Laser Pointer - How to Measure Beam Divergence

Beam divergence, measured in milliradians or mRad, is the change in laser beam diameter relative to distance. If the beam divergence is 0 mRad, the laser beam emitted is parallel and the beam diameter does not change with distance. If the beam divergence is positive (most laser pointers have positive divergences), the beam diameter increases with distance. That is, if you measure the laser beam diameter at 5 meters, it maybe only 1 inch. However, if you measure the diameter at a farther distance, say 100 meters away, the laser diameter maybe increased to 3 inches. In ideal situations, you want a small laser beam divergence where the diameter grows slowly with distance.

Calculating Laser Beam Divergence

Calculating the beam divergence of a high power green laser pointer is relatively simple. When following the steps below, make sure to wear laser safety goggles.

Step 1: Mount your laser pointer on a base making sure the laser pointer cannot be moved. Moving the laser pointer around at this point will effect your calculations.

Step 2: Place a piece of paper on a wall 500 centimeters away. This distance can be represented by L in the equation below (L=500).

Step 3: Measure the beam diameter at aperture or where the laser beam exits the laser pointer. The measurement should be in millimeters (mm). This measurement is represented by D(i) in the equation below.

Step 4: Point the laser pointer at the piece of paper 500 cm away. Measure the laser beam diameter as it appears on the piece of paper. Again, this should be measured in mm. This measurement is represented by D(f) in the equation below.

Step 5: After writing down the measurements taken in the steps above, plug the numbers into the follow equation:

Laser Beam Divergence = 2 arctan ( (D(f) - D(i)) / 2 L)