A Bluffer Guide to....CMOS


Mike Colyer TV-Bay Magazine
Read ezine online
i
I like to break my employers' cameras. For me, there is nothing more exciting than considering a daring location to install a micro camera system. It could be in a trackside breaking board at a motorsport event (culminating in slightly-too-extreme close up of, say, a Porsche bumper) - or a system installed a tad too high up a tree in a jungle (where a direct lightning strike can push the system somewhat over its 12 volt max). Thanks to modern CMOS camera design, however, not only can this be done safely (unless you are a camera) - but it can be done cheaply, often with impressive results.

CMOS (Complementary Metal Oxide Semiconductor) Sensors are now not only hidden in numerous broadcast, film and action cameras (and often shouted about in leaflets you collected at NAB) - but also in your mobile phone, CCTV systems and webcams. However, what exactly is a CMOS sensor, and how does it work?

The University of Edinburgh claims a paper written by some of their researchers in 1986 was the birthplace of the single-chip CMOS based video camera. The premise of this design is not unlike other imaging sensors (like CCD) - to turn photons of light in to an electrical signal. Consider fig.1. Here, we can see that the photons (light) travel through a micro lens to help focus the beam down onto a single pixel's photo-diode. The photons travel past the internal wiring and digitisers of the sensor down to the photodiode itself, which turns the photon into an analogue electrical signal.

Immediately after this, the analogue signal passes to a dedicated digitiser to turn the output into a series of binary values (1s and 0s). Finally, these binary numbers, along with the numbers from the other digitisers on the sensor, are sent on to another part of the camera for processing (referred to as the Imaging Pipeline).

At this point, some readers may be wondering why the photons must first travel past the wiring of the chip before hitting the photodiodes. It is clear that this method means some light will not make it to the photo site, which will lead to a degraded image. Imaging companies such as OmniVision and Sony have addressed this by switching the wiring and photodiodes around (as per fig.2.).

This style of sensor is referred to as "Back Illumined" and, naturally, provides better low light performance at the cost of being more complex to make and thus purchase.

It is also worth noticing the colour filter in figures one and two. Most CMOS sensors use a colour filter pattern known as the 'Bayer Pattern' to generate the initial colours to be processed by the imagine pipeline.

Fig.3. shows what the Bayer pattern looks like - effectively, it is a repeated 2x2 square featuring the colours Red, Green, Green and Blue. When Bryce Bayer patented the pattern in 1976, the rationale he offered behind the repeated green element was that green is the colour that the human eye is most sensitive to. As such, to ensure that cameras capture images closer to 'human vision', this filter is now used in virtually all CMOS sensors.

Some of the photodiodes, however (usually around the extremities of the sensor) have a black filter. This allows the system to measure 'dark current' - or how much of a false signal (often shown as noise in an image) is generated when there are no photons hitting the sensor at all. By using this technique, it then becomes possible to calculate where noise could exist in the illuminated photodiode readings and remove it, leading to an improved image quality.

CMOS technology is still evolving - with some camera manufacturers' already utilising "next generation" sensors, such a sCMOS (Scientific CMOS) - promising even greater latitude and low light response, aiding in the development of even more dramatic footage. Almost a shame that many of these sensors will only see the light of day once...

Mike Colyer is a Special Cameras and Vision Engineer who is currently studying towards a PhD in the field, investigating how small camera systems can be improved and made safer. He was awarded the RTS 'Young Technologist of the Year' award in 2015, a year after graduating Ravensbourne University with First Class Honours in Outside Broadcast Technology. He has worked with the cameras on shows ranging from "Formula One", across to reality shows such as "I'm A Celebrity Get Me Out Of Here" and strongly believes that special cameras pave the way to more immersive content.


Tags: iss125 | cmos | bluffers guide | ccd | Mike Colyer
Contributing Author Mike Colyer

Read this article in the tv-bay digital magazine
Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

Related Interviews
  • Grass Valley at BVE 2013

    Grass Valley at BVE 2013

  • Guntermann and Drunck CrossDisplay switching and CCD at IBC 2013

    Guntermann and Drunck CrossDisplay switching and CCD at IBC 2013

  • Guntermann and Drunck DP-HR range at IBC 2014

    Guntermann and Drunck DP-HR range at IBC 2014


Articles
Connect A Broadcast-Quality Miniature Camera To Your Computer!
Spencer Doran Upgrade any home working or live streaming setup with a broadcast-quality professional webcam
Tags: webcam | marshall | CV503 | USB | ZOOM | Streaming | Spencer Doran
Contributing Author Spencer Doran Click to read
Virtualising your Playout
Alison Pavitt

The buzz around virtualised playout is exponentially growing with increased deployments. But questions remain about the economic, logistical and technical benefits to the end user.

Conversation has shifted and amplified, with the debate around the private/public cloud escalating and new cloud-native solutions entering the market. So, what do you need to consider today if you are thinking about virtualising?

Tags: iss142 | playout | pebble beach | cloud | Alison Pavitt
Contributing Author Alison Pavitt Click to read or download PDF
How to engage students with live projects
Martin Andrews For many employers, how and what we teach at university is sometimes considered to be very mysterious. The responsibility for creating the structure and content within the curriculum is the responsibility of academics. In my experience, the best method for making sure that students and staff are kept up to date with current working practices is to regularly engage with ‘Live’ projects and ‘Real’ clients.
Tags: iss142 | ccitv | university portsmouth | charles haskell | vlogs | Martin Andrews
Contributing Author Martin Andrews Click to read or download PDF
An Argument for the Z Cam
Jake Ratcliffe Z Cam first dipped their toes into the cinema camera market back at NAB 2018 with the release of the E2 and since then they have proven themselves as one of the go-to camera manufacturers for affordable and unique camera solutions. Their family of cinema cameras has since evolved into a set of 6.
Tags: iss142 | cvp | cvpgroup | micro four thirds | e2c | zcam | full frame 6k | tilta | smallrig | 8sinn | zacuto | rvlvr | Jake Ratcliffe
Contributing Author Jake Ratcliffe Click to read or download PDF
Why Routing Control
Mark Davies Routing control sits at the very heart of all advanced control systems. A well-configured system that manages signal routes throughout a facility and groups multi-level actions into a single control surface allows for much richer and focused production. Using one-touch panels, operators could route video, audio and data signals between the various elements of a network, perhaps to route a camera signal to a vision mixer, as well as through other processors, before it arrives at its destination. Users could adopt routing control to trigger an external event, a complete reconfiguration of a multi-router setup, route several sources simultaneously, or make set routes or multiple routes via a single button press.
Tags: iss142 | es broadcast | tsl products | remote production | tallyman | Mark Davies
Contributing Author Mark Davies Click to read or download PDF