Panasonic Develops Technology for Highly Sensitive Image Sensors Using Micro Color Splitters

Osaka, Japan - Panasonic Corporation has developed unique "micro color splitters", which separate the light that falls on image sensors by exploiting light's wavelike properties. Applying them to actual image sensors allows bright color images to be achieved even under low-light conditions. This development makes color filters unnecessary by using the micro color splitters that control the diffraction¹ of light at a microscopic level. Panasonic has achieved approximately double the color sensitivity in comparison with conventional sensors that use color filters.

Image sensors are used in devices like smartphones, digital still cameras and video cameras, as well in security, vehicle parking, office, and healthcare applications - anywhere, in fact, that digital imaging is needed. Conventional color image sensors use a Bayer array², in which a red, green, or blue light-transmitting filter is placed above each sensor. These filters block 50 - 70% of the incoming light before it even reaches the sensor. Progress is being made in increasing the resolution of image sensors used in mobile and other devices by reducing pixel size, but demand for higher-sensitivity cameras is also increasing. Panasonic has developed a new technology that can be applied to existing or future sensors to enable them to capture uniquely vivid color images.

The developed technology has the following features.

1. Using color alignment, which can use light more efficiently, instead of color filters, vivid color photographs can be taken at half the light levels needed by conventional sensors.
2. Micro color splitters can simply replace the color filters in conventional image sensors, and are not dependent on the type of image sensor (CCD³ or CMOS⁴) underneath.
3. Micro color splitters can be fabricated using inorganic materials and existing semiconductor fabrication processes.

This development is based on the following new technology.

1. A unique method of analysis and design based on wave optics that permits fast and precise computation of wave-optics phenomena.
2. Device optimization technologies for creating micro color splitters that control the phase of the light passing through a transparent and highly-refractive plate-like structure to separate colors at a microscopic scale using diffraction.
3. Layout technologies and unique algorithms that allow highly sensitive and precise color reproduction by combining the light that falls on detectors separated by the micro color splitters and processing the detected signals.

Panasonic holds 21 Japanese patents and 16 overseas patents, including pending applications, for this development.

This development is described in general terms in the Advance Online Publication version of Nature Photonics issued on February 3, 2013.

More on the Technology

1. Unique method of analysis and design based on wave optics permitting fast and precise computation of wave-optics phenomena

FDTD⁵ is widely used to analyze light in wave form, but its heavy computation workload has up to now made it impractical for designing micro color splitters. On the other hand, BPM⁶ is an effective method of fast computation, but it has lower precision than FDTD and cannot accurately simulate color splitting. This prompted Panasonic to develop a practical and original design method that permits fast and precise computation of wave-optics phenomena. This technology allows the precise modeling of optical phenomena such as reflection, refraction, and diffraction by modeling spaces in regions with different optical constants and applying BPM to the spaces. This method can be applied not only to the design of micro color splitters, but can be extended to the design of other nano-scale optical processing systems.

2. Device optimization technologies leading to the creation of micro color splitters that control the phase of the light passing through a transparent and highly-refractive plate-like structure and use diffraction to separate colors on a microscopic scale

Color separation of light in micro color splitters is caused by a difference in refractive index between a) the plate-like high refractive material that is thinner than the wavelength of the light and b) the surrounding material. Controlling the phase of traveling light by optimizing the shape parameters causes diffraction phenomena that are seen only on a microscopic scale and which cause color separation. Micro color splitters are fabricated using a conventional semiconductor manufacturing process. Fine-tuning their shapes causes the efficient separation of certain colors and their complementary colors, or the splitting of white light into blue, green, and red like a prism, with almost no loss of light.

3. Layout technologies and unique algorithms that enable highly sensitive and precise color reproduction by overlapping diffracted light on detectors separated by micro color splitters and processing the detected signals

Since light separated by micro color splitters falls on the detectors in an overlapping manner, a new pixel layout and design algorithm are needed. The layout scheme is combined and optimized using an arithmetic processing technique designed specifically for mixed color signals. The result is highly sensitive and precise color reproduction. For example, if the structure separates light into a certain color and its complementary color, color pixels of white + red, white - red, white + blue, and white - blue are obtained and, using the arithmetic processing technique, are translated into normal color images without any loss of resolution.

Notes:

1. Diffraction

Behavior of light as a wave on the wavelength (nanometer) scale. Various phenomena occur when a wave encounters an obstacle.

2. Bayer array

The arrangement of color filters used in most single-chip digital imaging sensors used in digital cameras, camcorders, and scanners to create a color image. The filter pattern is 50% green, 25% red and 25% blue.

3. Charge Coupled Device Image Sensor (CCD sensor)

A type of solid-state image sensing device for digital imaging, used in digital video cameras of all types. It has higher sensitivity and lower noise than other sensing devices.

4. Complementary Metal Oxide Semiconductor Image Sensor (CMOS sensor)

A solid-state image sensing device for digital imaging using CMOS.

5. Finite-Difference Time-Domain method (FDTD)

FDTD is a versatile modeling technique used to solve Maxwell's equations by spatial and temporal discretization.

6. Beam Propagation Method (BPM)

A numerical analysis technique in electromagnetics for solving the Helmholtz equation under conditions of a time-harmonic wave.

Figure 1 Constitution and features compared with the conventional method

<Conventional method using a color filer>	<Developed method using a micro color splitter>

Figure2 Comparison of images using the same sensitive CCDs

About Panasonic

Panasonic Corporation is a worldwide leader in the development and manufacture of electronic products in three business fields, consumer, components & devices, and solutions. Based in Osaka, Japan, the company recorded consolidated net sales of 7.85 trillion yen for the year ended March 31, 2012. Panasonic's stock is listed on the Tokyo, Osaka, Nagoya and New York (NYSE:PC) Stock Exchanges. The company has the vision of becoming the No. 1 Green Innovation Company in the Electronics Industry by the 100th year of its founding in 2018. For more information on Panasonic, its brand and commitment to sustainability, visit the company's website at http://panasonic.net/.