Instruments4Chem

High Performance, Low Cost…

Charge Coupled Devices (CCD’s) - Overview


CCD’s are optical detectors frequently used in spectroscopic instruments and to capture stunning images on astronomical telescopes. The pixels in a CCD can be thought of as light-sensitive “buckets” that store electronic charges when being illuminated. These “buckets” accumulate charge in proportion to both the exposure time and the incident intensity. The analogy here is a good one since once a pixel (bucket) is full it can store no more charge and the pixel is said to be saturated.

If a CCD chip is placed into an optical system so that different colours of light fall at different pixels on the detector, then the accumulated charges measured at each pixel position reflect the spectrum of the incident radiation.

The most complicated aspect of CCD operation is the sequential movement of these charges through an amplifier stage and the subsequent analog-to-digital conversion step so that a digitized spectrum can be obtained. Fortunately, these functions are made easy with our CCD processing engine. Go here to see a description of this and here to find out how to make a complete spectrometer system.
Stacks Image 1258
In this photo, sunlight is dispersed onto a wall by a diffraction grating. The pixels in a linear CCD sensor placed lengthwise across this spectrum would register the light intensities of the different colours, or wavelengths of light that are present.
Stacks Image 5258

The Sony ILX511 CCD Detector


A typical example of a linear CCD is the Sony ILX511, which is an array of 2086 pixels, each 14 μm wide and 200 μm high. This pixel format is ideally suited to spectrometers with a narrow, but tall entrance slit.

A metal film covers the first 32 pixels and the last 6 pixels of the array so these do not respond to light, leaving 2048 effective pixels. The film-covered pixels are called “dark pixels”. This sensor is extremely sensitive, allowing very faint optical signatures to be recorded if a long enough exposure time is employed.

A CCD processing engine described here is specifically designed for use with the ILX511, however similar linear sensors can be easily accommodated.

For further information on the Sony ILX511 2048-pixel CCD linear image sensor, read the product data sheet that can be found at http://www.datasheetcatalog.com/datasheets_pdf/I/L/X/5/ILX511.shtml.