My experiment from before should prove or disprove it. Long solid cylinder, small hole just bigger than lasers diameter, laser, someway to measure light. Measure light at beginning, then measure light at end. If they are not the same, then that might be BLT in effect.
If however then you get the same results with the tube in a vertical orientation, then you know it is not BLT.
See how much better it is when we accept each others arguments?