Get PWM frequency of LED backlight

LED backlights use PWM for dimming, with a typical frequency of 0.1kHz to 1kHz. What I'd like is to detect the frequency using photo cells, have the arduino calculate the current average frequency over the past second(s), and display it on a simple LCD. The device should be completely self-contained, requiring no computer (other than for programming, obviously).

The main question I have is about the photo cells. I'm not sure if photo cells are fast enough to detect such a rapid change. I have noticed a "rise time" and "fall time" attribute in photo cell specs, ranging from ns to ms. Does this indicate reaction time?

The device would be put in a box, directly facing the LED backlight of a monitor, so there would be little to no interference.

If the project is feasible, I'd appreciate some advice on which arduino to use. Thanks!

What I'd like is to detect the frequency using photo cells,

Why?

I have noticed a "rise time" and "fall time" attribute in photo cell specs, ranging from ns to ms. Does this indicate reaction time?

Yes

If the project is feasible, I'd appreciate some advice on which arduino to use.

Any it doesn't matter for this.

If you could get into the display you could use the Arduino to measure the actual PWM via a pulse in.