Editing Beam diameter (section)

=== Knife-edge width ===
Before the advent of the [[charge-coupled device|CCD]] beam profiler, the beam width was estimated using the knife-edge technique: slice a laser beam with a razor and measure the power of the clipped beam as a function of the razor position.  The measured curve is the integral of the marginal distribution, and starts at the total beam power and decreases monotonically to zero power.  The width of the beam is defined as the distance between the points of the measured curve that are 10% and 90% (or 20% and 80%) of the maximum value.  If the baseline value is small or subtracted out, the knife-edge beam width always corresponds to 60%, in the case of 20/80, or 80%, in the case of 10/90, of the total beam power no matter what the beam profile.  On the other hand, the D4σ, 1/e<sup>2</sup>, and FWHM widths encompass fractions of power that are beam-shape dependent.  Therefore, the 10/90 or 20/80 knife-edge width is a useful metric when the user wishes to be sure that the width encompasses a fixed fraction of total beam power.  Most CCD beam profiler's software can compute the knife-edge width numerically.