I need to emit 20J over 5ms onto a target area of 962mm^2 from an LED source. The source as well as the target will reside in a completely encapsulated environment with reflective internal walls such that all of the energy will inevitably make its way to the target.
If I were to use a xenon flasher, rather than the LED bank, determining the output energy is quite easy: the bulb's data sheet provides the max input energy (electrical) to apply at the base to achieve the max output energy (in the form of light). Most xenon bulbs are 50% efficient at converting electrical energy into light energy so if I wanted 20J output I would purchase a bulb rated at 40J input and I would power it using the relationship E=0.5*C*V_c^2.
Does there exist a similar process for determining the output energy of an LED?