Sony mimics eye with curved sensor5 August 2014
Sony has created curved Exmor R CMOS sensors that have the same curvature as the human retina and should be up to twice as sensitive as a flat sensor.
On a normal sensor, light rays hit the pixels towards the sides at an angle, which can lead to distortions and reduce the light captured, whereas light rays hit all parts of the curved sensor straight on. That also means that lenses don’t have to be designed to correct for aberrations, vignetting or distortions (which means fewer elements can be used in a lens, allowing for lighter, less complex constructions, as the sensor works with the natural field curvature of the lenses).
However, it would mean that any lens would need to be designed with a fixed rear element group as it would be necessary to maintain the same distance between the rear element and the sensor (unless the curve were variable, which this isn’t).
The curved back-illuminated sensor (BIS), when fitted to an integrated lens, is claimed to be 1.4 times as sensitive as a traditional sensor in the centre.
According to Sony, this is “because the lens field curvature aberration was overcome in principle by the curved sensor itself, the curved BIS enables higher system sensitivity through design of a brighter lens with a smaller F number than is possible with a planar BIS,” so the lens can let in more light.
“At the same time, we controlled the tensile stress of the BIS chip to produce a curved shape that widens the energy band gap to obtain a lower dark current,” with one-fifth lower dark current than that of a planar BIS. This background dark current is always present even when the pixel is unlit, and lowering this residual current will also reduce the noise it induces, producing richer, cleaner blacks.
The sensor is also claimed to be twice as sensitive in the corners (where the pixels capture the same amount of straight light as at the centre).
The sensor is bent on a custom-made machine then fixed to a ceramic substructure to stabilise it.
This isn’t the first attempt at a curved sensor – there has been research done on this elsewhere, while Toshiba, Nikon and others have had patents relating to it. In the past Minox and others had cameras with curved film planes.
From a broadcast point of view, it may be some time before this curved lens design has any impact, as it is likely to be introduced first in mobile devices, which will benefit from the greater light sensitivity and simplified lenses, and in compact fixed-focal length cameras (such as any replacement for Sony’s RX1 – which is rumoured to appear early 2015). Of course, with more video being shot on mobile devices, and the use of small action cameras with fixed lenses, this sensor could be very useful. However, if it is introduced in an interchangeable lens camera it would require new lenses designed specially for it.