Tuesday, January 19, 2016

THE FUTURE OF SENSOR TECHNOLOGY AT LEICA







Just a short while ago it seemed that Leica had missed the changeover to digital photography, that the company might be on a track to oblivion like so many other great photographic companies that went out of business.  It was thought that they tried to survive with the old film oriented business model much too long, that they concentrated too much on their past glory.

Fortunately this has changed, and today Leica is without question one of the leading companies of the digital camera market.  However, in one respect Leica did indeed look to the past.  In order to be able to use virtually all of their past lenses on the new digital Leica cameras, some huge hurdles needed to be overcome.

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.

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.

Leica CMOS sensor


However, with the Leica M-System two different lens systems had to be equally usable. The short focal lengths of the M system have a main beam angle up to 35, while the R lenses with larger focal lengths have a much smaller beam angle. Consequently something special had to be done.  By reducing the distance between the photodiode and the microlens an enlarged pixel aperture was achieved,   At the same time the refractive index of the microlenses was changed to the extent that any microlens offset was no longer necessary.  This made the sensor less sensitive to the influences of the lens, and thus equally usable for both M and R lenses.

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.

CMOS sensor
Conventional CMOS sensor with flat microlenses and deep pixel wells

MAX CMOS sensor
Leica CMOS sensor with tall micro lenses and very shallow pixel wells, allowing for a larger pixel area


Designing their own sensors led to a welcome side effect, the possibility to cater to special requests without huge additional costs.  This was the case with the first Leica Monochrom, which carries essentially the same sensor as the Leica M9, however without the Bayer color filter.  The result was a senor where each individual pixel senses variations in illumination instead of being able to sense only one of the three colors necessary for color photography.  The result is a higher level of sensitivity and greater sharpness.  To this day Leica is the only company using black and white only sensors in one of their cameras.



The most widely sensors at this time are the CMOS sensors.  They use se so called system on a chip technology, where some of the necessary electronics are integrated in the sensor itself.  Compared to the older CCD sensors, these require substantially less wiring outside the sensor.  

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.

pixel diagram
The surface of a sensor.  If the non-sensitive areas can be made as small as possible, more surface area is gained to collect light. 
 Image courtesy of  Leica Store Miami


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 M240 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 6┬Ám2. The sensors of many competitors have noticeably smaller pixel areas.

Considerable research is being conducted about how to generate the different colors.  One approach would be to do away with the regular color filter patterns of the Bayer filter, as it is the case at present and to replace it with a stochastic arrangement of the pixels.  That would mimic the distribution of the silver halide crystals in photographic films.  While that sounds relatively simple, it is unfortunately a fact that the regular patterns of the Bayer filter currently are necessary to obtain a useful section of the data generated by the sensor.


The megapixel craziness has fortunately quieted down.  In the immediate future we will see sensors with less noise and higher dynamic range.  After that, according to Leica, another approach would be with curved instead of flat sensors.  This would allow for the design of substantially more compact lenses and also more compact camera systems.    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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10 comments:

  1. You wrote a lot about Leica sensors and what they have done to make them work for their cameras and lenses, but little about what they plan to do in the future.

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    1. Yes, a major part of the article was about current sensor technology and what Leica has done to make it work. But by outlining the difficulties I also indirectly pointed out where further improvements can be made. As with any manufacturer, information about future products can only be obtained in general terms. Leica is no different, like anybody else they guard their new products quite carefully. I was, however, able to point out the general direction they plan to take.

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    2. To expect exact information about future plans from any manufacturer of high tech equipment is naive.

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  2. I liked the article. It was quite informative and it clearly shows that Leica is not sitting on their laurels.

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    Replies
    1. Thank you. That Leica is not sitting on their laurels has been clearly shown by the wealth of new products they have introduced over the past few years.

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  3. Very well written article! Do you know whether any further steps were taken in order to produce the M 246?

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    Replies
    1. I am not sure what you mean by "produce the M 246". It is currently part of the line of the Leica M cameras and the current model of the M Monochrom. I have no information about future models of this camera. As usual, Leica is very tight lipped about this.

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    2. Sorry--long day yesterday! What I meant to ask was whether you had information on any steps taken in transitioning from the first MM, based on the M9, to the new MM, based on the M240 and its CMOS sensor.

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    3. Not to worry, I know the feeling. In order to add some of the new features of the Monochrom Typ 246 (live view, video...) it was necessary to switch to a M240 based camera with CMOS sensor. I just returned from a trip to Wetzlar a little while ago. I tried to get at least a little bit of info about new products or trends, but that was only met by a polite smile. As I mentioned already, they are very tight lipped about anything the future might bring. While there I received an over four hour VIP tour of the facility and saw a lot of things that are not typically part of the factory tour. In addition our guide Herr Roland Elbert gave a lot of detailed information about their production methods, but a look into the future was not part of that. As a matter of fact, some areas of the production were covered by huge murals to prevent anything from being seen that was not for public view at that time.

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    4. Thank you. That sounds like a fascinating tour of the Wetzler facilities. It's not surprising that they are tightlipped about upcoming projects. They did a fine job of springing the Leica Q on everyone!

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