If you are previewing and you see an intensity fluctuation, then you are most likely seeing the light source interacting with the camera's exposure. To test this is the case, perform the following:

• Use a constant light source that does not flicker with the AC power.
• Set the exposure to a multiple of a 60Hz cycle (or 50Hz depending on your country), ie 1/60 = 16.7ms. Any multiple of 16.7ms should reduce the fluctuation.