How do you define laser beam size?

To better understand the subtleties of laser beam size, let's approach it from 3 angles.

  • What does laser beam "size" mean?
  • Where is size measured?
  • How are you defining the “start” and “end” of the laser?

What are you measuring?

The cross-section of a laser is the "spot" you see when you shine it directly at a wall. Beam size essentially measures how wide that spot is.

The 3 main laser beam shapes are:

  • circle: defined by diameter
  • ellipse: defined by major axis diameter and minor axis diameter
  • rectangle: defined by width and height

Once you have your beam size, you can get more information about the laser beam using the area & power density calculator.

Where are you measuring?

Laser beams change as they propagate. Not (only) because the experimental setup has problems, but because even perfect textbook lasers diverge.

This means that where the beam size is measured has an impact on the measurement.

Agreeing on a standard

Because measurement location impacts beam size, standard beam size characteristics are measured at the focal point. 2 important metrics are:

  • Spot size: the width of the beam at the focal point.
    This is the same thing as the beam diameter, except spot size has to be measured at the focal point.
  • Depth of field: a distance before & after the focal point where the laser's power density is still substantial (see the explanations in our beam spot size calculator for details).
    Depth-of-field gives you an idea of how tolerant the system is to variations in working distance.

Laser specifications change over time for many reasons and it causes problems accross all industries. Learn about how laser output measurement solves numerous problems in YOUR industry. Download the guide below.


Gentec-EO's high-accuracy laser beam measurement instruments help engineers, scientists and technicians in all sorts of laser applications from the factory to the hospital, laboratory and research center. Learn about our solutions for these measurement types:

How are you measuring?

The boundaries of a laser are not clear-cut.

Some lasers have what's called a gaussian profile, where the intensity is highest in the center of the beam and then tapers off gradually according to a specific formula. Other lasers known as flat-tops have a more "on or off" intensity distribution. All lasers can have irregularities in their profile.

This fuzziness poses a problem when trying to decide where exactly beam width measurements should start and end.

Defining clear boundaries

There are 4 common beam width definitions, which each give a different answer to the question: "where does a laser’s spot end?"

  • FWHM
  • 1/e2
  • D86
  • D4σ


FWHM (Full Width Half Maximum) defines beam width as the distance between the 2 points where the beam's intensity falls to 50% of its maximum.


The 1/e2 definition of beam width is the distance between 2 points where the intensity falls to 1/e2 (13.5%) of the maximum

For an ideal gaussian beam, the 1/e2 and D4σ definitions give the same result.


The D86 beam width is the diameter of a circle that contains 86% of the laser's total power.


The D4σ beam width definition is the international ISO standard. It is computed automatically by the BEAMAGE-4M beam profiler.

D4σ defines beam width as being 4 standard deviations wide.

D4σ is the only beam width definition that takes into account how the intensity varies (instead of just looking for one specific cut-off threshold).

Laser beam measurement

Beam size is just one of the parameters that can be measured in lasers. At Gentec-EO, we specialize in laser power & energy measurement and beam profiling instruments. Explore laser beam measurement solutions.

Gentec Electro-Optics is specialized in laser beam and terahertz source measurement and analysis. With an outstanding 50-year track record of innovation, developing and providing state-of the-art technologies to the laser market, Gentec-EO has become The Expert of the laser beam measurement field. For all sorts of laser applications from the factory to the hospital, laboratory and research center, Gentec-EO offers the broadest range of off-the-shelf and custom solutions, and stands ready to serve you now and in the future.
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