I did not mention that photodiodes cannot be calibrated. I was assuming you had a home made photo sensing device and wanted to calibrate it. Manufactured photodiodes are undoubtedly pre-calibrated. Calibration is when you set two points of known values.
Lights off - zero intensity: photodiode produces a amount of current - calibrated as 0
Lights on - 100W light bulb: photodiode produces b amount of current - calibrated as x (where x is the known reference intensity - you can use physics to figure out what x is)
For the most part, what you do is increase the power of the light source. Say originally you dissipated 30W of energy into the light source, now you dissipate 60W into it. Electric power can easily be measured. Now your intensity is known. Intensity is a function of power and distance right.