Leica on the Future of Sensor Technology

In an article over on Germany's ColorFoto website, there's a discussion by "Leica on the future of sensor technology" (in German). A lot of interesting bits on the sensor of the M and using both R lenses and classic M lenses on it, some CMOS vs. CCD and even the future of the S system. Well worth checking out.

The article translated into English (via Google):

Leica on the future of sensor technology

In the development laboratories of Colorfoto, Leica discussed the future of sensors, signal processing, and the challenges of M-System.

Leica and digital photography - it is believed the grapevine in Internet forums, is primarily a story of missed opportunities. Too late, technologically obsolete and fixed solely on design and brand image are the Leica models, can be read there. In fact there was a time in which threatened to miss the boat because of financial turmoil the company. But with a new investor and successful in-house developments in the high-end area you have got things right again. This is evidenced not only the new headquarters, the Leica moved in May 2014..

On one point, however, it is actually turned on Leica backward: To ensure that all M and R lenses can be used on the digital Leica M from year 1954 one or the other chin-up was needed. Because with the statement of the analog lenses was not yet clear that behind the lens someday again a piece of glass would sit - that on the sensor. So you had to develop a sensor that compensates for this difference and it does not matter what angle the incident radiation. So we came to the conclusion to let develop its own sensor, instead of the bar to buy.

A pleasant side effect: Who can design its sensors individually, can operate at a reasonable cost and special requests - such as with the Leica M Monochrome. This principle is the sensor of the Leica M for use, but no color filter. The differences caused thereby compensates Leica from the beam path and adjust the software accordingly. This can provide brightness values ​​each individual pixel, rather than being responsible for one of three colors - so increase basic sensitivity and resolution of the sensor.

Sensors

What is to consider everything when developing a sensor? The CMOS is conceived as a "system on a chip", a part of the processing electronics is therefore already integrated, and the integration process proceeds rapidly. Plugged in ten years maybe behind each pixel, a processor? "I would not like to see," says Dr. Volker rooms, department manager at Leica Digital Imaging. "Because that would cost back light sensitivity."

The problem with the progressive integration: Additional electronics needs space. The first CMOS sensors have a fill factor of 25 percent, three quarters of the sensor area thus had to be sacrificed for the non-light sensitive electronics. We are now at a fill factor of 80 percent, to the attached lens may still increase. Further integration caused a deterioration of the filling factor - and gives an additional heat source in addition to the light-sensitive diode which actually should not rush. Carefully balancing is thus called for.

Next topic in sensor: How to distinguish colors? There are indeed quite different approaches - Keyword Foveon - but where does the journey? "A stochastic distribution of pixels - similar to the distribution of crystals in the film - could help solve the problems caused by the regular arrangement of the color filters in the Bayer Pattern," explains Dr. Zimmer. This sounds simple, but it is not. Because on the other hand need the regular structures namely for reading out the data. But there are also very different approaches. "Spherical sensors instead plan would make some things easier and significantly more compact lenses, for example, allow," says Oliver Giesenberg, Director Development & Engineering. Currently this is still a long way, but there are already promising approaches worldwide developer.

CMOS sensors

CMOS sensors have already implemented most of the steps of the signal processing on the pixel level and need as compared to the previously used CCD sensors less external wiring (Off Chip Circuitry).

Each pixel therefore contributes in addition to a photodiode in addition a plurality of transistors, which convert the charge of the diode changes in measurable voltages and transfer them to the vertical column bus. This technique makes it possible to read every single pixel with very high frame rates.

Therefore, until the transition to CMOS allows creating videos in high-resolution formats and thus the consistent features of cameras with video functions. A portion of the pixel area is needed for transistors, electrodes or register. The size of this part is, describes the fill factor, the ratio of the light-sensitive area to the total area of ​​the pixel. This ratio has increased in the past decade, from less than 40 percent to over 80 percent. However, the design also ensures problems: The near heat-producing electronics for the photodiode is a major cause of the noise. And because each pixel performs the analog-to-digital conversion itself and the individual components do not work exactly the same, it also leads to fluctuations in the conversion of charge into voltage. As a result, the uniformity of the same color regions of the image decreases. Each sensor manufacturers poised at sensor design, these individual factors to today.

Signal Processing

The signal processing is the immense power of digital technology more quickly - or can work at the same pace ever more complex actions - for example, image adjustments. But what I want at all correct? Does one really completely eliminate the noise? Or then only a heartless image is left? Dr. Zimmer puts it like this: "Each sensor has pixel errors that are corrected, and it turns quickly the question: What's correction and where do I start to invent something to" In addition to the objective criteria - an edge to an edge be shown that I can measure - there are also many subjective variables in image quality. Particularly striking are those voting in the skin color. Europeans like to see suntanned from, Asians prefer elegant pale.

Many of these subjective factors are influenced both by the optics as well as by the signal processing. "We have therefore clear specifications, such as should look a bokeh, or when the light sensitivity is more important than noise," says Stephan Schulz, Head of Product Management Professional Photo. In the M-series sometimes compromises in favor of backward compatibility is required in the S-Class - which was developed from the ground up as a digital system - could, however be consistently optimized quality of results. Setting limits also in the S-system factors such as weight and price, since the product is in direct competition with other medium format systems.