A Guide to CCD and CMOS Sensor Technology

By soumitra, Gaea News Network
Thursday, April 30, 2009

cmosPerhaps on a digital camera spec sheet you have noticed a point mentioning that your camera has CCD or CMOS Sensor technology. And if you wanted to learn about it from the salesperson of the stores, you were told that it is some kind of chip that converts optical image into electronic signals which makes the digital image. If you want to know more about these technologies, we are ready to help you with our knowledge. Here is a guide to these two sensor technologies of digital camera.

Brief introduction to image capturing technique

ARTICLE CONTINUED BELOW

inside_cam
Pic Source : Axis.com
The Lens or Lens system captures and converges the light onto the Image sensors which converts the optical image to electronic analog and sends it to Image processor. The image processor creates the digital version of the image and transfers it the compression cheap where it is converted to specific format such jpeg or tiff etc. The CPU makes any correction to the images if necessary and stores it to Flash Memory. DRAM is used by the CPU for processing purpose. We may also like to view the images or videos as they are captured through the Ethernet interface.

Now we will see the different technologies of the Image Sensors

Basic CCD Technology

The main concept CCD or Charge Coupled Device is based on the fact that a CCD shows photoelectric effect when light falls on it. The charge acquired by a two dimensional array of capacitors are transferred a charge amplifier when light of the captured scene falls on it. The charge amplifier converts the charge into voltage. By repeating the process the whole image is captured and transferred to the next stage for further processing.

Basic CMOS Technology

CMOS or Complementary Metal Oxide Semiconductor Sensors are created using the same manufacturing technology which are used to make microprocessors. These chips contain transistors at each pixel position which amplifies and moves the charge using more conventional wires which is a more flexible technology as every pixel can be read individually.

A Comparison

ccdandcmos

Power Consumption

CCDs use more power compared to CMOS Sensors. The power consumption of a CCD may be more than 10 times than similar quality CMOS Sensors. This limits the use of CCDs where power consumption is an important factor. CMOS Sensor are widely used as mobile camera, webcam for this reason.

Noise

CMOS Sensors are more susceptible to noise than the CCDs.

Low Light Sensitivity

CCDs have better sensitivity in low light situation than CMOS. This is because some part of CMOS chip has the transistors for signal amplification which does not have sensitivity to incident light. On the other hand CCD can have more light as the whole surface can be used for light interception.

Production Cost

CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors. CCD sensors are made in a non-standard process and more complex to incorporate into a camera.

Uniformity

It means that each pixel produces same response with same amount of light. The CMOS sensors have amplifiers built into each pixel position. Everyone of the cannot be the same. So the uniformity is low in case of CMOS chips.

Speed

All camera functions can be placed on a single chip in case of CMOS sensors which is not possible in case of CCDs. CMOS wins this battle.

Smear

CCDs suffer from vertical smearing on bright light sources. In this example you can see the bright column of light in the middle of the picture. The bright headlight of the car blooded the entire row of sensors on the CCD.
headlight-smear
Below is a picture taken by a camera with CMOS sensor.
cmos-smear
And this one is taken by CCDs

ccd-smear
You can see the greenish lines running from the lights in case of CCD Sensors. CMOS are immune to smear effect.
Pic Source : dvxuser.com

Dynamic Range

It is the ratio of a pixel’s saturation level to its signal threshold. The CCDs have a dynamic range to nearly double to that of CMOS chips.

Windowing

When applying digital zoom, only the selected portion of the image can be extracted from the CMOS chip which is not possible for the CCDs. So, Windowing cannot be extensively applied to them.

Responsivity

The amount of output energy delivered by the chip per unit input of optical energy is called the responsitivity of a chip. CMOS chips are marginally superior to the CCDs in this respect as they can provide signal amplification at pixel level.

Conclusion

Based on the above points we can conclude that though both of the sensors have their pros and cons, both of them have their own application. CCDs tend to be used in high-end cameras which focus on good light sensitivity and more megapixels. On the other hand, CMOS sensors are being used in low end compact camera which keeps an eye on battery life and low price tags. As more and more research is done on these technologies, it is now possible to create cheap CCDs and high quality CMOS Sensors.

Discussion

Hasitha Liyanage

imad
October 29, 2010: 2:17 pm

i have a problem with my photo machine exactelly with the CMOS or the CCD and it broke and have to buy a new CCD and i live in algeria

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