14.2 Data Capture Digital Imaging using CCDs

Capacitors  A capacitor is a device that stores charge.  Def’n of Capacitance - The ratio of the charge Q on one of the plates to the potential difference V between the plates.  C = Q/V  Unit is the Farad (1F = 1Cv -1 )

How they work

Charge Coupled Devices (CCDs)  A silicon semi-conductor microchip that can be used to electronically record an image focused onto its surface. The surface is divided into a large number of small areas called pixels  photography/digital/digital-camera2.htm

Builiding up a digital image using a CCD  The CCD is exposed to light energy (photons)  These strike the CCD and the light energy is converted into electrical energy (via the PE Effect)  The more incident photons on a pixel the more that pixel develops charge  The charge is collected in different pixels  The pixel behaves like a capacitor and stores the charge

Continued…  The p.d of each pixel is proportional to the charge built up during the exposure  This p.d can be measured and converted into a digital signal using binary code.  This information can be used to reconstruct the image as the information from each pixel has recorded the different light intensities in different parts of the image.

Limiting factors/Image parameters  Quantum Efficiency – A perfect CCD would emit one electron for every photon striking the surface. Practically, this is not achieved.  QE = N e /N p  Actual CCDs operate around 20-90% efficiency.  If QE is low not all photons striking the surface will register so it will reduce the quality of the image

Continued…  Magnification– Lenses must be used to focus the image of an object onto the CCD.  If the image formed is small then only a small number of pixels will be illuminated cause poor image quality  Magnification = Image Length / Object Length  Resolution- The amount of detail the image contains  Two points on an object may just be resolved if the points on the image are two pixels apart

Uses of CCDs  Digital Cameras  Video Cameras  Telescopes  Medical x-ray imaging  Scanners

Advantages of CCDs  Cheaper – doesn’t require film  Traditional film has low QE (~10%)  Easily enhanced and edited  Viewing can take place virtually immediately  Storage and archiving of a large number of photos is easy.