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CCD Sensor Cameras for Machine Vision Systems

The CCD array is the most common sensor type currently used in machine vision. There are many different CCD formats, the common ones ranging from 1/4 inch to an inch. Oddly this does not indicate the sensor size but is another throw back to the days of vacuum tubes.

CCD Sensor Operation

Interline transfer CCD diagramThis diagram illustrates the general layout of the most common type of CCD array, the Interline Transfer CCD. The CCD is composed of precisely positioned light sensitive semiconductor elements arranged as rows and columns. Each row in the array represents a single line in the resulting image. When light falls onto the sensor elements, photons are converted to electrons, the charge accumulated by each element being proportional to the light intensity and exposure time. This is known as the integration phase. After a pre determined period of time the accumulated charge is transferred to the vertical shift registers.

In cameras conforming to the video standards mentioned above the charge transfer to the vertical shift registers is accomplished in two stages. Initially the charge in the odd numbered rows is transferred, followed by the even rows. Next the charges in the vertical registers are shifted into the horizontal shift register and clocked to the CCD output. Consequently all the odd rows are clocked out first (odd field) followed by all the even rows (even field). The rate at which the charge from the horizontal shift registers is clocked out is governed by the number of elements (pixels) per row and the video standard the camera complies with.

CCD camera lens diagramAn inherent problem associated with the interline transfer CCD lies in the fact that the vertical shift registers running across the array are insensitive areas and as such act as blind spots. One way of overcoming this is to fabricate micro lenses over each element thereby increasing the effective area of the cell. The lenses also help with the smaller format CCD. Because of the electrical characteristics of the semiconductor substrate on which the CCD is formed each cell has an absolute minimum separation from adjacent cells. Therefore smaller CCDs require smaller cells. Reducing cell size reduces the amount of accumulated charge, using lenses increases the incident light.

Frame transfer CCDs diagramAnother way of overcoming the problem caused by the vertical shift registers is to do away with them and utilize a different charge transfer mechanism. Frame Transfer CCDs do exactly that. This type of CCD has a separate storage area into which the charge is directly transferred from each cell. This process has to be performed rapidly in order prevent blurring as transfer occurs during the exposure time. Once in the storage area the charge can be clocked out in a similar manner to the interline transfer device.

CCD Characteristics

There are a number of key CCD variables that characterise a particular CCD. These are the size classification, pixel size, spectral sensitivity and signal to noise ratio.

CCD arrays come in a number of size formats as shown in the table below:


Width (mm)

Height (mm)

Diagonal (mm)

















As you can see from the table the CCD size is generally specified in inches. It is a measure of the length of the diagonal. However just because you have say a 1" CCD it does not mean you will have a CCD with a 1" diagonal. The size classification is another throwback to the Vidicon tubes used in sensors of old. Instead a CCD is assigned to the class whose diagonal is at least the length of the sensor. For example the Dalsa 2M30 mega pixel camera has a CCD with a 14.8mm diagonal, this will therefore be classified as a 1" CCD. The CCD size classification is important when selecting an appropriate lens and when considering the required field of view.

The pixel size is important on two fronts. Firstly the smaller the pixel the less light will fall upon it. This has the effect of reducing the sensitivity and may have implications on the type of lighting system used or the integration time. Secondly the aspect ratio needs consideration. For example, in a gauging application where the objective is to determine the distance between two features in the image a square pixel could easily render different results to a rectangular pixel, the rectangular pixel effectively stretches the image in the x direction. The image with rectangular pixels can obviously be compensated for but it increases the complexity of the gauging algorithm and consequently the inspection time increases too. In these applications it is best to use cameras whose CCDs have square pixels.

The spectral sensitivity is a measure of the response of the CCD to differing wave lengths. The response of a CCD is different to that of a human eye. Human eyes respond to wavelengths in the range 400 to 700nm whereas a CCD typically responds from 200 to 1100nm. This obviously includes infra red light. In applications where you are interested in the visible spectrum care needs to be taken to shield the work piece from excessive infra red otherwise the image could become over brightened or the sharpness could be compromised due to chromatic aberrations in the lens. On the other hand your application may be best suited to inspection in the infra red region. By using an infra red light source and a suitable band pass filter on the camera lens the system would become very tolerant to changes in ambient light levels.

Typical CCD spectral response chart

The signal to noise ratio is a measure of the dynamic range of the CCD. Most cameras operating in normal conditions have a signal to noise ratio of just under 50dB. This is generally represented by an 8 bit brightness level. (8 bits gives 256 levels, or roughly 48dB) The signal to noise ratio is often limited by the Dark current produced by the CCD. The dark current is caused by thermal excitation and typically doubles for every 7°C rise in temperature at the sensor and increases linearly with integration time. When using cameras with high signal to noise ratios it is important to carefully consider thermal management if the full range is to be reliably utilised.
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