Philo Farnsworth is nearly universally given credit as the inventor of electronic television.
His first transmission of an image via his television system occurred in 1927, and it
was an incredibly big deal. But his invention had less to do with the television in your
house, and more to do with the camera in the studio. Today, we’ll be exploring how Philo
Farnsworth brought television out of the realm of mechanical contraptions and into the landscape
of pure electronics.
You see, the cathode ray tube, the device which effectively is the screen of an old-style
television, was invented in the 19th century, decades before Farnsworth made his contribution.
The trouble was no one had yet figured out how to turn a CRT into a television. When
Baird invented his mechanical television, the holes in the spinning disc served both
as a scanning device, creating a signal from an image focused onto it as the holes traveled
past, AND as a display device, reconstructing the image as a light source duplicated the
intensity and location of each part of the scanning device, with the holes moving the
light’s apparent location to recreate the image line by line. With the CRT being pretty
thoroughly researched, it wouldn’t take too much imagination to create an image with
a moving electron beam. But what was unknown was how to use a similar device to create
a signal to actually run a CRT display.
As I worked out in my last video, to use a mechanical scanning method like in the Baird
system to achieve equivalent image quality of a US CRT television, the scanning disc
would have to be impractically large, and it would have to spin impossibly fast. Even
if the lens focused on just a 3 centimeter wide target, a 525 line scanning disc would
be 5 meters tall, and the edges would still be breaking the sound barrier for 30 complete
frames per second.
There were other ways to use a mechanical scanning device, though. One of these was
the so-called “Flying Spot” system. With these systems, a mechanical disc in front
of an extremely bright arc lamp would project a scanning pattern on a performer’s face,
and a simple light sensor pointed at the performer would measure the light reflected off of him
or her for generating the signal, with only one spot hitting the performer at one time.
The projection meant the mechanical disc could produce a
much larger image than its physical size. The trouble with this approach, however, was
that the performer would have to be in absolute darkness, and you still had to work around
the poor resolution of mechanical systems on the receiving end. Plus, I imagine the
strobing effect as the light shone across your eyes was fairly unpleasant for television
talent of the day.
Before Baird had even demonstrated his mechanical television, Farnsworth theorized a way to
electronically convert an image into a transmittable electrical signal in 1921. This is part of
why he is often regarded as television’s inventor. He was what you might call a prodigy,
as in 1921 he was only 15. He grew up on a Utah farm, and the story goes that he came
up with his idea after observing the lines drawn in the fields from a plow, realizing
that by altering a line as it’s drawn over a distance, you could make an image from a
field of lines. Perhaps this is why to this day, each half of an interlaced frame of video
is called a field.
Farnsworth submitted a patent application in January of 1927. However, Farnsworth wasn’t
the only person trying to build an image dissector. In fact, the invention of television is very
complicated and to say that one person invented it is rather simplistic. For example, before
Farnsworth had his image dissector worked out, German professor Max Dieckmann and his
student Rudolf Hell applied for a patent for…. This, in 1925. In fact, Dieckmann was among
the first to demonstrate the display capability of the cathode ray tube back in 1906. And
in the following year, a Russian scientist named Boris Rosing used a CRT with experimental
“video” signals to create geometric shapes. See, there was a lot of experimentation going
on all over the world. The wikipedia entry on the history of television, which is linked
in the description, goes over this in much greater detail.
Hell and Dieckmann’s patent was granted to them in October of 1927, but they were
never able to produce a working model. Meanwhile, Farnsworth transmitted the first image with
his image dissector at his laboratory in San Francisco on September the 7th, 1927. Farnsworth’s
invention was very crude at this point, as it only really transmitted a line, and not
a whole screen of an image. However, it was a revolutionary proof of concept. On September
3rd, 1928, Farnsworth called a press conference, where he declared “Good news, everyone!
I’ve invented television”. His demonstration of actually functional electronic television
is generally accepted as the first, and it is for this reason that he’s often given
credit as the inventor of television.
Farnsworth’s image dissector was a vacuum tube much like a CRT, but instead of emitting
light, the image dissector was meant to detect it. Inside the tube was a coating of caesium
oxide, a photosensitive material. Caesium oxide has an interesting characteristic where
when photons hit it, it emits electrons. This meant that an image focused with a lens onto
the face of an image dissector would create a pattern of electron emissions in the shape
and intensity of the image itself. Then, this electron image would be scanned.
An electron beam very much like that which would be used to draw an image on the face
of a CRT television receiver, would methodically scan the face of the image dissector. Now,
as we all know, opposite attract, and likes repel--electrons really don’t like to hang
out together. What happens when the electron beam encounters an area of the image dissector
where bright light is hitting the surface? Well, the caesium oxide coating is itself
producing electrons, so the electron beam would bounce back at areas where light was
present.
Ordinarily, most of the beam would just be absorbed by the oxide coating or even the
tube’s glass. But any electrons sent by the gun that ran into a spot already filled
with electrons emitted from the caesium oxide would be reflected back into the tube. The
image dissector contained a detector of sorts that would collect these reflected electrons.
By amplifying the output of the detector, a signal could be produced which corresponded
to the exact image brightness of wherever the beam happened to be pointing on the surface
of the image dissector. Deflect the beam in a raster pattern using electromagnetic fields
from a deflection yoke, and you can scan the whole face of the dissector and generate a
signal from the entire image.
Here’s a more practical demonstration of what’s going on. This poster board represents
the target of the scanning electron beam. The image we’re looking to capture is a
simple white circle. A lens is focusing that circle onto the face of the image dissector.
The circle of light, which moves us all, by the way, will cause the caesium oxide coating
to emit electrons inside the tube wherever the light touches. We’ll represent electrons
in red. Therefore, the circle will be filled with red electrons. It should be noted here
that the resemblance to the Japanese flag was entirely accidental. It took me filling
in over 90% of this to realize, Oh, that’s the flag of Japan.
Anyway, the electron gun sits behind the caesium oxide (Pop sound) and it emits a string of
yarn, I mean an electron beam (NES Zapper sound effect). Deflector magnets bend the
electron beam, and will start scanning the image. For the first few lines, nothing extraordinary
occurs as the electrons are just absorbed by the target. But, once the beam reaches
the circle, it gets deflected back because electrons are already there. In other words,
where the light touches, well the electrons must never go there.
These deflected electrons get picked up by a collector electrode inside the tube, and
by monitoring the output from this electrode, you get a signal which varies in intensity
in proportion to the brightness of the spot the beam is currently scanning. So, reverse
the process. Here’s a television receiver. It’s electron beam is following along the
same path as the one in the image dissector, so if the scanning beam is aimed at one spot,
the electron beam in the television set will be pointed at the same spot. Whenever the
dissector detects light from the circle, the signal it produces will cause the television
receiver to spit electrons out from its own electron gun. This will make the phosphors
inside the tube glow in the same places that the dissector detected light, and at the same
relative intensity. After the scanning is complete, the television will have drawn that
circle. Do that really really fast and over and over again, and you’ve got yourself
some mighty fine television.
If you’re confused, and I wouldn’t blame you if you are, it might be helpful to check
out my previous video on how analog television works. That video discusses the raster pattern,
how the television synchronizes the image coherently via triggers built into the signal,
and more in greater detail. Hopefully you’ll be able to see that the television camera
tube is essentially the same thing as the cathode ray tube in a television set, but
with the electron beam used as a way to detect the presence of light, rather than to reproduce
it. You can find a link to the video down below, or hang around until the endscreen.
Farnsworth’s image dissector was a big deal, but it kinda sucked. Because the beam has
to move across the face of the dissector so quickly in order to produce an image, it has
only the tiniest fraction of a second to actually encounter an electron on the surface and be
deflected back. The caesium oxide coating wasn’t super great at producing electrons
from light, so television cameras that used Farnsworth’s image dissector needed an insane
amount of light in order to work. This meant studio lighting was absurdly bright--and hot--,
and it generally presented unfortunate limitations.
A much more practical device for producing television signals was the iconoscope. Here
comes another person into the fold. Vladimir Zworykin filed patents for a television system
in 1923 and 1925. If you’ve been paying attention, you’ll have noticed that these
years predate Farnsworth’s patent application. I told you this was complicated.
In 1923, Zworykin presented his idea to H P Davis, the general manager of Westinghouse,
where he worked, and in 1925 he demonstrated the first prototype. However, it barely worked,
and Davis wasn’t impressed. So unphased was Davis that he told Zworykin to work on
something useful. Wwll. Zworykin would later work for RCA.
The iconoscope functioned essentially the same as the image dissector, but there was
one key difference. And Zworykin wasn’t the one who discovered the principle that
would solve the problem. That was Hungarian engineer Kálmán Tihanyi. Too many people
here.
Anyway, the big deal with the iconoscope was that it didn’t rely solely on electrons
emitted from the Caesium Oxide. Rather, it used a sheet of mica which had tiny silver
particles coated with our friend Caesium Oxide on one side, and a thin film of plain old
silver on the other. The separation of these two sheets provided by the mica essentially
turned the thing into a giant capacitor, capable of storing electrons.
When operating, the iconoscope would first send a steady sweep of electrons across the
whole target. This would provide a uniform charge throughout the mica sheet. Light reflected
from subjects in the studio and subsequently focused through a lens onto the target would
cause the caesium oxide to emit electrons again in the spots hit with light, creating
an electron copy of the image, and this would cause the charge stored between the layers
to decay more rapidly than it would if no light were hitting it. The next time the target
is scanned, areas that weren’t hit with light will still have electrons in them, which
will resist the beam’s efforts to try and add more. This extra beam energy is reflected
back and picked up by the detector ring. Areas that were hit with light would quickly lose
their electrons, and the beam’s energy would instead be used to replenish these lost electrons.
The iconoscope produced an inverted signal, as dark areas reflected the beam strongly,
and bright areas didn’t reflect it much at all, but it was much, much, MUCH more sensitive
than Farnsworth’s image dissector because it could store electrons in the mica sheet,
and thus greatly increase the likelihood that the electron beam would actually be deflected
back to the detector.
One tricky bit about the iconoscope was that the light it was detecting and the electron
beam had to hit the mica sheet from the same side. This is why the tube is such a weird
shape. The electron gun can’t be in the line of sight of the target, so it’s tucked
below the target screen at an angle. In a working camera, it’s actually in the front,
resting below the lens. Electronics in the camera would adjust its scanning sweeps as
it went to compensate for the keystone shape it would naturally produce if the beam was
projected as a simple square at an upwards angle.
A side effect of the way the iconoscope works is an image that can never truly be black.
Electrons will decay from the mica screen without any light hitting it at all--light
simply accelerates this decay. The detector ring would pick these rogue electrons up,
and it would transmit as an entirely grey screen. There needed to be bright areas of
the picture to pull down the average emission of the mica screen and make the other areas
appear black--in other words, high contrast scenery was required. Lighting conditions
would have to be accounted for to keep the averaging effect of the iconoscope from producing
odd images.
Because this episode is a circus of who-did-what, let’s drop one more name. The iconoscope
design was immensely improved by accident in 1931 when Sanford Essig left one of the
mica plates in the oven too long. This broke up the silver layer into tiny globules, which
was responsible for increasing the resolution the iconoscope could detect immensely,
Therefore, a sharper image was produced. So, don’t forget about him.
Although the iconoscope was a great improvement over Farnsworth’s image dissector, it still
wasn’t that great. The images were noisy, of poor resolution, and it still required
a lot of light, though far less than the original disector did. RCA would develop the Image
Orthicon tube in the 1940’s, and this much more sensitive device would be used into the
1960s. The Image Orthicon tube combined principles from the iconoscope, image dissector, and
the original Orthicon tube.
The Orthicon tube (along with the Eurpoean CPS Emitron tube) contained deflector plates
that when calibrated correctly would reduce the velocity of scanning electrons coming
from the electron gun to near zero as they approached the target. This was immensely
helpful, because without these deflectors, electrons could still bounce back into the
detector ring without photoelectrons being present. This created for a noisy picture.
Slowing the electrons down before they hit the target meant almost no rogue electrons
would have enough gusto to make it back to the detector. Only those scanning electrons
that actually encountered another electron at the target would make their way back, as
the extra repelling force imparted by its photo-electric neighbor would help to push
it backwards. This greatly reduced the grainy image noise associated with ordinary tubes.
In addition to the electron slow-downy thing, the Image Orthicon tube used a neat physics
trick to amplify the effect of the photoelectrons and make the whole thing more sensitive. In
an image orthicon tube, the surface the light falls on to make the image, called the photocathode,
and the surface the electron beam scans to make a signal, called the target, are separated
by a fairly great distance. The photocathode is negatively charged, meaning electrons near
it will want to fly away from it, and in front of the scanning target is a wire mesh with
a slight positive charge used to attract the photoelectrons. This causes the photoelectrons
emitted from the photocathode to be accelerated towards the scanning target. The separation
of photocathode and target causes a dramatic increase in speed of the photoelectrons, which
results in a multiplication of the electrons generated from the image. This happens because
when an individual electron slams into the target at high speed, it causes a splash,
forcing many electrons out of the target. This is called secondary emission, and a wire
mesh behind the target with a slight positive potential traps these extra electrons. This
phenomenon is used to greatly increase the tube’s sensitivity.
You see, the extra electrons produced when the first photoelectron hit the target came
from within the target itself. Essentially, the high-speed of the photoelectron knocks
out a whole bunch of electrons when it hits the target. This causes a net loss of electrons
in the area of impact, giving that area of the scanning target a slight positive charge.
When the scanning beam from the electron gun runs over this area, the electrons it emits
are first used to refill those lost from the secondary emission event. The result is that
bright areas of the picture use the beam’s energy to recharge the target, and no electrons
are reflected back and detected. Dark areas of the image don’t displace electrons in
the target, so the beam is reflected back as electrons are already present, and a strong
signal is produced via the dynodes and electron multiplier at the base of the tube.
The image orthicon tube was a big deal for many reasons, not the least of which was that
cameras could be much smaller and less awkward as the scanning portion of the tube was no
longer in front of the imaging target. But it was also sensitive enough to capture scenes
lit by candlelight, and its logarithmic light sensitivity matches that of the human eye,
which made images produced from these tubes appear more natural. A fun little fact immortalized
by Wikipedia regarding the image orthicon tube is that it’s directly responsible for
the name of the award given by the Academy of Television Arts and Sciences. Image Orthicon
tubes were often referred to informally as “Immys”. The presidents of the Academy
at the time, Harry Lubcke, wanted to name the award after the Immy. But since the statuette
is female, the more feminine “Emmy” was chosen.
The inverted signals of the image orthicon and iconoscope--meaning bright areas produce
little to no signal and dark areas produce a strong signal--wasn’t a problem as this
is how television broadcasts were transmitted, anyway. It was up to your TV set to flip those
values around.
Phew. That was complicated. And I didn’t even mention Kenjiro Takayanagi’s 1926 demonstration
in Japan of a CRT-based television system. Sadly he doesn’t get much credit because
his camera was still mechanical using a Nipkow disc. There I said it. Nip-koff. OR, should
it be nip-ko as some have suggested? I don’t know, maybe you should argue about it in the
comments.
There are so many dots to connect here that I’m not going to claim I got it all correct.
For one thing, It’s hard to pin down how developments in Europe affected those in the
US and vice versa, particularly due to the patent dispute between Farnsworth and Zworykin
which made new developments dance around their respective technological claims. This is a
large part of why I didn’t go into much detail on European tubes and systems. And
all the names given to all the tubes is very confusing for creating a research timeline
in me head. I’m welcome to all comments that may set records straight, so leave ‘em
below. This video is long enough, so I’ll be ending it here. Stay tuned, as in the next
video, we’ll be looking at the next big thing in TV--Color.
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(Oh no!)
(Well...)
(Might as well change the yarn again)