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Image Sensor

A CMOS sensor is an image sensor that is used in digital cameras or camcorders. Its photosensitive parts convert the incoming light into voltage using transistors that are located on the pixel. A distinction is made between on-chip A/D conversion and off-chip A/D conversion.

CMOS sensors are active pixel sensors, which are responsible for light measurement as semiconductor detectors. Each element has an amplifier circuit for a signal readout. The use of CMOS technology makes it possible to integrate additional functions into the sensor chip, such as exposure control or contrast correction.

Image Sensor
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Where Is It Used?

Image sensors are used in digital cameras, video cameras, reflex cameras, and smartphones. But they are also becoming increasingly important in industrial image processing. They are additionally supplemented by microlenses on the pixels, which enlarge the light-sensitive surfaces. In addition, they are enhanced by Backside Illuminated CMOS sensor technology: this way, exposure is performed from the backside through a very thinly manufactured substrate. By integrating the readout electronics on the sensor, the designs become more compact, more power efficient and less expensive. In industrial image processing, a reduction of blooming effects and an increase in frame rates is noticeable. The blooming effect is a fringe of light on the image caused by bright and overexposed image areas.

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How Does a CMOS Sensor Work?

The abbreviation CMOS stands for "Complementary Metal Oxide Semiconductor". It is an electronic circuit based on the photoelectric effect that converts photons into electric charges. The transmission of this information happens for each individual photodiode via an amplifier so that each pixel is read electronically.

The Advantages and Disadvantages of Image Sensor Technology

The integrated evaluation of electronics per pixel provides numerous advantages. On the one hand, the power consumption is considerably reduced and on the other hand, the camera is smaller, since the evaluation electronics are located on the same chip. Compared to CCD sensors, higher frame rates are also achieved. In addition, the blooming effect is significantly reduced. In the NIR range, i.e. in the range of short-wave infrared radiation, a higher sensitivity is also achieved. In addition, the sensor can be read out flexibly by directly addressing the individual pixels.

As a disadvantage, in certain cases a poorer light sensitivity can be observed, resulting in increased image noise when images are taken with lower brightness.

The Use of Image Sensors in Pre-Press Inspection

CMOS sensors are used in industrial image processing. This image processing software is used in the context of artwork & prepress inspection by EyeC and meets the high demands of the printing industry for image quality. The ultra-fast scanners of the EyeC Proofiler software solution read the print samples and, thanks to the high-quality image sensors, detect all kinds of defects, such as color deviations, stains, or smudged passages. With this high-quality software, you can check texts, graphics, Braille, and 1D and 2D codes. Errors on all types of packaging, such as folding cartons, package inserts or labels, can be detected and corrected during pre-press, thereby optimizing the entire printing process.

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The Differences between CCD Sensors

The main difference between CMOS and CCD sensors is the way the pixels are read.  With the CCD sensor, the charge is transported over the whole chip and an analog converter converts each pixel into a digital value. In contrast, CMOS sensors have several transistors for each pixel: these amplify the charge, and the transport takes place via traditional lines. Currently, about 75% of the image sensors used are CCD sensors and around 25% use CMOS technology. The CCD has the advantage of lower readout noise and high imaging quality, while the CMOS technology scores with the elimination of any smear and blooming effects.

Both types of image sensors are used in a wide range of applications, for example in the field of medicine. We encounter CCD technology mainly in bioscience, where it is needed in microscopy technology to ensure high image quality. CCD sensors are also used in all kinds of applications that require long exposure times.

CCDs use a so-called global shutter mechanism, in which all pixels on the CCD are exposed simultaneously. Conventional CMOS technology, on the other hand, uses rolling shutter technology, in which the exposure of the pixels takes place line by line. This produces the dreaded distortion effects. In recent years, however, the global shutter mechanism has also been further developed for CMOS. It is used for 3D scans because of its low energy consumption and low cost.

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Ashley Madden

EyeC America

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