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
The brave new world of software based production
Boromy Ung In today’s rapidly evolving broadcast industry, the only constant media organizations can truly count on is change — and the need to adapt as rapidly and cost-effectively as possible. One of the biggest agents of change is the IP revolution, driving broadcasters to migrate their operations to all-software solutions running on commodity, IT-based technologies.
Tags: iss134 | chyronhego | graphics | sports | ott | Boromy Ung
Contributing Author Boromy Ung Click to read or download PDF
Sony HDC-4800 Review
Andy McKenzie First announced at NAB 2016, the Sony HDC-4800 is a studio camera system capable of shooting 4K/UHD at up to 8x or full HD at up to 16x. With a price point upwards of £250,000 it is a very high-end product with a wide feature set. In Sony's own words, "This is the future of live production, designed to satisfy the storytelling aspect of modern sports production.” Deliveries began in mid 2017 and, after careful preliminary evaluation, we invested in several systems for our hire fleet ahead of the FIFA World Cup in Russia.
Tags: iss134 | review | hdc-4800 | sony | finepoint | Andy McKenzie
Contributing Author Andy McKenzie Click to read or download PDF
AI in Media and Entertainment
David Candler Artificial Intelligence (AI) is a term appearing everywhere these days. What is happening in media and entertainment (M&E) that makes the industry ripe for AI? In other words, why does the M&E industry need AI?
Tags: iss134 | AI | wazee | David Candler
Contributing Author David Candler Click to read or download PDF
An Obituary to Timecode
Bruce Devlin - new A stoic and persistent character that stubbornly refused to change with the times, Timecode has finally passed on, but no-one has noticed. A long-lasting industry veteran, Timecode was brought into this world at an uncertain date in the late 1960s due to the needs of analogue tape workflows and the demand for synchronisation between audio and video devices. A joint activity between SMPTE and the EBU led to the work on Time and Control codes starting its journey to standardisation in the early 1970s.
Tags: iss134 | timecode | smpte | ebu | edit | Bruce Devlin - new
Contributing Author Bruce Devlin - new Click to read
GoPro HERO 7 Review
Tim Bearder When I heard I was filming a nature restoration project in the pouring rain this week I was excited. WHY? No Cameraman enjoys the rain, surely but this time I was enthusiastic because I knew this would be the perfect opportunity to try out the brand new GoPro Hero 7 Black Edition.
Tags: iss134 | gopro | hero 7 | review | liberal media | Tim Bearder
Contributing Author Tim Bearder Click to read or download PDF