Position Sensing Detectors (PSDs) and Charge Coupled Devices (CCDs) are really two different breeds of cats. Both have the ability to detect light but they do it in quite different manners. The PSD gives an output that is a function of the center of gravity of the total light quantity distribution on the active area. The CCD on the other hand detects the peak value of the light quantity distribution over the active area for each pixel and the values are put out sequentially.

PSDs are purely analog devices and rely on a current generated in a photodiode dividing in one or two resistive layers. This simple design gives the advantages of stability and reliability. The electronics needed for signal processing of the analog output is quite simple and can be implemented at low cost.

A CCD is basically an array of closely spaced MOS diodes. The light is recorded as an electric charge in each diode. Under the application of a proper sequence of clock voltage pulses the accumulated charges can be transferred in a controlled manner across the semiconductor surface to the output of the device. The much more complicated structure makes CCDs harder to manufacture and more prone to failures. The CCD gives a digital output.

PSDs will measure the position of the center of gravity of a light spot. That's about the only thing it can do, but it does it within nanoseconds with nanometers resolution. Accuracy of about 0.1% is achievable and the dynamic light-range is over several decades. Using stored reference points as a look up table can enhance this accuracy of the PSD several decades. Usually the optical components used along with the sensor will add distortion, which can be implemented into the look-up table and thus minimized. As the PSD provides the position-sensing information through the diodes' photo response, the device can be treated as a normal large area photodiode using standard method for signal processing such as for example using modulated light to avoid interference from stray light. A PSD can be manufactured to have any shape. Some odd examples are the helix, circular and spherical PSDs used for 2-D and 3-D angular measurements. For some applications (like for example surface inspection equipment) arrays s have been designed.

The CCD output contains information on the light quantity distribution all over the active area and thus describes a picture. A CCD is for example the normal choice for the picture catching element in video cameras. The CCD cannot measure the center of gravity of a light spot without additional digital signal processing and thus this type of measurement will not be as readily available as in the PSD. Sampling and digitally processing all the pixels will add some time and make it much slower than the PSD. On the other hand all the pixels have a mask defined positions so accuracy can be very high. However, in order to reach maximum accuracy and the highest resolution interpolation between neighboring pixels must take place. This further slows down the process. For light spots smaller than the distance between two adjacent pixels, interpolation is not possible and the signal is lost. This sets a lower limit for the spot size that can be used. The dynamic range of a CCD is limited and sudden shift in light intensity can give rise to blooming. This can be overcome by using the CCD sibling the CMOS arrays. These, newly introduced devices, overcome many of the CCDs weaknesses when it comes to dynamic range. An advantage of the CCD is that it like the human eye can store light received until time to measure it. This feature can come in handy when measuring minute light quantities.


As can be seen from the above the simplest, cheapest and fastest way to measure the position of the centroid of a light spot is to use a PSD. In many straightforward applications this is exactly what is done. Examples are alignment systems where the position of a reference laser beam relative to the PSD is measured. Such systems are used for alignment of everything from bridges to optical systems. As PSDs can be made to operate at very low temperatures (liquid nitrogen) this alignment method has also been applied to infrared optics where the infrared radiation from the PSD must be kept at a minimum. A weakness is that the PSD cannot differ between a direct beam and a reflected beam. It will just output the resulting center of gravity from the two spots. Using a CCD in this application gives the possibility to differ between direct hits and reflections by evaluating the signal strength in the light spots. Of course this will add to the complexity of the system and slow it down.

PSDs are widely used in displacement sensors systems using triangulation. Such a system can be made at a low cost using rather simple electronics. The downside is that the condition of the surface being measured can cause considerable variations in measurement values. Also the texture of the surface may distort the shape of the light spot used for measurements. This will shift the center of gravity of the light spot thus fooling the PSD. The CCD on the other hand detects the peak value and identifies this as the target position - uniformly and accurately. Using sophisticated signal processing such as optical filtering and synchronous detection together with a PSD may solve some "impossible" measurement tasks such as measuring the displacement of molten red hot iron or taking measurements inside the arc of a welding torch.

An area where the PSD will shine is in sensors where some sort of displacement needs to be monitored without loading the member moving. Examples are membranes in microphones, loudspeakers and pressure-sensors or optical fibers moving in wind sensors or accelerometers. Here nanometer resolution could be obtained using very simple analog circuitry.

If you need any further information please contact us directly.