Is this an ANSI Z-136 type exposure limit?I have the problem of making "at home" or almost, a measure of a laser's pulse energy for unit area of the target: those kinds lasts for ~ tens of milliseconds, up to some hundreds of ms and I should be able to verify that this energy "density" doesn't go beyond 40 J/cm^2 for a single pulse (required by local laws concerning aesthetics lasers).
If this is a regulatory requirement, you must measure the pulse energy with "approved" procedures and equipment. Because you don't appear to require detailed information about the pulse shape or duration (but you must check this!), a simple calorimeter measurement is likely sufficient. There are many available devices on the market, but you may have to ensure it is calibrated, etc.