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>   I assumed analog detection  is the right way to go.....or is it ?

Alan, I use a similar arrangement to measure propellor speed on
light aircraft, with a couple of differences

Inside the propellor head, approx 200mm diameter, are 6 strips
of adhesive aluminium foil on a painted black background. At
3000rpm this means 300 pulses/sec into b0 of a 16F628. INT
is the detection flag, and it works very well. The sensor collector
is pulled up to 12V (plane battery). Output from the resistor -
collector junction is normally high, and low over the foil. Pulses
go through a minimal filter and clamp, as shown

If you want to use an analogue input (ADC or comparator) I'd
suggest a low-high window to eliminate the false positive from
the transition. The difference between light and dark should be
distinct enough to do that successfully

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