Help in finding the RPM of small shaft

Hi guys. I have a question for anyone willing to help out.

I want to find the rotational speed of a small shaft using electronic device(s). The area on the end of the shaft is a hemisphere and the diameter is around 5mm, the material is possibly aluminum.

Any suggestions are welcome. Thanks.

Mark the shaft with a white line and use a reflective IR-barrier (transmitter/receiver), e.g. CNY70.

It has a focussing distance of 0.3mm.

Attached is the datasheet.

Boncuk