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48Mpixels in a cellphone camera

The unusual architecture of the new 48 Mpixels Sony sensor. The pixels are clustered in groups of 4 and the software in the camera works out the normal distribution that will then be used to render the image. Credit: Sony

The new digital camera sensor, designed for top of the line smartphones by Sony, is quite remarkable and it shows the kind of sophistication, and ingenuity, reached in this area.

Cramming 48Mpixels in the small surface available for a sensor targeting smartphones means having to reduce the pixel size considerably, in this case to 0.8µm. If there is plenty of light a small pixel area is not a big problem but when the light is limited then you get more noise than signal.

Here comes a first stroke of genius by the sensor’s designer: when the light is low pixels are clustered in group of four to get a much bigger pixel (you get the equivalent of 1.6µm size pixel) and that allows for more photons to be harvested and a higher signal to noise ratio.

To make this clustering possible, however, the colour filter (Bayer matrix) cannot be the usual one, otherwise you would get in the same bucket few photons of different wavelengths. Hence Sony has designed a Quad Bayer matrix where each colour filter covers 4 pixels. The drawback in this situation, of course, is that you no longer have 48Mpixels resolution but 12Mpixels resolution.  However, in low light situations his is good enough.

How can you get the 48Mpixels resolution with a Quad Bayer matrix? By using software. If you get plenty of photons (that’s when there is plenty of light) you can do some statistical distribution and convert, in software, the Quad Bayer into a normal one (see the figure).

Sony is also claiming that by using software is is possible to increase the dynamic range (to capture more hues), probably by switching off at the middle of the exposure two of the 4 pixels clustered under the Bayer filter. In this way those two pixel will receive only half of the photons and can be used to assess in software the high key tones (it is like to have two shots taken, one at the selected exposure, the other with one stop less. Clever!

Of course it remains to be seen the real quality delivered, and for that we need to wait till the end of the year when the sensor will start to be available in new smartphones.

About Roberto Saracco

Roberto Saracco fell in love with technology and its implications long time ago. His background is in math and computer science. Until April 2017 he led the EIT Digital Italian Node and then was head of the Industrial Doctoral School of EIT Digital up to September 2018. Previously, up to December 2011 he was the Director of the Telecom Italia Future Centre in Venice, looking at the interplay of technology evolution, economics and society. At the turn of the century he led a World Bank-Infodev project to stimulate entrepreneurship in Latin America. He is a senior member of IEEE where he leads the Industry Advisory Board within the Future Directions Committee and co-chairs the Digital Reality Initiative. He teaches a Master course on Technology Forecasting and Market impact at the University of Trento. He has published over 100 papers in journals and magazines and 14 books.