Calculating Laser Beam Divergence

The laser beam divergence, measured in mRad or milliradians, is the change in the laser's beam diameter relative to distance. Ideally, for any laser, you want the beam divergence to be zero. That is, the laser's beam diameter does not change with distance. If the beam diameter at aperture is 1mm, the beam diameter at a distance of infinity remains at 1mm. While this would be ideal, it is nearly impossible to accomplish.

For most lasers, the beam divergence is positive, that is, the laser's beam diameter increases with distance. For example, our green lasers have a beam diameter of 1mm at aperture and beam divergence of 1.2mRad.

You can actually quickly approximate how a laser beam will expand by assuming the divergence as millimeters per meter. That is, for our green lasers, assume the beam diameter to increase 1.2 millimeters per meter. At aperture, the beam diameter is 1mm. At a distance of 1 meter, the laser spot should measure 2.2mm (the original 1mm + 1.2mm). At a distance of 2 meters, the laser spot should measure 3.4mm (the original 1mm + 1.2mm + 1.2mm). Since it is nearly impossible to achieve a divergence of zero, aim for lowest divergence possible.

How to Calculate Beam Divergence

Calculating the beam divergence of your laser is quite simple. When following the steps below, make sure you are wearing safety laser goggles.

Step 1: Mount your laser on a base. Make sure the laser cannot be moved as this will effect your beam divergence calculations.

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

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

Step 4: Shine the laser beam at the piece of paper 500 cm away and 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 the measurements taken in the above steps down on a piece of paper, plug the numbers into the following equation:

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