When high speed is crucial the camera stops, the PSD starts!

Position Sensing Detectors (PSDs) and Charge Coupled Devices (CCDs → as used in cameras) are really two birds of another feather. Both have the ability to detect light but they do it in a different way:

• the analogue based PSD gives a direct output as function of central gravity point of the total light quantity distribution on the active area.
• the CCD detects the peak value of the light quantity distribution over the active area for each pixel and the values are sampled and processed sequentially

PSD

A PSD will measure the position of the centre of gravity of a light spot. That’s its most important feature. It does 1 or 2 pixels (1 or 2 dimensional) within ns with nm resolution. Accuracy of about 0.1% is achievable with a high dynamic light-range (several decades). This makes it suitable for a.o:

• vibration measurements
• high speed production line applications
• high speed profile measurements (triangulation)

CCD

The CCD by nature is suitable for imaging purposes, however it cannot measure the centre of gravity of a light spot without additional digital signal processing. Therefore, this type of high speed position  measurement will not be as readily available as in the PSD. Sampling and digitally processing all the pixels will add  time and make it (> 10.000 times) slower than the PSD (>MHz vs. <kHz). 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 neighbouring pixels must take place. This even further slows down the camera solution process.

Conclusion

An analogue based 1 or 2D PSD solution is by nature very fast compared to a CCD digital camera solution. In other words:

As ‘analogue is more or less infinite digital’. The advantages are evident!

Links

Seepos PSD signal processing tool
PSD applications
PSD school

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