Techniques of reading PFM from a sensor


I am using a UVtron sensor with an arduino to detect UV light.

The signal that is relayed from the sensor is Pulse Frequency Modulation(PFM). The more intense the source of UV the more frequent a pulse will be sent out and the pulse width does not vary.

I have been able to set off a simple alarm when the input pin sees a high state. What I want is to reliably read the frequency so I can set up a software filter to ignore lower frequency signals.

Any idea on how to accomplish this?

A thought just came to mind, what if I try to time the low spots in between pulses?