Question about photodiodes

Hi, I am working on a design to be able to sense the amount of light outputted by your typical photography flash and convert it to a voltage linear to the light output. I think I need to use a silicon photodiode but after that I am lost. The information on the flash I know is that it is rated at 150J for a full flash and the light output is in the 400-1000nm spectral range. I know placement of the photodiode is critical, but disregarding that for now, how can I relate flash power to actual light (radiant) power? In the photodiode datasheets I see sensitivities in A/W units but it doesn't make sense that the 150J (electrical) can be related to the light power. If it is, than I think the diodes would be destroyed from the light energy itself.

Also, I would think photovoltaic mode would be best, as I don't have to worry about the dark current, but I could be wrong and maybe photoconductive mode is best. Any information would be greatly appreciated. Thanks.

Generally a photodiode is operated in the photovoltaic mode in low noise application and Photoconductive mode when maximum speed is the concern.

THis doesn't seem feasible at all. How much light from the flash if going to strike the photodiode? 0.001%, 1%, 10% etc? It will be different depending on where the photodiode is placed. Moreover, you're also going to integrate a greaty deal of background light, unless you perform this in a darkened environment, or have a shutter exactly timed to the duration of your flash (you'll still integrate background light doing this, but not so much assuming the light from the flash incident on the diode is many times the light power of the background).

To calculate the power of light incident on the diode is fairly easy, you just need to know the area of the photodiode and the quantuim efficiency etc. which will be given with the data sheet. That will only give you the power of the light incident on the diode though, not the power odf your source.