A 'diffraction-free' light distribution does not change its transverse intensity
pattern as it propagates through a homogeneous medium. Hence, it would be extremely
useful as a probe in optical metrology, specifically for 'light sectioning'. Is such a
'light plane' possible? How much confined can light be, without being spreaded by
diffraction? We discuss these questions. It turns out that a light plane cannot be
generated by monochromatic, coherent illumination. However, it is possible to generate a
light plane incoherently, by a scanning axicon beam.
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