American Cinematographer - May 2011 - 63

transistors that convert the accumulated electron charge into a measurable voltage. These additional electronic components reside alongside the light-sensitive photodiode and occupy a certain percentage of the area of the photosite. This remaining percentageis the actual photosensitive area and mostly determines the actual sensitivity and dynamic range of the sensor. After converting the photosite’s stored charge into an electrical voltage, and before transferring the photosite’s analog electrical signal to a vertical column bus, this digital circuitry must reset the photodiode to begin the next integration cycle. The bus’s function is to also supply necessary timing signals to the photodiodes and send their readout information to the analog decoding and processing circuitry. A benefit of this grid structure of buses is that it allows each of the photosites in the array to be read as simple x and y coordinates. “With a CMOS imaging device, each photosite has an amplifier with a number of transistors, diodes and capacitors — a miniature circuit — that occupies the same [substrate],” explains Galt. “With CMOS manufacturing, it has been proven that the size of the features on any semiconductor have been following Moore’s Law, getting smaller and smaller.” Moore’s Law refers to Gordon Moore, the co-founder of Intel Corp., who stated, “Every 18 months, the density doubles, or the feature-sizes half.” This rapid pace of increasing technological complexity has allowed CMOS imagers to make significant qualitative steps. As Galt notes, “They have gone from having circuit features that you could see with the human eye to having features that you can only see with an electron microscope! “To grasp the manufacturing challenges this presents,” he continues, “I use an analogy to help people get a sense of how small the elements of these devices can get. On edge, a dollar bill is pretty consistently 100 microns thick. On the sensor we use for the [Panavision] Genesis, a photosite is only

A photomicrograph image of a cross-section of a pixel. Visible are the microlens above each photosite “well,” as well as the color filter photo-lithographically printed beneath the lens. (Image courtesy of Panavision.)

4.1 microns wide.” Zarnowski expands, “The initial promise of CMOS imagers was hindered by the fact that they were noisy [compared] to CCDs. This was due to both Fixed Pattern Noise and Temporal Noise, and as a result CMOS imagers had objectionable patterns that could be seen by the viewer and didn’t quite have the same sensitivity as CCDs. With the recent architecture changes made to eliminate FPN and the creation of overall lowernoise pixels — through the addition of pinned photodiodes — CMOS imagers are now living up to their potential.” “CCD technology was the superior technology in the ’80s and probably through the ’90s, but CMOS was on a real growth curve because it’s a technology that is used in all of the other semiconductor manufacturing operations,” agrees Glenn Kennel, president and CEO of Arri, Inc. “Unfortunately, all of the Moore’s Law’s improvements no longer apply to CCD because there just aren’t enough products being developed for them. CCD is still a viable technology, but it’s more expensive to make, and it isn’t improving at the same rate.
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In fact, it has reached a kind of plateau. CMOS imagers continue to improve and have now passed CCDs in sensitivity, dynamic range and frame-rate capabilities.” General Components of a Digital Camera For simplicity’s sake, we can say the digital camera has two discrete sections: the front-end imaging section and the back end for digital processing and output digital interfaces. In the imaging section, an image enters a lens and is projected onto a sensor. The sensor then samples this image opto-electrically with its photosites and generates an analog electrical signal. It is at this step when the resolution for the entire system is greatly determined and/or influenced by the number of samples on the sensor — in this case, the number of photosites. This analog sampling of the image is then “handed” to the digital section and subsequently subjected to separate digital sampling. “In the digital section, we have to grab that analog information and digitize it — it’s a second sampling step,”
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American Cinematographer - May 2011

Table of Contents for the Digital Edition of American Cinematographer - May 2011