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Hi - John Hess from FilmmakerIQ.com and today we’re going to look at the science and technology
of the digital sensors.
Before we get into the quantum mechanics of digital sensors, let’s look at the one of
the crowning achievements of 19th century chemistry - photographic film - which in many
ways is actually an analog to the processes underlying digital.
Film, at it’s most basic, is a thin sheet of cellulose, or plastic, covered with a very
thin coating of a Silver Halide Crystals suspended in a purified gelatin. When the shutter is
released and the film is exposed to light, those Silver Halide Crystals begin to break
down.
Photons - the quanta packets of energy that make up light strike and get absorbed by the
atoms in a Silver Hallide crystal - in this case Silver Bromide. This extra energy causes
the bond between the silver and a bromide to break resulting in a free electron, a positively
charged Silver ion and a Bromine atom. This free electron migrates in the crystal toward
the portion of the crystal called the sensitivity speck - a shallow electron trap that is either
a deformity or a piece of silver sulfide or gold added to the crystal in the manufacturing
process. At this Sensitivity speck, the electron bonds with a positively charged silver ion
to produce a free silver atom. The more intense the light hitting the crystal grain - the
more free silver atoms created.
This quantum dance occurs on every piece of grain in the exposed film, but after the initial
exposure the silver atoms on the grains are still too small to see. This “latent image”
needs to be developed. Amazingly, you only need four silver atoms arranged in a molecule
on a crystal to make the crystal developable. Using further baths and chemical processes,
the tiny invisible silver specks are “amplified” often billions of times creating a negative
image that we can use to reverse and create a positive print.
As silver halide is the key chemical that makes film possible, Silicon is what makes
digital possible. Silicon is a semiconductor - meaning it doesn’t conduct electricity
as well as a metal but it conducts better than insulators. This is because silicon has
4 electrons in its outer or valence shells. This causes silicon to create tetrahedral
crystals with other silicon atoms where each atom is satisfied with eight electrons in
their outer shell. Since the outer shell is now full, there are only a few free electrons
floating in the crystal lattice - it can conduct a little electricity but not easily.
But if you inject a small amount of a different element you can create different kinds conductivity
in the silicon crystal - a process called doping. Add a small amount of Phosphorus which
has 5 electrons in its valence shell and you create an N-type semiconductor. Phosphorus
fits in the tetrahydral crystal but it has one extra electron.
Contrarily, if you add an element without only 3 electrons in it’s outer shell, say
Boron, you create a P-type semiconductor. The missing electron is creates what is called
a “hole” - a space attracts a free electron.
Though P-type and N-type carry positive holes and negatively charged electrons respectfully,
the actual charge of the P and N type semiconductors by itself are both neutral because there is an equal number
of electrons and protons in the semiconductor crystal.
Now when you put a P-Type and an N Type semiconductor together you create one of the most basic
but important electronic component - a P-N Junction diode.
A diode is simply a device which allows current to flow in one direction but not the other.
When a P-type and N Type semiconductors come together some of the extra electrons from
the N Type moves in to fill the holes of the P-type creating a “depletion layer”. Here
the p-type material with it’s holes fill with electrons is negatively charged and the
n-type material, now missing it’s extra electron is positively charged.
So if we connect the positive terminal of a battery to the P-type side or Anode side
and the negative terminal to the N type or Cathode side we start to draw electrons out
of the holes in the depletion layer causing this depletion layer to shrink and the current
to flow through the diode. This is called forward bias.
Reverse the polarity of the battery and now the battery is adding more electrons into
the P-type side causing the depletion layer to increase. The negatively charged p-type
area now repels electrons and current cannot flow through the diode. This called reverse
bias.
When a diode is in reverse bias we can exploit a phenomenon called photoelectric effect.
Just as with silver hallide crystals - Silicon reacts when struck with photons of specific
energy - wavelenths between 190 and 1100 nanometers which happens to include visible light.
When a photon is absorbed by silicon crystal it creates an electron-hole pair. The electron
will move toward the positively charged N-type side of the diode. From here we can measure
the intensity of the light by measuring the number of electrons or the voltage.
So how do we get a bunch of photodiodes to create an image? The most intuitive way to
wire them up in a grid one photodiode per pixel and read out the voltage column by column.
That’s just what RCA tried but every time the a voltage passes a pixel, it picks up
a little bit of extra voltage - a problem called Capacitive Coupling resulting in a
lot of noise and striation in the image.
The solution to the problem would be to get rid of the wires entirely in the breakthrough
process developed at AT&T Bell Labs by Willard Boyle and George E. Smith in 1969.
The Charged Coupled Device or CCD uses no wires - instead, it uses a single piece of silicon - a large
photoelectric diode divided with insulating channel stops. This slab is then coated with
a thin layer of silicon oxide and tiny strips of charged aluminum are laid perpendicular
to the channel stops. So each pixel of this sensor is bound on the sides by a channel stop
with three aluminum strips running top middle and bottom.
And here’s what happens. An electronic shutter on the camera opens letting light onto the sensor.
The photoelectric effect creates electrons that migrate to the top of the silicon slab
where they are confined by channel stops and charged aluminum strips. The shutter closes
and the sensor holds the charge from the exposure. Now that charge is shifted row by row by alternating
the current along those thin aluminum strips and sent onto a serial shift register which
is then sent out to a capacitor connected to an amplifier and recorded onto the camera’s
memory. Once the entire charge is moved off, the sensor is ready for the next exposure.
This ingenious bucket brigade system of moving pixels produced clean and relatively noise
free video signal. But as with every piece of engineering, there is always a trade off.
Though the image was noise free - a tiny amount of the charge was lost each time the electrons
are shifted. This would become a problem with higher resolutions that required more and
more shifting. Also the read out of a CCD is slow - the information read in serial,
one pixel at a time - during which the electrical shutter had to be closed. And finally the
process of shuttling those electrons requires quite a bit of voltage - voltage that would
quickly drain a battery in say a small device. Still the CCD came to dominate the video market
in the 80s and 90s. But an alternative was brewing and ironically it would go back to
the same x-y grid concept that originally failed because of capacitive coupling..
CMOS or Complementary Metal Oxide Semiconductor is a technique of constructing integrated
circuits such SRAM, DRAM and microprocessors. The CMOS sensor or more accurately CMOS APS
for Active Pixel Sensors were described in papers at relatively the same time as the
invention of the CCD, but CMOS sensor lagged behind because of manufacturing reasons.
But as manufacturing processes became more sophisticated the CMOS APS became commercially viable as
a camera sensor in the mid 1990s.
Much like a CCD, a CMOS sensor is made out of single piece of silicon. But instead of
shuttling electrons row by row, each pixel has a small capacitor and signal amplifier.
Amplifying at the pixel level overcomes the Capacitive Coupling issue and is what makes
the sensor an Active Pixel sensor as opposed to a passive one. This amplified voltage is
sent down tiny wires onto a bus and processed in parallel to be stored in the camera’s computer.
Now despite the fact that each pixel has it’s own capacitor and signal amplifier, CMOS sensors
draw much less power because there’s no need to shuttle electrons like in a CCD. Because
the voltage data is read in parellel, CMOS sensors are capable of reading the data off
the sensor much faster.
But one of the drawbacks of CMOS sensors is higher noise. In CCD sensors you have only
one amplifier at the end of the imaging process - in a CMOS sensor, each pixel has an amplifier
and micro variations can cause noise issues.
Also there’s the common motion artifact that plagues practically all CMOS sensors
- rolling shutter. In a CCD, the image is recorded in a single instant. A CMOS sensor
records data line by line, the information collected from the top of the frame is slightly
ahead in time from the information from the bottom. This results in the jello effect where
fast pans look crooked or strange patterns developing in fast moving objects.
Now Rolling shutter isn’t always necessarily bad. By reading lines at different times,
you can give each pixel a bit extra time to capture photons - resulting in better low
light performance. Rolling shutter can also be mitigated with really fast readouts
or by including a bit of memory on each pixel that stores the voltage before being read out.
Despite these limitations, the advantages of the CMOS sensor, from its ease of fabrication
to low power draw make it ideal for both consumer and professional cameras.
Arthur C. Clark third Law states “Any sufficiently advanced technology is indistinguishable from magic.”
In explaining the quantum physics of camera sensors it feels a lot like describing magic.
I mean photons, electrons, measuring quantum particles smaller than the wave length of visible light
all done by a species who just a few centuries ago was unaware of microscopic
bacteria. And now we’re harnessing even more infinitesimally small particles - bending
them our will, storing them up and expending them to tell a visual and audio story that
changes our experiences: how we think and how we feel
And that power over nature is practically in reach of every human being on the planet whether they have a
superficial understanding of the mechanism underneath or not...
well that to me is nothing short of pure magic.
We stand on the shoulders of such giants. Use this power you have and
Go out there make something Great. I’m John Hess and I’ll see you at FilmmakerIQ.com