What are Image Sensors?

In this article, we will learn about the different image sensors which are used in digital cameras and modern-day smartphones. We will also learn how this image sensor works. and at the end, we will compare this image sensor in terms of the different aspects. Two types of imaging sensors are quite commonly used in the imaging CCD, which is the charge-coupled device and the CMOS that is a complementary metal-oxide-semiconductor. Both the type of sensor consists of millions of photosites or pixels. These photosites convert the incoming light into a charge or an electron. Although these two sensors are quite different, they are common in many aspects.

Image sensor
Image sensor

These are the following steps that are being followed in both the sensors. These sensors, first of all, convert the incoming light into a charge. The pixels or photosites are exposed to the light for a certain amount of time, and during this time, the charge will get collected in these pixels. Once the charge is being collected by this Pixel, this charge is transferred for further processing. And after the transfer, this charge is converted into voltage and this voltage is being amplified using the amplifiers. These are the common steps that you see in both the sensors. But depending on the sensor, the sequence might vary.

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How does CCD Image Sensor work?

First of all, understand how these charge-coupled device or CCD works. These image sensors consist of millions of pixels. As these Pixels are exposed to the incoming light, these pixels will convert the incoming light into the charge and the charge will get accumulated in these pixels. So once the charge is getting collected by these pixels, this charge is being transferred using this horizontal shift register and this charge is being transferred into the vertical shift register. In this shift register, one by 1, each charge is being converted into a voltage, and after the voltage conversion, each voltage is being amplified using the amplifier. Once the vertical shift register gets emptied, the same procedure is followed for the remaining charge and one by 1, each charge is getting converted into the voltage and after that, it is being amplified. Once the charge of each pixel is converted into voltage and amplified, the output signal is being converted into the digital form using the analogue to digital converter. So in this way, one by 1, the charge of each Pixel is getting converted into the voltage and the same procedure is repeated converted into the voltage and the same procedure is repeated, this is the basic principle of the charge-coupled device.

Image sensor
Image sensor

How does CMOS Image Sensor Work?

Now let us similarly see how this simple sensor works. In the case of the CMOS sensor, the fabrication technology is very similar to the fabrication technology of the integrated circuits because of that, many peripheral circuits can be integrated inside a single chip. In the case, of the CMOS sensor, the charge two voltage conversion, as well as the voltage amplification, is carried out in the Pixel itself. And as the charge, two voltage conversion and amplification is carried out in the Pixel itself. So the processing speed of the CMOS sensor will be much higher than the CCD image sensors. In the CMOS sensor, the voltage that is generated by each Pixel is being read in a line by line fashion. First of all, the first row of the Pixel is being activated using this Pixel Select switch then this Pixel Select switch connects this output voltage of this Pixel to the column line. And by activating the column Select switch one by 1, we can read the data of each Pixel, and the same procedure is repeated for the remaining lines. In this way, in the CMOS sensor, the data is being read in a line by line fashion. So this is the overview of how CMOS sensor works.

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Image sensor
Image sensor

Now compare these two sensors in terms of system integration. The coupled device or the CCD is very old technology. In this technology, it is not possible to integrate the peripheral components like timers and DC into the main sensor. For these peripheral circuits, you will require an additional chip and the overall size of the CD sensor will get large. While in the CMOS sensor, as the fabrication procedure is very similar to the fabrication procedure of the integrated circuit, it is possible to integrate these peripheral components into a single chip. In the CMOS sensor, it is possible to have a camera on-chip or a system-on-chip and because of that, the CMOS sensor is quite compact.

Now let us compare these two sensors in terms of this power consumption. In the CCD sensor, we require different types of power supplies for the different timing clocks. And not only that, the typical voltages which are being required for the CCD is in the range of seven to 10 volt. The overall power consumption of the CCD sensor will be high. While in the case of the CMOS sensor, requires a single power supply. The typical voltage which is required for the CMO image sensor is relatively low, that is 3.3 to 5 volt. So overall power consumption of the CMOS image sensor will be lesser compared to the CCD image sensors. In the power consumption scenario, the CMOS sensor is preferred over the CCD sensors.

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Now let us compare these two sensors in terms of the processing speed. In the CCD sensor, the charge that is generated in each pixel is converted into the voltage one by 1. So overall processing speed of the CCD image sensor will be lesser compared to the CMOS sensor, but this processing speed can be further increased by using the multiple shift resistor. So in this way, by doing the parallel processing, we can increase the processing speed of the CCD sensor, but in this case, we will require the additional amount of hardware.

Now let us compare this image sensor in terms of this image distortion. In the case of the CCD image sensor, when you expose the sensor for a longer time, then you will see the effect that is known as blooming. Nowadays, by using this and blooming technique, we can reduce this blooming. While in the case of the CMOS sensor the most common type of distortion is to be known as the rolling shutter. So as we have already read earlier, in the case of the CMOS image sensor, the pixels are being read in the line by line fashion because whenever any fast-moving object is being captured by the CMOS sensor, then this rolling chatter effect is quite noticeable.

Nowadays, many of the CMOS image sensors are also coming with this global shutter. So as we have already read when we require a fast processing speed as well as low power consumption. In such cases, the CMOS Image sensors are preferred over the CCD sensors, while in applications value require a high dynamic range and a low amount of noise like a space application. In such cases, the CCD image sensors are preferred over this CMOS sensor.

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