How Image Sensor Works | Comparison of CCD & CMOS Sensors

In this blog post, 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 these image sensors in terms of the different aspects.

 

So, two types of imaging sensors that are quite commonly used in imaging are 

 

1. CCD (Charge Coupled Device)
2. CMOS (Complementary Metal Oxide Semiconductor)

 

Both types of sensors consist of millions of photosites or pixels. So, these photosites convert the incoming light into the charge of an electron.

 

Although these two sensors are quite different, they are common in many aspects.

 

These are the following steps that are being followed in both the sensors.

 

These sensors, first of all, convert the incoming light into charge. So, the photosites or the pixels 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 upon the sensor the sequence might vary.

 

Now let’s, first of all, understand how this charge-coupled device or CCD works.

How CCD Image Sensor Works:

 

As I said earlier, this CCD consists of millions of pixels. As these pixels are exposed to the incoming light, these pixels will convert the incoming light into a charge.

 

And the charge will get accumulated in these pixels. Once the charge is getting collected by these pixels, these charges are being transferred using this horizontal shift register.

 

And these charges are being transferred into the vertical shift register. In this shift register, one by one each charge is converted into 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 one each charge is getting converted into 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 analog to digital converter.

 

In this way, one by one the charge of each pixel is getting converted into voltage. And the same procedure is repeated for the next frame. So, this is the basic principle of the charge-coupled device.

 

Now, let us similarly see how this CMOS sensor works.

 

How CMOS Image 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 to voltage conversion as well as the voltage amplification is carried out in the pixel itself. So, the processing speed of the CMOS sensor will be much higher than the CCD sensor.

 

In the case of 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. This pixel select switch connects the output voltage of this pixel to the column line.

 

And by activating the column select switch, one by one we can read the data of each pixel. And the same procedure is repeated for the remaining lines.

 

So, in this way in the case of the CMOS sensor, the data is being read in the line-by-line fashion. This is the overview of how CMO Sensor works.

 

So, as we have understood how CCD and CMOS sensors work, now let us compare these two technologies.

 

Comparison of CCD & CMOS image sensors:

First, let’s compare these two sensors in terms of system integration.

 

1. System Integration

 

This charge-coupled device or CCD is very old technology. So, in this technology, it is not possible to integrate the peripheral components like timers and ADC into the main sensor. For these peripheral circuits, you will require an additional chip. So, the overall size of the CCD sensor will get large.

 

While in the case of CMOS sensors, 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.

 

So, in the case of the CMOS sensor, it is possible to have the camera on-chip or system-on-chip. Because of that, the CMOS sensor is quite compact.

 

Now let’s compare these two sensors in terms of power consumption.

 

2. Power Consumption

 

In the case of the CCD sensor, we require the different types of power supplies for the different timing clocks. And not only that the typical voltages which are being required for this CCD are in the range of 7 to 10 V. So, but the overall power consumption of this CCD sensor will also be high.

 

While in the case of this CMOS sensor, requires a single power supply. And not only that the typical voltage which is required for this CMOS sensor is relatively low. (That is 3.3 V to 5 V) So, the overall power consumption of this CMOS sensor will be lesser compared to the CCD sensor.

 

So, in the applications where power consumption is the main criteria, in such scenarios, the CMOS sensors are preferred over CCD sensors.

So, now let’s compare these two sensors in terms of the processing speed.

 

3. Processing Speed

 

As I said earlier, in the case of the CCD sensor, the charge that is generated in each pixel is converted into the voltage one by one. so, the overall processing speed of this CCD sensor will be lesser compared to the CMOS sensor.

 

But this processing speed can be further increased by using the multiple shift registers. 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 have an additional amount of hardware.

 

While in the case of the CMOS sensor, as we have seen, the charge to voltage conversion is carried out in the same pixel. So, the processing speed of this CMOS sensor will be higher compared to this CCD sensor.

 

And further, we can increase the speed of this CMOS sensor by using the multiple-column select lines. So, in this way, by doing the parallel processing we can further increase the speed of this CMOS sensor.

 

So, now let’s compare these two sensors in terms of noise and sensitivity.

 

4. Noise & Sensitivity

 

As we have seen, in the case of the CMOS sensor, the charge-to-voltage converter circuit, as well as the amplification circuit, are integrated into the same pixel. So, overall, the fill factor of this CMOS sensor will be less compared to the CCD sensor.

 

And because of that, the sensitivity of this CMOS sensor will be less compared to the CCD Sensor. And because of that, the dynamic range of this CCD sensor is quite high compared to CMOS sensors.

 

And not only that in the case of this CMOS sensor, but the amplifiers which are being used in each pixel also are not identical. So, because of that, you will see the non-uniform amplification. And that will act as additional noise.

 

Alright so now let’s compare this image sensor in terms of the image distortion.

 

5. Image Distortion

 

In the case of the CCD sensor, when you expose this sensor for a long time, then you will see the effect that is known as blooming. So, nowadays, by using this anti-blooming technique we can reduce this blooming.

 

While in the case of the CMOS sensor, the most common type of distortion that is to be known is the rolling shutter. So, as we have seen earlier, in the case of CMOS sensors the pixels are being read in the line-by-line fashion.

 

Because of that whenever any fast-moving object is being captured by this CMOS sensor, then this rolling shutter effect is quite noticeable. Like, in the case of this helicopter, the wing of this helicopter is straight.

 

But when it is being captured by this CMOS sensor then it looks like it is having curvature. So, that effect is known as the rolling shutter effect.

 

While in the case of a CCD sensor, all the pixels are exposed at the same time. This effect is not seen in the case of CCD sensors.

 

If we want to remove this rolling shutter effect in this CMOS sensor then all the pixels should be exposed at the same time, which is known as the global shutter. Nowadays, many of the CMOS sensors are also coming with this global shutter.

 

Verdict

So, as we have seen when we require the fast processing speed as well as the low power consumption in such cases the CMOS sensors are preferred over the CCD sensors.

 

While in applications where you require high dynamic range and low amount of noise, like space application, in such cases the CCD sensors are preferred over this CMOS sensor.

 

So, I hope in this article you understood the different types of image sensors.

Know more about image sensors on Wikipedia’s official page

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