[vtech]

When you say it is a frequency, it IS a TTL pulse that can be used as a trigger to control another device such as a micro controller.
You are already looking at the right place.
Instead of going the discrete way, why not experimenting with any of the TSL12.13,14 etc. You have a photodiode & op-amp/driver already packaged & they are in the microsecond region. Depending on the video & it's luma(brightness) content, you just may have to tweak the TSLxx sensor's position or use some sort of tiny magnifier lense.
http://www.taosinc.com/category.asp?cateid=20

[ynwia]

The project is to detect the onset/offset of pictures on a computer monitor (CRT or LCD). Monitor starts with a totally dark background, then a 300x300 picture appears for a few hundred milliseconds then switches to darkness, then another 300x300 picture, then dark background again...

Once the circuit detects optical change(darkness to pic, pic to darkness), it either immediately outputs a TTL pulse or changes it current status like a flipflop. The response time should be as small as possible and 1 millisecond is definitely the upper limit. (microseconds or even nanoseconds would be ideal)

I already made a simple circuit with TAOS TSL230R. It kind of works except its output is frequency (not a single pulse or constant voltage) and I have no idea of its response time.

The options in my mind Idea are

1.Arrow Go analog. For example, Idea Photodiode->op amps->voltage regulator (7805 ).

2.Arrow Digital. For example, Idea Photodiode->op amps->microcontroller (pic 12f675 )

Which one do you suggest?
Any specific parts/circuit solutions to recommend ?

Any info is highly appreciated.