CCD & CMOS Image CCD & CMOS Image SensorsSensors

Marcus BowdenMarcus BowdenBruno GarciaBruno Garcia

COSC 3P92 - SeminarCOSC 3P92 - Seminar

ContentsContents

1.1. What is an Image sensorWhat is an Image sensor

2.2. Possible uses for Image SensorsPossible uses for Image Sensors

3.3. CCD Image SensorsCCD Image Sensors

4.4. CMOS Image SensorsCMOS Image Sensors

5.5. Main Advantages/Disadvantages Main Advantages/Disadvantages between CMOS and CCDbetween CMOS and CCD

6.6. Questions and commentsQuestions and comments

What is an Image Sensor?What is an Image Sensor? An Image Sensor is a photosensitive An Image Sensor is a photosensitive

device that converts light signals into device that converts light signals into digital signals (colours/RGB data).digital signals (colours/RGB data).

Typically, the two main types in common Typically, the two main types in common use are CCD and CMOS sensors and are use are CCD and CMOS sensors and are mainly used in digital cameras and other mainly used in digital cameras and other imaging devices.imaging devices.

CCD stands for Charged-Coupled Device CCD stands for Charged-Coupled Device and CMOS stands for Complementary and CMOS stands for Complementary Metal–Oxide–SemiconductorMetal–Oxide–Semiconductor

Mirror Covering Image Sensor Mirror Raised

Sensor Exposed

Uses?Uses?

Image sensors are not Image sensors are not only limited to digital only limited to digital cameras.cameras.

Image sensors are used Image sensors are used in other fields such as:in other fields such as:• Astronomy, most notably Astronomy, most notably

in the Hubble Space in the Hubble Space Telescope Telescope

• Machine vision/sensingMachine vision/sensing• UV SpectroscopyUV Spectroscopy• Etc.Etc.

How Image Sensors WorkHow Image Sensors Work Both CCD and CMOS Both CCD and CMOS

sensors work by employing sensors work by employing photosensitive circuitry photosensitive circuitry that reacts to light and that reacts to light and stores the analog signals stores the analog signals as digital data, namely an as digital data, namely an image.image.

They both use different They both use different methods to achieve this.methods to achieve this.

First we will take a look at First we will take a look at CCD image sensors.CCD image sensors.

CCDCCD A CCD, or a Charged-Coupled Device, is a A CCD, or a Charged-Coupled Device, is a

photosensitive analog device that records light as a photosensitive analog device that records light as a small electrical charge in each of its pixels or cells. In small electrical charge in each of its pixels or cells. In essence a CCD is an collection of CCD cells.essence a CCD is an collection of CCD cells.

The signal captured by the CCD requires additional The signal captured by the CCD requires additional circuitry to convert the analog light data into a circuitry to convert the analog light data into a readable digital signal.readable digital signal.

This is mainly layers of capacitors called Stages This is mainly layers of capacitors called Stages which act as a way to transport the analog signal to which act as a way to transport the analog signal to an array of flip-flops which store the data all an array of flip-flops which store the data all controlled by a clock signal.controlled by a clock signal.

This is the definition of an Analog Shift Register.This is the definition of an Analog Shift Register.

CCDCCD When light strikes a CCD, it acquires an electrical When light strikes a CCD, it acquires an electrical

charge according to how much light has hit the charge according to how much light has hit the particular CCD cell.particular CCD cell.

Each CCD cell can transfer its charge to its Each CCD cell can transfer its charge to its neighboring cell and then off to external circuitry.neighboring cell and then off to external circuitry.

The charge is then read off by an analog-to-digital The charge is then read off by an analog-to-digital converter as an integer on a range of 0 to 4095 converter as an integer on a range of 0 to 4095 for most modern DSLR cameras. Lower ranges for most modern DSLR cameras. Lower ranges exist, such as 0-255, for lower quality cameras.exist, such as 0-255, for lower quality cameras.

How CCDs Record ColourHow CCDs Record Colour Each CCD cell in the CCD array Each CCD cell in the CCD array

produces a single value produces a single value independent of colour.independent of colour.

To make colour images, CCD cells To make colour images, CCD cells are organized in groups of four are organized in groups of four cells (making one pixel) and a cells (making one pixel) and a Bayer Filter is placed on top of the Bayer Filter is placed on top of the group to allow only red light to hit group to allow only red light to hit one of the four cells, blue light to one of the four cells, blue light to hit another and green light to hit hit another and green light to hit the remaining two.the remaining two.

The reasoning behind the two The reasoning behind the two green cells is because the human green cells is because the human eye is more sensitive to green eye is more sensitive to green light and it is more convenient to light and it is more convenient to use a 4 pixel filter than a 3 pixel use a 4 pixel filter than a 3 pixel filter (harder to implement) and filter (harder to implement) and can be compensated after a can be compensated after a image capture with something image capture with something called white balance.called white balance.

Ex. A Bayer filter applied to the Ex. A Bayer filter applied to the underlying CCD pixelunderlying CCD pixel

A CMOS SensorA CMOS Sensor

CMOSCMOS

A CMOS, or Complementary Metal Oxide Semiconductor, each pixel has neighboring transistors which locally perform the analog to digital conversion.

This difference in readout has many implications in the overall organization and capability of the camera.

Each one of these pixel sensors are called an Active Pixel Sensor (APS).

CMOSCMOS

The imaging logic is integrated on a CMOS The imaging logic is integrated on a CMOS chip, where a CCD is a modular imager chip, where a CCD is a modular imager that can be replaced.that can be replaced.

Because of this, design of a new CMOS Because of this, design of a new CMOS chip is more expensive.chip is more expensive.

However, APSs are transistor-based, which However, APSs are transistor-based, which means that CMOS chips can be cheaply means that CMOS chips can be cheaply manufactured on any standard silicon manufactured on any standard silicon production line.production line.

Pros and ConsPros and Cons CCDCCD

Needs extra circuitry Needs extra circuitry to convert to digital to convert to digital signalsignal

High dynamic range of High dynamic range of lightinglighting

Less noise due to less Less noise due to less on-chip circuitry on-chip circuitry

CMOSCMOS

Higher cost to developHigher cost to develop

On-chip analog-to-On-chip analog-to-digital conversiondigital conversion

Lower complexity on Lower complexity on the sensor leading to the sensor leading to faster image capturefaster image capture

Reduced power Reduced power consumptionconsumption

Noise and Dynamic RangeNoise and Dynamic Range

Works CitedWorks Cited http://www.dalsa.com/corp/markets/CCD_vs_CMOS.aspxhttp://www.dalsa.com/corp/markets/CCD_vs_CMOS.aspx

http://electronics.howstuffworks.com/question362.htmhttp://electronics.howstuffworks.com/question362.htm

http://www.axis.com/products/video/camera/ccd_cmos.htmhttp://www.axis.com/products/video/camera/ccd_cmos.htm

http://en.wikipedia.org/wiki/Image_sensorhttp://en.wikipedia.org/wiki/Image_sensor

http://www.kenrockwell.comhttp://www.kenrockwell.com

http://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Spectroscopy

http://www.nasa.gov/worldbook/hubble_telescope_worldbook.htmlhttp://www.nasa.gov/worldbook/hubble_telescope_worldbook.html

http://www.sensorcleaning.com/whatisasensor.phphttp://www.sensorcleaning.com/whatisasensor.php

Tanenbaum, Andrew S., Tanenbaum, Andrew S., Structured Computer OrganizationStructured Computer Organization. Amsterdam: . Amsterdam: Pearson Education, 2008. Pearson Education, 2008.