The genesis of the modern image sensor can be traced back to the early 20th century when ‘image dissector tubes’, were pioneered by Philo Farnsworth, an American inventor and television pioneer. It was not until the 1960’s that significant strides were made in the development of solid-state image sensors, largely due to the advent of semiconductor technology.
One of the most noteworthy accomplishments in the field of image sensors is the invention of the charge-coupled device (CCD) in 1969 by Willard Boyle and George E. Smith, working at AT&T Bell Laboratories. The CCD is a semiconductor-based device that converts light into electrical signals, enabling the capture of high-resolution images. The invention of the CCD led to a revolution in the field of imaging, as it allowed for the development of digital cameras and other imaging devices, supplanting the earlier analog systems.
In the years that followed, the complementary metal-oxide-semiconductor (CMOS) image sensor emerged as a formidable competitor to the CCD. CMOS sensors were developed by Eric Fossum in the early 1990s, and these devices offered a range of advantages, including lower power consumption, higher integration of electronic components, and lower production costs. The ensuing competition between CCD and CMOS sensors led to rapid advancements in both technologies.
Important specifications that differentiate image sensors include resolution, pixel size, dynamic range, signal-to-noise ratio, and frame rate. The resolution refers to the total number of pixels within the sensor and is directly proportional to the level of detail captured in an image. Pixel size, on the other hand, influences the light-gathering capability of the sensor, with larger pixels often yielding better low-light performance. Dynamic range denotes the range of luminance levels the sensor can capture, while the signal-to-noise ratio is a measure of the image’s clarity, with higher values indicating reduced noise. Lastly, the frame rate is the frequency at which consecutive images are captured, which is crucial for high-speed and video applications.
In recent years, the focus of image sensor development has shifted towards achieving higher resolutions, improved dynamic range, and enhanced low-light capabilities. This has led to the emergence of back-illuminated sensors, which offer improved light-gathering efficiency, as well as the development of multi-layered, or ‘stacked’, sensors that aim to overcome the limitations of traditional planar designs.
The Charge-Coupled Device, or CCD, is a type of semiconductor-based image sensor conceived in 1969 by Willard Boyle and George E. Smith at AT&T Bell Laboratories. CCDs have been employed in an array of applications, ranging from digital photography and microscopy to astronomy and medical imaging.
The underlying principle of a CCD sensor is the conversion of incident light into electrical charges, which are subsequently measured and processed to produce a digital image. The sensor comprises an array of photosensitive elements, or pixels, fabricated from a silicon substrate. When photons impinge upon these pixels, they generate electron-hole pairs, which in turn produce an electrical charge proportional to the intensity of the incident light. Through a carefully orchestrated sequence of charge transfers, the sensor conveys the accumulated charges to an output node, where they are subsequently converted into voltage signals, digitised, and processed to yield the final image.
A salient feature of CCD sensors is their predilection for high image quality and sensitivity, owing to the manner in which they transport the accumulated charges. The precise and orderly transfer of charges from one pixel to another ensures minimal crosstalk, thereby reducing the amount of noise introduced into the image. Furthermore, the architecture of the CCD sensor allows for the use of a single output amplifier, which further contributes to the reduction of noise and the enhancement of image quality.
The advent of the CCD sensor has given rise to a plethora of applications in diverse fields. In the realm of digital photography, CCD sensors have been employed in early digital cameras, engendering a revolution in image capture and dissemination. The exceptional image quality and sensitivity of CCD sensors have rendered them particularly well-suited to scientific applications, such as astronomical imaging and spectroscopy. Here, the ability to capture faint signals with minimal noise is of paramount importance, and CCDs have played an indispensable role in advancing our understanding of the cosmos.
Another application of CCD sensors lies in the domain of medical imaging, where they have facilitated the development of sophisticated devices, such as digital radiography systems and confocal microscopes. In these applications, the high resolution and sensitivity of CCD sensors have enabled the acquisition of detailed and accurate images, thereby contributing to improvements in diagnostic accuracy and patient care.
The Complementary Metal-Oxide-Semiconductor (CMOS) sensor emerged as a potent competitor to the Charge-Coupled Device (CCD) sensor. Developed by Eric Fossum in the early 1990s, CMOS sensors have since permeated a diverse range of applications, from consumer electronics to cutting-edge scientific research.
At its core, a CMOS sensor is a type of solid-state image sensor that harnesses the principles of semiconductor technology to convert incident light into electrical signals, which are subsequently processed to produce a digital image. The sensor comprises an array of photodetectors, or pixels, fabricated on a silicon substrate. When photons impinge upon these pixels, they engender electron-hole pairs, the subsequent separation of which generates an electrical charge proportional to the intensity of the incident light. In contrast to the CCD sensor, which necessitates the sequential transfer of charges, the CMOS sensor allows for the direct measurement of charges within each pixel. This is accomplished through the integration of transistors, which serve to amplify and read out the charges, within the pixel itself.
The CMOS sensor boasts several advantages over its CCD counterpart, including lower power consumption, greater integration of electronic components, and reduced manufacturing costs. The direct readout of charges in CMOS sensors obviates the need for a complex sequence of charge transfers, thereby reducing the overall power consumption. Additionally, the architecture of the CMOS sensor enables the incorporation of ancillary electronic components, such as analog-to-digital converters and noise reduction circuitry, directly onto the sensor chip. This high level of integration not only reduces the complexity of the overall system but also contributes to a diminution in manufacturing costs.
The versatility and cost-effectiveness of CMOS sensors have engendered a wide array of applications in disparate fields. In the domain of consumer electronics, CMOS sensors have become the cornerstone of modern digital cameras, smartphones, and tablet devices, enabling the capture of high-quality images with minimal power consumption. The rapid proliferation of these devices has been facilitated, in no small part, by the advantageous attributes of CMOS sensors.
In the realm of scientific research, CMOS sensors have found utility in applications such as high-speed imaging and photon counting. The capacity for direct charge readout, coupled with the ability to integrate advanced electronic circuitry, renders CMOS sensors well-suited to capturing fast-moving events and discerning minute changes in light intensity. Examples of scientific applications that leverage CMOS sensors include the study of fluid dynamics, the observation of biological processes, and the detection of single photons in quantum optics experiments.
Other Sensor Technologies
The evolution of Charge-Coupled Devices (CCDs) and Complementary Metal-Oxide-Semiconductors (CMOS) has been accompanied by alternative sensor technologies. While these rival technologies exhibited promise, they ultimately failed to achieve widespread adoption, as they were unable to compete with the advantages offered by CCD and CMOS sensors.
One such technology is the Vidicon tube, an analog electronic device that was widely used in television cameras and closed-circuit television (CCTV) systems prior to the advent of CCDs. The Vidicon tube functions by converting light into electrical signals through a photosensitive layer deposited on a glass envelope. As incident light strikes the photosensitive layer, it generates an electrical charge, which is subsequently scanned by an electron beam, producing a voltage signal that is proportional to the light intensity. While Vidicon tubes were once prevalent, their susceptibility to image lag, low sensitivity, and limited dynamic range rendered them ill-equipped to compete with the superior performance of CCD sensors. Consequently, the Vidicon technology gradually faded into obsolescence.
Another imaging was the Image Orthicon tube. Developed in the 1940s, the Image Orthicon was widely used in television cameras and boasted superior sensitivity compared to its contemporaries. The Image Orthicon tube operates by converting photons into electrons within a photosensitive layer, which are subsequently accelerated onto a target electrode, producing an electrical charge. An electron beam then scans the target, generating a voltage signal that is indicative of the incident light intensity. Despite its initial success, the Image Orthicon was plagued by issues such as image distortion, high noise levels, and a propensity for producing halo-like artifacts around bright objects. The advent of CCD sensors, with their superior image quality and noise performance, heralded the decline of the Image Orthicon tube technology.
The Photoconductive Image Device (PID) is yet another example of a technology that struggled to compete in its time. PIDs are solid-state devices that employ photoconductive materials, such as selenium or amorphous silicon, to convert light into electrical signals. When light impinges upon the photoconductive layer, it induces a change in the material’s electrical resistance, which is subsequently measured and processed to produce an image. While PIDs exhibited potential in applications such as facsimile machines and document scanners, their limited resolution, sensitivity, and dynamic range, as well as their susceptibility to noise, ultimately hindered their ability to compete with CCD and CMOS sensors in the broader imaging market.
Back-illuminated sensors, also known as backside-illuminated (BSI) sensors, are a type of image sensor designed to improve light-gathering efficiency and overall image quality, particularly in low-light conditions. The key difference between back-illuminated sensors and traditional front-illuminated sensors lies in the arrangement of the photodiodes and the wiring on the sensor.
In a traditional front-illuminated sensor, the photodiodes are located beneath a layer of metal wiring and other components, which can partially block and scatter incoming light. This can result in reduced light-gathering efficiency and poorer low-light performance.
In a back-illuminated sensor, the arrangement of the sensor is flipped, positioning the photodiodes closer to the surface of the sensor, and moving the wiring and other components to the back. This design reduces the amount of incoming light that is blocked or scattered, allowing more light to reach the photodiodes and improving the sensor’s light-gathering efficiency.
Some advantages of back-illuminated sensors include:
- Improved low-light performance: With more light reaching the photodiodes, back-illuminated sensors can deliver better image quality in low-light conditions, with less noise and more detail.
- Higher sensitivity: The increased light-gathering efficiency allows back-illuminated sensors to have higher sensitivity, resulting in better image quality even when shooting at higher ISO settings.
- Smaller pixel size without sacrificing image quality: Since back-illuminated sensors can gather light more efficiently, they can maintain good image quality even with smaller pixels, allowing for higher resolution sensors without a significant loss in image quality.
- Better color reproduction: The reduced light scattering in back-illuminated sensors can lead to more accurate color reproduction, as the sensor can more effectively capture the incoming light’s color information.
Sony’s Exmor R technology is an example of a back-illuminated sensor, which has been used in various cameras and smartphones to achieve better image quality and improved low-light performance. Back-illuminated sensor technology has become increasingly popular, and many manufacturers have adopted it to improve the performance of their imaging devices.
Stacked Image Sensors
A stacked image sensor is an advanced type of CMOS (Complementary Metal-Oxide-Semiconductor) image sensor that has a layered design, where the pixel array and the processing circuitry are separated into distinct layers or “stacked” on top of one another. This design allows for faster readout speeds, higher resolution, and improved overall performance compared to traditional CMOS sensors.
In a traditional CMOS sensor, the pixel array and processing circuitry are located on the same plane, which can cause limitations in terms of readout speed, image quality, and noise reduction. Stacked image sensors address these issues by separating the photodiode layer (pixel array) from the processing circuitry layer. This arrangement enables the following benefits:
- Faster readout speeds: With the processing circuitry separated from the pixel array, data can be read out and processed more quickly, reducing the rolling shutter effect and enabling faster continuous shooting and high-speed video recording.
- Higher resolution: Stacked image sensors can achieve higher pixel counts without increasing the overall size of the sensor, as the processing circuitry doesn’t take up space within the pixel array. This allows for higher resolution images without compromising image quality due to pixel size reduction.
- Improved low-light performance: The layered design of stacked image sensors allows for larger photodiodes, which can capture more light and improve the sensor’s low-light performance. This leads to better image quality in challenging lighting conditions.
- Enhanced image processing: Stacked sensors enable more advanced image processing features, such as on-chip analog-to-digital conversion and high dynamic range (HDR) processing, which can result in better image quality and dynamic range.
Sony’s Exmor RS sensor is an example of a stacked image sensor technology, which has been utilized in various cameras and smartphones to achieve better image quality, faster readout speeds, and improved low-light performance. The adoption of stacked image sensor technology is expected to continue as manufacturers seek to improve the capabilities and performance of imaging devices in various applications.
Major camera image sensor manufacturers include:
- Sony: Sony is a leading global electronics company and a major player in the image sensor market. Their Exmor and Exmor RS series of CMOS image sensors are known for their high performance, low noise, and fast readout speeds, which are used in a wide range of consumer and professional cameras, including smartphones, mirrorless cameras, and DSLRs.
- Canon: Canon is a renowned Japanese multinational corporation specializing in the manufacture of imaging and optical products, such as cameras and lenses. Canon designs and produces their own image sensors, primarily CMOS, for their DSLR and mirrorless camera lines, as well as for some industrial applications.
- Samsung: Samsung, a South Korean conglomerate, is known for its consumer electronics, including smartphones and tablets. They produce their own image sensors under the ISOCELL brand, which can be found in Samsung’s smartphones and other devices. These sensors are known for their advanced pixel isolation technology that improves light sensitivity and color fidelity.
- OmniVision Technologies: OmniVision is a US-based company that designs and develops advanced digital imaging solutions, including CMOS image sensors. Their products cater to a diverse range of applications, including mobile phones, laptops, automotive, medical, and security devices.
- ON Semiconductor: ON Semiconductor is an American company that provides a wide range of semiconductor products, including image sensors. They offer a variety of CMOS and CCD image sensors for applications such as automotive, industrial, medical, and consumer electronics.
- Panasonic: Panasonic is a Japanese multinational corporation known for its wide range of consumer and industrial electronics. They produce image sensors, primarily CMOS, for various applications, including digital cameras, surveillance cameras, and automotive systems.
- STMicroelectronics: STMicroelectronics is a Swiss-Italian multinational electronics and semiconductor manufacturer. They produce a range of image sensors, including CMOS sensors, that are used in applications such as automotive, mobile devices, and IoT.