Sony's curved sensors may allow for simpler lenses and better images
The sensors inside digital cameras are - generally - flat. But curved sensors promise greater sensitivity, better image quality, and provide scope for simpler lenses.
Speaking at the VLSI symposium in Hawaii, Sony device manager Kazuichiro Itonaga showed a curved full-frame sensor and a smaller, curved 11mm diagonal sensor (which would be classed as 2/3"-type). The former is suited well for a fixed-lens full-frame camera (such as the RX1), while the latter is sized similarly to sensors in enthusiast compacts as well as the Nokia Lumia 1020. He did not detail the process but it appears they are fabricated flat, bent into shape using a custom machine, and then backed with ceramic to maintain the bent shape permanently.
|First public image of Sony's curved CMOS image sensor. [Source: Image Sensors World]|
But why a curved sensor? In a nutshell, a curved sensor is designed to combat the field curvature (also known as Petzval field curvature) associated with simple lenses imaging a flat plane on a flat imaging sensor. Field curvature results from the fact that the effective focal length light rays experience decreases the more off-axis light rays are from the lens' optical axis (the optical axis is defined by a line running from infinity through the center of the lens). In other words, the more off-axis a light ray is, the further in front of the focal plane (defined by focused on-axis rays) it falls. In fact, as you can see in the image below, a flat plane in the real world is imaged on a curved surface known as the 'Petzval surface', not a flat surface (labeled 'Theoretical image plane' below).
|Because of field curvature of simple lenses, a flat plane in the real world is imaged on a curved surface known as the 'Petzval surface'. Image sensors tend to be flat, therefore requiring corrections for this field curvature. [Source: astrosurf.com]|
Lens manufacturers go to great lengths to combat field curvature by introducing field flattening elements - necessarily making lens designs quite complex. Furthermore, these elements often introduce other aberrations; for example, there's a well known tradeoff between astigmatism and field curvature.
What if we could work with the field curvature inherent in lenses, as opposed to against it? That's exactly where curved sensor technology can help. Curved sensors allow simpler lens design, since light rays entering the lens from oblique angles don't have to be corrected to project onto a flat surface. They also combat peripheral light fall-off by improving light collection at the edges of the sensor, since the light enters the photosites at a less oblique angle (back in the days of film, this sort of fall-off was somewhat mitigated due to the fact that film dyes were more accepting of oblique light rays than deep pixel wells are). In fact, a 2x increase in photon collection efficiency is reported for the periphery, with a 1.4x increase in sensitivity in the center of the sensor as well. The latter may be due to a better ability to collect oblique light rays entering the peripheries of the lens. All of this means that there's a potential for the use of faster, brighter, simpler lenses without the significant peripheral sharpness and light fall-off we've become accustomed to.
Sony also found that the bending process improves the sensor's fundamental performance. The strain introduced into the sensor upon bending widened the energy band gap and decreased dark current (residual electronic signal present in the absence of any external light). Both of these achievements should contribute to increased image quality.
|A curved sensor mimics the retina of a human eye, which effectively allows for simpler lens design. It also realizes significant image benefits by avoiding traditional problems encountered when attempting to record oblique light rays using pixel wells of finite depth on flat sensors. From Sony's recent paper "A Novel Curved CMOS Image Sensor Integrated with Imaging System". [Source: IEEE Spectrum]|
You may have noticed that the bottom panel in the figure above resembles a human eye. Indeed, a curved sensor can also be thought of as a form of biomimicry. Itonaga himself stated that the level of curvature achieved in Sony's new sensor is similar to that found in the human eye. The human eye has a relatively simple lens and a curved photosensitive surface known as the retina (the analog of a camera sensor). This allows our eyes to image flat planes in the real world with higher acuity than if our retinas had been flat, by - to a degree - avoiding the field curvature explained above.
While the team from Sony showed off images from the curved sensors integrated with lenses, it did not provide comparisons with equivalent systems employing traditional, flat image sensors. One caveat we feel compelled to point out is that an image sensor with a given amount of curvature must be matched to a particular lens such that the curvature matches the inherent field curvature of that lens (which changes with focal length, aperture, etc.). This means that such curved sensors will probably only find their ways into fixed-lens cameras in the near future. That is, until a point in time when the curvature of the sensors can be adjusted on the fly (if ever).
That said, the potential for simpler lenses in such fixed-lens cameras is quite compelling. It's interesting to note that last year Sony was awarded a patent for a number of lens designs, including a rather simple 35mm f/1.8 lens with 4 elements in 3 groups, shown below.
|In 2013, Sony was awarded a patent for a number of lens designs, including the rather simple 35mm f/1.8 lens paired with a curved image sensor shown above. [Source: Egami]|
We note that while the idea of the curved sensor itself isn't new - the image sensor array at the Kepler space observatory is curved to match the field curvature of the telescope, and John A Roger's group at the University of Illinois showed a prototype hemispherical electronic eye camera back in 2008 - Sony is the first to show working curved image sensors of these sizes integrated in an imaging system. More importantly, it seems that the technology from Sony is not just technology in development, but in fact appears to be almost ready for production. According to Image Sensors World, Itonaga has stated: "The team has made somewhere in the vicinity of 100 full-size sensors with their bending machine. We are ready."