Electronic optical imaging sensors/Image sensors

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Electronic Optical Imaging Sensors/Image Sensors

Many electronic optical imaging sensors are CMOS-based image sensors. CMOS technology is commonly used in image sensors due to its efficiency, lower power consumption, and integration capabilities. The CMOS-based image sensors convert light into electrical signals, capturing visual information through an array of pixels. These sensors are widely used in various applications, including digital cameras, smartphones, automotive systems, and industrial imaging, due to their ability to integrate image processing and control circuitry directly on the chip, making them more compact and cost-effective compared to other technologies like CCD (Charge-Coupled Device) sensors.

The input of an imaging sensor is light (photons) from a scene, which may pass through a color filter array to capture wavelength-specific information at each pixel. The sensor converts this light into electrical signals, which are then processed and digitized to produce the output: a digital image composed of pixel values representing brightness and color.

Figure 0125a shows a detailed view of a CMOS image sensor layout, which is often used in high-resolution solid-state imaging devices. The key components and their roles in CMOS image sensors are:

Bayer Mosaic Filters: This color filter array is placed over the sensor pixels to allow only certain wavelengths of light (colors) to pass through. This layout captures color information by assigning red, green, and blue filters to the pixels in a specific pattern.
CMOS Active Pixel Sensor Color Imaging Array: This is the core of the CMOS sensor, where light is converted into electrical signals. Each pixel in this array is an active pixel, meaning it has its own amplifier, which helps improve the readout speed and reduces noise.
Analog Signal Processing: After the light is converted into an electrical signal, it undergoes analog processing to optimize the signal before digital conversion. This step is crucial for improving image quality by reducing noise and enhancing the signal.
Analog-to-Digital Conversion: The analog signal is converted into a digital signal, which is then processed further in the digital logic section. This conversion is essential for digital storage and further processing of the image.
Clock and Timing Control: This section coordinates the timing of various operations within the sensor, such as exposure and readout cycles, ensuring synchronized operation.
Pad Ring: This part of the sensor contains electrical connections that link the sensor to external circuits and systems. It allows the sensor to communicate with other components, facilitating data transfer and power management.
Digital Logic (Interface, Timing, Processing, Output): The digital processing unit handles tasks such as data formatting, noise reduction, and image enhancement. It also manages the interface and timing control for outputting the final image data.

CMOS image Sensor

Figure 0125a. Detailed view of a CMOS image sensor layout. [1]

An electronic optical imaging sensor consists of photonsensitive regions which are called pixels. Each pixel converts photons into electrons according to its incident light intensity, accumulates the generated electrons (charge collection) according to its well capacity during the exposure time, and transfers the collected electrons via a charge-to-voltage converter according to its bit depth and readout mechanism. Each of these processes, charge generation, collection, and transfer is also accompanied with various types of noise, which affects the quality of formed image. For instance, a dark current noise is caused by thermal effects during the exposure time and generates nonphotonic charges in the wells. The readout noise is added to the readout signal simultaneously while the collected charge is transferred from pixel to the on-chip amplifier and then, is converted into an analog voltage.

The cone-shaped cells inside human eyes are sensitive to red (R), green (G), and blue (B) which are the "primary colors". All other colors are combinations of these primary colors. In the conventional photography, the red, green, and blue components of light expose the corresponding chemical layers of color films. The new Foveon sensors are based on the same principle, and have three sensor layers that detect the primary colors as shown in Figure 0125b. In the color digital imaging, a mosaic of square tiles or "pixels" of uniform color basically are so tiny that it appears uniform and smooth to our eyes.

Principle of Foveon sensor

Figure 0125b. Principle of Foveon sensor.

[1] www.olympus-lifescience.com.

Integrated Circuits and Materials

An Online Book, First Edition by Dr. Yougui Liao (2018)

Electronic Optical Imaging Sensors/Image Sensors