Next up is determining how fast the spindle is spinning. I just followed Klass's method as I have exactly the same router - a kress fm 530. His instructions are here. He'd already done the hard work of finding a suitable sensor - the Vishay TCRT5000. I started off paining a thin white stripe on the plastic part of the spindle (sorry no photos as they all turned out pretty un clear). This didn't work, so I continued adding white paint and making the stripe wider and wider - I realised it didn't really matter how long the stripe was.

The resulting pulses looked like this, and worked out at the expected 30000RPM.