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The Leica M Sensor

The quest for ever higher megapixel resolution is still in full swing with no end in sight. People think that higher resolution (more megapixels) will assure better performance results.

Leica seems to have a more sensible approach quitting the pixel race keeping performance of Leica M cameras some of the best still available.

Instead of using the usual sensors from Sony, Leica latest sensors are made by CMOSIS, a renowned European specialist for advanced CMOS image sensors. They developed a high-resolution, high-dynamic-range CMOS image sensor exclusively for Leica Camera AG.

Guy Meynants, CTO at CMOSIS, Antwerp, Belgium, said “This is the first time that a CMOS image sensor for a 35mm high-end camera was designed, and is manufactured, in Europe for a European customer. Apart from the ceramic IC package the Leica MAX 24MP CMOS Sensor is a 100-percent European product.

The new custom-designed sensor chip is made by STMicroelectronics (STM) in Grenoble, France, using 300mm wafers in their IMG175 CIS technology with a pixel count of 6,000 x 4,000 pixels on a 6 x 6 µm² grid across the active area of 36 x 24 mm.

The result is an impressive linear dynamic range close to 76dB and noticeably less noise. Pixel data of the Leica MAX 24MP CMOS Sensor are digitized by patented low-power, high-speed 14-bit column AD converters. The sensor features an electronic rolling shutter with global reset and noise cancellation through both analog as well as digital correlated double sampling (CDS) resulting in low temporal and spatial noise and non-uniformities.

STMicroelectronics took special care to reduce crosstalk between neighboring pixels for a wide range of incident light angles. The sensor reduces spatial crosstalk by its very small distance between color filters and photodiodes. Microlenses with a strong curvature and high top height focus the incoming light rays in the center of each pixel’s photodiode. This allows the “Leica M” to accept the full range of high-quality lenses in the camera system.

These sensors are covered by a microlens array which covers each pixel with a small lens that directs the incoming light more directly to the individual pixels. To minimize geometric aberrations at oblique incident light rays, the microlenses of the image pixels have to be shifted slightly in the corners towards the center. This is referred to as the microlens offset.

When those pre-digital age lenses were designed, nobody thought that in the future an additional piece of glass might be placed behind these lenses. This is the glass cover over any of the digital sensors. It was necessary for Leica to develop a sensor which compensated for this, a sensor that was immune to the angle by which the light was reaching the sensor. For that reason Leica decided to develop a sensor that specifically addressed their needs instead of using existing sensor technology.

An advantage was gained by using a different design for the microlens covering the entire sensor. Instead of using the common flat lenses, Leica went to an elongated, parabolic design. That has the advantage that more of the incoming light will be redirected to the individual pixel areas and, especially at the corners, there will be no noticeable vignetting.

The former Leica CEO Alfred Schopf said “With CMOSIS, we are very pleased to have found a partner who made it possible for us to design and construct a sensor especially for Leica. Thanks to the special sensor technology and wide pixel aperture from CMOSIS, we can now, and for the first time, offer a digital system camera that is perfectly optimized for use with both M and R-Lenses. Leica is particularly proud of the fact that the Leica M employs a sensor Made in Europe, as a large proportion of the sensor is manufactured in France and Germany. At the same time, the extremely low power consumption of the sensor brings added benefits for both image quality and battery life.

It is no secret that individual pixel size does make a difference. The larger the individual pixels, the better they will perform. Regardless of the number of pixels on a sensor, not the entire surface area of each individual pixel is light sensitive. The goal was to keep the non-sensitive areas of each pixel as small as possible. If less of the surface of the sensor is taken up by supporting electronics overhead, then more surface area can be used to collect incoming light. This results in greater dynamic range and a higher initial sensitivity. However, this leads to another problem. The density of the electronics generated more heat which unfortunately causes a greater amount of noise in the image generated. However, that problem is being addressed and greatly eliminated. Besides, is it really necessary to eliminate all noise? Would that ultimately lead to characterless images?

The pixels need to be supported by a substructure and each individual pixel needs to be connected to the system by small wires. Canon, for instance uses wires with a size of 0.35 micron. Sony is definitely better with a size of 0.18 micron. Leica by far exceeds that with a size of 0.09 microns. As a matter of fact, when determining the specifications for the sensor, Leica demanded that the structure sizes be kept as small as technically possible.

It used to be the case that 75 percent of the sensor was taken up by non light sensitive electronics. This has been decreased to less than 20 percent with the new Leica CMOS sensors.

Another means to increase sensor performance is to make it as thin as possible. The Leica CMOS sensor in the Leica M10 is the thinnest ever developed. Each sensor contains several layers. By making each of them as thin as possible, the end result is a significant increase in performance because more of the incoming light can actually reach the photodiodes, the individual pixels.

An additional advantage was gained by using copper for the connecting material instead of aluminum, which is the common choice because the process of using copper is substantially more complex. Copper has a substantially lower electrical resistance than Aluminum, meaning that conducting layers with half the thickness could be used. In addition, to minimize thickness, instead of having four metal layers for the conductors typically employed on CMOS sensors, only two were necessary on the MAX CMOS chip.

Following these steps, Leica was able to use a pixel area of 6µm2. The sensors of many competitors have noticeably smaller pixel areas.

Leica is convinced that this approach will lead them back to their roots: high quality in a compact design. After all, that was Oskar Barnack’s goal when he designed the original Leica prototype, the Ur Leica, over 100 years ago.

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