Measuring light from xenon flashlamp?

What you need to do is get a photodiode, and capture the current through it as a signal when you fire the flash. You could use a digital scope for this.

Then you can use the datasheet for the photodiode to convert the current you see into a measure of light intensity.

You will need to adjust the results for the distance between the flash tube and the photodiode and you may also need to add neutral density filters between the flash tube and the photodiode to bring the light level down to a value that you can sensibly extrapolate the diodes data to (and you'll have to allow for them as well).

Expect a value in hundreds of thousands.

Remember that you'll be measuring light intensity, not total light output, so you may need to convert that again. You should really be working in standard units like lux and lumens, not candela and foot-candles.

edit: This article may give you some ideas of the numbers to expect.