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"Power Point Presentation of this article" courtesy Ed Dinkler (USA)

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1. What is a PSD?
PSD (Position Sensing Detector) is an opto-electronic device which converts an incident light spot into continuous position data. It provides outstanding resolution, fast response and excellent linearity, for a wide range of light intensities and simple operating circuits.
The basic feature and advantage of a PSD device is that it gives continuous data readings.

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2. What is a CCD?
CCD (Charge-coupled device) is a sensor consisting of an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbors.
The basic feature and disadvantage of CCD device is that it is a matrix of On/Off sensors that are switched on and off by lights.

3. Why PSD is more accurate than CCD?
Because CCD is a matrix of dots switched on and off, the resolution of CCD sensors depends on how many dots are located on the sensor.
Typically a linear CCD sensors has 1024 dots.
Enhanced versions of CCD sensors have 2048 dots.
Just imagine that modern digital cameras are using 2048x2048 = 4 millions pixels.

1024/2048 linear pixels correspond to a 10/11 bits digital resolution respectively.

PSD is a continuous sensor, therefore the digital resolution of a PSD depends not on the PSD itself (as it is "continuous" that is it has an infinite resolution) but on the digital system that "reads" the PSD.
FASEP alignment sensors are equipped with a 16-bit digital conversion system, that gives a resolution of 65536 points.
This gives PSD sensors 32 to 64 times more accuracy then competitor's products using CCDs.

4. Why PSD is faster then CCD?
Because linear CCDs have a low resolution, alignment sensors using CCDs have to be programmed to do multiple measures at every step to improve measurement accuracy and to lower measurement "noise".

To have the same accuracy of a PSD, CCD should perform no less then 32 measures and hence calculate the average measurement.

Of course, it takes a lot of time to do 32 measurements in a sequence (even if we are dealing with milliseconds).
Most designers would settle for 4 to 8 measurements to balance speed and accuracy.
In these case you end up having slower speed from the sensors (4 to 8 times slower), and still less accuracy (8 to 16 times) then PSD.

Also, PSD sensor need an expensive circuit design including Analog-to-Digital conversion.
CCD on the other side is already "digitalized" so all it needs it is "speed" to make more measurements at the same time to improve accuracy.