How you measure the beams diameter is arbitrary. You can measure it from maximum center intensity down to 0 edge intensity. Or full width half measure. It does not matter how you measure divergence because the divergence will remain the same as long as you start with a baseline measurement.
One way I found that I like is to acquire a picture frame glass. Mark off in an X,Y fashion two intersecting lines ticked every millimeter along both lines.
Start close to the aperture. Shine the laser through the glass and measure the spot size while looking through the glass from the opposite side. At 10 meters repeat. Repeat this every 10 meters to get an ever more accurate determination.
One way I found that I like is to acquire a picture frame glass. Mark off in an X,Y fashion two intersecting lines ticked every millimeter along both lines.
Start close to the aperture. Shine the laser through the glass and measure the spot size while looking through the glass from the opposite side. At 10 meters repeat. Repeat this every 10 meters to get an ever more accurate determination.
