have been an issue with antimony tri-sulphide technology, but it certainly affected IR-responsive cameras. A fluorescent light in the light box would solve the problem, but we were all satisfied just to know what the problem was. We didn't measure the gamma, we just looked at scenes and judged the quality by eye. You could really tell the difference between gamma corrected and unity gamma settings! Everybody was happy. IET sold a lot of ISIT cameras in 1974 when the permissible radiation level used in airport X-ray machines was severely reduced. The screens were too dim for anything but an intensified camera, and the high resolution of the silicon intensifier technology made the ISIT the technology of choice. Jesse had really wanted the company name to be Impossible Electronics, but someone in Los Angeles area had trademarked that name. We answered the phone, "This is Impossible!" This was amusing, because many of our customers were serious federal agencies such as the FBI, etc. Then again, their purchasing agents used assumed names such as Carlos Castanada, etc. Also, our business cards showed a flying saucer logo. It happened that there was a scene in the old Star Trek TV series, where Spock said "This is impossible," and we always wanted to put a recording of that on an answering machine, but it never happened. |