Slit Diameter Measurement

The Slit Diameter (Slit_D) is defined as the distance between the two slit positions having a certain amount of the beam peak energy. 
Normally the reference value is agreed to be 86.5% of the peak energy of the laser beam.
One can define both Horizontal and vertical slit diameter in a following manner:
#1 Consider 1 pixel wide slit that is scanning across the image from left to right.
#2 If we calculate the peak energy through that slit at each position, then after full scan we can get Maximum energy (peak energy).
#3 Now we repeat the same scan until we reach 13.5% energy of the peak. The position found we mark as Point_1.
#4 Making scan from the right side to left of the image we can similarly find Point_2 at 13.5% peak energy.

The Slit Diameter can be defined as the difference between Point_1 and Point_2.

Slit_D = P2 - P1

The profile in the drawing below represents the energy contained in each pixel column of the image.

To determine the Vertical Slit diameter, perform the same process with a slit that scans from the top of the image to the bottom.
 

Knife Edge Width Measurement

The Knife Edge Widths are widths in Horizontal (X) and vertical ( Y) axes.  Using ISO standard we can define these as follows:

#1 We calculate total power of the beam.
#2 We start summing power in vertical laser profiles from left to right direction. When the sum reaches 16% of the total energy, we mark corresponding position Point_1.
#3 We start summing power in vertical laser profiles from right to left direction. When the sum reaches 16% of the total energy, we mark corresponding position Point_2.
#4 By ISO definition, the Knife Edge Widths are defines as:
 
K = 2 * (P2 - P1)

The profile in the drawing below shows the energy contained in each pixel column of the image.

Vertical Knife width is we can determine in a similar manner summing horizontal profiles power.

Please refer to ISO documentation for more information.
 
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