The widespread adoption of imaging systems in industrial applications continues to expand not only by the development of new image sensor technologies and products, but also by advances in support platforms such as computer power and high-speed data interfaces. Today, the use of imaging systems is common in a variety of areas, such as wiring inspection, traffic monitoring/enforcement, surveillance, and medical and scientific imaging, resulting in improved imaging performance, read speed, and resolution due to advances in image sensor technology. . As image sensors are now designed with charge-coupled (CCD) and complementary metal-oxide-semiconductor (CMOS) technologies, reviewing these two platforms is useful for selecting the best image sensor for a particular application.
The development of electronic imaging technology began in the 1960s, when Nobel laureates Boyle and Smith developed the first CCD. These components operate by converting the photons into electrons using the intrinsic ability of the doped germanium and measuring the light intensity using the resulting pixel-level charge. Architecturally, the biggest advantage of this design is simplicity. The entire pixel area can be used to detect photons and store charge, providing maximum signal levels and supporting high dynamic range.
The same pixel area is used to transfer charge to a finite output where the charge is converted to a voltage. Over time, this architecture has been refined to include the Interline Transfer CCD design, which includes an electronic shutter of the pixel level, eliminating the need for a mechanical shutter in the camera design. Today, CCDs are custom-made semiconductor processes that are highly optimized for imaging applications and require external circuitry to convert analog output voltages into digital signals for subsequent processing. In general, CCDs are typically characterized by efficient electronic shutter capability, wide dynamic range, and excellent image uniformity.
In contrast, CMOS image sensor designs were originally developed using processes developed for the fabrication of mainstream semiconductor components, such as those used in logic chips, microprocessors, and memory modules. This has the enormous advantage that digital processing functions can be incorporated directly into the wafer to enhance image sensor functionality. CMOS image sensors do not transfer charge to a limited output like a CCD. Instead, they place a transistor in each pixel (or each set of pixels) to convert between charge and voltage. In this way, voltage (rather than charge) can be transmitted through the entire component, making image reading faster and more flexible. In addition, high-end processing can be directly coupled to the wafer, and if desired, the image sensor can output fully processed JPEG images, even H.264 video streams.
Although CCD image sensors have historically provided better imaging performance than CMOS components, the gap has been greatly reduced in recent years, and the image quality that CMOS image sensors can provide is now suitable for a variety of applications. This can be seen in the latest generation of CMOS components for industrial imaging, such as the PYTHON CMOS image sensor family from ON Semiconductor.
Although some of the imaging parameters that the best CCDs can provide may still surpass this series, the image quality of these PYTHON components is already suitable for online inspection, traffic monitoring/charge, motion analysis, and more. This makes other performance advantages of CMOS technology more significant, such as faster frame rates, lower power consumption, and area of ​​interest (ROI) imaging. Every performance is critical to increasing production and supporting these applications.
Because of these inherent advantages, it is expected that CCD image sensors will eventually die out, because CMOS technology continues to advance and will eventually eclipse CCD performance in all aspects. However, CCD and CMOS technologies will continue to evolve in the future. The CCD infrastructure indicates that certain regions will continue to maintain specific performance advantages, making CCD the technology of choice for industrial applications requiring the highest imaging performance.
Although image uniformity continues to improve with advances in CMOS technology, the highest performance standards are still used in CCD image sensor applications. This is a direct result of these technical architectures: although CMOS components have thousands of individual amplifiers (one for each column, or even one for each pixel), the CCD can route charge from pixels to a single amplifier, and the sensor does not need to be read by Any amplifier to amplify the change. The high uniformity of images is important for applications such as medical and scientific imaging, and even critical product inspections where the quantification of these applications is key to providing clear, unprocessed images. In addition, the use of CCDs tends to be more uniform than CMOS components when scaling to high resolution and large optical formats.
The analogy of CCD design also allows CCD cameras to “fine-tune†specific end applications to optimize specific imaging characteristics. For example, for astrophotography applications, camera manufacturers can choose to fully optimize the capabilities of the sensor (expanding the dynamic range) at the expense of anti-spray (which may not be as important for this application). Other scientific imaging applications can also benefit from the extremely low dark currents provided by CCDs and may require exposure times of more than an hour to detect very weak signals.
Due to architectural advantages such as this, ON Semiconductor continues to invest in CCD technology and products. An important example can be found in the recently announced new CCD technology platform, which combines the imaging performance of the Interline Transfer CCD with the extremely low sensitivity that can be obtained from the Electron Multiplier (EMCCD) output.
The combination of interline TransferEMCCD allows a camera to simultaneously capture a portion of an image scene (such as an alley) at very low light levels (down to moonlight or even starlight) while the other part is under bright light (street lights). This performance allows an independent camera to capture light-level images from daytime to starlight, unique to CCD technology because it takes advantage of the charge multiplication of the EMCCD output, which is a feature that CMOS components are limited to beyond the operating voltage range. Products incorporating this technology feature 1080p resolution and a 30 fps frame rate for applications such as ultra-light illumination monitoring, scientific imaging, and medical imaging.
Although we are trying to identify a "winner" when comparing CCD and CMOS technologies, it really hurts both because each technology is unique and offers different end-user advantages. While products using CMOS technology are clearly more widely available, CCD image sensors still retain some advantages in some respects, making them more suitable for certain applications than CMOS components. Therefore, rather than finding the best technology, it is better to determine the key performance parameters of the particular end-use scenario under consideration, and then combine these key requirements with the characteristics and performance of the different products.
While in some cases, products based on one technology may provide the best match, it may be particularly important to work with companies that offer both technologies in order to gain an objective view, while others may not be so clear. By acquiring information on a broad product lineup based on both CCD and CMOS technologies, end customers can identify and select products that truly fit their specific end application and become true winners.
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