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Curved image sensors for better resolution, and wide field of view cameras for cell phones to Miniature UAVs

Smartphone camera technology is growing in leaps and bounds in the past couple of years and continues to be a major point to differentiate their products.  More and more technologies are being incorporated including  larger sensors, better lenses, ultra-thin lens, optical image stabilization technologies, dual camera optical zoom technology that also improve low light performance and remove noise. CMOS image sensor has been the revolutionary technology for smartphones that through smaller pixels, higher sensitivity and lower noise at a decreased cost has resulted in cameras that produce fast and high-quality images.

 

The development of curved image sensors may be the biggest advance in camera technology in decades, allowing for simpler, flatter lenses with larger apertures as well as dramatically better image quality.  NIKON, Sony and Canon are reported to be in race to develop and market curved sensor camera that operates using lens designs with fewer elements, less weight, less light loss, less internal reflection, less distortion and less aberration, all at lower cost. At present Sony is leading the race and already has a patent for a 35mm F1.8 compact camera.

 

Leti (Grenoble, France) has also introduced PIXCURVE, new curving technology for optical components that improves performance, enhances field of view and compensates for optical aberrations. PIXCURVE is a proof of concept for optical components such as microdisplays, visible imagers and cooled infrared sensors used in mobile phones, telescopes, medical-imaging tools, IR sensors and other imaging applications. In addition to improving performance, the curving substrate technology minimizes the vignetting effect that reduces brightness on the borders of images, and it makes cameras, imagers, and microdisplays lighter and more compact.

 

Now in the Optical Society journal Optics Express, researchers from Microsoft Research and research-and-development laboratory HRL Laboratories LLC, report that their new method can create image sensors with three times more spherical curvature than reported previously. They have been able to incorporate one of the sensors into a prototype camera. Compared to today’s high-end commercial single-lens reflex camera (SLR) cameras, the camera with the new sensor produced higher resolution images across the entire field of view.

 

“When using curved sensors, it is possible to correct aberrations in a much more efficient way, making it easier to create very wide angle lenses that produce sharp images across the entire field of view or to create fast lenses that produce better images in low light,” said Neel Joshi, a member of the research team. “It is also more straightforward to make cameras that exhibit uniform illumination across the entire image.”

 

“Our approach to curving commercially available image sensors could make it possible to have a new class of camera that would be very small, but have image quality that would be comparable to image sensors found in much larger cameras,” said Brian Guenter, leader of the Microsoft Research team. “In addition to improving consumer cameras, curved sensors could be used to create better cameras for surveillance, head-mounted displays and advancements in autonomous vehicle navigation.

 

Curved FPA’s have many potential military and security applications too. Small surveillance camera Mounted in a room or across the street from a door or other place where someone might want to monitor the movement of people, vehicles, and so forth and needs to conduct this surveillance over a wide FOV without edge distortion The sensor and imaging systems of Miniature UAVs ,and ground robots  need high-resolution and large FOV. Curved FPAs could be used to advantage here

 

Curved Sensor Camera

“A curved image sensor can potentially improve the chief ray incidence angle (CRA) on the sensor, as well as the aberration balancing, image quality, packaging, and manufacturing tolerance sensitivity, “explains Dmitry Reshidko and José Sasian in SPIE. Digital image sensors become less efficient when the incident light is at higher obliquity or larger chief ray incidence angle (CRA) on the sensor. The field of view (FOV) of the mobile camera is large, and the CRA proportional to the FOV. “However, if the image sensor is curved, the light rays get to the sensor in straighter lines and CRA is significantly reduced. Moreover, decreased incidence angles on the sensor reduce crosstalk between adjacent pixels,”

 

Comparatively, curved sensors can be paired with simpler, flatter lenses with larger apertures – that means more light reaches the sensors. Small f-number lenses provide better quality and low-light imaging, and can accommodate a larger number of sensor pixels, leading to better resolution.

 

And the process of bending CMOS sensors has a surprising advantageous effect. When you bend a silicon sensor you alter its band gap, lowering the ambient noise level caused by the dark current which continues to move through a pixel in the absence of light.

 

Pixcurve at CES: New Curved Image Sensor Technology Reduces Size, Weight & Cost of Digital Cameras – and Improves Image Quality

CEA-Leti will demonstrate its new technology for image sensors at CES 2019 that replicates the curve of the human retina. This curved image sensor technology breakthrough, called Pixcurve, requires fewer lens elements in digital cameras, which shrinks camera size by half and lowers costs – while improving image quality. Form Factor Reducing the number of lens elements in digital cameras from 10 to six reduces the size of the final compound lens by 60 percent. The overall length of the optical system also is shorter.

 

“The digital camera market has taken a huge hit from the massive adoption of smartphones, even though these bigger cameras deliver better image quality than smartphone cameras,” said David Henry, manager of packaging and assembling at CEA-Leti. “Pixcurve, which also reduces camera sizes in smartphones, microdisplays and virtual-reality glasses, provides camera manufacturers with cost effective, compact, easy-to-assemble optical components they need to bring their products to new levels of performance in the digital era.”

 

Improved Performance Curved image sensors reduce—and in some cases completely eliminate—optical aberrations like curvature of field and the vignetting effect. They also deliver increased brightness and a wider field of view. Cost Reducing the number of lens elements and eliminating aspheric lens elements, which will be unnecessary, will lower the cost of systems integrating Pixcurve technology. Assembly Fewer components means quicker and easier assembly—a major advantage for manufacturers.

Microsoft Develops a Curved Sensor

In The Optical Society journal Optics Express, researchers from Microsoft Research and research-and-development laboratory HRL Laboratories LLC, report that their new method can create image sensors with three times more spherical curvature than reported previously. They have been able to incorporate one of the sensors into a prototype camera. Compared to today’s high-end commercial single-lens reflex camera (SLR) cameras, the camera with the new sensor produced higher resolution images across the entire field of view.

 

To make curved sensors, the researchers placed individual sensors cut from a thinned CMOS image-sensor wafer into custom-made molds and then used pneumatic pressure to push each sensor down into the mold. Other attempts at curving a sensor have typically involved gluing the edges down and trying to push on the center of the sensor. However, this creates points of high stress that cause the sensor to shatter before it reaches the target level of curvature.

 

The researchers coaxed significantly more curvature out of the sensors by letting them float freely during the bending process, which allowed stresses to dissipate gradually. They also used a specially shaped mold that very slowly builds stress around the chip’s edges as it is pressed into the mold. Microsoft contracted HRL Laboratories, which has semiconductor fabrication capabilities and equipment, to help solve some of the specific physics challenges involved in bending the sensors.

 

Tests showed that curving the sensors did not change any of their electrical or imaging characteristics. When used in a prototype camera with a specially designed f/1.2 lens, a curved sensor exhibited a resolution more than double that of a high-end SLR camera with a similar lens. Toward the edges of the image, the curved sensor was about five times sharper than the SLR camera.

 

Although most cameras exhibit decreased light detection around the corners of the imaging sensor, the researchers showed that the curved sensors lost almost no light. This was a significant improvement compared to the decrease of around 90 percent measured for the commercial SLR camera.
“We showed that you can take an off-the-shelf sensor, curve it and dramatically improve the performance of the optical system,” said Guenter. “This can be done with relatively low costs and effectively no downside.”

 

The new approach for creating curved sensors grew out of a question the researchers asked themselves about seven years ago: “What would an ideal camera be like?” They decided such a camera would take pictures under very low light, be very small, and produce extremely sharp pictures.

 

Curved sensors for mobile phones

Although the prototype camera reported in the paper is about the size of a small consumer camera, the researchers say that the lenses could be made small enough for mobile phones and tablets. It should also be possible to build machines that could mass produce these curved sensors, allowing the additional processing to be incorporated into existing sensor manufacturing in a way that would amortize well in volume production.

 

The researchers are now working to see if further improvements might produce sensors with even more curvature. They also want to experiment with curving sensors that operate in infrared wavelengths, which could be useful for telescopes, 3D spatial mapping, biometric authentication and various scientific applications. Although they caution that it is unlikely that commercial products featuring the curved sensors will be available soon, they are interested in working with other companies to further improve the sensors and to perform the strenuous robustness testing that would be needed to prepare for mass production.

Sony’s curved CMOS sensor boosts light sensitivity

Announcing its technology in 2014, Sony engineers reported that they have created a set of curved CMOS image sensors using a “bending machine” of their own construction.  The engineers claimed their sensor possesses higher sensitivity to light compared to current flat sensors, 1.4 times more sensitive at the center of the sensor, and twice as sensitive at the edges.

 

The change in geometry allow the sensor to work with a flatter and a larger aperture lens, allowing more light to reach the sensor. Bending the sensor induces strain on the sensor, alters its bandgap that results in lower “dark current” flowing through a pixel, thereby improving the image quality further.

 

Equally important is the development of mass manufacture method, Sony engineers have produced in the vicinity of 100 full-size sensors with their bending machine.

DARPA’s HARDI program for development of Curved FPA

Digital cameras use a focal plane array (FPA) to convert light energy into electrical energy that can be processed and stored. The FPA is a two-dimensional (2-D) array of photodetectors (or pixels) fabricated on an electro-optical material. Used in this way, the term pixel refers to the actual physical detector in a camera.

 

The size and the number of the individual pixels affect the resolution and the readout speed of the camera. The more pixels a digital camera has, the more detail it can record when a photo is taken. Smaller pixels improve resolution, if the camera resolution is not optics limited, but may reduce sensitivity (i.e., need more time for required energy deposition). Modern digital cameras contain FPAs that have pixel counts on the order of megapixels. For example, cameras with 2 megapixels are becoming obsolete, cameras with 5 megapixels are in decline but still a good value, and cameras with 10 megapixels are in the mainstream.

 

In current digital cameras, a design constraint of optical systems is that the image surface (or Petzval surface) must be planar so that the image can be recorded on a planar silicon FPA. Placing the sensors (pixels) on a plane surface can lead to off-axis (spherical) aberrations and can limit the field of view (FOV) of optical systems unless exotic optics are used.

 

To correct for these distortions, designers have to use additional optical elements, which complicate the design of the optics and increase the cost of the cameras. These off-axis aberrations including spherical, astigmatism, field curvature, and coma.  To correct for these distortions, designers use additional optical elements, which complicate the design of the optics and increase the cost of the cameras.

 

Curved FPAs with simple spherical lenses would provide improved performance (vs. the commonly used planar FPA). These curved FPAs would provide a large FOV with better resolution off axis, would require fewer lenses, and would eliminate the need for image post processing. The systems that employ these curved FPAs have superior optical properties, but the curved FPAs are more difficult to manufacture.

 

To meet this challenge, the Defense Advanced Research Projects Agency/Microsystems Technology Office (DARPA/MTO) has instituted a program called the Hemispheric Array Detector for Imaging (HARDI). The program’s goal is to develop curved FPA technology and combine it with appropriate lens systems to enhance military capability. To accomplish this goal of creating a curved FPA, the HARDI program is exploiting the properties of organic and hybrid organic/inorganic semiconductor materials.

 

Curved FPAs remove the need for complex lens systems to correct off-axis distortion, and this improvement alone reduces the weight, size, and complexity of lens systems. From the multiple specific applications of curved FPAs identified by IDA (cameras mounted on small robots, miniature unmanned aerial vehicles (UAVs), and small surveillance cameras),

 

In support of the HARDI program, the Institute for Defense Analyses (IDA) performed technical assessments and provided planning assistance. IDA has identified promising applications of spherical FPAs that could potentially be exploited for defense-related applications.

Evaluate the potential benefits of introducing curved focal plane arrays (FPAs) into military systems:
• Identify systems that can be simplified and substantially improved by combining curved FPAs with simple, small lightweight lenses
• Identify innovative applications by combining curved FPAs with simple commercial lenses (e.g., feasibility of using rail-mounted, moving curved FPAs to provide passive three dimensional (3-D) target imaging (detection, location, identification))
• Identify potential application for using Gradient Index of Refraction (GRIN) lenses with curved FPAs to provide wide field of view (FOV) optical systems (e.g., gun and weapon sights)

 

HARDI management decided to focus first on the cameras used on small robots for the Advanced Mine Detection System (AMDS) Program.

The small robots have a problem with

• Tunnel vision. Like looking through a straw. Reduces information input to the operator and prevents robot from using required information for maneuvering

− Current solution. The developers of the optics and cameras try to alleviate this problem by adding cameras, but this solution adds weight, cost, and complexity
− New solution.Wider field of view (FOV) of curved FPAs would help reduce the lens system complexity, cost, and weight

• Misidentification of objects. Results from fuzzy images

− Solution. This problem could be attacked/helped with improved offaxis resolution

 

 

References and Resources also include:

http://www.securityelectronicsandnetworks.com/articles/2016/07/07/nikon-joins-sony-canon-curved-cmos-sensor-development

http://spie.org/newsroom/6327-current-trends-in-miniature-camera-lens-technology

http://www.androidauthority.com/future-smartphone-camera-tech-2016-679967/

https://phys.org/news/2017-05-breakthrough-sensor-image-quality-captured.html

http://www.osa.org/en-us/about_osa/newsroom/news_releases/2017/breakthrough_curved_sensor_could_dramatically_impr/

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