Capturing light in a vacuum: The magic of tube video cameras

As a hacker, cameras are a strange rabbit hole to fall into, as we have well over a century of items to choose from, many of which can be had for relatively little money. In my case I have more of them than I care to mention, mostly film cameras and 8mm film cameras, but there are one or two that are completely different. My first interest in electronics came via PAL televisions, so it's hardly surprising that over time I've also acquired more than one clunky old tube-based video camera. These sets have long since been replaced by their semiconductor-based replacements, but as a reflection of the CRT-based TVs I know so well, they still hold a fascination for me. It's time for a fascinating descent into the world of analog video.

Electrons chase light, chase electrons

The basic operation of all but a few of the very first electronic camera tubes is that an electron gun paints its grid of electrons on a light-sensitive target, and the current flowing through the electron beam varies in proportion to the light at each point on the target. This allows a voltage to be generated which, in combination with the various synchronization pulses, results in a video signal that can be understood by a monitor. The different types of tubes have names such as Iconoscope, Emitron or Vidicon, and the main differences between these different types of tubes lie in the combination of materials and the design of their targets. Successive generations of tubes improved sensitivity and noise performance by first combining photoemissive layers with electron multiplying layers to amplify the video signal in the same way as a photomultiplier tube, and then using photoconductive targets to vary the target's conductivity depending on the light at one certain point.

Time for some real cameras

The tube camera I've had the longest is probably best when you have the lid off and can see the inside. It is an RCA security camera from the mid-1980s. My model was built very robustly in the USA and is the 625 line version for the European market. When you open it, you see another echo of the CRT monitor, with the same distraction and signal panels found at the other end of the chain. At the top is a sync generator panel, which is much more than a simple pair of oscillators. Instead, it is packed with circuitry to produce the full standard odd and even field sync times. Lifting out the sync field reveals the tube, in this case a vidicon with a photoconductive target surrounded by its magnetic focus and deflection coils. Since it's a monochrome camera, everything is pretty easy to understand.

When an analog color video camera is explained, it usually starts with a diagram of a light path with a few bean splitters and a set of filters to feed three different tubes with red, green and blue images. This resulted in high quality broadcast images, but with significant cost and complexity. When color home video equipment came onto the market in the 1970s, a demand for single-tube color cameras emerged, and to this end manufacturers developed a series of similar tubes with RGB strip filters over their targets. I discovered a couple of these cameras, both with Panasonic Newvicon tubes. These distinguish between red, green and blue parts of the image based on their amplitudes, and while the image is definitely in color, I'd be lying if I said it was broadcast quality.

Here in 2024, there's little reason to use a tube camera unless, like me, you're looking for a specific aesthetic. However, they remain a fun and forgettable piece of consumer electronics to experiment with, so grab one and give it a try if you want to see one. When you look at the entire camera and monitor system, you can see the beauty of analog television because all parts of the system are perfectly synchronized. Imagine an entire country's televisions tuned to the same channel, all synchronized to a fraction of a microsecond, and you'll see what I mean, although the idea of ​​everyone watching the same show together is now more than acceptable is ridiculous.

If you liked this, you can find more from the PAL Coalface here.

Headline: Kyle Senior, CC BY-SA 4.0.