Determining Spot SizeMinimum spot size for a given lens is obtained by balancing the effects of diffraction and spherical aberration. As an example, the spot size due to diffraction and spherical aberration for a 5.00” focal length meniscus ZnSe lens is plotted as a input beam diameter function in the graph below. A perfectly diffraction limited input beam (M2=1) is assumed. Also plotted is the sum of the aberration and diffraction spot sizes.
|Spot Size Aberration: Spot size due to aberration and diffraction|
The graph shows that spot size obtained by summing the aberration and diffraction contributions reaches a minimum value about 85µm at an input beam diameter approximately 25 mm. While simply summing the aberration and diffraction contributions may not be rigorously correct, it does provide what is probably a worst case estimate for actual spot size, and is generally an adequate criteria for choosing a lens. To summarize:
|Spot Size Equation|
The value of k is given for several materials at 10.6µm in the following table:
|Spherical Aberration Table|