Charge Coupled Device (CCD) Prepared By: Sagar Reddy DOEACC B Level

Contents  What is CCD ?  Applications  Components of CCD cameras  Types of CCDs  Advantages of CCD cameras  Summary

What is CCD ?  A charge-coupled device (CCD) is a sensor for recording images in digital cameras.  It consist an integrated circuit containing an array of linked, or coupled, capacitors acting as many small pixels.  The light falling on a pixel is converted into a charge pulse which is then measured by the CCD electronics and represented by a number.  The number usually ranges from 0 (no light) to 65,535 (very intense light).

A CCD chip

Applications  CCDs are used in digital cameras, optical scanners and video cameras as light-sensing devices.  CCD cameras used in astrophotography.  CCDs are typically sensitive to infrared light, which allows infrared photography, night-vision devices, and zero lux (or near zero lux) video- recording/photography.  CCD are use to take exposures of galaxies and nebulae,.

Applications  They commonly respond to 70% of the Incident light (meaning a quantum efficiency of about 70%,) making them more efficient than photographic film, which captures only about 2% of the incident light. As a result CCDs were rapidly adopted by astronomers

CCD Camera CCD camera mounted on telescope

Components of CCD cameras  The CCD camera consists of three main parts. Camera body CCD chip Electronics

Camera body  The camera body is the mechanical part of the camera. It houses the CCD chip and protects it against unwanted light from outside. Some CCD cameras can also be mounted on the telescope.

CCD chip  A CCD (Charge Coupled Device) chip is a light sensitive device, made of silicon. It is an array of light sensitive pixels. When light falls on a pixel, it will be converted to a charge. This charge is captured in the pixel. It can’t go to other pixels. The more light falls on the pixel, the more charge will accumulate in the pixel.  The amount of charge is a measure of the amount of light that felt on the pixel.

The CCD chip seen in the camera. CCD Chip inside the camera

an image of a TC245 CCD chip representation of a small part of the light sensitive surface of the CCD chip. You can see it's made out of pixels

How a CCD chip registers image?  The telescope projects an image on the CCD chip. High intensity areas will produce more charge in the pixels than low intensity areas.  The amount of charge in every pixel makes the image.The computer can read this charge and convert it to an image.

The value in every pixel gives the charge, which is related to the light intensity.

How is an image made with a CCD chip?  There are three main phases: Clearing phase Exposure or integration phase Readout phase

1) Clearing phase  Mostly you don’t have a shutter in your camera, this means that there is always light falling on the CCD chip.  When you want to make an image, all existing charge has to be removed from the pixels. The CCD chip can shift lines downward with electrical pulses.  All charge in a line will move to the line below of it. This happens with all lines of the chip on the same moment.

Clearing phase  The lowest line will move the charge to a clearing line, where it will be removed.  When all lines are shifted away, there is no charge left anymore. This phase takes a few ms.

Clearing phase Charge clearing

2) Exposure or integration phase  The telescope projects the image on the CCD chip, so charge will accumulate in the pixels.  The longer you wait, the more light will be registered by the chip.  This phase controls the exposure time, also called: integration time,  because the chip integrates the amount of light in a certain period. With lunar and planetary photography it usually takes a few tenths of a second, but it can take minutes or even hours

The CCD array after 1 and 2 seconds exposure. The charge in the pixels accumulated with a factor of two.

3) Readout phase  To get the image in your computer, it will have to read the amount of charge of every pixel.  There is a device, the charge detection node, in the CCD chip that is able to measure the charge of a pixel.  Every pixel must be shifted in this device. The lowest line is able to shift pixels to the left in the direction of the device. So these pixels can be measured one by one.

Readout phase  When all pixels of the line are measured, all lines are shifted one line down.  Then again the lowest line can be measured. This will be repeated until all lines are measured.  The computer can read the measurements through the camera electronics. This phase can take a few seconds.

When the computer has collected all measurements, it can show an image of it.

Electronics  The electronics of the camera are the intermediate between the CCD chip and the computer.  The computer must be able to control the camera through its electronics.  To shift the lines on the CCD chip, it needs clock signals or pulses.  They must be generated by the electronics or, by the computer.

Electronics  Another important task is the conversion of the analog output signal of the charge detection node to a digital format the computer is able to read.  The conversion is done by an ADC (Analog to Digital Converter) and takes most time of readout phase.

Electronics  The CB245 camera needs two electronic units. The preamplifier This unit is mounted on the camera body. It amplifies the analog signal from the CCD chip and converts the clock signals. The interface card This unit connects the computer to the preamplifier. It converts the computer signals and also the analog signal from the CCD chip to a digital format.

Electronics interface card camera and preamplifier

Software  The software on the computer lets you control the camera.  This is called the acquisition software.  You don’t have to worry about clock signals, conversions, clearing and so on, the software handles it.  You only have to tell the software when and how long it has to make an image.  It also gives you the possibility to save an image to the hard disk, for later use.

acquisition software (Astro -Snap)

The most used types of CCD chips  Full frame device This is the most simple device. It is used in the CB211 camera. This type can’t be used with lunar and planetary photography without a shutter. Making images of this kind of objects typically requires an integration time of a few tenths of a second. The problem is the readout time, which can take a few seconds. During readout, the chip is still receiving light, which will destroy the original image, before it’s completely read by the computer.

The most used types of CCD chips  Frame transfer device With this kind of chip, the lowest half of the chip is protected against light. The image is only made on the upper half of the chip. When you are finished making your image, the image will be transferred from the upper half to the lower half, where it will be protected from light. This will take only a few ms. There is enough time, in this area, to read the image. A frame transfer device can be used with lunar and planetary photography.

Frame transfer device The TC245 is divided in 2 parts.

The most used types of CCD chips  Interline transfer device One line of every two lines is protected from light. During integration, only one line receives light. At the end, this line is shifted to the protected line, where it will be read by the computer. An interline transfer device can be used with planetary photography, Lunar photography will be more difficult, because of it’s brightness. The bright light can influence the protected line. The protected area of a frame transfer device is safer.

intensified charged-coupled device (ICCD)  is a CCD that is fiber-optically connected to a micro-channel plate (MCP) to increase the sensitivity. micro-channel plate  In ICCD Cameras a photo-cathode in front of the MCP converts photons to electrons which are multiplied by the MCP.  After the MCP a phosphor screen converts the electrons back to photons which are fiber- optically guided to the CCD.

intensified charged-coupled device (ICCD)  ICCDs are used in night vision equipment.  Besides the gain in sensitivity the possibility of getting the MCP also offers the possibility to gate ICCD cameras very fast.

Advantages of CCD cameras  Instant review of pictures, with no wait for the film to be developed  If there's a problem with a picture, the photographer can immediately correct the problem and take another picture.  Only successful pictures need to be printed. This means you can take many shots of the same scene but with slightly different settings, then choose the best one. Doing this with film would be too expensive

Advantages of CCD cameras  Minimal ongoing costs for those wishing to capture hundreds of photographs for digital uses, such as computer storage and ing, but not printing.  Images may be copied from one media to another without any degradation.  Digital cameras can be much smaller than film cameras of equivalent quality.

Advantages of CCD cameras  Ability to capture and store hundreds of photographs on the same media device within the digital camera; by contrast, a film camera would require regular changing of film (after say, every 24 or 27 shots).  Digital image files can be backed up to CD- ROM or DVD-ROM  Immediate image review and removal, lighting and composition can be assessed without wasting storage space.

Summary a brief review of what happens in a CCD camera, from beginning to end:  You aim the camera at the subject and adjust the optical zoom to get closer or farther away.  When you press lightly on the shutter release.  The camera automatically focuses on the subject and takes a reading of the available light.

 The camera sets the aperture and shutter speed for optimal exposure.  You press the shutter release all the way.  The camera resets the CCD and exposes it to the light, building up an electrical charge, until the shutter closes.  The ADC measures the charge and creates a digital signal that represents the values of the charge at each pixel.

 A processor interpolates the data from the different pixels to create natural color. On many cameras, it is possible to see the output on the LCD at this stage.  A processor may perform a preset level of compression on the data.  The information is stored in some form of memory device (probably a Flash Memory card).

Memory device Flash Memory card A memory stick

Micro-channel plate  A micro-channel plate is a device which intensifies radiation or particles by multiplication of electrons in small channels under the presence of a high electric field.  Electrons which travel inside these channels scatter frequently with the channel walls producing more and more secondary electrons.  This process amplifies the original signal by several orders of magnitude depending on the electric field strength and the geometry of the micro-channel plate.